Fluent Inc. Logo return to home
next previous contents index

5.3 Turbo Operations

5.3.1 Overview

GAMBIT turbo operations allow you to model flow scenarios that involve turbomachinery components such as fans or turbochargers. The purpose of such operations is to create and mesh a turbo volume—that is, a model composed of one or more real volumes that together represent the flow environment in the region surrounding an individual turbomachinery blade. The turbo volume always includes boundaries that represent the hub, casing, inlet, outlet, and blade and may also include boundaries that represent a splitter—a turbomachinery component attached to the hub the purpose of which is to direct flow between the blades.

This section describes the functions and procedures required to perform basic turbo modeling. It also describes the operations that GAMBIT employs to create and mesh the turbo volume. Specifically, it includes:

Turbo Component Types

For GAMBIT modeling purposes, turbomachinery configurations are considered to consist of four component types (see Figure 5-17):

Figure 5-17: GAMBIT turbo components

The hub and casing define the inner and outer radial boundaries of the turbo flow environment. Blades are attached to the hub and are used to move or direct fluid through the environment. Splitters (not shown in Figure 5-17) are optional components that are attached to the hub and located between the blades. They are typically used to direct flow through the environment in proximity of the blades.

Turbo Functions

The purpose of the GAMBIT turbo operations is to create and mesh a model that represents a section of the flow environment surrounding an individual blade. Specifically, the GAMBIT turbo functions allow you to perform the following operations.

The following example illustrates the application of these operations to the creation and meshing of a simple turbo model. For detailed descriptions of the functions associated with the operations, see Section 5.3.2, below.

Turbo Modeling Procedure

In GAMBIT, the most basic procedure of modeling of a turbomachinery configuration such as that shown in Figure 5-17 involves the following steps.

Step
 Operation Description
1
 Define and create the turbo profile Specify edges that describe the shapes and locations of the hub, casing, blades, and (optionally) splitters and specify the axis of revolution. Employ the specifications to create a wireframe shell that is used to define the shape of the turbo volume.
2
 Create the turbo volume Create a set of one or more real volumes that encompasses the region surrounding an individual turbo blade.
3
 Assign turbo zone types Specify faces or sets of faces that comprise standard turbo zone types-for example, hub or casing.
4
 Decompose the turbo volume Apply a standard face decomposition template to the hub, casing, and any intermediate faces of the turbo volume and modify the decomposition, if necessary, to facilitate meshing.
5
 Mesh the turbo volume Create a mesh throughout the turbo model.
6
 View the turbo volume Display the turbo volume in a standard turbo view.

As a general example of the procedure described above, consider the turbo configuration shown in Figure 5-17, above. The configuration consists of a hub and casing the curved surfaces of which are aligned with the axis of rotation, eight turbo blades, and no splitters. Each turbo blade is curved, like an airplane wing and tapered toward the tip, and all are slightly skewed with respect to the axis of rotation. The base of each blade is flush with the curved surface of the hub, and a small clearance exists between the casing and blade tips.

The following sections employ the configuration described above to describe and illustrate the steps required to create and mesh a turbo volume. They also describe steps that are not immediately related to the modeling of this example but which illustrate general GAMBIT turbo modeling options.

Step 1—Defining and Creating the Turbo Profile

The first step in the GAMBIT turbo modeling process involves defining and creating a turbo profile. A turbo profile is a wireframe that GAMBIT uses to define the shape of a turbo volume. To define the turbo profile, you must specify edges that describe the turbo components and an axis of revolution that determines the general shape of the configuration. When you create a turbo profile, GAMBIT uses the edge and axis specifications to create the wireframe.

NOTE: In GAMBIT, turbo profile creation and turbo volume creation are separate operations, and each employs its own specification form. The operations are separated in GAMBIT so that you can manipulate the created profile, if necessary, and thereby control the shape and meshing characteristics of the turbo volume.

Defining the Turbo Profile

To define a turbo profile, you must specify the following information:

For the turbo configuration shown in Figure 5-17, the profile can be defined by the collection of real edges shown in Figure 5-18. The collection consists of 14 edges. Two of the edges describe the shapes and locations of the hub and casing. The other 12 edges comprise two sets of six edges each, and each set defines a planar cross section of the blade. The centermost vertices on the inlet and outlet tips of the blade cross-section edge sets are shown as the leading and trailing vertices, respectively, in Figure 5-18.

Figure 5-18: Edges used to define a turbo profile

In this example, the edges that define the hub and casing are aligned with the turbomachinery axis of revolution, which, in this case, is the x axis of the global coordinate system. As a result, they will produce hub and casing faces the curved surfaces of which are aligned with the axis of revolution.

Each of the blade cross sections shown in Figure 5-18 consists of six connected edges. Two of the edges in each set define the large curved surfaces on the pressure and suction sides of the blade; the other four edges in each set define small curved surfaces at the inlet and outlet blade tips. The blade cross sections in this example represent planar cuts through an individual blade at two distinct radial distances from the rotational axis.

NOTE (1): In general, GAMBIT does not require the blade-cut cross sections to be planar.
NOTE (2): Each set of edges that describes a blade cross section must consist of either two, four, or six edges. GAMBIT does not allow you to specify odd numbers of edges to describe a blade cross section, nor does GAMBIT allow you to specify more than six edges in a set.

Specifying Edges That Describe the Profile

In GAMBIT, the edges that describe a turbo profile are specified by means of their inlet vertices—that is, the vertices on the side that is nearest to the expected inlet to the flow region. For example, Figure 5-19 shows the vertex specifications required to define the turbo profile for this example in terms of the input fields on the Create Turbo Profile form. The Create Turbo Profile specifications for this example are as follows:

Figure 5-19: Turbo profile vertex specifications, no splitter

Figure 5-20 shows a set of vertex specifications similar to those shown in Figure 5-19 but including vertices to define a splitter (Splitters vertices) as well as a turbo blade.

Figure 5-20: Turbo profile vertex specifications, including splitter

NOTE: To facilitate creation of the turbo volume (see below), GAMBIT smoothes each profile edge by replacing its geometry definition with a simple NURBS representation. To perform the smoothing operation, GAMBIT generates sampling points at equal intervals in model space on the original geometry. You can control the smoothing operation by means of two default variables:
  • GEOMETRY.EDGE.NUM_SAMPLING_POINTS—specifies the number of sampling points
  • TURBO.GENERAL.SMOOTH_BLADE_PROFILES—specifies whether or not the edges are smoothed during turbo volume construction
For instructions regarding the specification of GAMBIT default variables, see Section 4.2.4 in the GAMBIT User’s Guide.

Specifying the Axis of Revolution

In addition to the vertex specifications described above, the turbo definition requires specification of an axis of revolution. The axis specification is made by means of the Vector Definition form, which is accessed by means of the Axis:Define pushbutton on the Create Turbo Profile form. (For a complete description of the Vector Definition form and its specifications, see Section 2.1.4 of this guide.)

In this example, the x axis of the global coordinate system serves as the axis of revolution for the turbo profile, therefore the correct Vector Definition form specifications are:

NOTE (1): The importance of the axis direction specification on turbo volume construction depends on whether or not the turbo configuration includes a splitter. The dependence can be stated as follows.
  • For configurations that do not include a splitter, the axis direction does not affect the turbo volume construction in any way. (NOTE: The axis direction does affect the graphical orientation for the turbo views (see "Step 6—Viewing the Turbo Volume," below).)
  • For configurations that do include a splitter, the axis direction must be specified such that the splitter follows the turbo blade on a rotational path defined by the right-hand rule. For example, in Figure 5-20 (above), the Positive X axis specification defines the rotational direction shown in the figure—in which the splitter follows the turbo blade. An axis specification of Negative X would define a rotational direction opposite that shown in the figure, and the splitter would precede the blade, therefore Negative X constitutes an invalid axis specification for this example.
NOTE (2): In this example, the hub edge is located at a distance of 10 units from the x axis of the global coordinate system (not shown). The distance between the axis of revolution and the hub and casing edges, in conjunction with the pitch specification, determines the angular width of the hub and casing faces, respectively. GAMBIT does not allow you to specify an axis of revolution that is coincident with either the hub or casing edges.

Creating a Turbo Profile

When you apply the specifications described above and execute the Create Turbo Profile command, GAMBIT creates the turbo profile shown in Figure 5-21. The turbo profile is a wireframe configuration that GAMBIT uses to define the turbo volume. In addition to the blade cross-section edge sets, which are identical to those specified in the turbo profile definition, it consists of the following components:

Figure 5-21: Turbo profile

GAMBIT uses the inlet and outlet rail edges to define the inlet (front) and outlet (back) faces, respectively, of the turbo volume. Similarly, GAMBIT uses the medial edges to define the periodic (side) faces of the turbo volume. The bottom and top faces of the turbo volume are defined by the hub and casing edges, respectively, and by the uppermost and lowermost blade cross sections (see below).

Inlet and Outlet Rail Edges

The inlet and outlet rail edges are real, circular edges that are formed by revolving vertices about the axis of revolution. The innermost and outermost rail edges are formed by revolving the endpoint vertices of the hub and casing edges, respectively (see Figure 5-21). If the turbo profile specification includes more than two sets of blade cross-section edges, GAMBIT creates intermediate rail edges, as well. The intermediate rail edges are formed by the following process.

  1. Project the leading and trailing vertices for each intermediate blade cross section onto imaginary discs defined by the innermost and outermost inlet and outlet rail edges, respectively.
  2. Create a vertex at each projection point.
  3. Revolve the projected vertices about the axis of revolution to create the intermediate rail edge.

As a result of this process, each blade cross section is associated with its own corresponding set of inlet and outlet rail edges. For example, if the defining sets of edges for this example include a third set of blade cross-section edges located halfway between the other two, the resulting turbo profile appears as shown in Figure 5-22.

Figure 5-22: Turbo profile for three blade cross-section edge sets

Medial Edges

Medial edges are virtual edges the center portions of which pass through the approximate middles of the blade cross sections (see Figure 5-21). They are used in the turbo volume creation process to define the shapes of the periodic faces that serve as the side boundaries for the turbo volume. The endpoint vertices of each medial edge lie on (and are hosted by) the inlet and outlet rail edges corresponding to the edge.

Because medial edges are virtual edges, you can change their shapes by sliding their endpoint vertices along their respective host rail edges. By sliding the endpoint vertex of a medial edge and thereby changing its shape, you can alter the shapes of the periodic faces for the turbo volume.

NOTE: When you slide the endpoint vertex of a medial edge, GAMBIT changes the shape of the edge subject to the constraint that the medial edge must pass through both the leading and trailing tips of the blade cross section that corresponds to the edge.

Step 2—Creating the Turbo Volume

A turbo volume is a set of one or more real volumes that, together, enclose the flow region immediately surrounding the turbomachinery blade. Figure 5-23 shows a simple turbo volume based on the turbo profile shown in Figure 5-21. The turbo volume shown in Figure 5-23 consists of a single real volume that encompasses a blade-shaped void. (For examples of turbo volumes that consist of more than one real volume, see "Specifying the Tip Clearance" and "Specifying the Number of Spanwise Sections," below.)

Figure 5-23: Example turbo volume

Figure 5-24 shows a turbo profile similar to that shown in Figure 5-23 but including a splitter the characteristics of which are defined by the Splitters specifications shown in Figure 5-20.

Figure 5-24: Example turbo volume, including splitter

NOTE: GAMBIT creates passage-to-passage turbo volumes. In a passage-to-passage turbo volume, the turbo volume represents a section of the flow region that completely encompasses the turbo blade, and the blade is represented by a blade-shaped void in the center of the volume. The shapes of the periodic (side) faces of the turbo volume represent projections of the turbo profile medial edges. If the turbo profile includes a splitter, the splitter also manifests as a void in the turbo volume, and the void is completely encompassed by the boundaries of the model.

Turbo Volume Specifications

The shape, size, and characteristics of any turbo volume are determined by its turbo profile and by the following specifications:

The pitch determines the angular (lateral) width of the turbo volume. The tip clearance specifies the distance between the casing and the tip of the blade. The number of spanwise sections specification determines the number of individual volumes that comprise the entire turbo volume.

Specifying the Pitch

The pitch determines the angular width of the turbo volume. GAMBIT allows you to specify the pitch angle either directly (in degrees) or in terms of the total number of blades attached to the turbomachinery hub.

As an example of the effect of the pitch specification on the width of the turbo volume, consider the turbo volumes shown in Figure 5-25, each of which is based on the turbo profile shown in Figure 5-21. Figure 5-25(a) shows the turbo volume created with a pitch angle of 45° (8 blades). Figure 5-25(b) shows the turbo volume created with a pitch angle of 36° (10 blades).

Figure 5-25: Effect of pitch on turbo volume size

The 8-pitch turbo volume is wider than the 10-pitch turbo volume, but in both cases, the void representing the turbo blade is located in the center of the volume.

Specifying the Tip Clearance

The tip-clearance specification allows you to account for clearance between the turbo blade tips and the casing. You can specify the tip clearance either in terms of a uniform distance from the casing or by means of an edge the swept shape of which truncates the tip of the turbo blade.

GAMBIT turbo volumes that include a tip clearance are composed of three or more real volumes that represent two distinct regions of the model. Two of the volumes define the region of the turbo volume between the blade tip and the casing. The other volume (or volumes) define(s) the region immediately surrounding the blade.

Turbo Volume Regions

When you specify a tip clearance, the resulting turbo volume consists of two distinct regions (see Figure 5-26):

Figure 5-26: Turbo volume with tip clearance

The clearance and blade regions represent, respectively, the parts of the model domain that exist above and around the turbo blade. The clearance region consists of two real volumes and extends in thickness between the tip of the blade and the casing. The blade region consists of one or more real volumes that surround the void that represents the turbo blade and extends in thickness from the hub surface to the bottom of the clearance region. Figure 5-27 illustrates the three volumes that make up the clearance and blade regions for the turbo volume shown in Figure 5-26—that is, the plug volume, outer plug volume, and outer blade volume.

Figure 5-27: Clearance and blade regions

Clearance Region

The clearance region consists of two connected volumes: the plug volume and the outer plug volume. The plug volume (Figure 5-27(b)) constitutes the region immediately above the blade tip. Its shape represents an extension of the blade tip through the clearance region, and its thickness extends from the blade tip to the casing surface. The outer plug volume (Figure 5-27(c)) constitutes the portion of the clearance region outside the perimeter of the plug volume.

Blade Region

The blade region (Figure 5-27(d)) consists of one or more volumes that together constitute the part of the model immediately surrounding the void that represents the turbo blade. For the example shown above, the blade region consists of a single outer blade volume. It is possible, however, to split the blade region horizontally into a set of separate, connected volumes in order to facilitate meshing operations, in which case the blade region consists of more than one volume. The number of volumes included in the blade region is determined by the number of spanwise sections specified when creating the turbo volume (see "Specifying the Number of Spanwise Sections," below).

Tip-clearance Options

GAMBIT provides two options for specifying the tip-clearance:

The distance specification is an absolute distance measurement that constitutes a uniform thickness applied to the clearance region. The tip-edge inlet option specifies the leading vertex of an edge that defines the shape of the underside of the clearance region.

Specifying the Distance Option

When you specify a tip-clearance distance, the resulting clearance region is of uniform thickness across the entire turbo volume (see Figure 5-26, above). As a result, the contours of the face that constitutes the underside of the clearance region are projections of those of the casing surface.

Specifying the Tip-edge Inlet Option

As an alternative to specifying a uniform thickness for the tip clearance, GAMBIT allows you to specify an edge the swept surface of which constitutes the underside of the clearance region. To specify such an edge, you must specify its endpoint vertex on the inlet side of the turbo profile.

As an example of the effect of the tip-edge inlet specification, consider the set of edges shown in Figure 5-28. This set of edges is identical to that used to define the turbo profile described above (see Figure 5-32) but includes an extra edge the purpose of which is to define the underside of the clearance region. In this example, the tip edge slopes downward toward the inlet side of the turbo profile.

Figure 5-28: Edges used to define a turbo profile, including tip edge

Figure 5-29 illustrates the difference between the distance and tip-edge inlet options on the configuration of the turbo volume for this example. If you specify the tip clearance by means of the distance option, GAMBIT creates a turbo volume such as that shown in Figure 5-29(a) (which is identical to that shown in Figure 5-26, above). In this turbo volume, the plug and outer plug regions are of uniform thickness across the turbo volume, and the blade-tip shape follows the contours of the casing surface. If you specify the tip clearance by means of the tip-edge inlet option and specify the tip edge shown in Figure 5-28, GAMBIT creates the turbo volume shown in Figure 5-29(b). In this case, the clearance region narrows from the inlet to the outlet side of the turbo volume, and the blade tip is not aligned with the casing surface.

Figure 5-29: Effect of distance and tip-edge inlet options on clearance volume

Specifying the Number of Spanwise Sections

When you create a turbo volume, GAMBIT allows you to automatically split the turbo volume into horizontal (spanwise) sections, which can aid in meshing the turbo volume as a whole, especially for complex blade configurations. To specify the number of spanwise sections for the turbo volume, you must input a value for the Spanwise sections specification on the Create Turbo Volume form (see "Create Turbo Volume" in Section 5.3.2, below).

Each spanwise section consists of a separate real volume, and each volume is connected to those above and/or below it. Figure 5-30 shows a turbo volume for this example divided into two spanwise sections.

Figure 5-30: Example turbo volume divided into spanwise sections

GAMBIT creates the spanwise sections such that the turbo volume as a whole is divided into sections of equal radial width. As a result, the outermost spanwise section can differ in thickness from the others due to volume taken up by a clearance region. For example, in Figure 5-30, the spanwise section nearest the casing is slightly thinner than the section adjacent to the hub because of the volume occupied by the clearance region.

NOTE (1): Because the spanwise sections are equally spaced between the hub and casing, rather than between the hub and underside of the clearance region, it is possible to create a turbo volume in which one or more spanwise sections intersects the underside of the clearance region itself. Although such turbo volumes represent collections of valid real volumes, they can render unusable the GAMBIT turbo operations that are designed to facilitate meshing-such as the Decompose Turbo Volume command-and thereby render the turbo volume difficult to mesh. Consequently, it is sometimes necessary to limit the number of spanwise sections in order to prevent the creation of such intersected volumes.
NOTE (2): The number of spanwise sections is independent of the number of blade cross sections specified in the turbo profile. For example, the turbo volume shown in Figure 5-30, above, can be created using either the profile shown in Figure 5-21 or that shown in Figure 5-22.

Step 3—Assigning Turbo Zone Types

In addition to the operations described above, GAMBIT provides special tools that aid in meshing and viewing the turbo volume. Although you can mesh and view the turbo volume by conventional means, the special tools facilitate such operations by splitting the volume according to the predefined template decomposition and by displaying views of the turbo volume that cannot be created by any of the conventional GAMBIT graphics operations.

To employ the special tools described above, you must assign turbo zone types to certain faces the turbo volume. The zone-type assignments define boundaries of the turbo volume according to six predefined categories:

Figure 5-31 shows an example assignment of turbo zone types for the turbo volume shown in Figure 5-23, above.

Figure 5-31: Example turbo volume zone-type assignments

Specifying the Hub and Casing Faces

The hub and casing turbo zone types are assigned to the faces on the hub and casing, respectively, of the turbo volume (see Figure 5-31(a)). For any turbo volumes that involves a tip clearance, the casing zone-type assignment must include the uppermost face of the plug volume in addition to any faces on the uppermost surface of the outer plug volume (see Figure 5-27, above).

Specifying the Inlet and Outlet Faces

The inlet and outlet turbo zone types are assigned to the faces on the influx and outflow sides, respectively, of the turbo volume. The number of faces included in the inlet and outlet turbo zone-type assignments depends on the construction of the turbo volume. If the turbo volume does not include either spanwise sections or a tip-clearance volume, the inlet and outlet zone-type assignments each consist of a single face (see Figure 5-31(b)). If the turbo volume does include spanwise sections and/or a tip-clearance volume, the inlet and outlet turbo zone-type assignments each consist of multiple faces.

Specifying the Pressure and Suction Faces

The pressure and suction zone types are assigned to the faces that surround the void that represents the turbo blade (see Figure 5-31(c) and (d)). They designate the sides of the blade that are subject to high and low pressure, respectively, in actual operation of the turbomachinery blade.

The number of faces included in each zone-type assignment depends on two factors:

For example, each blade cross-section employed in this example includes six edges (see Figure 5-32). As a result, the void in the turbo volume that represents the blade is bounded by six faces-three on each side of the blade. Therefore, for the turbo volume shown in Figure 5-31, the pressure and suction turbo zone-type assignments can each contain as many as three faces. If the turbo volume includes spanwise sections (such as those shown in Figure 5-30), the pressure and suction zone-type assignments can contain up to six faces each.

NOTE (1): As noted above, GAMBIT allows you to specify up to three faces each for the pressure and suction zone-type assignments for the turbo volume shown in Figure 5-31. However, GAMBIT requires only one face each for the zone-type assignments. If you specify more than one face for a pressure or suction turbo zone-type assignment, the specified faces are merged into a single face during subsequent turbo decomposition operations (see "Step 4—Decomposing the Turbo Volume," below). For example, if you specify three faces each for the pressure and suction turbo zone-type assignments, the volume resulting from the turbo decomposition includes only two boundary faces for the void that represents the turbo blade.
NOTE (2): The pressure and suction turbo zone-type assignments must include only side faces that constitute boundaries on the void that represents the turbo blade. GAMBIT does not allow you to include side faces of the plug volume when assigning turbo pressure and suction zone-types.

If the turbo profile includes a splitter, the pressure and suction faces on the splitter must be included in the pressure and suction zone-type specifications, respectively (see Figure 5-32).

Figure 5-32: Pressure and suction zone-type assignments, including splitter

Executing the Define Turbo Zones Command

When you execute the Define Turbo Zones command, GAMBIT automatically creates boundary zones associated with each of the six turbo zones listed above. In addition, GAMBIT creates a periodic zone that includes the side faces of the turbo volume. GAMBIT automatically associates solver-specific boundary zone designations, such as WALL or OUTLET, with the turbo zones. By default, GAMBIT uses FLUENT 5/6 boundary zones types. If you specify the FIDAP, FLUENT/UNS, or RAMPANT solver, however, GAMBIT assigns boundary zones associated with the specified solver.

NOTE: When you assign turbo zone types to a turbo volume, GAMBIT automatically creates mesh links between the faces that comprise the periodic group.

Step 4—Decomposing the Turbo Volume

GAMBIT turbo decomposition operations allow you to modify the turbo volume in order to facilitate meshing. When you decompose a turbo volume, GAMBIT splits edges and faces according to a predefined "H-type" template. In addition, GAMBIT sets face vertex types to accommodate face map meshing schemes, applies default meshing parameters to edges of the turbo volume, and creates mesh links between source faces, where appropriate, in order to ensure that mesh characteristics are consistent throughout the model.

NOTE: You must assign turbo zone types before decomposing a turbo volume. GAMBIT will not decompose a turbo volume for which turbo zone types have not been assigned.

As an example of the effect of decomposition on a turbo volume, consider the turbo volume shown in Figure 5-23, above. If you assign turbo zone types as shown in Figure 5-31 and decompose the turbo volume, GAMBIT modifies the turbo volume as shown in Figure 5-33.

To create the decomposition shown in Figure 5-33, GAMBIT merges the pressure- and suction-face sets to create individual pressure and suction faces, respectively, and splits the hub and casing faces into four connected faces each. In addition, GAMBIT sets the hub and casing face vertex types to accommodate map meshes and links the horizontal faces and edges to facilitate consistent meshing throughout the turbo volume.

Figure 5-33: Example turbo volume decomposition

NOTE (1): For turbo volumes that include splitters, the decomposition is configured differently from that shown in Figure 5-33. For a description of the decomposition for turbo volumes that include splitters, see "Decomposition Template," below.
NOTE (2): As noted above, GAMBIT creates mesh links between the source faces when decomposing a turbo volume. In addition, GAMBIT assigns default edge meshing parameters to the edges involved in the decomposition. You can change the default meshing parameters by means of default variables available on the Edit Defaults form. For a list of the available parameters, their default settings, and a description of their correspondence to edges resulting from a turbo decomposition, see "Default Grading Parameters" in Step 5, below.
NOTE (3): As a first step in decomposing a turbo volume, GAMBIT splits the edges that define the blade and splitter cross sections, as well as the edges that define the boundaries of the periodic (side) faces. To determine the locations at which to split such edges, GAMBIT employs adjustable default parameters. You can specify the parameters, and thereby control the split locations, by means of the Edit Defaults form (see "Default Splitting Parameters," below).

Automatic Geometry Linking

In addition to the mesh links described above, GAMBIT links the virtual vertices on all source faces so that if you adjust the position of a vertex on one face, GAMBIT automatically adjusts the position of the corresponding vertices on the other faces. For example, if you move vertex a in Figure 5-33 to the center of its host edge, GAMBIT moves vertex a' to the center of its host edge, as well.

Tip Clearance and Spanwise Sections

If you decompose a turbo volume that includes a tip clearance and/or spanwise sections, GAMBIT splits the underside of the tip clearance region, as well as all horizontal faces associated with the spanwise sections, in the pattern used to split the hub and casing. For example, if you decompose the turbo volume shown in Figure 5-30, GAMBIT creates the decomposed volume shown in Figure 5-34.

Figure 5-34: Decomposed turbo volume including tip and sections

Decomposition Template

As noted above, GAMBIT employs an H-type decomposition template when decomposing a turbo volume. Figure 5-35 shows the H-type template for a turbo volume that does not include a splitter (such as that shown in Figure 5-23, above).

Figure 5-35: Decomposition template

To perform the decomposition, GAMBIT splits each of the horizontal faces into four connected faces by means of four virtual edges. As noted above, the splitting edges are virtual edges, therefore you can slide their endpoint vertices along their host edges and thereby modify the meshing characteristics the turbo volume.

For turbo volumes that include splitters, the decomposition results in the splitting of faces as shown in Figure 5-36.

Figure 5-36: Decomposition template—turbo volume including splitter

In this case, each of the horizontal faces is split into six faces by seven virtual edges.

Default Splitting Parameters

GAMBIT employs a set of default parameters to determine the split points for the edges that define the blade and splitter cross sections, as well as the edges that define the boundaries of the periodic (side) faces. You can adjust the default parameters, and thereby specify the split-point locations, by means of the Edit Defaults form as follows:

  1. Select Edit from the GAMBIT main menu bar.
  2. Select Defaults… from the Edit menu to open the Edit Defaults form.
  3. Select the TURBO tab on the Edit Defaults form.
  4. Select and modify the appropriate default variable.
The following subsections describe the split-point parameters that GAMBIT uses when decomposing a turbo volume. (NOTE: The default specifications differ according to whether or not the turbo volume includes a splitter.)

Turbo Blade Only

Figure 5-37 shows a horizontal surface for a decomposed turbo volume that includes a turbo blade but does not include a splitter. The surface consists of four faces that surround a void region representing the blade. The faces are bounded by 18 edges, 12 of which—a through l—are directly associated with the periodic and blade surfaces of the turbo volume.

Figure 5-37: Edge designations for split-point defaults—turbo blade only

Prior to decomposition, the surface shown in Figure 5-37 consisted of a single face with a blade-shaped void. The face was bounded by six edges, four of which were directly associated with the periodic and blade surfaces of the turbo volume. The four edges and their definitions with respect to those shown in Figure 5-37 can be defined as follows:

GAMBIT determines the decomposition split points for edges such as those shown in Figure 5-37 by means of a set of default variables that specify the lengths of the post-split edges relative to those of the edges being split. For example, GAMBIT determines the length of edge c in Figure 5-37 by means of a default variable named, SPLIT_PARAMETER2, the value of which constitutes the fraction of edge A represented by the length of edge c. By default, GAMBIT assigns a value of 0.1 to SPLIT_PARAMETER2, therefore it splits edge A such that the length of edge c represents ten percent of the original length of the edge A.

Table 5.1 shows the relevant split-parameter default variables and formulas that determine the lengths of the edges shown in Figure 5-37.

Table 5.1: Default split parameters—turbo blade only

Edge(s)
 Default Variable
Value
 Formula
a, b
 SPLIT_PARAMETER1
0.1
  
c, d
 SPLIT_PARAMETER2
0.1
  
e, f
 SPLIT_PARAMETER3
0.1
  
g, h
 SPLIT_PARAMETER4
0.1
  
i
  
 
 A - (a + c)
j
  
 
 B - (e + g)
k
  
 
 C - (f + h)
l
  
 
 D - (b + d)

NOTE: Each default variable shown in Table 5.1 specifies the lengths of two edge splits in the decomposition operation. For example, the SPLIT_PARAMETER1 variable specifies both the length of edge a relative to that of edge A and the length of edge b relative to that of edge D.

Turbo Blade and Splitter

Figure 5-38 shows a horizontal surface for a decomposed turbo volume that includes a turbo blade and a splitter. The surface consists of six faces that surround void regions representing the blade and splitter. The faces are bounded by 29 edges, 21 of which—a through u—are directly associated with the periodic, blade, and splitter surfaces of the turbo volume.

Figure 5-38: Edge designations for split-point defaults—blade and splitter

Prior to decomposition, the surface shown in Figure 5-38 consisted of a single face with a blade- and splitter-shaped voids. The face was bounded by eight edges, six of which were directly associated with the periodic, blade, and splitter surfaces of the turbo volume. The six edges and their definitions with respect to those shown in Figure 5-38 can be defined as follows:

The SPLIT_PARAMETER default variables described above for the turbo-only edge splits also control edge splits for turbo volumes the include a splitter. Table 5.2 shows the relevant split-parameter default variables and formulas that determine the lengths of the edges shown in Figure 5-38.

Table 5.2: Default split parameters—blade and splitter

Edge(s)
 Default Variable
Value
 Formula
a, b
 SPLIT_PARAMETER1
0.1
  
c, d
 SPLIT_PARAMETER2
0.1
  
e, f
 SPLIT_PARAMETER3
0.1
  
g, h
 SPLIT_PARAMETER4
0.1
  
i
 SPLIT_PARAMETER5
0.4
  
j, k
 SPLIT_PARAMETER6
0.1
  
l, m
 SPLIT_PARAMETER7
0.1
  
n
 SPLIT_PARAMETER8
0.4
  
o
  
 
 A - (a + p + c)
p
  
 
 A - (a + o + c)
q
  
 
 B - (e + g)
r
  
 
 C - (f + i + h)
s
  
 
 D - (j + l)
t
  
 
 E - (k + m)
u
  
 
 F - (b + n + d)

NOTE: GAMBIT calculates the combined length of edges o and p by the formula: (o + p) = A - (a + c). GAMBIT locates the split point between edges o and p such that it coincides with the split point between edges n and u on edge F.

Performing the Decomposition

The GAMBIT turbo volume decomposition operation is invoked by means of the Decompose Turbo Volume specification form. To open the Decompose Turbo Volume form, click the Decompose Turbo Volume command button on the Turbo toolpad. (NOTE: As noted above, you must assign turbo zone types before decomposing a turbo volume. GAMBIT will not decompose a turbo volume for which turbo zone types have not been assigned.)

The Decompose Turbo Volume specification form contains a display field that shows the H-type decomposition template to be applied to the turbo volume. When you click Apply on the form, GAMBIT automatically decomposes the turbo volume according to the template.

Step 5—Meshing the Turbo Volume

A GAMBIT turbo volume is composed of one or more real volumes each of which can be meshed according to standard GAMBIT meshing techniques. For example, you can mesh the turbo volume shown in Figure 5-37 without modification by means of the Tet/Hybrid or Stairstep volume meshing techniques. Alternatively, you can apply a Pave mesh to the casing face and employ the Cooper volume meshing technique to mesh the volume.

Although it is possible to mesh turbo component volumes without modification, the decomposition techniques described above facilitate meshing by creating mappable source faces the shapes and dimensions of which can be adjusted as necessary to improve element quality. In addition to creating mappable source faces, decomposition facilitates meshing by creating interior edges that can be specially graded or refined in areas of interest.

Effect of Decomposition on Turbo Volume Meshes

Decomposed turbo volumes such as that shown in Figure 5-33, facilitate meshing by increasing user control over the meshing characteristics of the source faces-in this case, the hub and casing faces. Figure 5-39 illustrates the effect of such control by contrasting meshes created for a non-decomposed and decomposed turbo volume. In both cases, the meshes shown in Figure 5-39 represent the results of the Cooper volume meshing scheme wherein the casing and hub faces serve as source faces for the scheme.

The casing face for the non-decomposed turbo volume (Figure 5-39(a)) can be meshed only by means of the Pave meshing scheme because it does not meet the topological criteria required for either the Map, Submap, or Tri Primitive schemes. By contrast, each of the faces that comprise the casing surface for the decomposed turbo volume (Figure 5-39(b)) does meet the criteria required for the Map meshing scheme, therefore each face can be individually mapped.

Figure 5-39: Effect of decomposition on volume mesh

In this example, the decomposed-volume mesh contains hexahedral elements that are more regular than those of the mesh for the non-decomposed volume. In addition, the decomposed volume provides flexibility with regard to adjusting and customizing the mesh.

Modifying Meshes on Virtual Source Faces

The internal edges that are used to split the hub and casing faces on the decomposed volume are virtual edges, therefore you can adjust their positions by sliding their endpoint vertices along their respective host edges. By sliding the vertices, you can adjust the shape of the existing mesh or modify the orientations of the edges prior to meshing. As an example of the effect of adjusting the split-edge orientations for a decomposed turbo volume, consider the meshed turbo volume shown in Figure 5-40.

Figure 5-40: Effect of adjusted decomposition on volume mesh

In this case, the split-edge endpoint vertices hosted by the boundary edges on the blade-shaped void have been moved toward the tips of the blade, thereby eliminating sharp corners in the face geometries. As a result of these changes, the mesh shown in Figure 5-40 demonstrates generally superior element quality in the regions surrounding the blade tips than does the mesh shown in Figure 5-39(b). (NOTE: In Figure 5-40, the edges used to split the source faces are graded toward the blade, and the interval assignments on the blade tip are adjusted to reduce the number of small, highly-skewed elements.)

Automatic Face Mesh Links on the Turbo Volume

Figure 5-41 shows the types of face-mesh links that exist on a decomposed turbo volume. When you create a turbo volume, GAMBIT automatically creates mesh links between periodic faces of the volume (see faces A-A' in Figure 5-41(a)). When you decompose a turbo volume, GAMBIT also links casing-surface faces to the corresponding faces on the hub surface and on any other source surfaces, such as those that represent the underside of the clearance region or the intermediate faces between spanwise links (see faces A-A', B-B', C-C', and D-D' in Figure 5-31(b)).

Figure 5-41: Face mesh links on a decomposed turbo volume

As an example of the effect of mesh linking on a turbo volume, consider the face meshes shown in Figure 5-42. In this case, the meshing of the casing face closest to the inlet side of the turbo volume results in the automatic meshing of the corresponding face on the hub surface. The edge grading parameters (with respect to interval ratio and number of intervals) are identical for each meshed face.

Figure 5-42: Linked-face meshing on decomposed turbo volume

Default Grading Parameters

When you decompose a turbo volume, GAMBIT automatically applies default interval-count and grading parameters to the edges that result from the decomposition operation. You can adjust the default parameters by means of the Edit Defaults form as follows:

  1. Select Edit from the GAMBIT main menu bar.
  2. Select Defaults… from the Edit menu to open the Edit Defaults form.
  3. Select the TURBO tab on the Edit Defaults form.
  4. Select and modify the appropriate default variable.

The following subsections describe the general GAMBIT default meshing parameters for edges resulting from a turbo decomposition operation. (NOTE: The default specifications differ according to whether or not the turbo volume includes a splitter.)

Turbo Blade Only

Figure 5-43 shows a source surface for decomposed turbo volume that includes a turbo blade but does not include a splitter. The source surface consists of four faces and includes 16 edges: a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, and p.

Figure 5-43: Edge designations for decomposition defaults—turbo blade only

Default Interval Counts

GAMBIT directly assigns default mesh interval counts for nine of the edges shown in Figure 5-43. For example, the GAMBIT default variable PTP_SINGLE_INT1 determines the default interval count for edge a. The default interval counts for the other edges can be represented by formulas involving the edges for which GAMBIT assigns the interval counts. For example, the default count for edge k cannot be directly set by means of a GAMBIT default variable. Rather, it is determined by the formula: k = a+f+g+i. Table 5.3 shows the relevant default variables and formulas that determine the default interval counts for the edges shown in Figure 5-43.

Table 5.3: Default interval settings—turbo blade only

Edge
 Default Variable
Count
Formula
a
 PTP_SINGLE_INT1
6
  
b
 PTP_SINGLE_INT2
50
  
c
 PTP_SINGLE_INT3
6
  
d
 PTP_SINGLE_INT4
6
  
e
  
 
b
f
 PTP_SINGLE_INT5
6
  
g
 PTP_SINGLE_INT6
20
  
h
    
g
i
 PTP_SINGLE_INT7
20
  
j
    
i
k
 PTP_SINGLE_INT8
20
  
l
    
b
m
 PTP_SINGLE_INT9
20
  
n
    
k
o
    
l
p
    
m

Default Grading Ratios

In addition to assigning default interval counts, GAMBIT assigns default grading parameters to some of the edges shown in Figure 5-43. For example, the GAMBIT default variable, PTP_SINGLE_GRAD1, determines the default grading schemes employed on edges a, c, d, and f. Table 5.4 lists the edges for which default grading schemes can be specified, along with the GAMBIT default variables associated with the settings. In addition, Table 5.4 shows the default grading types and grading ratios employed for each set of edges. (NOTE: All edges shown in Figure 5-43 and not listed in Table 5.4 are assigned a uniform default grading of unity (1). The default grading value for such edges cannot be altered by means of any GAMBIT default variable.)

Table 5.4: Default grading parameters—turbo blade only

Edge
 Default Variable
Ratio
 Grading
a
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from f
b
 PTP_DOUBLE_GRAD2 1.01 Double-sided
c
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from d
d
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from c
e
 PTP_DOUBLE_GRAD2 1.01 Double-sided
f
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from a
g
 PTP_DOUBLE_GRAD3 1.1 Single-sided, away from a
h
 PTP_DOUBLE_GRAD3 1.1Single-sided, away from c
i
 PTP_DOUBLE_GRAD3 1.1 Single-sided, away from f
j
 PTP_DOUBLE_GRAD3 1.1 Single-sided, away from d
k
 PTP_DOUBLE_GRAD4 1.0 Single-sided, away from g
l
 PTP_DOUBLE_GRAD2 1.01 Double-sided
m
 PTP_DOUBLE_GRAD4 1.0 Single-sided, away from h
n
 PTP_DOUBLE_GRAD4 1.0 Single-sided, away from i
o
 PTP_DOUBLE_GRAD2 1.01 Double-sided
p
 PTP_DOUBLE_GRAD4 1.0 Single-sided, away from j

Vertex Types

Vertex types (and boundary-layer vertex types) are defined as appropriate on the edges that define the turbo blade. All other vertex types are set to End.

Turbo Blade and Splitter

Figure 5-44 shows a source surface for decomposed turbo volume that includes both a turbo blade and a splitter. The source surface consists of six faces and includes 28 edges: a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v1, v2, w, x, y, z, and aa.

Figure 5-44: Edge designations for default grading—turbo blade and splitter

Default Interval Assignments

GAMBIT assigns default interval counts for 16 of the edges shown in Figure 5-44. The default interval counts for the other edges can be represented as formulas involving the edges for which GAMBIT assigns the interval counts. For example, the default setting for edge p cannot be directly set by means of a GAMBIT default variable. Rather, it is calculated by: p=h+m+q+s. Table 5.5 shows the default variables and formulas that determine the default interval counts for the edges shown in Figure 5-34.

Table 5.5: Default interval counts—turbo blade and splitter

Edge
 Default Variable
Count
Formula
a
 PTP_DOUBLE_INT1
6
  
b
 PTP_DOUBLE_INT2
50
  
c
 PTP_DOUBLE_INT3
6
  
d
 PTP_DOUBLE_INT4
6
  
e
 PTP_DOUBLE_INT5
25
  
f
 PTP_DOUBLE_INT6
25
  
g
 PTP_DOUBLE_INT7
6
  
h
 PTP_DOUBLE_INT8
6
  
i
    
e
j
 PTP_DOUBLE_INT9
6
  
k
 PTP_DOUBLE_INT10
6
  
l
  
 
e
m
 PTP_DOUBLE_INT11
6
  
n
 PTP_DOUBLE_INT12
20
  
o
    
n
p
    
h+m+q+s
q
 PTP_DOUBLE_INT13
20
  
r
    
q
s
 PTP_DOUBLE_INT14
20
  
t
    
s
u
 PTP_DOUBLE_INT15
20
  
v1
    
f
v2
    
e
w
 PTP_DOUBLE_INT16
20
  
x
    
u
y
    
f
z
    
e
aa
    
w

Default Grading Ratios

In addition to assigning default interval counts, GAMBIT assigns default grading parameters to some of the edges shown in Figure 5-44. Table 5.6 shows the default grading parameters employed for each edge in the figure. (NOTE: All edges shown in Figure 5-44 and not listed in Table 5.6 are assigned a uniform default grading of unity (1). The default grading value for such edges cannot be altered by means of any GAMBIT default variable.)

Table 5.6: Default edge grading settings—turbo blade and splitter

Edge
 Default Variable
Setting
 Grading
a
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from g
b
 PTP_DOUBLE_GRAD2 1.01 Double-sided
c
 PTP_DOUBLE_GRAD1 1.2Single-sided, away from d
d
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from c
e
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from d
f
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from g
g
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from a
h
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from m
i
 PTP_DOUBLE_GRAD3 1.05 Double-sided
j
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from k
k
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from j
l
 PTP_DOUBLE_GRAD3 1.05 Double-sided
m
 PTP_DOUBLE_GRAD1 1.2 Single-sided, away from h
n
 PTP_DOUBLE_GRAD4 1.1 Single-sided, away from a
o
 PTP_DOUBLE_GRAD4 1.1 Single-sided, away from c
p
 PTP_DOUBLE_GRAD4 1.1 Single-sided, away from g
q
 PTP_DOUBLE_GRAD4 1.1 Double-sided
r
 PTP_DOUBLE_GRAD4 1.1 Double-sided
s
 PTP_DOUBLE_GRAD4 1.1 Single-sided, away from m
t
 PTP_DOUBLE_GRAD4 1.1 Single-sided, away from k
u
 PTP_DOUBLE_GRAD5 1.0 Single-sided, away from n
v1
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from n
v2
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from o
w
 PTP_DOUBLE_GRAD5 1.0 Single-sided, away from o
x
 PTP_DOUBLE_GRAD5 1.0 Single-sided, away from p
y
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from p
z
 PTP_DOUBLE_GRAD3 1.05 Single-sided, away from t
aa
 PTP_DOUBLE_GRAD5 1.0 Single-sided, away from t

Vertex Types

Vertex types (and boundary-layer vertex types) are defined as appropriate on the edges that define the turbo blade. All other vertex types are set to End.

Step 6—Viewing the Turbo Volume

In addition to the standard graphics-window viewing options for display of a turbo volume, GAMBIT allows you to view any turbo volume in a special cascade-view format. In a cascade-view display format, a turbo-volume radial surface—that is, the hub surface, casing surface, or any upper or lower surface associated with a spanwise section-is displayed as if projected onto a flat, two-dimensional plane. Figure 5-45 shows cascade views of the hub and shroud for the simple turbo volume (no splitter) in the example outlined above.

Figure 5-45: Cascade views of hub and casing for example turbo volume

Displaying a Cascade View

When you invoke a cascade view, by means of the Cascade option on the View Turbo Volume specification form, GAMBIT displays the cascade view in all of the graphics-window quadrants specified as active on the View Turbo Volume specification form. When you turn Off the cascade view, GAMBIT returns the affected graphics windows to the model perspectives that they displayed prior to the turning on of the cascade view.

For a description of the options available on the View Turbo Volume specification form, see "View Turbo Volume," in Section 5.3.2, below.

NOTE: GAMBIT does not allow you to resize, reorient, zoom, or otherwise alter the cascade view of a turbo volume.

Effect of Profile Axis Direction

As noted in "Step 1—Defining and Creating the Turbo Profile," above, the turbo profile axis direction affects the graphical orientation for turbo views. The effect of the axis direction on the turbo view orientation depends, in part, on whether the turbo configuration flow direction is axial, radial, or mixed.

As an example of the effect of turbo profile axis on the turbo view, consider the turbo configurations shown in Figure 5-46. Figure 5-46(a) shows the simple, 8-blade turbo volume employed to illustrate the turbo modeling procedure outlined above, in which flow enters and exits the blade array in the -z direction (straight flow direction). Figure 5-46(b) shows the configuration for a low-speed centrifugal compressor in which flow enters the blade array in the -z direction and exits in the radial direction (mixed flow direction).

Figure 5-46: Effect of profile axis direction—example configurations

Figure 5-47 and Figure 5-48 show the effect of turbo profile axis direction on the turbo view of the hub for the blade configurations shown in Figure 5-46(a) and Figure 5-46(b), respectively.

Figure 5-47: Turbo view of hub—straight flow configuration

Figure 5-48: Turbo view of hub—mixed flow configuration

5.3.2 Turbo Commands

The Tools/Turbo subpad includes the following commands.

Symbol Command(s) Description
Create Turbo Profile Creates a wireframe structure that can be used to create a turbo volume
Slide Virtual Vertex Changes the position of a virtual vertex along a host edge or face
Create Turbo Volume Creates a set of one or more volumes that models the flow environment surrounding a turbo blade
Define Turbo Zones Assigns boundary faces of the turbo volume to predefined turbo zone types
Decompose Turbo Volume Splits and modifies edges and faces of a turbo volume in order to facilitate meshing
Split Edge (Virtual)
Merge Edges (Virtual)
Split Face (Virtual)
Merge Faces (Virtual)
Split Volume (Virtual)		 Splits (virtually) existing edges, faces, or volumes or merges existing edges or faces
Create Boundary Layer
Modify Boundary Layer		 Creates or modifies boundary layers associated with the turbo volume
Mesh Edges
Mesh Faces
Mesh Volumes		 Generates meshes for edges, faces, or volumes
Link Edge Meshes
Unlink Edge Meshes
Link Face Meshes
Unlink Face Meshes		 Generates meshes for edges, faces, or volumes
View Turbo Volume Displays a turbo volume in either of two standard views

Four of the commands listed above are specific to GAMBIT turbo functionality. They are:

All other commands on the Turbo subpad are also accessible by means of either the Geometry or Mesh subpads. They are included on the Turbo subpad in order to facilitate turbo operations. The following sections describe the each of the four Turbo commands and reference the appropriate sections of this guide that describe the other functions listed above.

Create Turbo Profile

The Create Turbo Profile command allows you to define a set of specifications that describe the shape of a turbomachinery blade and the general configuration of its flow environment.

Specifying the Turbo Profile

A turbo profile is a set of specifications that describes the shape of a turbomachinery blade and the general configuration of its flow environment. To define and create a profile in GAMBIT, you must specify the following information:

The hub and casing contours define the axial shapes of the turbomachinery hub and casing, respectively. In GAMBIT, each is specified by means of single edge.

To define the overall shape of the blade, you must specify at least two blade cross sections. Each cross-section definition consists of two, four, or six of edges each of which represents a planar cut of the blade at a given distance from the axis. Splitter cross sections (optional) define the shape of any splitters included in the turbo configuration and are defined in the same way that blades are defined-by specifying sets of two, four, or six edges that describe the splitter cross section.

In GAMBIT, the edges for each of the turbo specifications listed above are specified in terms of their endpoint vertices on the inlet side of the turbo configuration. Specifically, the required vertex specifications (as defined on the Create Turbo Profile form) are as follows:

As an example of turbo profile specification, consider the set of edges shown in Figure 5-49. The set consists of 26 edges. Two of the edges represent the shapes of the hub and casing. Twelve of the other 24 edges comprise two sets of six edges each, and each set defines the blade cross section at a distinct level. The remaining edges also comprise two sets of six edges each, and each set defines the cross section of a splitter.

Figure 5-49: Edges used to define a turbo profile

When you define a turbo profile and execute the Create Turbo Profile command, GAMBIT creates a wireframe configuration that serves as the basis for a turbo volume. For example, if you define a turbo profile by means of the edges shown in Figure 5-49 and execute the Create Turbo Profile command, GAMBIT creates the profile shown in Figure 5-353.

Figure 5-50: Example turbo profile

The turbo profile consists of the following elements:

For a description of these elements and their use in defining the shape of the turbo volume, see "Step 1—Defining and Creating the Turbo Profile," in Section 5.3.1, above.

Using the Create Turbo Profile Form To open the Create Turbo Profile form (see below), click the Create Turbo Profile command button on the Tools/Turbo subpad.

The Create Turbo Profile form includes the following specifications.
Hub Inlet specifies the endpoint vertex at the inlet end of the edge or set of connected edges that describe(s) the hub profile.
Casing Inlet specifies the endpoint vertex at the inlet end of the edge or set of connected edges that describe(s) the casing profile.
Axis -----------------------------------------------------------------
Define
 opens the Vector Definition form, which allows you to specify a vector that defines the axis of revolution for the turbo profile (see "Using the Vector Definition Form" in Section 2.1.4 of this guide).
Blade Tips specifies the vertices located at the leading (inlet) ends of the edge sets that describe the blade shape. (NOTE: Each vertex specified in the Blade Tips list box must represent a separate set of edges that describe the blade cross section.)
Splitters (Optional) specifies the vertices located at the leading (inlet) ends of the edge sets that describe the splitter shape. (NOTE: Each vertex specified in the Splitters list box must represent a separate set of edges that describe the splitter cross section.)

Slide Virtual Vertex

For GAMBIT turbo modeling, the Slide Virtual Vertex command allows you to slide the endpoint vertices of the turbo-profile medial edges along their host rail edges, thereby adjusting the positions of the edges. By adjusting the positions of the linking edges, you can affect the shapes of the periodic faces that GAMBIT creates when it creates the turbo volume.

You can also use the Slide Virtual Vertex command to adjust the positions of linked vertices created during face-split operations associated with turbo decomposition. By adjusting the positions of such vertices, you can alter the shapes of the turbo-volume source faces to facilitate meshing or alter the shape of the mesh for previously-meshed source faces.

Sliding Spanwise-Linked Vertices

The Turbo version of the Slide Virtual Vertex form includes a Move With Links option that allows you to slide entire sets of linked vertices by means of a single operation. If you select the Move With Links option and slide a vertex, GAMBIT automatically slides all of the other linked vertices along their respective host edges. If you unselect the Move With Links option and slide a vertex, GAMBIT does not slide the other linked vertices.

NOTE: If you slide a vertex the host edge of which is periodically linked to another edge, GAMBIT moves the corresponding vertex on the linked edge regardless of whether or not you select the Move with links option. (For a description of edge linking operations, see Section 3.2.3 of this guide.)

As an example of the effect of the Move With Links option, consider the turbo profile shown in Figure 5-51. If you select the Move With Links option and slide a vertex on one of the inlet rail edges in the clockwise direction, GAMBIT automatically slides the other vertices on the inlet rail edges in the clockwise direction, as well.

Figure 5-51: Linked inlet vertices in a turbo profile

Using the Slide Virtual Vertex Form To open the Slide Virtual Vertex form (see below), click the Slide Virtual Vertex command button on the Geometry/Vertex subpad.

For a description of the Slide Virtual Vertex form and its use, see Section 2.2.2 in this guide.

Create Turbo Volume

The Create Turbo Volume command allows you to generate a turbo volume based on the currently-defined turbo profile. A turbo volume is a set of one or more real volumes that enclose the flow region surrounding a turbo blade. (NOTE: There are two general types of turbo volumes: passage-to-passage and blade-to-blade. The current version of GAMBIT creates only passage-to-passage turbo volumes such as that shown in Figure 5-50, above. For a description of the differences between the passage-to-passage and blade-to-blade volumes, see "Turbo Volume Types," in Section 5.3.1.)

Specifying the Turbo Volume

To create a turbo volume, you must create a turbo profile and specify the following information on the Create Turbo Volume form:

The Pitch determines the swept angle represented by the turbo volume. The Tip Clearance specifies the distance between the casing and the tip of the blade. The Spanwise Sections specification determines the number of horizontal sections (excluding the tip-clearance region) into which the turbo volume is divided.

For descriptions of the specifications listed above and their effects on the turbo volume creation, see "Turbo Volume Specifications," in Section 5.3.1," above.

Using the Create Turbo Volume Form

To open the Create Turbo Volume form (see below), click the Create Turbo Volume command button on the Tools/Turbo subpad.

The Create Turbo Volume form contains the following specification.
Pitch specifies the swept angle represented by the turbo volume.
Blade count
Angle 		specifies the units represented by the numerical value in the Pitch text input field. For the Blade count option, the Pitch must be specified as an integer value. For the Angle option, the angle must be expressed in units of degrees. (NOTE: Angle = 360/Blade count.)
Tip Clearance: specifes that the turbo volume includes a tip-clearance between the top of the blade and the casing. The tip clearance distance can be specified as either an absolute distance or by means of an edge that defines the tip clearance. (For a description of the two types of specifications and their effects on the tip-clearance region of a turbo volume, see "Specifying the Tip Clearance," in "Step 2—Creating the Turbo Volume," above.)
Distance specifies an absolute distance that represents the tip clearance.
Tip edge inlet specifies the leading (inlet) vertex on an edge that defines the shape and location of the tip-clearance volume.
Spanwise Sections: specifies the number of horizontal sections (excluding the tip-clearance region) into which the turbo volume is divided during creation.

Define Turbo Zones

The Define Turbo Zones command allows you to assign turbo zone types to faces of the completed turbo volume. Zone-type specifications are required for turbo volume decomposition and viewing operations and can also be used to assign solver-specific zone types to internal and external boundaries on the turbo volume.

Specifying Turbo Zones

Turbo zone-type assignments define boundaries of the turbo volume according to six predefined categories:

Figure 5-52 shows zone-type specifications for the simple turbo volume with no tip-clearance region shown in Figure 5-23, above.

Figure 5-52: Turbo volume zone-type specifications

The number of faces included in any individual zone-type specification depends on the configuration of the turbo volume. For description of the rules that govern turbo zone-type specifications, see "Step 3—Assigning Turbo Zone Types," in Section 5.3.1, above.

Using the Define Turbo Zones Form

To open the Define Turbo Zones form (see below), click the Grid command button on the Tools/Turbo subpad.

The Define Turbo Zones form contains the following specifications.
Hub specifies the face that comprises the turbo-volume hub.
Casing specifies the face(s) that comprise(s) the turbo-volume casing.
Inlet specifies the face(s) that comprise(s) the turbo-volume inlet.
Outlet specifies the face(s) that comprise(s) the turbo-volume outlet.
Pressure specifies the face(s) that comprise(s) the pressure side of the turbo blade.
Suction specifies the face(s) that comprise(s) the suction side of the turbo blade.

Decompose Turbo Volume

The Decompose Turbo Volume command automatically decomposes a turbo volume according to a predefined "H-type" template. The decomposition operation splits turbo-volume faces and sets face vertex types to facilitate meshing (see "Step 4—Decomposing the Turbo Volume," above).

Using the Decompose Turbo Volume Form

To open the Decompose Turbo Volume form (see below), click the Decompose Turbo Volume command button on the Tools/Turbo subpad.

The Decompose Turbo Volume form includes an option button that contains only the H option and a diagram that illustrates the H-type decomposition template. To decompose the current turbo volume, click Apply.

Split/Merge Geometry Operations

The Split/Merge Geometry command button allows you to perform the following operations.

Symbol
 Operation Description
Split Edge (Virtual) Splits an existing edge into two virtual edges (see "Split Edge" in Section 2.3.5)
Merge Edges (Virtual) Merges two or more existing edges into a virtual edge (see "Merge Edges (Virtual)" in Section 2.3.5)
Split Face (Virtual) Splits an existing face into two virtual faces (see "Split Face" in Section 2.4.7)
Merge Faces (Virtual) Merges two or more existing faces into a virtual face (see "Merge Faces (Virtual)" in Section 2.4.7)
Split Volume (Virtual) Splits an existing volume into two virtual volumes (see "Split Volume (Virtual)" in Section 2.5.7)

The commands listed above are identical to their counterparts invoked by means of the Geometry toolpad except that they permit only virtual operations. For example, the Split Edge specification form accessed by means of the Geometry/Edge subpad includes three split-type options: Real connected, Real disconnected, and Virtual connected. By contrast, the Split Edge (Virtual) specification form invoked by means of the Tools/Turbo subpad permits only Virtual connected split operations. Similarly, the Split Face (Virtual) and Split Volume (Virtual) commands permit only virtual split operations on faces and volumes, respectively. Aside from this restriction, the specifications and options available on the Split/Merge Geometry specification forms invoked by means of the Tools/Turbo toolpad are identical to those of their counterpart Geometry toolpad forms.

For descriptions of the commands listed above, see the corresponding referenced sections in this guide.

Create/Modify Boundary Layers

The Create/Modify Boundary Layers command button allows you to perform the following operations.

Symbol
 Operation Description
Create Boundary Layer Creates a boundary layer attached to an edge or face
Modify Boundary Layer Modifies the definition of an existing boundary layer

For descriptions of the commands listed above, see Section 3.1.2 in this guide.

Mesh Edges/Faces/Volumes

The Mesh Edges/Faces/Volumes command button allows you to perform the following operations.

Symbol
 Operation Description
Mesh Edges Creates mesh nodes on edges (see Section 3.2.1)
Mesh Faces Creates mesh elements on faces (see Section 3.3.1)
Mesh Volumes Creates mesh elements on volumes (see Section 3.4.1)

For descriptions of the commands listed above, see the corresponding referenced sections in this guide.

Link/Unlink Edge/Face Meshes

The Link/Unlink Edge/Face Meshes command button allows you to perform the following operations.

Symbol
 Operation Description
Link Edge Meshes Creates hard links between edges (see "Link Edge Meshes" in Section 3.2.3)
Unlink Edge Meshes Deletes hard links between edges (see "Unlink Edge Meshes" in Section 3.2.3)
Link Face Meshes Creates hard links between faces (see "Link Face Meshes" in Section 3.3.6)
Unlink Face Meshes Deletes hard links between faces (see "Unlink Face Meshes" in Section 3.3.6)

For descriptions of the commands listed above, see the corresponding referenced sections in this guide.

View Turbo Volume

The View Turbo Volume command allows you to display cascade surfaces of a turbo volume. Cascade surfaces are radial sections of the turbo volume defined by the hub, casing, and spanwise-section faces. When you view a cascade surface of the turbo volume, GAMBIT displays the cascade view in any graphics quadrants specified as active on the View Turbo Volume form. When you turn Off the cascade view, GAMBIT returns all quadrants in which the view was displayed to their display state prior to display of the cascade view.

Specifying the Turbo View

To view a cascade surface of the turbo volume by means of the View Turbo Volume form, you must specify the following options and/or information:

The display-status specification determines whether or not GAMBIT displays a turbo view in one or more of the graphics quadrants. The active-window specification determines the quadrants in which the view is to be displayed.

GAMBIT provides two display-status options:

If you select the Cascade surface option and click Apply on the View Turbo Volume form, GAMBIT displays a cascade surface in all of the graphics quadrants that are specified as active on the Windows button bar (see below). If you select the Off option, GAMBIT returns any quadrants in which a cascade surface is currently displayed to the model view they displayed before the View Turbo Volume command was activated.

Using the View Turbo Volume Form

To open the View Turbo Volume form (see below), click the View Turbo Volume command button on the Tools/Turbo subpad.

The View Turbo Volume form contains the following specifications.

Cascade surface: turns on the turbo view display.
Hub specifies display of the hub surface.
Casing specifies display of the casing surface.
Spanwise specifies display of one of the intermediate, spanwise-sectioning surfaces.
Off turns off the turbo view display.
Windows
 		(quadrant command buttons) enable or disable any or all quadrants with respect to display of the turbo view.


next previous contents index © Fluent, Inc. 12/21/01