/****************************************************************
 * FUNCTION gas - specifies the working gas properties          *
 * FILENAME:       gas.c                                        *
 * PROGRAMMER:     izzi urieli                                  *
 * DATE:           95/12/02                                     *
 * LAST MODIFICATION: 96/08/16 by I Urieli                      *
 * INCLUDE:        <stdio.h>, <string.h>, "define.h"            *
 * GLOBAL VARIABLES:                                            *
 *    FILE *infile - pointer to the input data file             *
 *    FILE *outfile - pointer to the output data file           *
 *    FILE *printfile - pointer to the print output file        *
 *    double rgas - gas constant (Joules/kg.K)                  *
 *    double cp - specific heat at const pressure (J/kg.K)      *
 *    double cv - specific heat at const volume (J/kg.K)        *
 *    double gama - ratio (cp / cv)                             *
 *    double mu0 - dynamic viscosity at ref temp t0 (kg.m/s)    *
 *    double t0 - reference temperature (273 K)                 *
 *    double t_suth - Sutherland constant for dynamic viscosity *
 *    double prandtl - Prandtl number (0.71)                    *
 * PROTOTYPE:      void gas(void);                              *
 * PRE:  global variables infile, outfile and printfile have    *
 *       been specified                                         *
 * POST: global variables rgas, cp, cv, gama, mu0, t0, t_suth,  *
 *       and Prandtl have been specified                        *
 ****************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include "define.h"

void gas()
{
   char mygas[3];

   fgetc(infile); /* remove "return" char from input buffer */
   do {
      printf("enter gas type: h2 (hydrogen), he (helium),ai (air): ");
      fflush(stdin);
      fgets(mygas, 3, infile);
      if (strcmp(mygas, "h2") == 0) {
         fprintf(outfile, "\n%s\n", mygas);
         printf(" gastype is hydrogen\n");
         fprintf(printfile, "\n gastype is hydrogen\n");
         gama = 1.4;
         rgas = 4157.2;
         mu0 = 8.35e-6;
         t_suth = 84.4;
      }
      else if (strcmp(mygas, "he") == 0) {
         fprintf(outfile, "\n%s\n", mygas);
         printf("\n gastype is helium\n");
         fprintf(printfile, " gastype is helium\n");
         gama = 1.67;
         rgas = 2078.6;
         mu0 = 18.85e-6;
         t_suth = 80.0;
      }
      else if (strcmp(mygas, "ai") == 0) {
         fprintf(outfile, "\n%s\n", mygas);
         printf(" gastype is air\n");
         fprintf(printfile, "\n gastype is air\n");
         gama = 1.4;
         rgas = 287.0;
         mu0 = 17.08e-6;
         t_suth = 112.0;
      }
      else {
         printf(" gastype %s is undefined\n", mygas);
         strcpy(mygas, "un");
      }
   } while (strcmp(mygas, "un") == 0);
   cv = rgas / (gama - 1.0);
   cp = gama * cv;
   t0 = 273.0;
   prandtl = 0.71;
}