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    /*
       DEMO1.I
       1-D hydro code written in Yorick language

       $Id: demo1.i,v 1.1 1993/08/27 18:50:06 munro Exp $
     */
    /*    Copyright (c) 1994.  The Regents of the University of California.
                        All rights reserved.  */

    /* ------------- Set a few parameters --------------------------- */

    /* Set initial density and temperature.  */
    rho0= 1.1845e-3;             /* g/cc dry air at NTP */
    RT0= 298.16;                       /* Kelvin at NTP */

    /* Set number of zones for hydro calculation, and
       total length of column.  */
    n_zones= 200;
    column_length= 100.0/*cm*/;

    /* A conversion factor.  */
    R_gas= 8.314e7;    /* erg/K/mole ideal gas constant */

    /* Gamma-law gas equation of state parameters.  */
    gamma= 1.4;                 /* Cp/Cv for air is 7/5 */
    gammaM1= gamma - 1;
    A_bar= 1.1845e-3/*g/cc dry air*/ * 24.5e3/*cc/mole*/;
    K_to_v2= 1.e-6*R_gas/A_bar;     /* (cm/ms)^2/Kelvin
                                       for dry air      */
    /* Local temperature units are (cm/ms)^2. --
       Note dependence of temperature units on A_bar.  */
    RT0*= K_to_v2;

    /* ------------- Set initial conditions --------------------------- */

    /* Set initial conditions for hydro calculation.  */
    func reset
    {
      extern RT, z, v, M, p;

      RT= array(RT0, n_zones);           /* temperature */

      z= span(0, column_length, n_zones+1);     /* zone
                                   boundary coordinates */

      v= array(0.0, dimsof(z)); /* zone bndy velocities */

      /* The column consists initially of n_zones zones
         of equal size containing equal amounts of gas. */

      M= rho0*abs(z(2)-z(1));         /* mass/area/zone */

      /* The pressure array includes a pressure before and
         after the column, p(0) and p(n_zones+1), which
         will be used to set pressure boundary conditions.
         (Velocity boundary conditions will be applied by
         setting v(0) and v(n_zones).)
         Note that the units of this pressure are
              g*(cm/ms)^2/cc.  */
      p= array(rho0*RT0, n_zones+2);
    }

    /* ------------- Set boundary conditions --------------------------- */

    func sound
    /* DOCUMENT sound

         Set up the initial conditions for evolve to launch a weak sound wave.

       SEE ALSO: shock, evolve
     */
    {
      extern bc0_v, bc0_time, bc0_p, bc0_Z;

      bc0_v= 0.1*sin(span(0, 2*pi, 100));
      bc0_time= span(0, 1.0, 100);
      bc0_p= bc0_Z= [];
      reset;
    }

    func shock
    /* DOCUMENT sound

         Set up the initial conditions for evolve to launch a strong wave, which

         steepens into a shock as it propagates.

       SEE ALSO: sound, evolve
     */
    {
      extern bc0_v, bc0_time, bc0_p, bc0_Z;

      bc0_v= 10.0*sin(span(0, 2*pi, 100));
      bc0_time= span(0, 1.0, 100);
      bc0_p= bc0_Z= [];
      reset;
    }

    sound;

    func nobc  {
      extern bcN_v, bcN_p, bcN_Z;
      bcN_v= bcN_p= bcN_Z= [];
    }

    func hardbc  {
      extern bcN_v, bcN_p, bcN_Z;
      bcN_v= 0;
      bcN_p= bcN_Z= [];
    }

    func softbc  {
      extern bcN_v, bcN_p, bcN_Z;
      bcN_p= rho0*RT0;
      bcN_v= bcN_Z= [];
    }

    func matchbc  {
      extern bcN_v, bcN_p, bcN_Z;
      softbc;
      bcN_Z= rho0*sqrt((gammaM1+1)*RT0);
    }

    /* ------------- Define the main function --------------------------- */

    /* The DOCUMENT comment will be printed in response to:  help, evolve */

    func evolve (time1, time0)
    /* DOCUMENT evolve, time1
             or evolve, time1, time0
         Step the hydro calculation forward to TIME1,
         starting with the initial conditions in the
         RT, z, and v arrays at time TIME0 (default 0.0
         if omitted).  The calculation also depends on

         the constants M (mass/area/zone) and gammaM1
         (gamma-1 for the gamma-law equation of state).

         The pressure array p is updated in addition to
         the primary state arrays RT, z, and v.

         Boundary conditions are specified by setting
         either a boundary pressure or a boundary
         velocity at each end of the fluid column.
         bc0_v   - Boundary velocity at z(0), or []
                   if z(0) has pressure BC.
         bc0_p   - Boundary pressure beyond z(0).

         bc0_time  - If bc0_v or bc0_p is an array,

                     bc0_time is an array of the same
                     length specifying the corresponding
                     times for time dependent BCs.
         bc0_Z   - Acoustic impedance at z(0) if bc0_v
                   is nil (default is 0).
         bcN_v, bcN_p, bcN_time, and bcN_Z have the same
         meanings for the z(n_zones) boundary.

         The worker routines OutputResults and
         TakeStep must be supplied.
     */
    {
      if (is_void(time0)) time0= 0.0;

      for (time=time0 ; ; time+=dt) {
        dt= GetTimeStep();

        SetBoundaryValues, time, dt;
        OutputResults, time, dt;
        if (time >= time1) break;
        TakeStep, dt;
      }
    }

    /* ----------------- compute time step --------------------- */

    func GetTimeStep (dummy)
    {
      dz= abs(z(dif));
      dv= abs(v(dif));
      cs= sqrt((gammaM1+1)*RT);

      return min( dz / max(courant*cs, accuracy*dv) ) * dt_multiplier;
    }
    /* Set reasonable default values for courant and
       accuracy parameters.  */

    courant= 2.0;  /* number of cycles for sound signal
                      to cross one zone -- must be >=2.0
                      for numerical stability */

    accuracy= 3.0; /* number of cycles for zone volume to
                      change by a factor of ~2 -- must be
                      >1.0 to avoid possible collapse to
                      zero volume */
    dt_multiplier= 1.0;

    /* ----------------- set boundary conditions ------------------ */

    func SetBoundaryValues (time, dt)
    {
      vtime= time + 0.5*dt;  /* velocity is 1/2 step
                                ahead of pressure, z */

      /* boundary at z(0) */
      if (!is_void(bc0_v)) {
        /* velocity BC */

        v(1)= BCinterp(bc0_v, bc0_time, vtime);

      } else if (!is_void(bc0_p)) {
        /* pressure BC */

        p(1)= BCinterp(bc0_p, bc0_time, time);
        /* acoustic impedance BC */
        if (!is_void(bc0_Z))

          p(1)-= sign(z(2)-z(1))*bc0_Z*v(1);
      }

      /* boundary at z(n_zones) (written here as z(0)) */
      if (!is_void(bcN_v)) {

        v(0)= BCinterp(bcN_v, bcN_time, vtime);

      } else if (!is_void(bcN_p)) {

        p(0)= BCinterp(bcN_p, bcN_time, time);
        if (!is_void(bcN_Z))

          p(0)+= sign(z(0)-z(-1))*bcN_Z*v(0);
      }
    }

    func BCinterp (values, times, time)
    {

      if (numberof(times)<2) return values(1);

      else return interp(values, times, time);
    }

    /* ----------------- produce output ----------------------- */

    func OutputVPlot (time, dt)
    {
      extern cycle_number;
      if (time==time0) cycle_number= 0;
      else cycle_number++;
      if (!(cycle_number%output_period)) {
        fma;
        plg, v, z;
        zx= sqrt((gammaM1+1)*RT0)*time;
        if (zx>max(z)) {

          /* make a dotted "ruler" at the location a sound wave
    	 would have reached */
          zx= 2*max(z)-zx;

          if (zx<min(z)) zx= 2*min(zx)-zx;
        }

        pldj, zx, min(bc0_v), zx, max(bc0_v), type="dot";
      }
    }
    output_period= 8;  /* about 50 frames for wave to
                          transit 200 zones */

    /* OutputResults can be switched among several possibilities */
    OutputResults= OutputVPlot;

    /* ----------------- 1-D hydro worker ----------------------- */

    func TakeStep1 (dt)
    {
      /* velocities 1/2 step ahead of coordinates */
      z+= dt * v;
      dv= v(dif);
      dz= z(dif);

      /* Compute artificial viscosity.  */
      q= array(0.0, n_zones+2);

      q(2:-1)= q_multiplier * M * max(-dv/dz, 0.0)*abs(dv);

      /* Apply 1st law of thermodynamics to compute
         temperature change from work p*v(dif) done
         on zone.  Note that p is not time-centered
         properly here.  */
      RT-= (dt * gammaM1/M) * (p+q)(2:-1) * dv;

      /* Update the pressures from the updated densities
         M/z(dif) and temperatures RT.  Note that p(0)
         and p(-1) updated by SetBoundaryValues.  */
      p(2:-1)= M * RT/dz;

      /* Apply Newton's 2nd law to update velocities.  */
      v-= (dt/M) * (p+q)(dif);
    }
    q_multiplier= 1.0;

    TakeStep= TakeStep1;

    /* ----------------- simple interface ----------------------- */

    func demo1
    /* DOCUMENT demo1

         run the 1-D hydrocode demonstration.  An X window should pop up

         in which a movie of a wave propagating down a shock tube appears.
         Use the 'sound' and 'shock' commands to set two different interesting
         initial conditions; the default is 'sound'.

       SEE ALSO: sound, shock, evolve
     */
    {
      window, 0, wait=1;
      reset;
      evolve, 5;
    }

    /* ----------------- end of demo1.i ----------------------- */