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    /*
       DEMO4.I
       Airfoil demo

       $Id$
     */
    /*    Copyright (c) 1995.  The Regents of the University of California.
                        All rights reserved.  */

    func demo4 (mono=)
    /* DOCUMENT demo4
             or demo4, mono=1
         solves for the flow past a 2D airfoil using Kutta-Jakowski theory.
         The colors represent static pressure (darker is lower pressure),
         red lines are streamlines.
         Solutions for various angles of attack are shown by animation.
         With the mono=1 keyword, only the streamlines are shown.  (On a
         monochrome terminal, the pressure makes the streamlines invisible.)
     */
    {
      require, "movie.i";
      local nx, ny;
      imax= 50;
      movie, display;
      display, imax;
    }

    func display(i)
    {
      attack= 28.*double(imax-i)/(imax-1);
      solve, attack, 2.0, 0.2;
      if (i==1) {
        extern cn, cx;
        cn= 0.8*min(pressure);
        cx= max(pressure);
      }
      limits, -2.8, 2.4, -2.4, 2.8;
      plmesh, z.im,z.re;

      if (!mono) plf, zncen(pressure), cmin=cn,cmax=cx;

      plc, potential.im, marks=0,color="red", levs=span(-2.5,3.5,33);

      plg,z.im(,1),z.re(,1), marks=0;
      return i<imax;
    }

    func solve (attack, chord, thick)
    {
      extern w, z, potential, velocity, pressure; /* outputs */
      attack*= pi/180.;
      a= 0.5*chord;
      w= get_mesh(a, attack);

      /* the log(w) term adds just enough circulation to move the rear
         stagnation point to the trailing edge -- the Kutta condition */

      potential= jakowski(w, a) + 2i*a*sin(attack)*log(w);
      dpdw= 1.-(a/w)^2 + 2i*a*sin(attack)/w;
      emith= exp(-1i*attack);
      a= a*emith;
      b= thick*emith;  /* could add camber here too someday? */
      w-= b;
      a-= b;
      z= jakowski(w, a);
      dzdw= 1.-(a/w)^2;
      velocity= conj(dpdw/dzdw);
      pressure= 0.5*(1.0-abs(velocity)^2);
    }

    func get_mesh (a, attack)
    {
      /* get a mesh in the w-plane
         -- the plane in which the airfoil is a circle
         the mesh splits at the point which will become the trailing edge */
      if (is_void(nx)) nx= 120;
      if (is_void(ny)) ny= 30;
      a= abs(a);

      theta= span(1.e-9, 2*pi-1.e-9, nx) - attack;
      r= a/span(1.0+1.e-9, 0.25, ny)(-,);
      return r*exp(1i*theta);
    }

    func jakowski (z, a)
    {
      /* Jakowski transform - circle of radius a into slot of length 4a */
      return z + a*a/z;
    }

    func ijakowski (z, a)
    {
      /* Inverse Jakowski - slot back into circle (unused here) */
      z*= 0.5;
      a= complex(a);

      sgn= sign((z*conj(a)).re);
      return z + sgn(z);
    }