title: Streamline plot of cylinder
body: <p>I have made this plot of streamlines around a cylinder with a radius of 1. Is there a way to remove whats inside the cylinder and maybe even high lite the cylinder with a different colour?</p>
  
  <p><a href="https://i.stack.imgur.com/O35AU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O35AU.png" alt="enter image description here"></a></p>
  
  <pre><code> clear
      % make axes
      xymax = 2;
      x = linspace(-xymax,xymax,100);
      y = linspace(-xymax,xymax,100);
      % note that x and y don't include 0
      [X,Y] = meshgrid(x,y);
      R = sqrt(X.^2 + Y.^2);
      sin_th = Y./R;
      cos_th = X./R;
      U = 1;
      a = 1;
      psi = U*(R - a*a./R).*sin_th;
      figure
      contour(X,Y,psi,[-3:.25:3],'-b');
  </code></pre>