function dxdt = v(t, x);
  % compute 2D inviscid cylinder velocity as function of x
  % input: vector x has x(1) = x coord, x(2) = y coord
  
  V0 = 1;  % scale of velocity
  a  = 1;  % radius of circle
  
  % compute polar coordinates
  r = sqrt(x(1)^2 + x(2)^2); 
  theta = atan2(x(2), x(1));        
  
  dxdt = [V0*(1 - (a/r)^2 * cos(2*theta)) ; 
         -V0*(a/r)^2 * sin(2*theta)];
       
end