# How to use the streamline function for a circular area?

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Xinchen Zhang on 14 Oct 2019
Commented: Xinchen Zhang on 14 Oct 2019
Hi all,
Let's say, 'x' and 'y' in the mat file are the coordinate of the circular area, consist of 64 ununiformly distributed points on radial lines and 180 uniformly distributed angles. 'ux' and 'uy' in the mat file are the velocity on x and y directions. But when I use 'streamline(x,y,ux,uy,0.1,0.1)' or 'stream2(x,y,ux,uy,0.1,0.1)' to plot the streamline of the circular area, it always gives me the error message
"Error using griddedInterpolant
Interpolation requires at least two sample points in each dimension."
So I would like to know how to use the streamline function for a circular area.

darova on 14 Oct 2019
You can just make new grid/mesh for your data (standard rectangular, corners are NaN)
Here is a way:
clc,clear
[m,n] = size(x);
x1 = linspace( min(x(:)), max(x(:)), 50 ); % x boundary
y1 = linspace( min(y(:)), max(y(:)), 50 ); % y boundary
[X,Y] = meshgrid(x1,y1); % new mesh
U = griddata(x,y,Ux,X,Y); % new Ux
V = griddata(x,y,Uy,X,Y); % new Uy
t = linspace(0,2*pi,20);
[sx1,sy1] = pol2cart(t,5e-3); % start points
cla
streamline(X,Y,U,V, sx1, sy1)
hold on
plot(sx1,sy1,'*r') % start points of streamlines
plot(x(end,:),y(end,:),'.-r') % boundary of a region
hold off
axis equal

#### 1 Comment

Xinchen Zhang on 14 Oct 2019
It works perfectly. Many thanks darova.