Streamline xy(20x1) & velocity component(400x1)

Hi I have input the following codes and the same error message seem to keep appearing, which say

'??? Error using ==> stream2 at 47, U,V must all be size 2x2 or greater.'

the following are my codes, please advice where did I make a mistakes? Thanks.

x=xyRp(:,1);
y=xyRp(:,2);
u=VelRp(:,1);
v=VelRp(:,2);

[sx, sy] = meshgrid(linspace(min(x),max(x),400), linspace(min(y),max(y),400));

[x y] = linspace((min(x), max(x), 400), (min(y), max(y), 400))

hhh = stream2(x, y, u, v, sx, sy);

[Walter Roberson]

Please use better tags; see http://www.mathworks.co.uk/matlabcentral/answers/43073-a-guide-to-tags

Answer 1

[Walter Roberson]

You say your velocity component is 400x1, but you index it as (:,1) and (:,2) which implies that either it is something-by-2 (at least) or else that it is 1 x 400 instead of 400 x 1. If it is 1 x 400 then u and v would come out as single elements. If it is something-by-2 then u and v would come out as column vectors and column vectors are not at least 2 wide to meet the 2 x 2 requirement.

[Louis]

Actually the velocity component file is Vel(400x2), and i break it into u(400x1) and v(400x1). the same apply to the xy(20x2), which was break into x(20x1) & y(20x1). If that's the case, is it possible to do stream2(x,y,u,v,startx,starty)?

Answer 2

[Sean de Wolski]

The error is occuring because x and y are vectors and thus their size is not greate > [2 2]

I don't understand what you're doing with linspace either? Calling it with two output arguments should throw an error.

[Louis]

Hi, I am not so sure whether do i need to use linspace in this case, as the u and v are both 400x1, but my x and y are only 20x1 each. Can you advise on how I can use these data and plot a stream2. Thank you.

[Sean de Wolski]

You need to use meshgrid to generate every pair of xy points:

[xx yy] = meshgrid(xy(:,1),xy(:,2));

Then you need to reshape the u/v vectors to be 20x20 which is the size xx and yy will be. With out knowing how those vectors (u/v) are ordered (and working on the big assumption that this is where the 400 values come from, i.e. each pair of u/v corresponds to one xx/yy pair), it is not possible for me to tell you the correct way to go about reshaping. Perhaps you could shed some light on that. I.e.

is u/v ordered:

x(1)y(1)
x(1)y(2)

or

x(1)y(1)
x(2)y(1)

Or some compltely other way?