# Reshaping the velocity data to match with generated meshgrid of x and y to further plot contour of velocity?

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Muhammad Atif on 17 Aug 2021
Commented: Muhammad Atif on 18 Aug 2021
Hi,
I have some CFD data on a plane. The data include x,y,z,u,v, and w information. I have read the data and further doing proper orthognal decomposition on this data but I know how to do that.
Here I have created a very simple matrix to understand what reshape and other functions does.
%Test code%
xmin=1; xmax=10; ymin=1; ymax=5;
x=linspace(xmin,xmax,10);
y=linspace(ymin,ymax,10);
[X,Y]=meshgrid(x,y);
U=10.1:0.1:11;
V=U';
U_reshaped= reshape(U(zeros(10^2,1), X(:), Y(:)), [10 10]);
This is the code I am trying on my large CFD data. However this last command "reshape" is not working. I ma getting following error below. I have copied this command from another example where it is working perfectly fine in the same format.
"Index in position 1 is invalid. Array indices must be positive integers or logical values.
Error in Untitled (line 22)
U_reshaped= reshape(U(zeros(10^2,1), X(:), Y(:)), [10 10]);"
I am very new to matlab. I would highly appriciate any help.
Thanks

Cris LaPierre on 18 Aug 2021
Edited: Cris LaPierre on 18 Aug 2021
Your error is referring to this command: U(zeros(10^2,1))
You have already defined U, so the zeros are being interpretted as indices. You cannot use 0 as an index. In MATLAB, indexing begins at 1.
Muhammad Atif on 18 Aug 2021
Hi Chris,
Thank you for your reply. I actually copied this command from someone else code where he also defined U matrix before which was about 33000 by 1 and he was using this remesh command to generate a 200 by 200 grid "U_reshaped= reshape(U(zeros(200^2,1), X(:), Y(:)), [200 200]);" Here both X and Y are also 200 by 200 matrix. and the command was working fine generating U_reshaped 200 by 200.
Anyway I used griddata command to populate U,V and W on meshgrid. code is below. It seems working fine but only problem is that the boundary is not captured very smoothly and scaled differently as comapred to CFD results. ANy help on capturing the boundary properly would be highly appriciated.(see pictures)
X1=min(X):0.1:max(X); Y1=min(Y):0.1:max(Y);
[xmesh, ymesh] = meshgrid(X1,Y1);
[x1 y1 u1]=griddata(X,Y,U,xmesh,ymesh);
[x1 y1 v1]=griddata(X,Y,V,xmesh,ymesh);
[x1 y1 w1]=griddata(X,Y,W,xmesh,ymesh);
Thanks