How to calculate vorticity and rotation of the vector field

Hello,

I need to calculate the vorticity and rotation of the vector field with the curl function, but I get only Infs and NaNs results. I have 4000 snapshots of a 2D flow field, each snapshot is 159x99 vectors, containts x and y coordinates in mm and U and V components in m/s. The x and y variables are 159x99 double, the Udatar and Vdatar variables are 4000x159x99 double. The included MAT file contains the first snapshot of the flow field.

For the beginning I want to calculate the curl for the first snapshot. Thanks for your help.

My code:

close all; clear;
input_folder=uigetdir;
cd(input_folder);
load('PIVkolmo_example.mat'); 
I=159; J=99; IJ=[159 99];     
% Ures1=reshape(Udatar(1,:,:),I,J);
% Vres1=reshape(Vdatar(1,:,:),I,J);
[curlz1,cav1] = curl(x,y,Ures1,Vres1);  

[VBBV]

If you calculate the vorticity vector, just divide them by 2. Or multiply by 0.5. Since you obtain vorticity as = 2xrotation

[VBBV]

Obtain x and y matrices using meshgrid and then apply curl

[VBBV]

Check also size of Ures1 and Vres1

[Petr Michalek]

Ures1 and Vres1 are both 159x99 double

[Petr Michalek]

Maybe I got it right now, as the x and y coordinates are already a grid of uniformly spaced and monotonic points, it is ok to change the code to [curlz1,cav1] = curl(Ures1,Vres1); and I get results for the 1st snapshot.

[Bjorn Gustavsson]

Then you just have to remember to scale the curl-components with the spacing of x and y.

[Petr Michalek]

How exactly do I scale the curl and cav with the spacing of x and y?

[Bjorn Gustavsson]

From the help of curl:

    [CURLZ, CAV] = curl(U,V) assumes 
          [X Y] = meshgrid(1:N, 1:M) where [M,N]=SIZE(U).

Since curl is a differential operator you simply have to divide by dx and dy, in the correct places...