# How to calculate vorticity and rotation of the vector field

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Petr Michalek on 2 Sep 2021
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);
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);
Bjorn Gustavsson on 3 Sep 2021
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...

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