Why are the patterns always along the 45 degree

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dry6211 dry6211
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Why are the patterns always along the 45 degree

Dear pivlab users,

In one of my analysis recently, I found the vorticity map always has some patterns along the 45 degree (or 135 degree), like in the image attached. But the particle movement in my video are almost random, and there cannot be any preferences along the specific 45 degree. I guess this might be related to the way the data is averaged and interpolated, but I don't know how. Can anyone help me solve this puzzle?

Thank you so much in advance!!

Best,
Ruoyu Dong
William William
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Re: Why are the patterns always along the 45 degree

Hi, this is most likely related to the bilinear interpolation of the colormap. You can change it to bicubic, but it will be slower. Search for this line in PIVlab_GUI.m :

PIVlab_GUI.m wrote
dispvar = imresize(in,[height width],'bilinear');

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dry6211 dry6211
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Re: Why are the patterns always along the 45 degree

Dear William,

Thank you so much for your reply!! But I found the situation does not change much. Actually when I draw the contour plot by myself and without resizing the colormap, there are still some patterns along the 45 degree, as in the figure below. So I think this is related to the calculated vorticity data itself. Is that possible that the algorithm in PIV might cause this?

Best,
Ruoyu

William William
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Re: Why are the patterns always along the 45 degree

In reply to this post by dry6211
Hi,
I don't think that this looks unnatural... You have data points in a cartesian coordinate system, and drawing contours around similar values. There will always be horizontal, vertical and diagonal connections, and this is what I see...
The vorticity in PIVlab is calculated by the Matlab built-in function "curl". Maybe you can check that out.

Please consider donating for the PIVlab project:
http://paypal.me/pools/c/81Y8wBVi4y

The developer needs to update to a recent Matlab release in order to keep the project running.

William William
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Re: Why are the patterns always along the 45 degree

In reply to this post by dry6211
This might help you investigating the issue:
http://folk.uio.no/jks/matpiv/html/node31.html

Please consider donating for the PIVlab project:
http://paypal.me/pools/c/81Y8wBVi4y

The developer needs to update to a recent Matlab release in order to keep the project running.

dry6211 dry6211
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Re: Why are the patterns always along the 45 degree

Yes, I guess maybe I'm thinking too much. Thank you for your kind help!!

Ruoyu