Simple strain rate

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Benjamin Benjamin
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Simple strain rate

Dear William,

I am currently using PIVlab to image horizontal flow in a viscous media (glucose syrup), and it's doing a great job. I saw that it was possible to extract "simple strain rate" from the velocity field. I would like to understand what is computed during the process and what is the physical significance of this parameter (any reference on the topic is welcome).

Thanks.

Benjamin
William William
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Re: Simple strain rate

Hi, this is how shear and strain are calculated in PIVlab:

function out=strain(x,y,u,v)
hx = x(1,:);
hy = y(:,1);
[px junk] = gradient(u, hx, hy);
[junk qy] = gradient(v, hx, hy); %#ok<*ASGLU>
out = px-qy;

function out=shear(x,y,u,v)
hx = x(1,:);
hy = y(:,1);
[junk py] = gradient(u, hx, hy);
[qx junk] = gradient(v, hx, hy);
out= qx+py;

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.

Benjamin Benjamin
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Re: Simple strain rate

Ok, thanks for the rapid reply!
Longde Longde
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Re: Simple strain rate

In reply to this post by William
Hi, William,

I am right now using PIV analysis as well. This is an awesome tool for capturing particle movement in granular system. I am wondering if there is any post process when plotting simple shear and strain rate contour? I tried to extract ASCII files and use the equations as you said and plot them but I cannot achieve the same result. Well, in my plot it doesn't look smooth or too much noise. If you did post process, would you mind to tell me what kind of method is that?

Thank you,

Longde
William William
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Re: Simple strain rate

I am pretty sure that there is no postprocessing of the data. However, there is some postprocessing during image display: If your PIV images are e.g. 500x500 pixels, you might e.g. get 30x30 vectors after the analysis. Usually, the resulting map for any derivative would hence be 30x30 pixels too, but this is scaled up to 500x500 pixels (bilinear interpolation) to make the display look better.

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.

ljin31 ljin31
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Re: Simple strain rate

Ah, that makes sense. I will try this and see. Thanks for the rapid reply! :)

Longde
Ryan Ryan
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Re: Simple strain rate

In reply to this post by William
Hi,

I am wondering what is simple strain standing for? If it is volumetric strain it should be px+qy right?

Thanks,
Ryan