The velocity around the cylindrical rod surface

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Jason Jason
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The velocity around the cylindrical rod surface

Dear William and all PIVlab users,

In PIVlab tutorial, we apply a mask that should be applied to exclude a cylindrical rod to exclude areas in the image from the analysis. In the end of tutorial, the velocity can be determined. Use Cartesian coordinates cut intercorrelation windows and analysis the vector field, these vector is not enough to describe the flow around the cylindrical surface. Vector layout in Cartesian coordinates, it can not describe a circle. How can I find out the velocity around the cylindrical rod in the PIV analysis?

Thanks for any suggestion!

Jason
William William
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Re: The velocity around the cylindrical rod surface

Hi,
I am not sure if I understand the question correctly... The vectors are placed on a grid, yes. But fluid is a "continuum" without jumps in the data (in subsonic flows), therefore interpolation between the grid points is acceptable. You can hence draw a circle around the rod to derive the velocity of the interpolated data. Was that your question...?

Am 7. Februar 2015 10:16:31 MEZ, schrieb "Jason [via PIVlab - Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB]" <[hidden email]>:

>
>
>Dear William and all PIVlab users,
>
>In PIVlab tutorial, we apply a mask that should be applied to exclude a
>cylindrical rod to exclude areas in the image from the analysis. In the
>end
>of tutorial, the velocity can be determined. Use Cartesian coordinates
>cut
>intercorrelation windows and analysis the vector field, these vector is
>not
>enough to describe the flow around the cylindrical surface. Vector
>layout in
>Cartesian coordinates, it can not describe a circle. How can I find out
>the
>velocity around the cylindrical rod in the PIV analysis?
>
>Thanks for any suggestion!
>
>Jason
>
>
>
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>To start a new topic under PIVlab - Time-Resolved Digital Particle
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Jason Jason
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Re: The velocity around the cylindrical rod surface

Thanks for your responses

I will try it.
The fluid is continue flow, but the flow field around the rod is not continue(the velocity is null in the rod). Using interpolation between the grid points can accept a series of velocity, but these interpolation is not consider the boundary condition around the surface(Velocity is zero along the normal direction). Are there had better method to find the velocity near the rod?

Thanks for any suggestion!
 
Jason
William William
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Re: The velocity around the cylindrical rod surface


Velocities near objects are hard to resolve with these kind of images. If you would like to resolve the boundary layer of the rod, you'd need to zoom in a lot with the camera and use a stronger seeding with much smaller particles and a thinner laser sheet. But still, you cannot measure the velocity directly on the surface of the rod (well it is zero anyway...).

Please consider donating for the PIVlab project:
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The developer needs to update to a recent Matlab release in order to keep the project running.

Jason Jason
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Re: The velocity around the cylindrical rod surface

PIV software can help us to determine velocity at the grid in the image. Should we use polar coordinate cutting the intercorrelation windows (Set coordinate origin at the center of the rod), and then derive the velocity around the rod boundary. Is it determined the velocity around the rod?

Thanks again!

Jason
William William
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Re: The velocity around the cylindrical rod surface

Actually, the problem probably is not the coordinate system, but the inability of PIV to calculate velocities close to bodies. But you're right, it would be very cool if the user could enter a list of coordinates with the the spots where he wants to derive velocities.

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.