## Simulation CFD

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# How to get reattachment point

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Hello

I was just trying 2D backward step and wanted to understand how to get the reattachment point.

Going by first principles, I plotted the contours of X-velocity and used the range as -1e-8 mm/s to +1e-8 mm/s, forcing the contour to be divided into two parts and the intersection of the separation line with the bottom wall edge is the reattachment point.

Yet, I think the best way would be to to plot the wall shear stress along the bottom edge and take the point where the shear stress is zero. Or, the X-velocity changes sign. But how to plot the variable on a line? I know XY plot lets you do that but it plots it on bunch of points and you may never know if you are choosing the points on the edge!

Regards

Omkar

Solved! Go to Solution.

# Re: How to get reattachment point

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BTW, by the method mentioned above, I got the result as XR/h=6.688 as against the experimental 6.67. (0.27% difference).

Regards

OJ

# Re: How to get reattachment point

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Omkar,

YOu mentioned all the methods we'd use. XY plot is at times one of the easiest. If you did the cad you would know the XYZ locations of the bottom edge. You can then either give enough data points, or change the number of Divisions (from the default 20) to increase the resolution and assess the reattachment point

Apolo

# Re: How to get reattachment point

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Agreed. Essentially, you are describing how to create line with two points and this seems a logical way. Yet I wish if there were a tool to create a line in SimCFD. Perhaps can be an enhancement request? This would be handy in situations.. like radial or axial velocity profiles etc.

That said, I wonder if it is possible to plot wall shear on the bottom edge (for 2D) or on a line (read:bunch of points) at bottom wall (for 3D).

Regards

OJ

# Re: How to get reattachment point

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Omkar

The XY plot tool is how we'd generate a line. You can either zoom in and click to defin the start and end of that line

Or type in an XYZ location to define the start and end.

Once youv'e defined the start/end set the divisions to ~100 and you'll get a nice plot of the data along that line

Apolo

# Re: How to get reattachment point

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Thanks, Now I realise that I can create a line by two coordinates, with many divisions in between as I want (which essentially is a line), and plot shear stress magnitude (tau) which is wall shear stress. This gives me the exact reattachment point without all the hassel of contours and migrating to the point manually!

Cheers

OJ