[bolus13]

Hi All,

I calculated a hydraulic flow through a channel including the free surface. I exported the data from CFX to EnSight.

Now from the result I wish to calculate the depth averaged velocity in the channel.

Would anyone know if this is possible in EnSight?

Thanks

[kevincolburn]

Are you attempting to obtain a scalar value on the free surface, which is the depth averaged velocity of just the velocity which is below that triangle on the free surface? In other words, conceptually, from each triangle on the free surface, shoot a line vertically downwards, and obtain the depth averaged velocity along that ray, and place that value on the triangle on the free surface?

[bolus13]

if I understand you correctly, yes that is what I would like. To end up with a plane showing the depth average velocity at each location.

Attached is an image of the flow field. It is a free surface flow from a channel into a reservoir. 6 meter water depth. I have added 5 clips in the image.

The mesh is unstructured, which makes it a bit more difficult.

[kevincolburn]

Okay. Take a look at the "Radiograph_grid()" and "Radiograph_mesh()" calculator operations in the UserManual. I suspect that the Radiograph_mesh() option will be the starting point allowing you to integrate the velocity along a ray cast from the each element in the vertical direction. You may also need to compute the radiograph_mesh of the depth of the water column in order to finally obtain the depth average velocity.

For example, you may need to utilize the velocity / depth as a variable calculation first. If you then utilize the Radiograph_mesh() to integrate the velocity / depth variable, this might provide the quantification that you are looking for.

-Kevin

[bolus13]

Hi Kevin,

Thanks for the answer. This seems to work. I applied the Radiograph mesh from the water level surface downards (z=-1) and it showed good result. This result indeed needs to be divided by the water depth.

Now I am unsure how to obtain the depth, which variable should I use in this case. I looked for the z-coordinate, but I can not find it in my data. I do have a variable called 'Coordinates', but it does not give its separate components.

Can you advise?

[bolus13]

In the meanwhile I found the answer. I used the water volume fraction as the variable to calculate the water depth. This value is 1 everywhere below the water line. With this I have been able to calculate the depth average velocity.

I do have one more question. Why does radiograph_mesh take much longer than radiograph_grid. With radiograph mesh it took several hours to perform the calculation.

[kevincolburn]

Previous answer/additional information:

For any vector quantity or Coordinate variable, you can use the UserDefinedFunctions (tab just to the right of the PreDefinedFunctions in the Calculator dialog) to extract out components of a vector or Coordinate. So, you could extract out the Z coordinate by : Cz = Coordinates[Z].
You could get the X component of a vector via similar : X_comp = Velocity[X]

I believe that the main difference is that the "grid" option simply places its locations at fixed locations normal to the surface (quick to determine their position, and interpolate those), as opposed to the "mesh" option which tries to integrate through each element in a normal direction... which requires lengthy connectivity and directional searches. So, I believe that the "mesh" option is heavily bound by connectivity searches while the "grid" option more simply places location for the interpolation to take place and is independent of connectivity related speed/computation.
-kevin

[bolus13]

Thanks, that makes sense now.

I do have one more question on saving my work. In what way can I just save my work and continue with it next day. At the moment I save my session file, but each time I open it it needs to redo the radiograph-grid calculation, which takes quite a while. How do I speed this up?

[kevincolburn]

The Session file is probably the best solution for you in terms of time/disk. You can use the "Archive" file save (File --- > Save -- > Archive). This writes what is in memory at the moment to disk. This means that you get back all of the resultant operations from the RadioGraph operations without having to physically do them again. However, the archive files are large, machine specific, and dataset specific.