How to calculate and justify the first layer thickness and y+ value in Altair SimLab?
Hello everyone,
I am working on a problem like the one shown in the image.
In this case, wind speed enters in the X+ direction and exits in the X- direction. I am seeking guidance on how to calculate the boundary layer, specifically how to determine the first layer thickness, the total number of layers, and the growth rate. This boundary layer will be applied to the region that I am showing in the following image.
Based on this tutorial SL 2060 Atmospheric Boundary Layer Problem - Flow Over Building, I have calculated the first layer thickness using an aspect ratio based on the values shown in the video. However, I would like to obtain a clearer and more concise way to justify this value for my specific case. I have also read that the calculation can be done by finding the y+ value, but I haven’t been able to find that value in Altair SimLab.
I would appreciate any guidance or advice you can provide. Thanks in advance for your help!
Best regards,
Jorge
Best Answer
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I see HyperMesh CFD has an option for setting the first boundary layer thickness based on a desired Y+, but SimLab does not. That really is a just a general guideline, as the estimation is based on a flat plate or inside a tube, so very few cases would actually have that in all locations. You can calculate this yourself based on the definition of Y+ on this page:
In AcuSolve the data is stored/calculated at the mesh nodes, rather than the cell center, so the Y (dimensional distance) when calculating Y+ would be the 1st layer height.
Typically we start with an initial mesh, review the resultant surface Y+ values in post-processing, then determine where the first layer might need to be adjusted. A general guide would be growth rate in the 1.1 to 1.3 range - with most applications using 1.2 - 1.3. You can also see how results of interest change as those 1st layer and growth ratio values change. The number of layers in itself is generally not critical - more how the boundary layers blend into the interior. We don't want a large change in cell size from the boundary layers to the interior.
2
Altair EmployeeAnswers
-
I see HyperMesh CFD has an option for setting the first boundary layer thickness based on a desired Y+, but SimLab does not. That really is a just a general guideline, as the estimation is based on a flat plate or inside a tube, so very few cases would actually have that in all locations. You can calculate this yourself based on the definition of Y+ on this page:
In AcuSolve the data is stored/calculated at the mesh nodes, rather than the cell center, so the Y (dimensional distance) when calculating Y+ would be the 1st layer height.
Typically we start with an initial mesh, review the resultant surface Y+ values in post-processing, then determine where the first layer might need to be adjusted. A general guide would be growth rate in the 1.1 to 1.3 range - with most applications using 1.2 - 1.3. You can also see how results of interest change as those 1st layer and growth ratio values change. The number of layers in itself is generally not critical - more how the boundary layers blend into the interior. We don't want a large change in cell size from the boundary layers to the interior.
2