[snappyHexMesh] Atmosferic Boundary Layer

alicezanella · June 11, 2020, 05:23

Hello there!

I'm currently doing a simulation of a tall building and I have a few doubts, I hope someone can answer them.

I've tried multiple times to do 2 dfferent boundary layers in SHM: one for the ground and one for the building. But it seems that I'm only able to have different layers for them, whereas I would like to completely separate them (like defining different Thickness, Expansion ratio,...). Do you know if it's possible?
What should be the value of the first cell height for the ground? The wind is 10 m/s.
Using a ABL inlet condition, like in the Turbine siting tutorial, should I still be using a boundary layer on the ground?

Thank you in advance.

Alice

Lisandro Maders · June 19, 2020, 21:01

Hello,

1. I didn't get exactly what you meant.. do you want or do you NOT want to have different boundary layers for each of them? I would recommend that you have same number of layers if both patches are connected, while it is fine to have different sizes and grow rates for them.

2. Use the CFD-Online calculator for that (https://www.cfd-online.com/Tools/yplus.php) You have to specify the desired y+ value, which depends on the turbulence model and wall function approach used. There are several posts discussing this (a quick search will tell you which y+ size you should use).

3. One thing has nothing related to another. Using the ABL gives you a better inlet profile,but you still need to resolve the boundary layer on every no-slip wall you have in the domain.

Cheers!

June 11, 2020, 05:23	Atmosferic Boundary Layer	#1
alicezanella New Member Alice Zanella Join Date: Oct 2019 Posts: 9 Rep Power: 7	Hello there! I'm currently doing a simulation of a tall building and I have a few doubts, I hope someone can answer them. I've tried multiple times to do 2 dfferent boundary layers in SHM: one for the ground and one for the building. But it seems that I'm only able to have different layers for them, whereas I would like to completely separate them (like defining different Thickness, Expansion ratio,...). Do you know if it's possible? What should be the value of the first cell height for the ground? The wind is 10 m/s. Using a ABL inlet condition, like in the Turbine siting tutorial, should I still be using a boundary layer on the ground? Thank you in advance. Alice

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June 19, 2020, 21:01		#2
Lisandro Maders Member Lisandro Maders Join Date: Feb 2013 Posts: 98 Rep Power: 13	Hello, 1. I didn't get exactly what you meant.. do you want or do you NOT want to have different boundary layers for each of them? I would recommend that you have same number of layers if both patches are connected, while it is fine to have different sizes and grow rates for them. 2. Use the CFD-Online calculator for that (https://www.cfd-online.com/Tools/yplus.php) You have to specify the desired y+ value, which depends on the turbulence model and wall function approach used. There are several posts discussing this (a quick search will tell you which y+ size you should use). 3. One thing has nothing related to another. Using the ABL gives you a better inlet profile,but you still need to resolve the boundary layer on every no-slip wall you have in the domain. Cheers!