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Numerical scheme for DES and SAS in CFX

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Old   March 20, 2018, 06:56
Smile Numerical scheme for DES and SAS in CFX
Tingyun YIN
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Hi, everyone.

When we switch on DES or SAS model in CFX, an additional option occur in the solver control plane. It's called 'CDS blending'. As is well know, different numerical schemes could be applied to different regions when using DES or SAS model: CDS for LES region and Second order for RANS region. So, here are my questions:

1. When we switch on the default DES model in CFX, the default numerical scheme is hybrid scheme? The advection scheme option is available only for RANS region because CDS is used for LES region as default. So, when 'CDS blending' is activated, there are two functions: The first one is to prevent the grid-induced separation (as default, minimum RANS blend= BF1=0, maximum RANS blend= BF2=1) and avoid oscillations due to the central difference scheme (as default, Maximum Courant Number=5, Limiter Exponent=1).

2. If the description of question 1 is correct, so, when we switch on the default SAS model and activate 'CDS blending' function, why minimum RANS blend and maximum RANS blend still occur? As we all know, no grid-induced separation occurs when using SAS model. So, what should I do? Don't activate them? To avoid oscillations of CDS, Maximum Courant Number=5, Limiter Exponent=1 can be activated.

3. If the description of question 1 is wrong, so, when we switch on the default DES model, we have to activate the 'CDS blending' to activate the hybrid scheme?

Last edited by Tingyun YIN; March 20, 2018 at 07:17. Reason: Questions supply
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cfx, des, numerical scheme, sas

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