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Low Y+ vs High Y+ CFD simulation for determining DRAG

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Old   September 18, 2017, 16:34
Default Low Y+ vs High Y+ CFD simulation for determining DRAG
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Suraj Pawar
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I was trying to determine the drag of underwater vehicle. I have also done CFD analysis of open water performance of propeller and I got good results by maintaining low Y+(nearly 1) over the blade surface.

But, when I run the simulation for determining drag of underwater vehicle I found that CFD model is under predicting the drag when I use low Y+ over the body surface. The CFD results gives good results when wall Y+ value is higher (around 30) over the body surface. I am comparing my results with drag obtained from ITTC 1957 line and using form factor.

My understanding was that when I have low Y+ value over the body surface, the velocity will get resolved till wall boundary by solving momentum equation. Also for first grid point from wall boundary u+=Y+ relation is used. And when Y+ is higher then log law is used for determining the velocity of first grid point from wall. (Please correct me if I am wrong). If my understanding is correct then CFD results with low Y+ should be more accurate because velocity near wall is determined by solving momentum equation instead of using log profile of velocity.

I am using K-w SST turbulent model with all Y+ treatment in StarCCM+.

Thank you in advance.
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Old   September 19, 2017, 00:58
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Uwe Pilz
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I am not familiar with StarCCM+.

If you run a RANS simulation the turbulence in the near of the wall is modeled. The model works fine if you have an y+ of around 30. If you have a too small y+ you don't have any turbulence model in the Prandtl layer.

To get valid results with low y+ you have to choose a solver which can model the turbulence effects in the boundary layer directly.
Uwe Pilz
Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)
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Old   September 19, 2017, 03:15
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Timofey Mukha
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The k-omega sst model has been originally developed to work with meshes that resolve the velocity up to y+ around 1, so it should work fine. No idea how exactly the concrete implementation in StarCCM works though.

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