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#1 |
New Member
Sagar More
Join Date: Jan 2014
Location: pune,india
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I am carrying out horizontal axis wind turbine CFD simulation in Fluent to check effect of surface roughness of blades. It’s a steady state simulation using moving reference frame with k-omega SST model and air as working fluid.
Wind velocity=8 m/s Angular velocity= 52.359 rad/s (500 rpm) Relative velocity=52.967 m/s Reynolds number=159790.244 Chord length=0.045 m Desired y+= 1 Using these conditions first cell height comes to be 5.48048E-06 m (for external aerodynamics based on turbulent flat plate). I want to check torque output for roughness height up to 30 micron. My questions are; 1. Literature suggests that centroid of first cell (half of first cell distance) should lie above roughness height. So how to model roughness height larger than half of initial cell height? Is this requires shifting of law of the wall? If yes please describe. 2. How to specify equivalent sand-grain roughness height? |
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#2 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
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Quote:
2. The roughness height parameter that you need to input in Fluent is the equivalent sand-grain roughness (it is not the RMS roughness in materials science). Or is your question the other way around (knowing rms, how to compute equivalent sand-grain roughness)? |
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#3 |
New Member
Sagar More
Join Date: Jan 2014
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Thanks for your prompt reply.
Does this mean fluent automatically ignores cells whose centroid lies below roughness height and considers cells whose centroid lies above roughness height (thus "virtually" shifting wall)? If so, then will fluent start measuring y+ from a virtually shifted position of the walll? I want to use low Reynolds number (k omega SST) model.My worry is about maintaining y+ value as one. |
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#4 |
Senior Member
Lucky
Join Date: Apr 2011
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The y+ of the wall adjacent cell is still computed from the wall adjacent cell, cells are not ignored. The virtual wall shift is a simple modification to the calculated y+ of the first cell. Hence "virtual" rather than actual/explicit. I don't mean to argue about semantics but since you are asking how Fluent is going to perform the task, it helps if you use the definition of "virtually shifting the wall" per Fluent's definition and not your own definition. Actually I agree that your definition of virtual wall shift makes more sense.
I believe the virtual wall shift is done for the k-omega SST model but I recommend reading Fluent User Guide 6.3.14.2.9 Law-of-the-Wall Modified for Roughness since different approaches are used for different models. |
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Tags |
k omega sst, law of the wall, roughness height, y plus selection |
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