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David September 27, 2002 10:59

Problem about 3D blunt body high Re simulation

During my learning Fluent by myself, I met a problem when calculating a square building model immersed in boundary layer, which dimensions were B:D:H=1:1:4.The result mean pressure seemed unreasonably large or small. My calculation tried to simulate the real wind tunnel test, and all the physical parameters that the numerical model needed were set the same as the test. The model was 15cm15cm60cm, and the field was 400cm180cm180cm. I had tried several mesh arrangements, including unstructural tetrahedral mesh, unstructural hexahedral mesh, hybrid mesh, and partitional structural mesh; with/without boundary layer mesh. The top velocity of the model was about 13m/s,the Reynolds number based on H(model height)and top velocity was about 5.27*10^5; RNG k-epsilon turbulence model& non-equilibrium wall function, SIMPLE & second order upwind scheme were adopted. All the dimensionless convergence criterions were set 10^-5. The result maximum mean pressure coefficient Cp was greater than 1 or lower than 0.5, which obtained by dividing by a reference dynamic pressure at the top of the model. Obviously it was unreasonable. But another case I tried, which dimensions were B:D:H=1:1:1 and Reynold number was 2.4*10^4, obtained good result.

I thought it was maybe caused by the mesh generation near the wall of model in Gambit. During the mesh generation prior to calculation, the first cell position was satisfied with Y+ equal to 30, which calculation was according to several approximate formulas. These problems frustrated me a few days:

1. What the problem of my simulation exists?

2. How to determine the near wall mesh generation? Is there a proper formula to estimate the near wall mesh distance prior to calculation?

Any suggestions would be great appreciated.

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