[murali666]

I am doing both RANS (k-w SST) and LES simulations to simulate a 3D vector field around a bridge pier.
I am using Hexa mesh in the whole flow domain developed in ICEM and providing the inflation layers near to the walls (bottom surface and cylinder) to accurately simulate the near wall effects. These inflation layers significantly effecting the mesh aspect ratio.

The mesh metrics of the developed mesh are
Aspect ratio = 1 (min) - 300 (max)
Erikson skewness = 0.8 (min) - 1 (max) (Recommended range of values as per ICEM manual is 0.5 to 1)
Mesh Quality = 0.8 (min) - 1 (max) (close to 1, better the mesh)
Determinant 3*3*3 = 0.92-1 (close to 1, better the mesh)

All the mesh metrics other than aspect ratio are in the acceptable range. The mesh elements with the worst aspect ratio are present in inflation layer region. The improvement of aspect ratio in that region leads to a significant increase in the number of mesh elements, such high number of mesh elements are not necessary in the regions away from the wall.

So please explain me the importance and aspect ratio in both RANS (k-w SST) and LES simulations and what will the suggested action in the above explained situation.

[alainislas]

Hi, from my experience is totally normal to have aspect ratios (AR) of 300 in the inflation layers. Depending on the y+ value the AR may increase up to few thousands (or even more). The important thing is to capture the intrinsic physics happening in the boundary layer, e.g. velocitiy and temperature gradients, shear stresses, skin frictions, etc ... To exploit the full capacity of kw-SST you should go for y+ ~ 1-5 (viscous layer), so AR will tend to increase (for LES, y+ <1 is obligatory). Furthermore, for LES mesh requirements are more strict in the sense of spatial resolution in the bulk, see the theory behind filtering, energy spectrum, and turbulence scales.

Again, high AR in the inflation layers is not critical, unless you working on DNS or LES at moderate Re numbers, keep the cell count reasonable. I'd concern more about non-orthogonality (<0.70), and/or min angle (>18), but since you using hexa I asume these are in conformance.

[murali666]

Thank you so much for your response,
The min angle of the mesh is 53(min) - 90(max).
Is Y plus values of 1-5 are sufficient for kw-SST simulations?
Is there any procedure to determine the min size of the mesh for LES simulations?

[alainislas]

(1) Yes, y+ 1-5 are OK for kw-SST.
(2) For LES, check your Re number and calculate a characteristic cell size based on the Taylor length scale (largest eddies). Time steps should satisfy a CFL condition CFL <1.0 too. A good reference for LES index of quality is provided by (Celik, 2005) https://doi.org/10.1115/1.1990201. Depending on the SGS viscosity, it will help you to estimate how much TKE your mesh is resolving. Again, see the theory behind filtering, energy spectrum and turbulence scales.