Learning Forum

[ehsan.sadeghi]

I have a rectangular duct with a flap. My objective is to do a DAVA simulation by first calculating the sources in ANSYS Fluent then propogate the sound into a room by using Harmonic Acoustic simulation. As for CFD part of the study for a max frequency of 2000 Hz I have calculated the minimum cell size of 17mm. 

I start with a RANS study then switch to LES. In RANS study using the relation (inregral length scale/cell length)<5 i see that 99% of the domain fall below this criteria which means that energy spectrum will be highly underresolved. 

My question is which criteria should I use to generate and assess the mesh quality for acoustic study? 17mm or the LES mesh size criteria?

[Bhanu Gupta]

The mesh resolution should be based on physics to be resolved. So the mesh size calculation for maximum frequency resolution to resolve minimum wavelength(Acoustics) Vs mesh size to resolve turbulent energy resolution for scale resolving turbulence models(Turbulence), which every is minimum size should be used. 

[ehsan.sadeghi]

Thank you for your response. To achieve a value of f<5, I will need to further refine my mesh significantly. I also have a question regarding the mesh quality near the walls. As I mentioned earlier, my goal is to use LES results to extract pressure fluctuations on the walls of the duct for harmonic acoustic analysis. However, based on my RANS results, I have noticed that the mesh resolution near the wall for LES is not fine enough (f<5) in this case. Since it is crucial to accurately capture the pressure fluctuations near the wall, I would appreciate your suggestions on how to refine the mesh in those areas. Would it be recommended to use prism layers in simulations of this nature?

[Bhanu Gupta]

Regarding capture Frequency(f) <5 you would not require refining the mesh but having longer signal length which means more flow time for recording the data(Atleast 0.2sec matured signal for f=5 Hz resolution). LES would definately require resolving BL and hence can be expensive, I would suggest using Stress Blended Eddy Simulation(SBES) turbulence model. SBES is scale resolving turbulence model resolving core flow using LES and near wall modeling using SST KW. This is one of the recommended turbulence model for Acoustics without resolving the boundary layer.

[ehsan.sadeghi]

SBES model is the one in K-Omega model right? 

one further question, from what I read in near wall region SBES will solve RANS model. So in this region what y+ is recommended for acoustic simulations?

Thanks