Setting up a Flow-Induced Noise Problem in COMSOL
When setting up and solving an aeroacoustic flow-induced noise problem in COMSOL Multiphysics, using the built-in functionality of the Acoustics Module, four main steps are necessary:
1
Solve the flow problem using an LES/DES model to resolve the structure of the turbulence. This problem is solved on an appropriate mesh for the flow with appropriate time stepping settings. This typically consists of several steps, for example, first solving a stationary RANS model to get a good initial condition for the transient LES/DES model. Then an initial LES/DES model has to be solved to get a fully developed turbulent flow. In this step the solution only needs to be stored at the end time. Finally, the LES/DES model is run with the fully developed turbulent flow as initial condition. Here, the solution is stored with an appropriate time resolution defined in the Output times.
A time resolution Δt (for the output times) will lead to a maximum frequency resolution of fmax = 1/Δt/2 in the frequency domain acoustics problem. Moreover, the length of the simulation (Tend) will lead to a frequency resolution of
Δf = 1/Tend. Create parameters for these values.
2
Map the solution from the CFD mesh to the acoustics mesh. This is done by using the Transient Mapping study step. The input to this study step is only the Time step Δt used in the LES/DES study. In the Transient Mapping study step it is only the Aeroacoustic Flow Source Coupling that is solved for.
3
Transform the mapped flow sources to the frequency domain using the Time to Frequency FFT study step. In the Study Settings for the study step, the End time is set to Tend and Maximum output frequency is set to fmax. In the Time to Frequency FFT study step it is also only the Aeroacoustic Flow Source Coupling that is solved for.
4
Finally, solve the acoustic problem in the frequency domain. In the Frequency Domain study step it is only the Pressure Acoustics, Frequency Domain physics that is solved for. The Values of variables not solved for settings should point to the Time to Frequency FFT study.
Cavity Flow Noise tutorial available in the COMSOL Application Gallery at: https://www.comsol.com/model/cavity-flow-noise-105021
For further details about LES and DES see the CFD Module User’s Guide:
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The Creeping Flow, Laminar Flow, Turbulent Flow, Large Eddy Simulation, and Detached Eddy Simulation Interfaces
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Theory for the Large Eddy Simulation Interfaces
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Theory for the Detached Eddy Simulation Interfaces