Background Fluid Flow Coupling
The Background Fluid Flow Coupling () is a one-way multiphysics coupling between a Fluid Flow interface and either the Linearized Navier-Stokes, Linearized Euler, or Convected Wave Equation interfaces. The coupling should be solved for in a separate study using the dedicated Mapping () study.
The multiphysics coupling feature and mapping study, ensures that the fluid flow variables are mapped correctly from the fluid flow mesh to the acoustics mesh. The two mesh are typically different because of different resolution requirements, or in the case of the Convected Wave Equation interface because different shape function orders are used. The mapping also per default adds a small amount of smoothing of the mapped flow solution, this ensures that no unphysical numerical noise is introduced to the acoustics model through the reactive terms in the governing equations.
Helmholtz Resonator with Flow: Interaction of Flow and Acoustics. The Application Library path: Acoustics_Module/Aeroacoustics_and_Noise/helmholtz_resnoator_with_flow
Coupled Interfaces
Select the Source fluid flow interface and the Destination acoustics interface.
Variables to Map
Select the variables that need to be mapped, this depends on the physics selected. For most cases the background mean flow velocity and pressure need to be mapped. If the floe is compressible also map the density and if the flow is nonisothermal also map the temperature. For Linearized Navier Stokes model the turbulent viscosity can also be mapped, this ensures that acoustic waves are correctly attenuated as they propagate through regions of high turbulence.
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Select Map the pressure (selected per default), Map the velocity (selected per default), Map the density, Map the temperature, or Map the turbulent viscosity as necessary.
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For the velocity select Use no slip boundary on no slip walls if the mapped variables should have a true no slip condition applied. This may modify the flow solution slightly near walls. When combined with the linearized Navier-Stokes physics this can be necessary to get consistent no slip conditions at walls.
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The Use symmetry on symmetry walls option, selected per default, ensures that the mapped flow field is symmetric for symmetry conditions.
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For the pressure, velocity, density, temperature, and turbulent viscosity variables the option Constrain the (variable) on exterior boundaries option sets up a constrain on all exterior boundaries, where the mapped variable is set equal to the source fluid flow variable.
Smoothing
Select the Smoothing method as Isotropic diffusion (the default) or None. For the isotropic diffusion option set the (numerical) Diffusion constant (the default: 1e-2). It is recommended to always use a small amount of smoothing to ensure smooth gradients of the background mean flow variables. To verify that the amount added is adequate, for example, compare the mapped variables and the fluid flow variables in a plot.
Special care should be taken if perfectly matched layers (PMLs) are present in the acoustic model. In this case make sure to disable the PMLs (either in the model builder tree or in the study) when setting up and solving the fluid flow problem. Remember to enable the PMLs again (if disabled in the model tree) when solving the acoustics problem.
The coupling feature does not, in this version of COMSOL Multiphysics, support mapping from the High Mach Number Flow physics interfaces. Coupling can be achieved using Manual Mapping as described in the Modeling with the Aeroacoustics Branch section.