Background Fluid Flow Coupling
The Background Fluid Flow Coupling () is a unidirectional 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 meshes 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.
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Helmholtz Resonator with Flow: Interaction of Flow and Acoustics. The Application Library path: Acoustics_Module/Aeroacoustics_and_Noise/helmholtz_resonator_with_flow
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Ultrasonic Flowmeter with Piezoelectric Transducers. The Application Library path: Acoustics_Module/Ultrasound/flow_meter_piezoelectric_transducers
When coupling a Fluid Flow interface to the Convected Wave Equation, Time Explicit interface it is important to have consistent settings for the Geometry Shape Function and the Discretization of the physics. The Automatic setting for the Geometry shape function (in the Curved Mesh Elements section on the Components node’s settings) results in a linear geometry representation th fit the Fluid Flow interface. This can lead to numerical errors when solving the Convected Wave Equation, Time Explicit physics as the default is to use fourth-order (quartic) spatial discretization of the dependent variables. Errors typically occur when curved boundaries are present in the model or when using multiphysics pair couplings. To remedy this change the Geometry shape function to Quadratic Lagrange.
If the settings in a model are inconsistent, a warning is given in the solver with the text:
The geometry shape function order is not consistent with the discretization of the dependent variables.
Consider increasing the geometry shape function order to Quadratic Lagrange in the Curved Mesh Element section in the Component node's settings.
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 flow is compressible also map the density. If the flow is nonisothermal also map the temperature. For linearized Navier–Stokes models 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/forced no-slip condition applied. This may modify the flow solution slightly near walls and should be used with care. When combined with the linearized Navier–Stokes physics this may 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. The smoothing represents numerical consistent stabilization comparable to source term stabilization. It is consistent in the sense that it scales with mesh and discretization. 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 using the Modify model configuration for the study step option) 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.