Selecting an Aeroacoustics Interface
When modeling a muffler with an internal nonisothermal flow, a jet engine, or a flow sensor you should consider which physics interface to use. The influence the background mean flow has on the acoustic behavior in an aeroacoustic model can be modeled in several ways. The effects that need to be included typically depend on the Mach number (Ma). A rule of thumb says that for a Mach number below 0.1 (Ma < 0.1) the convective effects of the background flow need not to be included, above they do. Other considerations are of course also important; for example, whether viscous and thermal losses are important, if the background flow has large gradients, or if the flow is turbulent or has vorticity.
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Mach number less than 0.1 (Ma < 0.1)
In this situation the convective flow effects can normally be neglected and Pressure Acoustics can be used. Only the background temperature distribution T = T(x) and background pressure distribution pA = pA(x) need to be included. This can be done directly in the Pressure Acoustics interface as a Model Inputs. The effects are included by making sure that the material properties depend on the local pressure and temperature; the spatial variations in the speed of sound is c = c(pA,T) = c(x) and in the density ρ = ρ(pA,T) = ρ(x), respectively.
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Mach number greater than 0.1 (Ma > 0.1):
In this case the convective effects of the flow can probably not be neglected. Modeling this type of system requires the use of one of the Aeroacoustics interfaces: linearized potential flow, linearized Euler, or linearized Navier–Stokes. The choice depends on the assumptions about the flow that can be made.
Note that when the background mean flow velocity u0 is set to zero, the linearized Navier–Stokes equations reduce to the thermoviscous acoustic equations. However, in thermoviscous acoustics the default discretization is P1, P2, P2 and no stabilization is applied.