The Acoustic Streaming from Pressure Acoustics and Acoustic Streaming from Thermoviscous Acoustics multiphysics interfaces are used to model the physical phenomena acoustic streaming where an acoustic field can induce a fluid flow. The multiphysics couplings Acoustic Streaming Domain Coupling and Acoustic Streaming Boundary Coupling are used to compute the forces, stresses, and boundary slip velocities that the acoustic field induce in the fluid, generating flow. The interfaces couple a frequency domain acoustic field to a stationary or time-dependent fluid flow. The acoustic streaming phenomenon is important in microfluidics and lab-on-a-chip systems for applications such as particle handling, mixing of fluids, and microfluidic pumps.

The Acoustic Streaming Domain Coupling is used to compute all the forces and stresses in the bulk of the fluid domain, while the Acoustic Streaming Boundary Coupling is used to compute a slip-velocity at a fluid boundary due to boundary contributions. For some simulation models it is sufficient to include either the Acoustic Streaming Domain Coupling or Acoustic Streaming Boundary Coupling while for other models it is necessary to include both multiphysics couplings, see Modeling Acoustic Streaming for modeling advice.

The coupling features couples a frequency domain acoustics interface to a stationary or time-dependent single-phase fluid flow interface. The acoustic interface can either be the Pressure Acoustics, Frequency Domain interface or the Thermoviscous Acoustics, Frequency Domain interface, while it has to couple to a single-phase fluid flow interface. Typically the Laminar flow or the Creeping flow interfaces. The interface can couple to a turbulent flow interface but it is advice to take extra caution since the derivation of the source terms does not take turbulent flow into consideration.

The acoustic streaming is a nonlinear phenomenon that occurs due to nonlinearities in the Navier–Stokes equations. When linearizing the Navier–Stokes equations the acoustic fields are the first-order fields in a perturbation theory. The acoustic streaming fields are the time-averaged second-order fields of the perturbation scheme. Thus, the time-varying second-order fields are neglected together with higher-order terms. Therefore, nonlinear effects as higher-order harmonics and nonlinear behavior of the acoustic field are not included in the derivation of the acoustic streaming fields. For a more detailed introduction to the theory see the section Theory Background for the Acoustic Streaming Multiphysics.