The Normal Displacement condition adds an acceleration source at a boundary. See the Normal Displacement in the The Pressure Acoustics, Frequency Domain Interface.

Note that the Normal Displacement condition contributes with the Wall feature. It specifically inherits the defined Mechanical condition (No slip or Slip (perfect)) and Thermal condition (Isothermal or Adiabatic) from the Wall feature. For specific combinations of mechanical and thermal conditions with the source, add a separate Wall feature on the source boundary. When No slip is used together with the source, the formulation — used in the SLNS formulation for thermoviscous acoustics — can result in a solution that looks unphysical near the moving wall. However, these artifacts are only local effects that do not influence the overall results.