Acoustic Heating Produced in the Boundary Layer

Instantaneous acoustic heating of a viscous fluid flow in a boundary layer is the subject of investigation.
The governing equation of acoustic heating is derived by means of a special linear combination of conser-
vation equations in the differential form, which reduces all acoustic terms in the linear part of the final
equation but preserves terms belonging to the thermal mode. The procedure of decomposition is valid in
a weakly nonlinear flow, it yields the nonlinear terms responsible for the modes interaction. Nonlinear
acoustic terms form a source of acoustic heating in the case of the dominative sound. This acoustic source
reflects the thermoviscous and dispersive properties of a fluid flow. The method of deriving the governing
equations does not need averaging over the sound period, and the final governing dynamic equation of
the thermal mode is instantaneous. Some examples of acoustic heating are illustrated and discussed, and
conclusions about efficiency of heating caused by different waveforms of sound are made.