Gravity waves and forcing efficiency

Convection generates acoustic and gravity waves which, in addition to propagating far from their source, may also do work in the vicinity of the forced region from which they originate. This work may be thought of as a kind of resistance to the forcing that is imposed by the dynamical response of the surrounding fluid. This resistance may affect the amplitude of the response and/or encourage the forcing to assume a more efficient configuration.

In order to investigate the effect of the dynamical response associated with acoustic and gravity waves on the energetic efficiency of imposed heat and mechanical forcing, this paper presents calculations in an idealised 2-d linear compressible model. Two types of forcing distributions are considered --- spatially isolated forcing, and an ensemble of forcing elements that may generate a dynamical response that propagates horizontally and interacts with neighboring forcing elements. The efficiency of an isolated forcing is shown to depend strongly on the characteristics of the background flow. For example, a maximum efficiency occurs for forcing moving at a flow relative speed for which the propagating dynamical response is in phase with the moving forcing.. The efficiency of an ensemble of forcing elements is shown to differ significantly from that corresponding to an isolated forcing.