Unsteady flow diagnostics using weak perturbations

Abstract

The examination of the motion of very weak waves generated from small sources on the boundary of a flow domain gives information on how the domain shape influences the flow, both from spatial and temporal perspectives. In the study of shock wave dynamics the shock itself generates a weak wave when passing over a small step, or surface irregularity. The basic principle is that if a particle produces a series of point disturbances in a flow field, the induced perturbations will propagate outwards at the local sonic velocity whilst at the same time being convected along with the local flow velocity. A number of issues may be identified for an unsteady flow. Firstly, the flow field at later times may be influenced by perturbations produced at earlier times. Secondly, if the positions of the perturbations can be monitored as a function of time, then the trajectory and velocity of the particle may be deduced. Thirdly, if a perturbation arises from a point on a boundary, then its influence, if any, on any particular part of the flow can be established. A number of examples are presented to illustrate the value of the technique and its potential to uncover the mechanisms responsible for the formation of certain flow patterns in high-speed compressible flows.