A steady-unsteady visualisation technique for wake-flow studies

A flow-visualization technique in which the test body is pulled through water is described. Initially, dyed and undyed parts of the water are separated by a plane horizontal interface; as the body emerges from below this interface, it carries dyed fluid upwards in its wake. A succession of photographs reveal the nature of the flow. In principle, the flow is steady in a frame fixed relative to the body, and unsteady in one relative to the tank and the initial interface. Interpretation of the experimental results must therefore proceed by way of an analysis in which the steady state equations are solved for velocity and turbulence quantities but an unsteady state equation is solved for the dye concentration. The flow behind a wedge is studied. This particular flow is unsteady even in the body fixed coordinate frame, which is strongly oscillatory.