Pitfalls in the Interpretation of Delta Wing Flow Visualisation

摘要

This paper illustrates some pitfalls in the interpretation of flow visualisation images of delta wing leading-edge vortices. Numerical simulations of tracer particle trajectories in around straight and helical vortices are presented, representing unburst and burst delta wing vortices respectively. For straight vortices, the appearance of the 'smoke-ring' effect in wind tunnel testing is analysed, and shown to be unrelated to the size or presence of a viscous inner core in the vortex. The role of the swirl-induced radial pressure gradient in the vortex centreline trace seen in water tunnels is demonstrated. For helical (burst) vortices, the crossflow topology visualised with upstream tracer injection is shown to depend on the vortex core size and the helix pitch. Typical vortex helices are close to a flow regime which can produce visualised flow structures resembling axisymirietric bubble breakdowns. It is postulated that the intermittent open-ended bubble burst seen in delta wing vortices is in fact an artefact of the tracer flow visualisation technique, as transient disturbances temporarily reduce the pitch of the fundamental helical burst structure.