Near resonantly enhanced schlieren for wake flow visualisation in shock tunnels

A new variant of the resonantly enhanced schlieren or shadowgraph technique has been developed for visualising flows with small density gradients using seeded lithium metal as the resonant species. The novelty of the technique lies in the use of a diode laser as the light source for the visualisation rather than systems based upon solid-state-pumped dye lasers or spectral lamps. We present time-resolved visualisations of near-wake flows around a cylinder in a hypersonic free stream in a shock tunnel, showing flow structures that cannot be resolved using a conventional standard schlieren system. Furthermore, a method of removing, at least partially, the limitation related to line-of-sight visualisation is demonstrated.