Introduction

The group has recently established a series of major advances in laser Doppler anemometry (LDA), arising from a new method of beam alignment which permits very considerably increased resolution of the flow. All three velocity components are now routinely measured at 50 µm resolution, a unique capability for long throw optics. This has led to a series of new results, including the effects of suction on turbulence development behind isolated roughness elements,the first complete measurement of the 3D boundary layer flow on a rotor, and the measurement of the emergence of longitudinal vortices in a separated shear layer on a delta wing.

Recently the LDA system has been applied to the study of the development of discrete frequency and broad-band noise from aerofoils. This latter was originally motivated by problems of rotor noise, especially for wind turbines, but is now starting to generate results which call in to question some of the established views on the development of flows on aerofoils at low Reynolds Number. Future work will examine further the impact of the acoustic environment on the transition and reattachment process in laminar separation bubbles at low Re.

Vorticity

The group undertakes work on a wide variety of vortex dominated flows, originally concentrating on separated flows on slender bodies at high angles of attack, but now looking at a variety of other applications. Vortex flows are of importance to the aerodynamics of flaps and spoilers, and of rotors, all areas in which the group has an established record of expertise. Following on from a series of visualisation studies of delta wing vortices at low speeds, recent LDA studies have revealed the complex steady and unsteady sub-structures to be found within the feeding shear layers. Further work planned to clarify the underlying instability mechanisms.

The phenomenon of vortex breakdown sets a limit on the achievable flight envelope of many swept-wing aircraft, due to a combination of lift-loss and the appearance of non-linearities and time-ependencies in the aerodynamic characteristics. A novel approach to the prediction and modelling of these effects has been developed, covering onset criteria, the form of the breakdown, the magnitude and frequency content of the consequent lift-loss and airframe buffeting, and the time-dependency of its response to wing motion. Initial results are promising, and further theoretical and experimental work is planned to validate and extend the analysis.

Rotary flows

The group has a number of rotary wing and propeller test rigs, which are primarily used in conjunction with the general-purpose low speed wind tunnel. These rigs have been used to investigate aerodynamic interactions between contra-rotating rotors, optimisation of helicopter tail rotor scissors angle, blade vortex interactions, three dimensional staff effects, teeter rotor instability, interaction of the rotor downwash with a fuselage, Magnus effect wind turbines, the stall behaviour of wind turbines, and the effect of inflow turbulence levels on performance and noise. The 3D shadowgraph flow visualisation method has been developed to provide non-intrusive real time visualisation of rotor blade trailing vortex behaviour. Current studies are concerned with the analysis and design of advanced autogiro configurations.