Study of vortex stability in swept wing configurations using theory, experiment and simulation

Swept wing planforms enable highly-agile and high-lift flight in aircraft and UAVs. The most important flow feature on such planforms is the unsteady leading-edge vortex (LEV) which is created owing to flow separation from the edges of the wing. This creates a low-pressure area on the suction side of the wing, enabling high lift and maeuverability. LEVs however are subject to instabilities which could lead to vortex breakdown. A better understanding of the instabilities and the development of flow-control methods to alleviate or eliminate them could contribute to an extension of the flight envelope in delta-wing based UAVs and aircraft. This project aims to develop new analytical and low-order numerical methods for representing vortex-based flight on swept-wing configurations. The novel aspects of the proposed research are to study the flow in a fully unsteady sense, and to develop a dynamical model for the system based on critical points (such as half saddles, full saddles and nodes) in the flow. To support the development of theoretical and numerical models, the critical points in 3D LEVs will be systematically studied using experiments. Experimental work will be conducted using the University of Glasgow's subsonic wind tunnel facilities. Advanced flow diagnostic techniques (stero-Particle Image Velocimetry, smoke flow visualisation, 3D Laser Doppler Anemometry, and unsteady Pressure Sensitive Paints) and a 6 component sting balance will be used. The locations and characteristics of vortex enhancement, breakdown and trajectories in particular will be looked for. The experimental setup includes a custom-built pitch-plunge-surge facility developed that is comprised of three linear actuators, controlled through LabVIEW, to create the different motions to investigate leading edge separation Transient CFD simulations (using an in-house OpenFOAM implmentation) will also be used to mutually validate experiments and to support model development. Further understanding of the interaction mechanisms between LEVs will allow wings to be designed to operate more efficiently and safely. Understanding of the conditions in which different types of stable LEV configurations exist will aid in avoidance and control of vortex breakdown events, and enable safe operation within the envelope of conditions where favorable LEVs exist. Using the knowledge gained, flow control for sustaining stable vortices will be investigated in the final phase of the project. Passive (vortex generators, surface roughness elements) and active (suction/blowing, plasma actuators) strategies will be considered.