Friday, January 27, 2012
Energy Exchange and Control of Unsteady Lift in Gusting Flows
David Williams, Professor, Mechanical and Aerospace Engineering, Illinois Institute of Technology
Unpowered gliders achieve 400 mph flight speeds by extracting energy from spatial velocity gradients. Energy extraction from unsteady gusting flows is also possible by the so-called Katzmayer effect, and has been studied as a method to extend the range and endurance of micro-air-vehicles. The aerodynamic loads on vertical axis wind turbines are inherently unsteady during their operation, even when the incident flow is steady. In all of these cases the flight mechanics models used to predict the performance of the flight vehicles are usually based on quasi-steady aerodynamics with the assumption that unsteady flow effects are small. We explore the validity of that assumption using an unsteady flow wind tunnel to produce controlled longitudinal and vertical disturbances simulating the velocity components of a gust. Measurements of the fluctuating lift response and the rate of energy exchange between the flow and a nominally two-dimensional wing will be presented. An example of gust suppression using active flow control on a three-dimensional wing will be shown to highlight the importance of correctly modeling the unsteady aerodynamics.