An Experimental and Analytical Study of the Stability of Counter-Rotating Vortex Pairs with Applications for Aircraft Wake Turbulence Control

Aircraft trailing vortex wakes are commonly referred to as `wake turbulence' and may pose a flight safety hazard to other aircraft that may encounter the wake. This hazard is of critical interest during the take-off and landing stages of flight, where aircraft are in the closest proximity to one another. During these flight stages, it is common for transport aircraft to be in a high-lift, or flaps down, configuration. In an effort to study these wakes a generic four-vortex wake is generated experimentally, such that the results are independent of a specific wing loading condition.

Three principle objectives served to focus the research project that is presented in this dissertation. The first two objectives were to develop an improved understanding of the wake configurations that were conducive to large instability growth rates and to subsequently use quantitative methods to identify the instability modes that dominate the far-field wake dynamic. With a clear understanding of the physics of an unstable aircraft wake, the third objective of the research project was to use this newly attained information to recommend methods for a reliable wake control strategy.

A compilation of flow visualization results shows a design space of counter-rotating wake configurations, defined by the circulation and span ratios, where rapidly amplifying instabilities are consistently seen to exist. This design space is also seen to encompass rigidly-translating wake systems. A combination of quantitative flow visualization estimates, hot-wire anemometry and an analytical stability analysis was successful in identifying two forms of bending wave instability, namely the long and short-wavelength modes. Having identified two bending instability modes in the experimental wake, it was possible to suggest a strategy by which these modes could be exploited for the control of aircraft wakes.