Turbulence suppression by active control
Airflow around aircraft is described mathematically by three components consisting of a mean flow, a dynamic but periodic component, and a random turbulence component for which Reynolds stresses are defined. The majority of work on 'taming' turbulence with the aim of reducing skin friction has focused on changes in the mean flow that result in changes to Reynolds stresses. Chinese and European researchers initiated the EU-funded project 'Manipulation of Reynolds stress for separation control and drag reduction' (MARS) to examine the problem differently. They concentrated on the effects of active flow control on the periodic component. This radically new approach allowed them to demonstrate the ability to control individual dynamic structures larger in scale and lower in frequency than in the turbulent shear layer. The performance of control devices such as plasma actuators and oscillating surfaces on dynamic structures that influence Reynolds stress was explored in wind tunnel set-ups. Detached eddy simulation and Reynolds-averaged Navier–Stokes models also provided insights into critical flow parameters. Experimental investigations and numerical simulations complemented each other for extracting flow details. Under certain conditions, unsteady flows were achieved, and the influence of the periodic component on turbulence Reynolds stresses was investigated. The findings offered MARS researchers a better understanding of the effects of flow control on turbulence Reynolds stress. These are responsible for a major part of momentum transfer in wall-bounded turbulent flows and hold the key to skin friction. In addition, MARS researchers identified candidate devices for further development to effectively reduce skin friction and thereby drag opposing the aircraft's motion under real flight conditions. The next generation of active airflow control devices could ensure more efficient air transportation with fewer emissions of harmful gases into the environment.
Keywords
Turbulence, flow control, aerodynamic performance, Reynolds stress, drag reduction
