Reynolds stress is the most important quantity affecting the mean flow as it is responsible for a major part of the momentum transfer in the wall bounded turbulent flow. It has a direct relevance to both skin friction and flow separation. Manipulation of the Reynolds stress can directly lead to changes in the viscous stress at the wall so as to effectively control the flow for effective flow control. However, there is a lack of current understanding of the inter-relationship between the various flow control devices and the Reynolds stresses in the flow field they produced.
An improved understanding can potentially significantly improve the effectiveness of flow control as the Reynolds stresses are closely related to the flow behaviour at the surface for effective separation control or drag reduction.
A variety of control devices are available and new ones are invented, but which one for what purpose is an open question yet to be fully answered. The MARS project proposes to reverse that process and considers the long term goal of controlling dynamic structures that influence the Reynolds stress that changes the mean flow. This radical approach recognises we are still some way away from hardware to implement it at flight scales but if successful, would establish a first important step towards our ultimate ambition.
The focus of MARS will be on the effects of a number of active flow control devices on the discrete dynamic components of the turbulent shear layers and the Reynolds stress. From the application point of view, MARS provides a positive and necessary step in the right direction wherein it will demonstrate the capability to control individual structures that are larger in scale and lower in frequency compared to the richness of the time and spatial scales in a turbulent boundary layer. MARS will investigate active flow control means rather than passive controls.
Turbulence suppression by active control
Numerical experiments have reached a maturity allowing EU-funded scientists to confirm wind tunnel measurements of active flow control devices. They proved that efficient manipulation of natural instability phenomena can improve the aerodynamic performance of aircraft.
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' (http://www.cimne.com/mars/ (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.