This paper describes an experimental comparison of two passive approaches for controlling the shock interaction with a turbulent boundary layer: low-profile vortex generators and a passive cavity (porous wall with a shallow cavity underneath). This investigation is the first known direct comparison of the two methods wherein the advantages and disadvantages of both are revealed. The experiments were conducted with a normal shock wave in an axisymmetric wind tunnel. The shock strength (M = 1.56-1.65) was of sufficient magnitude to induce a large separation bubble, thus causing substantial boundary-layer losses. The low-profile vortex generators were found to significantly suppress the shock-induced separation and improve the boundary-layer characteristics downstream of the shock. However, the suppression of the separation bubble decreased the extent of the low total pressure loss region associated with the lambda foot shock system which results in a lower mass-averaged total pressure downstream of the shock. The passive cavity substantially reduced the total pressure loss through the shock system (and thus wave drag) by causing a more isentropic compression over a larger lateral extent. However, the boundary-layer losses downstream of the shock were significantly increased.

Shock/boundary-layer interaction control with vortex generators and passive cavity

Turbulent drag reduction research at NASA langley: progress and plans☆

Flow control can be applied to shock wave/boundary layer interactions to achieve two different goals; the delay of shock-induced separation and/or the reduction of stagnation pressure losses, which cause wave drag or inlet inefficiencies. This paper introduces the principles and main techniques for both approaches and assesses their relative suitability for practical applications. While boundary layer suction is already in wide use for separation control, the most promising novel device is the micro-vortex generator, which can deliver similar benefits to traditional vortex generators at much reduced device drag. Shock control is not yet used on practical applications for a number of reasons, but recent research has focused on three-dimensional devices which promise to deliver flow control with improved offdesign behaviour. Furthermore, there are some indications that a new generation of control devices may be able to combine the benefits of shock and boundary layer control and reduce shock-induced stagnation pressure losses as well as delay shock-induced separation.

SBLI control for wings and inlets

Several passive separation control techniques for controlling moderate two-dimensional turbulent flow separation over a backward-facing ramp are studied. Small transverse and swept grooves, passive porous surfaces, large longitudinal grooves, and vortex generators were among the techniques used. It was found that, unlike the transverse and longitudinal grooves of an equivalent size, the 45-deg swept-groove configurations tested tended to enhance separation.

Turbulent flow separation control through passive techniques

This paper presents an overview of some of the recent developments that have taken place in the understanding, prediction and control of two-dimensional shock-wave-turbulent-boundary-layer interaction at high speeds. Following a brief description of the upstream influence phenomena, detailed discussions of incipient and fully separated flows at supersonic and transonic speeds are presented. A brief account of certain gross unsteady features of shock-separated flows is given next. Typical examples demonstrating the current ability to predict these complex flows are also included. Finally, a review of techniques using suction and tangential blowing for controlling shock-separated flows is presented.

Shock-wave-turbulent-boundary-layer interaction and its control - A survey of recent developments

An investigation of the passive shock wave/boundary layer interaction control for reducing the drag in 12 percent thick circular arc and 14 percent thick supercritical airfoils was conducted in a 3 in. x 15.4 in. transonic wind tunnel at transonic Mach numbers. A porous surface with a cavity beneath it was positioned on the area of the airfoils, mounted on the test section bottom wall, where the shock wave occurs. The static pressure distributions over the airfoil, the wake impact pressure data for determining the profile drag, and the Schlieren photographs for porous surface airfoils are presented and compared with the results for solid surface airfoils. With the porous surface the normal shock wave for solid surface was changed to a lambda shock wave system, and the wake impact pressure data indicated an appreciable drag reduction for both airfoils with the porous surface at transonic speeds while causing little or no loss of lift. The effect of porosity and cavity size is investigated and off-design performance is discussed.

L. Bahi

Papers

Passive shock wave/boundary layer control for transonic airfoil drag reduction