Shock wave boundary layer interaction (SWBLI) leads to separation at Mach numbers above M = 1.3. The process of separation usually becomes unsteady and causes buffeting at airfoils and shock oscillations in the internal flows. In such cases it is important to use control methods which are able to suppress or reduce separation and to diminish the unsteady effects. Reduction of shock movement is a very important aspect, which results in dumping of the vibrations induced by unsteady forces.
Normal shock wave boundary layer interaction is still an important problem in compressible aerodynamics and still finds a lot of attention. In many research centres there are taken a lot efforts to find uncomplicated, effective and easy in steering flow control method [e.g. 1]. In most cases those methods rely on energizing the boundary layer upstream of the critical place (e.g. shock wave, separation), making it more resistant to the adverse pressure gradient and therefore less likely to separate. One of the flow control method is based on streamwise vortices production in the flow. The streamwise vortices are produced by means of solid vortex generators (vane types, small wedges) or air-jet vortex generator. The jet in flow act like an obstacle, deflecting the incoming flow and in this way starts creating streamwise vortex. The same vortical structure can be also produced replacing the jets by the rods. In some applications it can be more convenient to install the rods instead of jets, especially in externals flows (e.g. on the airfoils).
This paper presents an experimental investigation concerning the application of rod vortex generator (RVG) for the shock wave – boundary layer interaction control. In many applications the shock waves induce separation and lead to strong unsteady effects. In these two context it is proposed to use vortices generated by the rods for the interaction control. This streamwise vortices generation method is useful because it allows to apply a simple method of switching it on and off. Such a vortex generator can be build in MEMS technology and the application in the external flows seems to be much easier than the fluidic jests which need channels for the air supply. The rods have to be introduced in appropriate angles in relation to the main stream. There have been analyzed the rod configuration affects on the intensity of the generated vortices. The results of the analysis was taken into account in the experimental investigations.
An experimental program has been carried out in the transonic wind tunnel in two different type of the test sections: rectilinear nozzle and curved wall nozzle. The rectilinear nozzle characterizes zero longitudinal pressure gradient in the investigated area. A normal shock wave and the vortex generator were located in the area of a uniform velocity. The flow in the curved nozzle accelerates (positive longitudinal pressure gradient) along the lower wall creating a local supersonic region terminated by a shock wave. The shock is the strongest at the convex wall and becomes weaker away from the wall, vanishing before reaching opposite concave wall. Thus the flow pattern in interaction area is very similar to external flow e.g. on the airfoil. The Mach numbers chosen for tests were in range M=1.43÷1.45 which correspond to fully separated flow.
RVG’s in the test sections were located in one row across the whole width of the test section The rods were located upstream of the interaction area in a distance of 10 × δ (boundary layer thickness). To investigate some aspects of the influence of RVG’s on SWBLI the following measurements was carried out:
- static pressure distribution along the centre line of the test wall,
- boundary layer profile upstream of the interaction area
- boundary layer profile in two locations downstream of the interaction area
- schlieren visualization of the shock wave structure, position and oscillation
- oil flow visualization at the test wall to display separation size and structure
- high speed movie visualization of the shock oscillation.
The very promising results were obtained from these experimental investigations in the context of SWBLI control by RVG’s. Reduction of separation size and shock wave unsteadiness has been obtained in the flow with vortex generators. Also one can observer the traces of vortices in boundary layer profile in the interaction area. The most important is that a new method of streamwise vortices generation, using an appropriately inclined rods, can replace in the flow control the air-jet vortex generators [2] especially in the external flows.
BIBLIOGRAFY
[1] Pearcy, H., Shock induced separation and its prevention by design and boundary layer control. Edited by C.V. Lachmann, Pergamon Press, 1961, 1166-1344
[2] Szwaba R., Comparison of the influence of different air-jet vortex generators on the separation region, Aerospace Science And Technology, 2011, vol. 15, 45-52
