Supersonic and Hypersonic Aerodynamics

Participants:  F.F.J Schrijer,  F. Scarano

Activities:

High-speed flight and Re-entry Aerodynamics

This subject deals with the aerodynamics of hypersonic speeds in particular, where one encounters significant aerothermodynamic effects, in that the temperatures may become so extreme that normal air will dissociate or even ionize. Aerothermodynamics implies that the aerodynamics and, in its general case, the thermodynamics of not only nitrogen and oxygen molecules must be considered, but of quite a number of species. The special treatment of aerodynamic problems incorporating high enthalpy phenomena starts already if local temperatures of about 800 K occur, since at that level the vibrational modes of molecules may be excited. At approximately 2000 K oxygen will dissociate, at 6000 K nitrogen, whereas ionization starts at about 9000 K. These temperatures are very realistic in hypersonic flight conditions, such as occurring during the atmospheric re-entry of space vehicles. At present in the Aerodynamics Laboratory of the Aerospace Department the experiments are performed using so called “cold hypersonics”, meaning that real high-enthalpy problems occurring for example in stagnation point regions are not simulated. In spite of this limitation sufficient experimental possibilities for research remain. The work then involves problems where phenomena like viscosity and heat transfer are important. In such cases the Mach number (simulation of flow geometry), Reynolds number (simulation of viscosity), Prandtl number (simulation of heat conduction) are relevant parameters. Specific topics of interest are hypersonic transition, flow separation and shock-boundary layer interaction. Some instances are described below:

· Plug nozzle project: flow along an expansion-streamline body extension behind a 2D nozzle. This exhaust system is applied at the X-33, Venture Star (numerical and experimental).

· DART project: aimed at building, testing and flying of a small re-entry vehicle. 

· Hypersonic flow over flaps and rudders of re-entry vehicles (numerical); this a DART-related project. · Experiments on the DART model have been carried out in the hypersonic windtunnel, including the use of quantitative infrared thermography to determine surface heat transfer. Further experiments on hypersonic transition induced by roughness (surface imperfections) have been performed for ESTEC in relation to the European “EXPERT” re-entry demonstrator project.

· In the framework of the European space project FLTP (Future Launchers Technology Program) the Aerodynamics group takes part in base flow analysis. It involves the interaction of a rocket exhaust and the external flow.  An extended study is foreseen to investigate more realistic conditions, by comparing results where the exhaust jet is cold (Aerodynamics Laboratory) or hot, stagnation temperature of 2000 K (Prins Maurits Laboratory of TNO).

· Shock-shock interactions: the mechanical and thermal loading of a stagnation region on a vehicle (nose, wing and fin leading edges, engine intake lip) can increase dramatically when the curved bow-shock is intersected by a (relatively weak) oblique shock, originating from other parts of the vehicle.

Base-flow Aerodynamics

The subject of “Base Flow Aerodynamics” has been receiving more interest the past few years. Investigation is needed for the base flow behind launchers like the Ariane 5 (Fig. 1) as well as for re-entry vehicles. For launchers the interaction between the rocket exhaust jet and the transonic/supersonic external flow deteriorates the performance, and base flow is the trigger. For future rocket nozzle technologies, such as the plug nozzle (Fig. 2), the performance is even more sensitive to the external flow and therefore the base flow has even more impact. The aerodynamic stability of re-entry vehicles is a further important item, and because base flow is sensitive to time-dependent aspects this should be investigated in more detail. Therefore, there are quite a number of spaceflight applications in which the base flow plays an important part.

In order to facilitate the understanding of the base flow parameters, a generic form for base flow applications has been chosen. The critical parameters for this generic shape are: 1. Approaching freestream Mach number 2. Character of the boundary layer (laminar or turbulent) 3. Boundary layer thickness in proportion to the base height 4. Base heating

To further increase of the base pressure, which decreases the base drag, one can think of different geometrical modifications, like boattails, additional cavities, sting and discs, or application of base bleed and base combustion. But first of all, a proper theory is needed for understanding the first four critical parameters. Therefore, a semi-analytical theory will be developed.

For this purpose experimental studies are also of the highest interest, like PIV investigation to reveal the unsteady features of the base flow. For its numerical investigation the existing ENS-code will be improved with these unsteady features and appropriate turbulence models.