Numerical and Experimental Investigations of Laminar/Turbulent Boundary Layer Transition

Accurate and reliable prediction of heat loads at control surfaces of re-entry vehicles in the hypersonic regime is still an important subject of CFD simulation. Surface overheating in transitional flow caused by instabilities and vortices especially in the separation region leads to significant uncertainties in thermal protection systems and wasting of material and weight. Recent investigations at the RWG facility (Rohrwindkanal Göttingen) of the DNW and corresponding CFD computations by the Institute of Design Aerodynamics deal essentially with the heat flux at the control surfaces of the NASA X-38 experimental vehicle. All computations and testing are carried out at M=6, 40° angle of attack and 20° flap deflection angle in a Reynolds number range from 2.1 Mio to Re=8.8 Mio, corresponding to the Reynolds number range under flight conditions. One aim of the CFD study was, apart from code validation in transitional and separated flow, an investigation of predicted three-dimensional instabilities in and after the separation region of the deflected flaps, which lead to the mentioned overshoot in heat fluxes. For resolution of instability effects, different local meshes in the region of control surfaces have been extracted from the original X-38 mesh and extensive studies of the influence of wake and further distant flow have shown sufficiency of grids in the flap environment. Satisfying mesh convergence was shown by local node doubling in all directions in laminar and turbulent cases, as well as for further refined meshes in the laminar range. WTT measurements in Göttingen have been carried out at the DLR Ludwieg tube facility, a low cost blowdown facility for high speed and high Reynolds number flow. For the present testing two X-38 models with a scaling of 3.33% have been manufactured at the DLR Göttingen. Pressure and thermocouple measurements are carried out on a brass model with different inserts for the respective instrumentation. A second model made by rapid prototyping technique was used for liquid crystal thermovisualisation related to the well known thermophosphor method. By this means cost reduction of 65% and time reduction of 75% compared with the customary manufacturing was possible. Comparison of CFD results and WTT data in the flap region and at the vehicle center line shows very good agreement of pressure distributions in the whole Re number regime. Measurements of heat flux distributions suggest the assumption of transition phenomena downstream separation since simulations with fixed transition lead to acceptable results for all cases without transition in front of the flaps. That approach is supported by the mentioned stability analysis of the concave boundary layer flow under the separation vortex.