Experimental Study On Supersonic Flow Over Protuberances Mounted On A Flat Plate And Double-cone Geometries

The studies of supersonic flow over protuberances mounted on a flat plate are crucial in aeronautics & astronautics and the design of weapons, which are also of great importance to the boundary later development and control. Double-cone geometries are typical axisymmetric models, and thus investigation of the flow field is meaningful to the development of high-speed vehicles and the accumulation of experimental data. The flow structures of supersonic flow over protuberances and double-cones are quite complex and of large gradient, bringing great challenges to the flow testing techniques. The characteristics of supersonic flow over protuberances and double-cones are studied from two aspects, namely structures and temporal evolution, by the methods of NPLS and schlieren. Feasibility of the methods of eliminating or weakening the scattering light from the walls of the test model is analyzed theoretically.In the beginning of this paper, the requirements of the flow diagnostics on the wind tunnel and test techniques are analyzed. I case of the flow field being influenced by other factors, the flow in the experimental section should be of low turbulent intensity and have no strong waves. Thus it is decided that the experiments should be done in the Ma=3.0 low-noise supersonic wind tunnel using NPLS as the testing technique. As to the strict requirements on the flow quality, an introduction of high quality supersonic nozzle design technique based on B-curve is also given.Together with the NPLS technique used in this paper, many flow field measuring techniques based on laser and imaging system are easily influenced by the scattering light from the walls of the model. The Doppler frequency displacement and polarization characteristics of both the scattering light from the nano-particles and the walls of the model are carefully studied. When the nano-particles are illuminated by linear polarization light of the laser, the scattering light is also linear polarized. Moreover the light received by the CCD camera has the largest frequency displacement at the edge of the field of view while at the center it is almost zero. Therefore, if the method of eliminating scattering light from the walls based on the frequency displacement is taken, it needs to put the zone concerned near the edge of the field of view and illuminate the zone at angle to get larger displacement. As to the walls not specially handled, for the effects of the degrees of roughness, the scattering light from the walls will not be pure linear polarized light. Thus by setting a polarizing disc before the camera, some light could be eliminated. At the same time, several traditional methods of eliminating or weakening the scattering light from the walls are introduced.NPLS and the schlieren technique is used to test the flow field of circular protuberances mounted on a flat plate departing from the floor of the wind tunnel. Cylinders with various heights namely 1mm, 2mm and 4mm are measured and the diameters are all 4mm with the Reynolds number of 7.7×106/m, while the thickness of boundary layer is about 1.2mm at the mounting position. In the NPLS figures, expansion wave, three-dimensional bow detached shock wave, reattachment shock wave and separation region ahead of the cylinders are clearly visualized. The lower the circular protuberance is, the closer the reattachment shock wave will be to the protuberance. Special attention is paid to the study of the near wake and the influence of the cylinder height on the wake flow, showing that vortices in the wake flow of the 1mm high cylinder are isolated and simple, while to the 4mm high cylinder are complex and mixed-up with less periodicity. The lower cylinder has a relatively longer laminar section and transition section. The square protuberance has a greater influence on the boundary layer. When coming to the square protuberance, boundary layer has developed and become turbulent, meanwhile the thickness of the boundary layer after reattachment is growing faster than the circular protuberance of the same height.Supersonic flow over double-cones is typical axisymmetric flow, and in this paper flow field of six different geometries is studied. By analyzing the interaction zone of the separation shock wave, the shock wave of the first and second cone, it can be concluded that axisymmetric shock/shock interaction is weaker than the 2D interaction. The bigger the angle difference of the first and second cone, the lager the separation zone at the corner will be and the greater interference will be to the boundary layer, accordingly faster will the boundary layer grow.