Experimental Investigation Of Supersonic Flow Over A Compression Ramp And Its Flow Control On Separation

As a typical aerodynamic configuration, compression ramp can be seen widely in supersonic aircraft, the interactions between shock wave and boundary layer which are caused by compression ramp have important effects on the stability and safety of aircraft. The research of supersonic compression ramp flow is a fundamental aspect of turbulence mechanism, and it is important for the design and optimization of aircraft. There are unsteadiness and three-dimensionality in supersonic compression ramp flow, which present challenges for experimental studies. In this thesis, flow visualization, quantitative structure, velocity field and flow control of supersonic compression ramp with laminar, transitional and turbulent upstream boundary layers are studied in experiments which are carried out at Mach 3.0.At first, the supersonic wind tunnel used for studies in this thesis is introduced, the measuring techniques such as Nano-tracer Planar Laser Scattering(NPLS) and Particle Image Velocimetry(PIV) are described. At the same time, the primary principles of NPLS and PIV system are illustrated, and the tracing ability of nano-particles, the resolution ratio of testing system are also discussed.Flow structures of supersonic laminar, transitional and turbulent compression ramp are studied respectively under different corner angles via NPLS technique, fine flow structures of compression ramp are obtained, time averaged flow field and the evolution of coherent structures are analyzed. When the corner angle is 23°, flow separation does not occur in all three compression ramp flows. When the corner angle is 25°, a typical separation occurs in the laminar boundary layer, a shock is induced by the fast development of laminar boundary layer, hairpin vortex, compression wave, separation shock and reattachment shock can be seen clearly in flow field; An incipient separation occurs in the transitional boundary layer, the length of separation region is shorter than that in laminar boundary layer, there is not any induced shock caused by transitional boundary layer; While the boundary layer in turbulent compression ramp is always attached to the wall. When the corner angle is 28°, the laminar boundary layer separates further, the length of separation region increases significantly and the flow structures are complex; A typical separation occurs in the transitional boundary layer and the length of separation region increases obviously, a series of compression waves merge to form the reattachment shock at the end of separation region; Separation also occurs in the turbulent boundary layer, but the length of separation region is much shorter and flow structures in separation region are relatively simple.Quantitative characteristics of flow structures in supersonic compression ramp are studied based on NPLS image, the intermittency of boundary layer, the angle of coherent structure and the fractal dimension of boundary layer are analyzed. The intermittency which is obtained from upstream boundary layer shows similar behavior to that found by Klebanoff in incompressible boundary layer. Being effected by adverse pressure gradient, the intermittency of boundary layer in separation region is totally different. Coherent structures in all boundary layers decrease along the normal direction, while they increase along streamwise in transitional and turbulent boundary layers. The angle of coherent structures in all boundary layers increase along normal direction. In laminar compression ramp flow, the fractal dimension of boundary layer is significantly different in different stage, while the change of fractal dimensions in transitional and turbulent compression ramp flow is very small along streamwise, and it is not effected by the variations of corner angle.The structures of velocity field in supersonic laminar, transitional and turbulent are studied respectively by PIV technique. When the corner angle is 25°, reverse flow occurs in the separation region of laminar compression ramp, the shear layer forms and increases gradually, the U component velocity presents layered structures, the V component velocity embodies the effects of shock wave and flow contraction, and the length of separation region is 62.6 mm; While the range of reverse flow in transitional compression ramp is smaller, and the shear layer is much thinner, the U component velocity also presents layered structures, the V component velocity presents a oblique “v” area, and the length of separation region is 24.5 mm; What is totally different is that flow separation does not occur in the turbulent compression ramp, the shear layer is much thicker, and there is intense speed shearing effect in the shear layer. When the corner angle is 28°, the range of reverse flow in laminar compression ramp increases distinctly, there is large velocity gradient in the recirculation region, the oblique “v” area in the V component velocity moves upstream, and the length of separation region is 86.0 mm; In transitional compression ramp the reverse flow becomes more prominent, and the shear layer becomes thicker, the distribution of U component velocity is a continuation of layered structure, the V component velocity embodies the effects of shock wave and reverse flow structure, the length of separation region is 65.4 mm; Although flow separation occurs in the turbulent compression ramp, the range of reverse flow near the wall is very small, the U component velocity still presents layered structures, the V component velocity embodies the effects of separation shock and flow contraction, and the length of separation region is 31.1 mm. Moreover, Proper Orthogonal Decomposition of velocity field structures is carried out based on the PIV measurements, the primary characteristics and details of velocity field are analyzed.At last, microvortex generators are used in experiments to control flow separation in supersonic laminar and turbulent compression ramp, fine flow structures and velocity field are obtained via NPLS and PIV techniques. Flow Visualization results indicate that whether the upstream boundary layer is laminar flow or turbulent flow, the wake flow of microramps is composed of two regions including counter-rotating vortex pairs and large scale coherent structures which show obvious intermittency. The low speed area in separation region of 25° laminar compression ramp decreases distinctly and the shock foot in flow field of 28° turbulent compression ramp moves downstream, the range of reverse flow decreases significantly in both two compression ramps. It can be concluded from the quantitative analysis of velocity field that the length of separation region in laminar and turbulent compression ramp can be reduced effectly under the control of microvortex generators.