Experimental Investigation Of Supersonic Boundary Layer And Shock Wave/Boundary Layer Interaction

The studies of supersonic boundary layers and shock wave / boundary layer interactions are crucial in the design of high-speed aerospace vehicles and weapons systems, which are also important for understanding the mechanism of compressible turbulence. The high speed, three-dimensional and highly unsteady natures of such flows present special challenges for experimental studies. In this thesis, the spatiotemporal structures, dynamic behaviors and density distributions of a flat-plate boundary layer and an incident shock wave / turbulent boundary layer interactions at Mach 3 were studied experimentally.The supersonic wind tunnel and measurement techniques used for the experimental studies in this thesis are described. The basic principles and setups of the relevant measurement techniques such as Nanoparticle-based Planar Laser Scattering (NPLS), supersonic Particle Image Velocimetry (PIV) and NPLS-based Density Technique (NPLS-DT) are present. And the performance parameters and measurement errors of these techniques are also discussed here.The coherent structures of a Mach 3 transitional and fully developed turbulent boundary layer in the streamwise-wall-normal plane and streamwise-spanwise planes at different heights were visualized using NPLS technique. The spatial structures and temporal evolutions of the supersonic flat-plate boundary layer were investigated. Large discrepancies of the coherent structures between the different stages of laminar- turbulent transition were identified, as well as between the initial stage and the fully developed stage of a turbulent boundary layer. Besides, the coherent structures reveal a characteristic of rapid translation and slow distortion. Individual hairpin vortex and hairpin packets were identified from NPLS images on the basis of the hairpin model.Λ-shaped vortices were found in a staggered pattern in the streamwise-spanwise plane, which indicated the H-type transition in the present supersonic boundary layer. Based on the digital image processing, the intermittency of the supersonic turbulent boundary layer was determined from the NPLS images with a high spatial resolution, which showed a similar behavior to that found in low-speed incompressible boundary layers. The fractal dimension of the supersonic boundary layer was also calculated, which is nearly one in the laminar stage and is increased from 1 to 1.5 in the process of laminar-turbulent transition. In the turbulent stage, the fractal dimension is kept constant at 1.5.The velocity measurement of supersonic boundary layers is difficult, due to the presence of high speed and strong velocity gradients. With the aid of supersonic PIV, the streamwise velocity distributions of a transitional boundary layer were acquired. The mean streamwise velocity profile is consistent with the Blasius profile in the laminar stage, and then it is becoming fuller. In the turbulent stage, the mean velocity profile is close to the Spalding profile. The growth rate of the boundary layer thickness, calculated from mean velocity profile, is relatively large in the laminar stage, and then become smaller in the stage of the laminar-turbulent transition. After a certain time when developed into turbulent boundary, the thickness increases quickly again. The mean and turbulent properties of a supersonic turbulent boundary layer were also investigated. Using different velocity decompositions, the hairpin packets were identified from the instantaneous streamwise velocity fields. Low- and high-speed streaks are observed in the instantaneous spanwise velocity fields in the near wall region, similar structures are not found in the outer layer of the boundary layer.The measurement of density field in supersonic flows is also very difficult. In this thesis, the density measurement of a flat-plat turbulent boundary layer at Mach 3 was achieved using the NPLS-DT technique. The experimental results were in good agreement with previous data. Due to the high spatial resolution of the density data, the frequency spectral of density fluctuations was analyzed. And the low-frequency fluctuations are on the same order of magnitude as the high-frequency fluctuations.For a thorough investigation of compressible turbulent flows, experimental methods for the simultaneous measurements of two or more parameters are urgently needed, which can help us to understand the correlations between different flow parameters. In this thesis, a new technique for the simultaneous measurements of instantaneous whole-field density and velocity fields of supersonic flows has been developed based on the NPLS technique. The basic principle of this technique is descried in detail. And this technique has been applied to a flat-plate turbulent boundary layer at Mach 3. A good agreement between the fluctuations of mass flux and density was found. However, no similar behavior was observed between the fluctuations of velocity and density. Based on this new technique, the Reynolds stress distributions of the supersonic turbulent boundary layer were obtained. The effect of density fluctuations on the behavior of Reynolds stresses can be neglected in a supersonic turbulent boundary layer at Mach 3, but the effect of density variations across the boundary layer is relatively large.The organized structures of an incident shock wave / turbulent boundary layer interaction (SWTBIL) at Mach 3 were also visualized using NPLS technique in the streamwise- wall-normal and streamwise-spanwise planes, respectively. The large-scale coherent structures within the incoming boundary layer affect the spatial organization of the reflected shock wave. However, the time scale of the large-scale unsteadiness observed in SWTBLI is much larger than the characteristic time scale of the incoming boundary layer. The unsteadiness of SWTBLI becomes more important as the shock strength increases. The velocity distributions of SWTBLI were also investigated using supersonic PIV. As the incident shock wave strength increases, the reversed-flow can be observed in the mean streamwise velocity field. And a relationship between the streamwise velocity fluctuations within the incoming boundary layer and reflected shock wave positions are found in the instantaneous spanwise velocity fields. In the separated zone, the tornado-shaped vortices can also be identified associated with the instantaneous streamwise and spanwise velocity fields.