Experimental Investigation Of Spatiotemporal Structures Of Supersonic Mixing Layer

In this paper, laminarized and low noise supersonic mixing layer wind tunnels were designed and a new quantitative supersonic flow imaging technique was developed, with which the spatiotemporal structure, signal characteristic, dynamic behavior, and light transmission characteristic of supersonic mixing layer were studied in detail.The aerodynamic and structural design methods of supersonic mixing layer wind tunnel were proposed. In order to ensure the laminarization of nozzle outflow, an adjustable contour nozzle design method based B-Spline curve was proposed, the reliability of which was validated by numerical simulation method. The performance of the wind tunnel was tested and calibrated with schlieren and pressure measurement system, the result shows that the pressure of low speed layer can be adjusted continuously, laminarized and pressure matched supersonic mixing layer can be realized, and the structure of test section is convenient for quantitative flow imaging.In order to solve the problems encountered in fine structures measurement and flow visualization of supersonic mixing layer, the nanoparticle-based planar laser imaging (NPLI) method was developed and systematically studied. In NPLI, nanoparticles and pulsed planar laser were used as tracer and illumination respectively, with which the complex supersonic flow structures can be imaged at high spatiotemporal resolution and high signal-to-noise ratio (SNR). The factors that influence the performance of NPLI were investigated in detail. The flow-following ability of nanoparticles in supersonic flows was studied, and the diameter of nanoparticle was measured based on oblique shock wave calibrating method. The laser scattering performance of nanoparticles was analyzed according to light scattering theory. The results show that the NPLI technique yields better flow-following ability than particle-based method and more intensive scattered light than molecule-based method, which meets the requirements of supersonic flow imaging on the flow-following ability and SNR.The NPLI technique and apparatus facilitate the experimental studies of supersonic mixing layer. The streamwise NPLI images reveal the whole structure of supersonic mixing layer from laminar region to the generation and breakdown of vortex. A new type vortex structure was discovered in spanwise NPLI images. By comparing and analyzing of the NPLI images, the spatial structures and temporal evolution of mixing layer were investigated. The results show that the quality of the inflow severely influences the streamwise structures; the three-dimensional (3D) vortex structures reveal characteristics of rapid translation and slow distortion; the interaction between shock wave, boundary layer and vortexes exists in pressure-unmatched flow field. The flow control experiments show that single wavelength and 3D disturbance evidently enhance the mixing efficiency.A new density measuring method of supersonic flow fields was proposed, with which the density distribution of mixing layer can be measured quantitatively at high spatiotemporal resolution. The 3D density distribution of mixing layer was reconstructed according to the spatial characteristic of vortexes. By utilizing Fourier and wavelet transformation, the frequency spectral and multi-resolution analysis of density signal fluctuation in mixing layer were realized. The results show that the frequency characteristic corresponds with vortex structures; the detail and approximate characteristics of signal are reflected by wavelet coefficient at different scales; the coefficient map of continuous wavelet transformation reveals the self-similarity of the signal. The high spatiotemporal resolution characteristic of NPLI images facilitates studying fractal characteristics of supersonic mixing layer. The results show that the interface of mixing layer exhibits excellent self-similarity at different scale; the fractal dimension can quantify the fragmental characteristic of mixing layer; the fractal dimension of fully developed mixing layer does not varies with different flow structures, which also reveals the self-similarity.The performance of hardware and algorithm of supersonic PIV system were analyzed. The flow-following ability problem of tracer particles encountered in supersonic PIV measurement was solved by utilizing particle generator of NPLI. The velocity distribution of supersonic mixing layer was studied with this PIV system. The results show that the laminar section of supersonic mixing layer is mainly influenced by the boundary layer of inflow and dominated by streamwise shear of velocity; the region dominated by vortexes reveals periodical structure; the rolling velocity of vortexes is asymmetric; the vector field of relative velocity and the relative streamline correspond with topological structure of particle imagesBackground oriented schlieren (BOS) is a kind of newly developed quantitative flow imaging technique. By analyzing the principle of BOS, a BOS apparatus was developed, which can be used to study the supersonic flow structures and its aero-optic characteristics. With this apparatus, the spatial structure and temporal evolution characteristic of supersonic mixing layer were studied. The results show that the periodic structures widely exist in streamwise and spanwise BOS images of mixing layer at different convective Mach number, which embodies the aero-optic aberration of the flow field. The time-correlated BOS images reveal the aero-optic dithering characteristics that influenced by the movement of vortexes.