The distorted wavefront due to the density fluctuation of the supersonic mixing layer is measured by the nano-based planar laser scattering technique, with the spatiotemporal resolutions as 10(-1) mm and 1 micros (time interval between two exposures), respectively. The optical path difference (OPD) is analyzed from the viewpoints of correlation and structure functions. Depending on the structure of the flow field, the type of the correlation function is Gaussian or exponential. The power index of the structure function is about 1.1, and the consistency over many cases is quite good. The far-field propagation of the Gaussian beam is simulated by Fourier transform, and the Strehl ratio is calculated with the rms of the OPD. The beam center and beam spread about the center in the far field are calculated, and are dominated by the statistics of the tilt of the distorted wavefront. The results indicate that aero-optical aberrations can severely limit the performance of airborne laser systems.
The parameters of instability wave of supersonic mixing layer (M c =0.5, M 1 =3.5/M 2 =1.4) are measured by flow visualization and calculated by means of two-dimensional direct numerical simulaitons of the compressible Navier-Stokes equations. In both cases of the mixing layer with harmonic disturbance or not, the comparative results indicate that the wavelength of the two-dimensional wave is equal to the vortex spacing in the streamwise direction because the difference between them is less than 1%. However, the measured convective velocity of the large-scale structure deviates from the theoretical isentropic prediction about 10%. Here the measured value confirms that M c,2
Employing the nanoparticle-based planar laser scattering (NPLS), supersonic flow over a delta-winged vortex generator on a flat plate was experimentally investigated in a supersonic quiet wind tunnel at Ma = 2.68. The fine structures of the flow field, shock waves, separation vortices, wake, and boundary layer transition were observed in the NPLS images. According to the time-correlation of the NPLS images and the measurement results of particle image velocimetry, the structural model of the flow field was improved further, and coherent wake structures were observed, which is of significance theoretically and in engineering application.
The evolution of preexisting nano-voids of different size in single crystal copper is investigated by means of molecular dynamics (MD) simulations. The results show that void collapse depends upon the size of the void. Numerous dislocation loops emerge for large void and phase transformation takes place in the sample. When the shock wave passes the void from left to right,dislocation loop emerges in the right of the void first for small void. However,loops emerge in the right and the left at the same time when the size of the void increases to a critical size,which is decided by the distribution of the stress near the void. Once dislocation loop emerges and grows,the frontal dislocation loop in the right of void grows faster than that in the left for the same void. When the size of the void increases,the velocity of frontal dislocation loops is essentially unvaried,and the angle between the direction leading from the centre of the void to the initial position of collapse and the direction of impact increases.
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