Analysing the structure of the optical path length of a supersonic mixing layer by using wavelet methods

The nano-particle-based planar laser scattering （NPLS） technique is used to measure the density distribution in the supersonic mixing layer of the convective Mach number 0.12, and the optical path difference （OPL）, which is quite crucial for the study of aero-optics, is obtained by post processing. Based on the high spatiotemporal resolutions of the NPLS, the structure of the OPL is ana]ysed using wavelet methods. The coherent structures of the OPL are extracted using three methods, including the methods of thresholding the coefficients of the orthogonal wavelet transform and the wavelet packet transform, and preserving a number of wavelet packet coefficients with the largest amplitudes determined by the entropy dimension. Their performances are compared, and the method using the wavelet packet is the best. Based on the viewpoint of multifractals, we study the OPL by the wavelet transform maxima method （WTMM）, and the result indicates that its scaling behaviour is evident.

Inhibition of the aero-optical effects of supersonic mixing layers based on the RVGAs'control

The infrared imaging windows of the hyper/supersonic optical dome are encountering severe aero-optical effects[AOEs],so a flow control device,the ramp vortex generator array[RVGA]is proposed based on the ramp vortex generator to inhibit the supersonic mixing layers’AOE,which is done by the nanotracer-based planar laser scattering technique and ray-tracing method.The experiments prove that under different pressure conditions,RVGA can reduce the mean and standard deviation of the root mean square of the optical path difference[OPDrms]and reduce the supersonic mixing layers’thickness and mixture a great deal.The AOE of the pressure-matched mixing layer is the weakest.Higher RVGA results in better optical performance.RVGA has the potential to be applied to supersonic film cooling to reduce aero-optical aberrations.

Ignition, flame propagation and extinction in the supersonic mixing layer flow

The supersonic mixing layer flow,consisting of a relatively cold,slow diluted hydrogen stream and a hot,faster air stream,is numerically simulated with detailed transport properties and chemical reaction mechanisms.The evolution of the combustion process in the supersonic reacting mixing layer is observed and unsteady phenomena of ignition,flame propagation and extinction are successfully captured.The ignition usually takes place at the air stream side of braid regions between two vortexes due to much higher temperature of premixed gases.After ignition,the flame propagates towards two vortexes respectively located on the upstream and downstream of the ignition position.The apparent flame speed is 1569.97 m/s,which is much higher than the laminar flame speed,resulting from the effects of expansion,turbulence,vortex stretching and consecutive ignition.After the flame arrives at the former vortex,the flame propagates along the outer region of the vortex in two branches.Then the upper flame branch close to fuel streamside distinguishes gradually due to too fuel-riched premixed mixtures in the front of the flame and the strong cooling effect of the adjacent cool fuel flow,while the lower flame branch continues to propagate in the vortex.

Existence of shocklets in a two-dimensional supersonic mixing layer and its influence on the flow structure

The spatial evolution of a T-S wave and its subharmonic wave, introduced at the inlet in a 2-D supersonic mixing layer, was investigated by using DNS. The relationship between the amplitude of the disturbance wave and the strength of the shocklet caused by the disturbance was investigated. We analyzed the shape of the disturbance velocity profile on both sides of the shocklet, and found that the existence of shocklet affected appreciably the disturbance velocity. The effects on the high speed side and low speed side of the mixing layer were found to be different.

Binary diffractive lens with subwavelength focusing for terahertz imaging

The converging lens is one of the key components in high-resolution terahertz imaging.In this Letter,a binary diffractive lens is proposed for the scanning imaging system working at 278.6 GHz,in which a convergent beam with a waist diameter of0.65 mm is generated,and 1 mm lateral imaging resolution is realized.This low-cost terahertz lens,constituted by concentric rings with different radii,is optimized by stimulated annealing algorithm and fabricated by three-dimensional printing.Compared with the conventional transmissive convex lens,higher resolution and enhanced imaging quality are achieved via smaller focal spot of the illumination beam.This type of lens would promote terahertz imaging closer to practical applications such as nondestructive testing and other scenarios.