A correction method for aero-optics thermal radiation effects based on gradient distribution and dark channel

In this paper, a new algorithm is proposed to remove the effects of aerodynamic optical thermal radiation from a single infrared image. In this method, the joint probability model of gradient distribution is introduced by studying the "global smoothing and local fluctuation" characteristics of the bias field. A prior L0 norm of dark channel is introduced to constrain the latent clear image. Finally, the split Bregman method is used to solve the optimization problem. The effectiveness of the proposed method is verified by a series of experiments, and the results are compared with the results of the existing methods for the correction of thermal radiation effects.

Universal form of the power spectrum of the aero-optical aberration caused by the supersonic turbulent boundary layer

Based on the measurement of one-dimensional （1D） optical path difference （OPD） of the supersonic turbulent bound- ary layer, an analytical form for the power spectrum of the two-dimensional （2D） OPD is obtained with its structure function and under the locally homogeneous isotropic assumption. The universality of this spectrum is argued, and its validity is checked by the comparison with experimental result. The potential applications of this model in theoretical and numerical studies are emphasized. Another contribution of this work is around the application of correlation function to analyzing the statistics of OPD. Based on our results and other results published elsewhere, we show that the OPD is often not stationary, and one should be cautious about using this tool.

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.

Temporal evolution of the optical path difference of a supersonic turbulent boundary layer

The density distribution of a supersonic turbulent boundary layer is measured with the nanoparticle-based planar laser scattering technique, and the temporal evolution of its optical path difference （OPD） in a short time interval is characterized by proper orthogonal decomposition （POD）. Based on the advantage of POD in capturing the energy of a signal, a temporal evolution model is suggested for the POD coefficients of the OPD. In this model, the first few coefficients vary linearly with time, and the others are modeled by Gaussian statistics. As an application, this method is used to compute the shortexposure optical transfer function.

Recognition of vortex structures in turbulent refractive index field using wavelet transformation

When the light wave propagates through the hypersonic flow field, the aero-optic distortion happens. It is necessary to recognize the vortex structure for studying the light propagation model. A new vortex structure recognition method is proposed in this paper. Firstly, the refractive index field, which is transformed from the turbulent density field, is changed to gray scale images with a- bundant texture information equivalently. Then, wavelet transform is performed to decompose these images and the entropy values of the wavelet base coefficients are calculated. Comparing the entropy value to a given threshold, the modules with lower entropy are considered to be the large-scale vortex modules while those with higher entropy are small-scale vortex modules. The computer simulation results show that the proposed method is valid to recognize the vortex structures. This paper provides basis for investigation on the optics propagation model in terms of the turbulence vortex structures.

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.

An experimental study of aero-optical aberration and dithering of supersonic mixing layer via BOS

The optical performance of supersonic mixing layer is heavily deteriorated by the aero-optical aberration and dithering of coherent structures, but current measuring methods limit the spatiotemporal resolution in relevant studies. A high resolution whole-field aero-optical aberration and dithering measuring method based on the Background Orient Schlieren (BOS) technique was studied. The systematic structure, sensitivity and resolution of BOS are analyzed in this paper. The aero-optical aberration and dithering of streamwise structures in supersonic mixing layers were quantificationally studied with BOS. The aberration field of spanwise structures revealed the ribbon-like aberration structures, which heavily restrict the optical performance of a mixing layer. The quantifications of aero-optical aberration and dithering are very important in studying aero-optical performance of supersonic mixing layer.

Aero-Optical Characteristics of Supersonic Flow over Blunt Wedge with Cavity Window

The optical rays that form the image of an object and propagate a supersonic flow over a vehicle are refracted by the density variations.A numerical analysis of the aero-optical characteristics of supersonic flow over blunt wedge with a cavity window is carried out.A hybrid method of Reynold averaged Navier-Stokes and direct simulation Monte Carlo(RANS/DSMC) is employed to simulate the flowfield.Refraction factor is introduced to evaluate the flowfield's aero-optical characteristic.The results show that mean flow's aero-optical effects are mainly caused by the shock wave,the expansion wave and the turbulent boundary layer.Fluctuation flow's aero-optical effects are mainly caused by the turbulent boundary layer and the shock wave induced by the cavity window.The aero-optical effects at the leading side of window are caused by the mean density variations,while the effects at the trailing side are caused by the density fluctuations.Different draft angles of the cavity window are investigated.The airborne optical devices of supersonic vehicle should be mounted in the middle of the cavity window with a large draft angle.

Aero-optical effects simulation technique for starlight transmission in boundary layer under high-speed conditions

Aero-optical effects for starlight transmission in the high-speed flow field will reduce the accuracy of the star sensor on an aircraft.Numerical simulations for aero-optical effects usually require plenty of calculations,which cause difficulties when designing a celestial navigation system for a high-speed aircraft.In this study,an Aero-Optical Simulator For Starlight Transmission(AOSST)in the boundary layer is developed.It effectively reduces the computational burden compared to that of the widely used CFD simulation,and it achieves satisfactory accuracy.In this simulator,gas ellipsoids satisfying certain design rules are used to simulate coherent density structures in boundary layers.Design rules for the gas ellipsoids are found from published experimental and high-fidelity CFD simulation results.The generated wavefront distortion by AOSST is anchored with the scale law for aero-optical distortion in the boundary layer by determining some control parameters,which enables the simulator to output reliable results over a wide range of flight states.Four numerical examples are provided to verify the performance of AOSST.The results demonstrate that AOSST is able to simulate the directional dependence of aero-optical distortions in boundary layers,the variation trend of distorted wavefront shapes with Reynolds number,and the grayscale distribution on the disturbed star map.

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.

Satellite-to-aircraft quantum key distribution performance estimation with boundary layer effects

Remarkable progress has been made in satellite-based quantum key distribution [QKD],which can effectively provide QKD service even at the intercontinental scale and construct an ultralong-distance global quantum network.But there are still some places where terrestrial fiber and ground stations cannot be constructed,like harsh mountainous areas and air space above the sea.So the airborne platform is expected to replace the ground station and provide flexible and relay links for the large-scale integrated communication network.However,the photon transmission rate would be randomly reduced,owing to the randomly distributed boundary layer that surrounds the surface of the aircraft when the flight speed is larger than0.3 Ma.Previous research of airborne QKD with boundary layer effects is mainly under the air-to-ground scenario in which the aircraft is a transmitter,while the satellite-to-aircraft scenario is rarely reported.In this article,we propose a performance evaluation scheme of satellite-to-aircraft QKD with boundary layer effects in which the aircraft is the receiver.With common experimental settings,the boundary layer would introduce a ~31 dB loss to the transmitted photons,decrease ~47% of the quantum communication time,and decrease ~51% of the secure key rate,which shows that the aero-optical effects caused by the boundary layer cannot be ignored.Our study can be performed in future airborne quantum communication designs.