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.     

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.     

不同光线入射角度下超声速湍流边界层气动光学效应的实验研究

利用基于纳米粒子的平面激光散射技术获取超声速(Ma=3.0)湍流边界层的密度分布,采用光线追迹方法计算其对应的光程差分布,并结合边界层气动光学相似律验证实验结果的可靠性.着重研究了光线入射角度对超声速湍流边界层气动光学效应的影响,并对其内在机理进行了分析.研究表明,气动光学效应对光线入射角度的依赖性源于光线在流场中的传输路径,传输路径的不同导致了光线在流场中的传输距离以及对应密度脉动互相关结果的差异.光线倾斜入射导致其在流场中传输距离增长,进而气动光学效应出现恶化.光线入射方向与壁面垂直方向之间的夹角越大,气动光学效应越显著,而且不同时刻的差异性增加,气动光学效应校正的难度增加.超声速湍流边界层中大量具有特定方向的涡结构导致了湍流边界层气动光学效应的各向异性.当光线倾斜向下游入射时,光线传播方向与流场中的涡结构具有较好的一致性,体现为此方向上密度脉动互相关系数较大,故气动光学效应比较严重.而当光线倾斜向上游入射时,相关系数较小,故气动光学效应较弱.     

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 analysed 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.     

Experimental study of a supersonic turbulent boundary layer using PIV

Particle image velocimetry was applied to the study of the statistical properties and the coherent structures of a flat plate turbulent boundary layer at Mach 3. The nanoparticles with a good flow-following capability in supersonic flows were adopted as the tracer particles in the present experiments. The results show that the Van Driest transformed mean velocity profile satisfies the incompressible scalings and reveals a log-law region that extends to y/ =0.4, which is further away from the wall than that ...     

Constrained large-eddy simulation of supersonic turbulent boundary layer over a compression ramp

A supersonic turbulent boundary layer over a compression ramp is numerically investigated using the constrained large-eddy simulation (CLES) method. The compression corner is characterised by a deflection angle of 24°. The free-stream Mach number is Ma_∞ = 2.9, and the Reynolds number based on the momentum thickness of inlet boundary layer is Re_θ = 2300. The mean and statistical quantities, such as mean velocity, wall pressure and Reynolds stresses, are thoroughly analysed and compared with those from traditional large-eddy simulation (LES), experimental measurement and direct numerical simulation (DNS). It turns out that CLES can predict the friction coefficient, wall-pressure distribution, size of separation bubble, Reynolds stresses, etc. more accurately than traditional LES, and the results are in reasonable agreement with the experimental and/or DNS data. Also discussed are the effects of specific parameterisations of the Reynolds constraint and interfacial positions separating the constrained and unconstrained regions on the performance of the CLES method.     

Direct numerical simulation of a supersonic turbulent boundary layer on a flat plate and its analysis

In the past three decades, considerable progress has been made in the investigation of incompressible turbulent boundary layer through experiments, DNS and theoretical works, including: (1) the statistics characteristic and structure of turbulence; (2) the co-herent structures in turbulent flows; (3) turbulence modeling and the large eddy simula-tion (LES). In contrast, the progress was very slow for the compressible, in particular, the super-sonic turbulent boundary layer. Recent works on d…     

Effect of wall temperature on hypersonic turbulent boundary layer

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer.     

The mechanism of breakdown in laminar-turbulent transition of a supersonic boundary layer on a flat plate-temporal mode

Recently, more and more attentions have been paid to the transition and turbulence of compressible flows, especially supersonic flows, but progress is very slow. In general, attentions were paid to the evolution of disturbances in laminar flow region, and the mechanism of breakdown process in laminar-turbulent transition remains unclear. The conventional idea of what leads to laminar-turbulent transition is that the transi-tion starts from the amplification of disturbances, and when the distur…     

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 dither...     

The investigation of coherent structures in the wall region of a supersonic turbulent boundary layer based on DNS database

Through temporal mode direct numerical simulation, flow field database of a fully developed turbulent boundary layer on a flat plate with Mach number 4.5 and Reynolds number Reθ =1094 has been obtained. Commonly used detection meth- ods in experiments are applied to detecting coherent structures in the flow field, and it is found that coherent structures do exist in the wall region of a supersonic turbulent boundary layer. The detected results show that a low-speed streak is de- tected by using the Mu-level method, the rising parts of this streak are detected by using the second quadrant method, and the crossing regions from a low-speed streak to the high-speed one are detected by using the VITA method respectively. Notwithstanding that different regions are detected by different methods, they are all accompanied by quasi-stream-wise vortex structures.     

Predetermined control of turbulent boundary layer with a piezoelectric oscillator

With a piezoelectric(PZT) oscillator, the predetermined controls of the turbulent boundary layer(TBL) are effective in reducing the drag force. The stream-wise velocities in the TBL are accurately measured downstream of the oscillator driven by an adjustable power source. The mean velocity profiles in the inner and outer scales are reported and the skin friction stresses with different voltage parameters are compared. Reduction of integral spatial scales in the inner region below y+of 30 suggests that the oscillator at work breaks up the near-wall stream-wise vortices responsible for high skin friction. For the TBL at Reθof 2183, the controls with a frequency of 160 Hz are superior among our experiments and a relative drag reduction rate of 26.83% is exciting. Wavelet analyses provide a reason why the controls with this special frequency perform best.     

Dynamic model of coherent structure in the wall region of a turbulent boundary layer

Based on the theoretical model for a single coherent structure in the wall region of a turbulent boundary layer, we studied the interaction of two coherent structures by direct numerical simulation in order to explain the mechanism for the formation of low-speed streaks.     

Interaction between strain and vorticity in compressible turbulent boundary layer

The interaction of strain and vorticity in compressible turbulent boundary layers at Mach number 2.0 and 4.9 is studied by direct numerical simulation(DNS)of the compressible Navier-Stokes equations.Some fundamental characteristics have been studied for both the enstrophy producing and destroying regions.It is found that large enstrophy production is associated with high dissipation and high enstrophy,while large enstrophy destruction with moderate ones.The enstrophy production and destruction are also correlated with the dissipation production and destruction.Moreover,the enstrophy producing region has a distinct tendency to be‘sheet-like’structures and the enstrophy destroying region tends to be‘tube-like’in the inner layer.Correspondingly,the tendency to be‘sheet-like’or‘tube-like’structures is no longer obvious in the outer layer.Further,the alignment between the vorticity vector and the strain-rate eigenvector is analyzed in the flow topologies.It is noticed that the enstrophy production rate depends mainly on the alignment between the vorticity vector and the intermediate eigenvector in the inner layer,and the enstrophy production(destruction)mainly on the alignment between the vorticity vector and the extensive(compressive)eigenvector in the outer layer.     

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer (TBL) is actively controlled by the synthetic jet generated from a circular hole. According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry (TR-PIV) system, the average drag reduction rate of 6.2% in the downstream direction of the hole is obtained with control. The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream, while the reverse vortex decays rapidly. From the statistical average, it can be found that a low-speed streak is generated downstream. Induced by the two vortex legs, the fluid under them converges to the middle. The drag reduction effect produced by the synthetic jet is local, and it reaches a maximum value at x⁺=400, where the drag reduction rate reaches about 12.2%. After the extraction of coherent structure from the spatial two-point correlation analysis, it can be seen that the synthetic jet suppresses the streamwise scale and wall-normal scale of the large scale coherent structure, and slightly weakens the spanwise motion to achieve the effect of drag reduction.     

Simulations of turbulent asymptotic suction boundary layers

A series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times.     

带喷流超声速光学头罩流场气动光学畸变试验研究

超声速光学头罩在大气层内飞行时, 需要在光学窗口表面顺来流方向进行喷流冷却, 致使窗口上方流场更为复杂. 目标光线穿过窗口上方流场, 受到激波、膨胀波、混合层、湍流边界层等流场结构引起的变密度场影响而产生波前畸变, 导致成像出现偏移、抖动、模糊等气动光学效应. 本文对马赫数3.8来流条件下有无喷流时超声速光学头罩流场引起的气动光学波前畸变进行了试验研究. 基于纳米示踪的平面激光散射技术, 首先对流场图像进行密度校准获得高时空分辨率密度场, 然后采用光线追迹法计算得到波长532 nm平面光波垂直于光学窗口穿过流场后的光程差(optical path difference, OPD)分布, 并对窗口上方近壁区有无喷流状态的流场结构引起的 OPD分布进行了研究. 发现无喷流时, 流场结构相对较为简单, 窗口上方有较长的回流区和层流区, 而有喷流时窗口上方出现复杂的剪切层、混合层及湍流边界层, 流动很快就转捩为湍流结构, 其引起的气动光学畸变要明显高于无喷流状态. 无喷流状态相隔5 μs的流场引起的光程差均方根值分别为0.0348 和0.0356 μm, 有喷流状态的光程差均方根值分别为0.0462 和0.0485 μm. 关键词： 超声速 喷流 气动光学 光程差     

The evolution of synthesised vortices in turbulent boundary layer

Particle image velocimetry technique has been used to investigate the evolution of synthesised vortical structures in the turbulent boundary layer. Synthetic jet actuator is implemented on the flat plate surface to synthesise various vortical structures by operating the actuator at varying operating parameters. The vortices are issued into the boundary layer and their evolution and subsequent interaction with the relatively less energetic near wall fluid is studied. The investigation is based on the quantitative measurements that are made both on the central and parallel lateral planes. Finally, the enhancement of the wall shear stress resulted in by the passing vortices is measured to evaluate the effectiveness of the actuator towards flow separation control.     

湍流边界层中下扫流与“反发卡涡”

用氢气泡法观测湍流边界层的下扫流和有关的流动结构.实验中发现一种新型涡结构，它的特征与典型的发卡涡正好相反.发卡涡的头部指向下游，而它的头部指向上游； 发卡涡的两腿之间，由于涡的诱导产生上升流，而它则在其两腿之间，由于涡的诱导产生下扫流. 关键词： 湍流边界层 流动显示 流动结构 发卡涡     

机体表面湍流边界层噪声特性及预测方法研究

湍流边界层噪声是飞机巡航过程中的主要外部噪声源,对舱内噪声水平的影响尤为重要。因此,对飞机机体表面湍流边界层噪声的研究具有重要意义。本文通过试验获得了某型民机巡航过程中的湍流边界层噪声,试飞工况为3500ft/0.78、3500ft/0.7、2500ft/0.67、1500ft/0.66。对实测数据进行分析,发现湍流边界层噪声与动压、边界层厚度等参数有关。同时,利用计算流体力学的方法得到了飞机机体表面的压力分布,并分析了压力梯度对湍流边界层噪声的影响。最后,基于工程预测方法对湍流边界层噪声进行了预测,对于不存在逆压梯度的区域,预测结果与试验结果吻合较好,仅部分频段存在一定偏差。通过对模型的参数进行优化,改善了预测结果。