Direct Numerical Simulation Research for Supersonic Shock Wave/Turbulent Boundary Layer Interactions

激波/边界层干扰问题是可压缩湍流领域中一个基本的流动现象，具有十分重要的工程应用背景。干扰区内的流动非常复杂，其中包括边界层大尺度的分离与再附过程、激波低频振荡现象、大尺度涡结构的生成以及强压力脉动载荷等。本文采用直接数值模拟对来流马赫数Ma = 2.25，33.2&deg; 激波角的入射激波/平板湍流边界层干扰流动的分离特性、再附边界层结构以及摩擦阻力的生成演化机制进行了研究。此外，还对来流马赫数Ma = 2.9，24&deg; 偏转角的压缩-膨胀折角构型中的复杂流动进行了直接数值模拟，探讨了膨胀角高度变化对离心不稳定性以及干扰区的非定常运动特性的影响规律，还对壁面压力脉动场的空间分布结构进行了分析。主要的工作和研究成果如下： (1) 入射激波与平板湍流边界层干扰流动的分离泡结构具有三维空间分布特征，整体上呈现扁平的单峰造型。通过定量对比三个不同展向站位的分析结果，研究了展向三维结构对分离区非定常运动特性以及瞬态分离微团的几何形态的影响规律。研究发现，分离泡的非定常特性表征为低频的、大尺度的膨胀和收缩运动。壁面压力脉动的功率谱密度结果表明，展向三维结构对非定常运动的特征频率影响较小。通过经验模态分解方法对分离泡面积脉动信号进行了低频重构，从中提取出了膨胀和收缩过程，借此条件统计分析分离泡呼吸运动对瞬态分离微团几何形状的影响。此外，应用本征正交模态分解对流向速度场进行了低阶近似，分析发现分离泡呼吸运动与低阶模态密切相关，前十个低阶模态重构出的流场能够精确复现分离泡低频的、大尺度呼吸运动。再附边界层内湍流运动的统计特性研究发现，边界层外层的相干结构强度明显增加、具有较大的空间尺度。壁面剪切应力脉动与流向速度脉动的时-空相关性分析表明，涡结构倾角在再附边界层的恢复过程中逐渐减小。通过空间两点的振幅调制相关性，对边界层内、外层结构间的尺度干扰效应进行了分析，提出了一种可能的机制来解释激波干扰区下游的强调制作用。 (2) 考虑激波入射角分别为33.2&deg;&nbsp;和28&deg;&nbsp;的两种情况，对比研究激波干扰强度对壁面摩擦阻力生成、演化特性的影响规律。研究发现，摩擦阻力在流动产生分离时的恢复速率较为缓慢；而边界层在保持附着的情况下，在一个边界层厚度的流向范围内快速恢复到来流充分发展湍流边界层的水平。本研究将最新提出的平均摩擦阻力分解方法应用在激波干扰区内，定量地考察摩擦阻力在不同激波入射角情况下的演化规律。研究发现，与充分发展湍流边界层不同，再附边界层内湍流相关项的贡献显著增强，而压力梯度引起的空间发展项在很大程度上抑制了摩擦阻力的产生。此外，通过二维经验模态分解技术对湍流脉动实现尺度分解，定量评估不同展向空间尺度的湍流运动对摩擦阻力生成的贡献。结果表明，在分离区和再附区来自外层大尺度结构的贡献占主导地位，而内层小尺度结构的贡献相当有限。然而在弱干扰的情况下，外层大尺度结构的贡献显著减少，导致外层大尺度和内层小尺度运动的贡献相当。 (3) 对压缩-膨胀折角干扰构型中的激波/湍流边界层干扰问题进行了直接数值模拟研究。考虑了膨胀角高度为 / = 4.25, 1.22 两种情况。当膨胀角高度较高时，流动规律与传统的压缩折角构型一致；而对于高度较低的情况，激波干扰区受下游膨胀角的存在影响较大，分离区的长度显著减小。壁面压力脉动的功率谱密度结果表明，干扰区非定常运动的低频成分被抑制。极限流线确认了G&ouml;rtler涡在再附区附近存在。通过对边界层内流线的曲率半径和G&ouml;rtler 数分布分析发现，离心不稳定性在压缩角附近区域得到保留，但在膨胀角附近区域消失。膨胀折角下游的流动结构相对复杂，可以观察到附加的激波串结构和新的流向涡生成。此外，本研究还在膨胀角上、下游流动之间发现了信号之间的负反馈机制。</p

Characteristics of wall-shear stress fluctuations in shock wave and turbulent boundary layer interaction

The wall-shear stress (WSS) fluctuations in the interaction of an oblique shock wave with a flat-plate turbulent boundary layer are investigated by means of direct numerical simulation (DNS) at Mach 2.25. The numerical results agree very well with previous experiments and DNS data in terms of turbulence statistics, wall pressure, and skin friction. The fluctuating WSS characteristics, including probability density function (PDF), frequency spectrum, space-time correlation, and convection velocity, are analysed systematically. It is found that the positively skewed PDF shape of the streamwise WSS fluctuations is significantly changed due to the presence of a separation bubble, while the PDF shape of the spanwise component is slightly affected, exhibiting a symmetric behaviour across the interaction. The weighted power-spectrum density map indicates that the low-frequency unsteadiness associated with the separated shock - exhibits little influence on the spectrum for either component, and no enhancement of the low-frequency energy is observed. A significant reduction in the spatial extent of the two-point correlation is observed, causing spanwise elongated coherence for the streamwise WSS fluctuations in the separation region. Moreover, the elliptic behaviour of the space-time correlations is essentially preserved throughout the interaction, and this is accompanied by a sudden reduction of the convection velocity in the separation bubble

Shock wave and turbulent boundary layer interaction in a double compression ramp

Direct numerical simulations of shock wave and supersonic turbulent boundary layer interaction in a double compression ramp with fixed ramp angles of 12 degrees and 24 degrees at Mach 2.9 are conducted. The characteristics of the shock interactions are investigated for four different length between the two ramp kinks, corresponding to L-c = 0.9 delta(ref), 1.8 delta(ref), 2.7 delta(ref), and 3.6 delta(ref) (delta(ref) being the upstream turbulent boundary layer thickness). The influence of increasing L-c on flow structures, unsteadiness, Reynolds stress, turbulence kinetic energy, and Reynolds stress anisotropy tensor is assessed. The size of the separation region is significantly decreased and reattached flow appears between the two ramp kinks. Streamwise vorticity contours and streamline curvature show the decreased spanwise width and increased spanwise coherency of Gortler-like vortices. Analysis of fluctuating wall pressure indicates that the low-frequency unsteadiness is strongly suppressed in the interaction region. Profiles of Reynolds stress components and turbulence kinetic energy exhibit different turbulence evolution across the interaction, leading to substantial differences observed in the anisotropy invariant map. It is found that the near wall region is characterized by decreased anisotropy, becoming closer to the axisymmetric compression state, while a significant increase of turbulence is identified in the outer region, following the axisymmetric expansion limit. Moreover, downstream of the interaction, turbulence in the near-wall region experiences a faster recovery and the influence of L-c is found to be marginal. The main effect of L-c is observed in the outer region, an increase of L-c resulting in a monotonic decay of turbulence intensities and an inward movement of turbulent structures

Properties of the compressibility and transport motion in wall-bounded turbulent flows at Mach 8

The compressibility effect and transport motion in highspeed turbulent boundary layer (TBL) is a fundamental problem because they dominate the average and statistical characteristics. Using the statistical methods and flow visualization technology, flat-plate TBLs at Ma(infinity) = 8 with high- and low-wall temperatures, T-w/T-infinity = 10.03 and 1.9, are investigated based on the direct numerical simulation (DNS) datasets. Compared with previous studies, this study considers relative higher Mach number and strong cold wall temperature condition at the same time. First, the turbulent Mach number and turbulent intensity show that the compressibility effects are enhanced by the cooling process. Second, the high-order statistical moments and structure parameters confirm cold wall that causes stronger compressibility and the corresponding increased intensities of local streamwise and wall-normal transport motions. Finally, for uncovering the relationship between the compressibility effect and turbulent transport, more indepth visualization analyses of velocity streaks are performed. It is found that 'knot-like' structures are generated when cooling the wall, and they lead to stronger intermittent, which results in the rapid increase of local compressibility effect and the wall-normal transport motion. Our research sheds light on providing a theoretical basis for further understanding the compressibility effects of TBL at high Mach number

激波/湍流边界层干扰分离泡直接数值模拟

采用直接数值模拟(DNS)方法对来流马赫数为2.25、33.2°激波角的入射激波/平板湍流边界层干扰分离泡进行了数值研究。在验证了计算结果可靠性的基础上，通过分析比较3个不同展向站位分离泡的非定常运动特性、分离微团几何特征和相干结构等，定量考察了三维展向结构差异的影响规律。研究发现，分离泡存在复杂的三维结构，其流向长度明显大于法向高度和展向宽度，整体上沿展向呈现中间高两边低的扁平型单峰结构。分离泡面积脉动预乘功率谱结果表明，分离泡的非定常运动表征为大尺度低频膨胀/收缩过程，其展向三维结构对峰值频率的影响较小，且分离泡两侧略滞后于中间。采用经验模态分解(EMD)方法对分离泡低频膨胀/收缩过程进行了条件统计分析。统计结果表明，膨胀和收缩运动对分离微团几何特征没有实质影响，各展向站位分离微团高度/长度比值的概率峰值出现在0.1附近，同时分离微团面积和法向高度近似满足二次方分布。此外，流向速度脉动场的本征正交分解(POD)分析指出，分离泡的非定常运动与低阶模态密切相关，而高阶模态的贡献相对较小。采用前10个低阶模态可以准确重构出分离泡的低频膨胀/收缩过程

Wall heat flux in supersonic turbulent expansion flow with shock impingement

We perform direct numerical simulations to investigate the characteristics of wall heat flux (WHF) in the interaction of an oblique shock wave at an angle of 33.2 & DEG; and free-stream Mach number M & INFIN; = 2.25 impinging on supersonic turbulent expansion corners with deflection angles of 0o (flat plate), 6o and 12o. The effect of the expansion on the WHF characteristics is analysed by comparing it to the interaction with the flat plate under the same flow conditions and a fixed shock impingement point. In the post-expansion region, the decreased mean WHF is found to collapse onto the flat plate case when scaled with the mean wall pressure. The statistical properties of the WHF fluctuations, including probability density function, frequency spectra, and space-time correlations, are comparatively analysed. The expansion causes an increase in the occurrence probability of negative extreme events, an enhancement of high-frequency energy, and an inhibition of intermediate-frequency energy. The increased expansion angle also results in a faster recovery of characteristic spanwise length scales and an increase in convection velocity. We use the mean WHF decomposition method in conjunction with bidimensional empirical mode decomposition to quantitatively analyse the impact of expansion on scale contributions. It is demonstrated that the presence of the expansion corner has no significant impact on the decomposed results, but it significantly reduces the contribution associated with outer large-scale structures

超声速压缩-膨胀构型激波与湍流边界层干扰直接数值模拟研究

采用直接数值模拟方法对来流马赫数2.924&deg;压缩-膨胀构型中的激波与湍流边界层干扰问题进行了数值研究。系统探究了膨胀角法向高度对干扰区内的若干基本流动现象的影响,如分离泡、激波非定常运动、湍流边界层的演化特性等。研究发现,随着膨胀角法向高度减小,膨胀效应导致分离泡流向与法向尺度均急剧减小。物面压力脉动信号预乘谱结果表明,膨胀效应抑制了分离激波的低频振荡运动。相较于压缩折角构型,压缩-膨胀构型下游存在复杂的膨胀波系结构,导致下游湍流边界层的内层、外层出现相反的恢复过程。</p

Decomposition of mean skin friction in incident shock wave/turbulent boundary layer interaction flows at Mach 2.25

The evolution characteristics of the mean skin friction beneath the supersonic turbulent boundary layer that interacts with incident shock waves at Mach 2.25 are analyzed using Direct Numerical Simulation (DNS). The separated and attached boundary layers in the interaction region that respectively correspond to 33.2 degrees and 28 degrees incident shock angles are considered. The mean skin friction recovery rate for the separated boundary layer is much gentler and distinctly less than that for the attached case where the skin friction completes its recovery within one boundary layer thick-ness. The novel mean skin friction decomposition method for compressible flows proposed by the recent research is applied in the interaction region to investigate the internal evolution characteristics quantitatively. The results reveal that the three decomposition components are distinctly unequal between the two cases. The contributions of the turbulent motions at different scales to the associated term are focused on using empirical mode decomposition technology. It indicates that the outer large-scale structures dominate separation and reattachment regions, while contributions from inner small-scale structures are limited. In contrast, contributions from the outer large-scale structures are dramatically reduced in the attached case, which results in the outer large-scale and inner small-scale motions being of equal importance.(c) 2023 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/)

激波/湍流边界层干扰压力脉动特性数值研究

激波/湍流边界层干扰问题广泛存在于高速飞行器内外流动中,激波干扰会导致局部流场出现强压力脉动,严重影响飞行器气动性能和飞行安全.为了考察干扰区内脉动压力的统计特性,对来流马赫数2.25,激波角33.2°的入射激波与平板湍流边界层相互作用问题进行了直接数值模拟研究.在对计算结果进行细致验证的基础上,分析比较了干扰区外层和物面脉动压力的典型统计特征,如脉动强度、功率谱密度、两点相关和时空关联特性等,着重探讨了两者的差异及其原因.研究发现,激波干扰对外层和物面压力脉动的影响差异显著.分离区内脉动以低频特征为主,随后再附区外层压力脉动的峰值频率往高频区偏移,而物面压力脉动的低频能量仍相对较高.两点相关结果表明,外层和物面脉动压力的展向关联性均明显强于其流向,前者积分尺度过激波急剧增长随后缓慢衰减,而后者积分尺度整体上呈现逐步增大趋势.此外,时空关联分析结果指出,脉动压力关联系数等值线仍符合经典的椭圆形分布,干扰区下游压力脉动对流速度将减小,外层对流速度仍明显高于物面

Direct numerical simulation of a supersonic turbulent boundary layer over a compression–decompression corner

A direct numerical simulation of the interaction between a shock wave and the supersonic turbulent boundary layer in a compression&ndash;decompression corner with a fixed 24 deflection angle at Mach 2.9 is conducted. The characteristics of the shock interactions are investigated for two heights between the compression and decompression corners, corresponding to H=dref &frac14; 4:25; 1:22, where dref denotes the reference turbulent boundary layer thickness. A classic shock wave/turbulent boundary layer interaction flow is reproduced in the higher case. For the lower case, the size of the separation region is significantly decreased, and the low-frequency unsteadiness is slightly suppressed in the interaction region, as assessed by analyzing the mean and fluctuating wall pressure. Flow patterns near the reattachment line show the existence of the G&euro;ortler vortices. By analyzing the curvature radius and G&euro;ortler number distribution, it was found that a strong centrifuge instability is reserved in the compression corner region and reversed in the decompression corner region due to the convex streamline curvature. The downstream flow of the decompression corner is relatively complex where the additional shocklet and new stream wise vortices are observed. A negative response mechanism is found regarding fluctuating wall-pressure signatures between the upstream and downstream of the decompression corner</p