凹腔诱发高超声速边界层转捩机理及气动加热研究

Sandwich-like heat protection tiles are very important for the Thermal Protection System (TPS) of the modern hypersonic aero-vehicles. The locally high pressure always leads to the formation of cavity with gaps, which induce the hypersonic boundary layer transition in advance. The extremely strong dynamic loads and heating augmentation on the aft wall of the cavity can easily destroy the thermal structures and eventually lead to the failure of the flight. The researches on cavity-induced transition (CIT) in hypersonic boundary layer and aero-heating augmentation are rare and mainly focus on the experiments in wind-tunnel. The mechanisms of CIT and heating augmentation are very difficult to predict, due to the deficiency of quantitative data from the measurements and very expensive computational resource requirements. At the same time, the simulation of aero-heating is one of the great challenges for modern computational fluid dynamics. Therefore, the mechanisms of CIT are urgently required to deeply investigate. .Based on the minimum dispersion and controllable dissipation (MDCD) high-order scheme and hundreds million grid numbers, direct numerical simulations (DNS) of CIT will be implemented. The formation, development, evolution and interaction of small-scale flow structures around the cavity, gaps, steps are hoped to describe in space. The frequency and amplitude characteristics of pressure and heating rate fluctuations are analyzed through the Fast Fourier Transform methods (FFT). Several main modes will be obtained by the Dynamic Mode Decomposition (DMD). Then, the time histories of the small-scale flow structures are hoped to explore. Some geometric and freestream flow parameters are investigated and their effects on the CIT and heating augmentations will be quantitatively achieved. The geometrical parameters are the length-depth ratio, width-depth ratio, swept angle, gap, and so on. The flow parameters are boundary layer thickness, adverse pressure gradients and disturbances of the freestream, and so on. The existing correlations will be corrected through the DNS results from our project. It will greatly help to explore the CIT mechanisms, improve the prediction accuracy of aero-heating rate and direct the design of TPS.

三明治式隔热瓦是现代高超声速飞行器热防护系统的重要组成部分，因局部受力会形成凹腔（含缝隙），促进流动转捩，导致极强动载荷和气动加热，甚至破坏防热结构，最终飞行失败。凹腔诱发的高超声速边界层转捩（CIT）和相应的气动加热研究，以风洞实验为主，定量数据少，难以揭示转捩机制和定量获得气动加热，急需深入研究。.在色散最小耗散可控高阶格式、局部加密的亿级网格基础上，直接模拟CIT，获得凹腔、缝隙、前后台阶等处小尺度流动结构的产生、发展、演化及相互作用的空间分布特征；用快速傅里叶变换获得压力和热流脉动频谱特征，用动模态分解获得各模态频率分布特征，获得小尺度流动结构的时间演化历程。研究凹腔几何（如长深比、宽深比、后掠角、缝隙等）和来流（边界层厚度、逆压梯度、来流扰动等）参数对转捩机制和气动加热的定量影响，修正现有气动加热经验关系式。有助于揭示转捩机理，提升气动热预测精度，指导热防护系统设计。

在高超声速飞行器中，由于隔热瓦的存在，其表面不可避免地会存在各式各样的凹腔和缝隙。凹腔会触发转捩，从而引起热流的急剧增大，这会影响到热防护系统设计，进而影响到整个飞行器的载荷。.本研究基于有限体积方法，采用四阶MDCD-WENO插值方法的Roe格式，直接数值模拟了高超声速凹腔非定常流动，主要研究凹腔强制转捩机理及各种参数对转捩的影响规律。首先模拟了并对比了与风洞实验状态相同的基准凹腔，发现本文计算结果精度高，具有很强的可靠性。同时选择了时间步长为Δt=0.25（外流流过凹腔约420Δt）和网格总数为8500万的精细网格。其次，模拟了深度分别为基准凹腔1.5倍，2倍和2.5倍的凹腔。结果表明，基准凹腔为闭式凹腔，1.5倍，2倍和2.5倍深度凹腔均为开式凹腔。在闭式凹腔内形成了类似Götler涡的流动，其在逆压梯度下失稳是闭式凹腔诱导转捩的主要原因。同时，闭式凹腔下游马蹄涡的失稳也诱导了转捩。基准凹腔的转捩宽度约为68mm。开式凹腔与外流的界面处形成了较强的剪切层，剪切层失稳为开式凹腔诱导转捩的主要原因。并且越深的凹腔对剪切层的干扰也越强，因此1.5倍深度凹腔未诱导转捩，2倍和2.5倍深度凹腔诱导了转捩，其转捩范围分别约为32mm和48mm。第三，模拟了宽度分别为基准凹腔0.25，0.5，0.75，1.5倍的凹腔。研究发现，0.5倍宽度凹腔出现了间歇转捩，其转捩状态下的流动结构与较宽的凹腔类似，未转捩状态与平板类似。研究发现，0.5倍宽度凹腔内的流向涡比基准凹腔更窄，且涡的位置与尺度在缓慢发生变化。在凹腔中前部，下方的两个涡不断膨胀并失稳，诱发了整个凹腔的转捩，后迅速恢复不转捩时的状态，转捩停止。间歇转捩的周期约为1.53 ms，间歇因子约为0.42.0.25倍宽度凹腔触发了稳定转捩，转捩机理和基准凹腔类似；0.75和1.5倍凹腔均为触发转捩。第四，模拟了后掠角为15°、30°，带缝凹腔，攻角为-10°、-12.5°，壁面条件为600K和绝热壁等多个不同算例，均为触发转捩。最后，本文还定量获得了凹腔及附近壁面的气动热与气动力的影响规律。基准凹腔引起的热流增长最显著；热流的增长幅度与诱导转捩的能力成正相关。.本项目成果利于认识高超声速气动加热机理，便于推广应用至气动热防护设计。