与超声速/高超声速飞行器有关的边界层转捩预测方法及湍流计算的研究

To develop super/hypersonically flying vehicles which can sustainably fly in atmosphere is a matter for the state security. As experiments for such R&D works are very difficult to conduct, some are even not possible at the moment, so the R&D works must largely rely on CFD. While in applying CFD to develop the super/hypersonically flying vehicles, it will be face four difficult problems, one of which is the transition prediction and turbulence computation. As different from the R&D of supersonically flying vehicle, thermal-protection design is a key issue for hypersonically flying vehicle’s design. Because of the heat transfer coefficient at the wall for laminar boundary layer is quite different from those for turbulent boundary layer, the precondition of the correct design of the thermal-protection is the correct prediction of transition location, and the accurate prediction of the heat transfer coefficient at the wall of the turbulent boundary layer. Otherwise, the design may be either too conservative, leading to over-weight of the vehicles, or insufficient thermal-protection, leading to the destruction of the vehicle. Recently, much more effort has been put to the transition prediction and turbulence computation both in China and oversea, and some progresses have been achieved, but is still far from enough for technical demands. According to the problems mentioned above, and based on what we have achieved, the goals of our program are: to make the transition prediction method more scientifically based, so to reduce its relying on experiments, including flying test, which is vital for Chinese engineers, because there are little relevant experiments done in China, and such situation is unlikely to change very soon; and for turbulence computations, to develop methods which are easy to use while at the same time, can satisfy the precision demand from engineers.

研制在大气层中长时间飞行的高超声速飞行器，事关国家安全。由于实验非常困难，研发很大程度上依赖计算流体力学。将计算流体力学用于高超声速飞行器面临着四大难题，其中之一就是转捩预测和湍流计算。在高超声速飞行器的研制中，热防护是关键问题之一。由于层流边界层与湍流边界层的传热系数相差很多，因此正确设计热防护的前提是准确预测转捩的位置，同时正确计算湍流边界层的传热系数。如果计算结果偏于保守，则大大增加飞行器重量，如果估计不足，则不能满足防热要求，导致飞行器损坏。近年来，国内外在高超声速边界层的转捩预测和湍流计算上都加大了研究力度，也取得了一定进展，但离实际需求还有相当距离。本项目拟针对上述有关问题，在已有的研究基础上，完善转捩预测方法，以减少对实验或飞行实验的依赖。这一研究对我国特别重要，因为我国的有关实验非常少，而且近期不会有大的改进。在湍流方面，希望建立能满算精度，又易行的计算方法。

本项目面向国家航空航天领域的重大需求，采用理论分析和数值计算结合方法，研究超及高超声速空气动力学领域的关键科学难题-------转捩和湍流。.主要研究内容：.1）结合超及高超声速飞行器设计的需求，考虑边界层对外界环境中各种扰动波的不同感受性特征，为改进和完善转捩位置预测奠定基础。.2）为解决复杂和接近实际流动转捩预测，研究改进和完善稳定性分析方法和转捩位置预测方法。同时，根据PSE的结果，完善eN方法，提高在超声速，高超声速三维边界层中预测转捩位置的能力。.3）以平板、尖锥、钝锥、椭球锥、后掠体及飞行体等复杂边界层为研究对象，研究不同马赫数下扰动的非线性演化规律，揭示超音速边界层转捩过程中起主导作用的非线性过程以及相应的的非线性演化特征。.4）针对复杂的高雷诺数流动进行高精度直接数值模拟，并根据直接数值模拟提供的流场细节，对相应的湍流模式进行探索研究。.重要结果及其科学意义.1、给出了强来流湍流激发Gortler涡的过程以及Gortler涡的二次稳定性完整的数学理论，在向建立转捩位置和来流扰动特征之间的定量关系迈进了重要一步。.2、来流中扰动与激波作用后，在激波后会产生慢声波，而慢声波在中性曲线下枝直接激发不稳定波，不需要另一个外来扰动（如壁面粗超元）参与，这是新且有效的感受性机理。此外，还可区分什么类型的扰动在何处有效地激发了不稳定波，比已有的结果更细致.3、对于第二模态波的激发，头部附近激波后的快声波起主导作用。机理是同快声波激发快模态，再由快模态通过模态转换激发。两者对比发现，第二模态比第一模态感受性效率低，但增长慢。因此，最终触发转捩的扰动要看两者感受性和演化共同竞争的结果。.4、对于弱非平行性，提出了抛物化方法PSE、EPSE、iEPSE。对于强非平行性，如，局部粗糙元和吹吸等，提出“局部散射理论”。与eN方法相结合预测粗糙元对边界层转捩位置的影响。