机器学习技术在气动优化中的应用

近年来优化设计在气动设计中发挥了越来越多的作用,但实用性和效率是制约其发挥作用的两大障碍。在大型客机超临界机翼设计中,通过"人在回路"（依靠人的经验在优化进行过程中实施必要干预）等努力,取得了较好的效果,机器学习技术逐步得到发展。提出了利用机器学习技术模拟人在优化过程中的合理行为和作用机制,以深层次利用信息和知识,改善优化的实用性和效率。梳理了机器学习技术在气动优化中应用的发展脉络,并结合工作实践介绍了机器学习在优化设计中的典型应用。进一步探讨了深度学习在气动优化中的可能应用方式。     

叶轮机非定常气动设计的缘线匹配(Ⅰ)理论与实施方法

针对缘线匹配展开理论与实施方法探讨，分析了缘线匹配潜在效益，并给出缘线匹配的两种实施方法：基于非定常流动束缚于拟流回转面假设的基元流动展向积分方法和全优化方法。分析指出：缘线匹配具有通过沿展高调协叶轮机相邻叶排非定常流动相位关系实现气动、气弹、气动噪声和热传导性能全方位、多目标优化潜力，为叶轮机非定常设计提供了新的设计自由度。     

二元喉道倾斜矢量喷管调节方法

利用数值模拟方法,对二元喉道倾斜矢量喷管的调节方法进行了研究。研究结果表明,当喉道注气流量不变,随着扩张段注气流量逐渐增大,喷管内出现三种典型流动状态。当扩张段注气流量比较小时,喉道处声速线的形状和位置没有明显变化,在扩张段注气口前面形成亚声速区域;当喷管扩张段注气流量比较大时,喷管中出现了强激波系和上下贯通的亚声速区域,这时喷管内的流动损失最大,推力系数最低;进一步增大扩张段注气流量,声速线会从喷管喉道移动到喉道注气口和扩张段注气口之间,出现典型的喉道倾斜现象。单独控制流量系数可通过保持扩张段注气流量不变,改变喉道注气流量来实现;矢量角的控制可通过保持喉道注气流量不变,改变扩张段注气流量来实现。     

Effect of pitch angle on initial stage of a transport airplane ditching

Airworthiness regulations require that the transport airplane should be proved to ensure the survivability of the ditching for the passengers. The planned ditching of a transport airplane on the calm water is numerically simulated. The effect of pitch angle on the impact characteristics is especially investigated by a subscaled model. The Reynolds-averaged Navier-Stokes (RANS) equations of unsteady compressible flow are solved and the realizable j-e equations are employed to model the turbulence. The transformation of the air-water interface is tracked by volume of fluid (VOF) model. The motion of the rigid body is modeled by dynamic mesh method. The initial ditching stage of the transport airplane is analyzed in detail. The numerical results show that as the pitching angle increases, the maximal normal force decreases and the pitching motion becomes much gentler. The aft fuselage would be sucked down by the water and lead to pitching up, whereas the low horizontal tail prevents this trend. Consequently, the transport aircraft with low horizontal tail should ditch on the water at an angle between 10 and 12 as a recommendation.     

A low-diffusion robust flux splitting scheme towards wide-ranging Mach number flows

This paper develops a low-diffusion robust flux splitting scheme termed TVAP to achieve the simulation of wide-ranging Mach number flows. Based on Toro-Vázquez splitting approach, the new scheme splits inviscid flux into convective system and pressure system. This method introduces Mach number splitting function and numerical sound speed to evaluate advection system. Meanwhile, pressure diffusion term, pressure momentum flux, interface pressure and interface velocity are constructed to measure pressure system. Then, typical test problems are utilized to systematically assess the robustness and accuracy of the resulting scheme. Matrix stability analysis and a series of numerical cases, such as double shear-layer problem and hypersonic viscous flow over blunt cone, demonstrate that TVAP scheme achieves excellent low diffusion, shock stability, contact discontinuity and low-speed resolution, and is potentially a good candidate for wide-ranging Mach number flows.     

Consequences of the application of the streamer fluid model to the study of the sprite inception mechanism

We present the results of a streamer-fluid model used to investigate the electrodynamical coupling between the troposphere and upper atmosphere due to the penetration of lightning electric fields into the mesosphere and the lower ionosphere, generating sprites. The model solves the continuity equation for electrons and ions coupled to Poisson equation. The dominant physical response of the atmosphere is the formation of a screening-ionization wave. The wave shields the atmosphere above it from the action of the lightning field and, together with the conductivity reduction below it due to attachment, the wave amplifies the total field below it, allowing for the penetration of intense electric fields in the mesosphere as it propagates downwards into regions of higher density that compress the wave. This is the key physical mechanism for sprite inception. We evaluated the effects of the thundercloud charge geometry, lightning current waveshape, atmospheric conductivity, via different electron density profiles, and the effect of ionization, attachment and electron mobility coefficients in the electrical breakdown process, related to halo production, and sprite streamer initiation. The results showed that electrons with higher mobility are more efficient in shielding the lightning electric field before breakdown, causing delay, and they contribute to the formation of the streamer seed after breakdown, anticipating the sprite streamer inception. Similarly, a higher effective ionization rate, produced by modifications in the attachment and ionization coefficients, anticipates sprite inception. The simulations with 6 different electron density profiles, and therefore conductivities, spanning 4 orders of magnitude, showed that the altitude of breakdown and sprite initiation, as well as their time delays from the lightning discharge are directly related to atmospheric conductivity: higher conductivities produce halo and sprite inception at lower altitudes with longer delays and may hinder sprite formation. We document that variations of 30 times in the lightning current leads to sprite initiation altitudes in the range 66.0–73.5 km, with delays between 1.550 and 34.500 ms, while variations of 4 orders of magnitude in the conductivity profile lead to initiation altitudes 61.0–70.6 km, with delays in the range 3.825–9.825 ms. Consequently, we suggest that lightning characteristics dominate over atmospheric parameters in determining sprites’ initiation altitude and delay. The simulation of a −CG, with a constant current of 30 kA, did not produce a sprite seed, confirming an asymmetry in the response of the atmosphere to positive and negative lightning. This is due to the free electron drift direction that is away from the screening ionization wave, preventing the formation of the streamer seed for the great majority of −CGs. The same does not apply to halos, which depend on the occurrence of breakdown and can be produced by discharges of both polarities.     

Effect of Martian atmosphere on aerodynamic performance of supersonic parachute two-body systems

Supersonic flows around parachute two-body systems are numerically investigated by solving the compressible Navier-Stokes equations. In the present study, both rigid and flexible parachute models are considered, which comprise a capsule and a canopy. The objective of the present study is to investigate the effects of the Martian atmosphere on the unsteady flows produced by these parachute two-body models and the structural behavior of the flexible canopy. It was found that in the Martian atmosph...     

一次调节通道流阻特性的数值模拟

为了研究冷却剂在层板发汗冷却推力室一次调节通道中的流动特性，引入了粗糙粘度μR的概念及其计算式，用有效粘度μ1,eff代替分子粘度μ，导出了冷却剂在一次调节通道中层流流动的N—S方程并对其进行了数值求解。计算结果表明，利用μ1,eff＝μR μ代数模型计算得到的流动结果与试验数据吻合较好，说明冷却剂在Dh≈0．16mm--18mm的一次调节通道中层流流动时，可不计微小尺度的效应，但应考虑壁面粗糙度对流动的影响。     

燃气非平衡流再生冷却流动传热数值模拟

为准确预测液体火箭发动机推力室身部再生冷却换热状况,采用数值模拟方法,对燃气、推力室壁和超临界气氢进行三维流动和换热耦合计算。采用6组分9步反应动力学模型计算燃气的非平衡化学反应,采用DO模型计算燃气辐射换热,考虑超临界气氢物性随温度和压力的变化。获得了室壁温度场、燃气及冷却剂流场。结果表明,Redlich-Kwong方程、Peng-Robinson方程、Lucas法、TRAPP法能分别准确计算超临界氢的密度、定压比热容、粘度、导热系数,采用燃气非平衡流计算所得流场值更符合实际情况。     

层板模型流阻特性的研究

设计了不同内部流动结构的单、双层多孔层板,并针对这些层板敢式风洞上进行了流阻实验,得出了绕流柱形式、开孔率、孔间距、绕流柱高度以及单、双等敏感因素对层板流阻特性的影响：方形扰流柱阻力最小,圆形次之,菱形阻力最大;开孔率越大,孔间距越小,流阻越小;扰流柱越高,流阻越小;双层扰流柱的层板阻力明显高于单层扰流柱。