跨介质航行器波浪环境入水流场演变和运动特性研究

跨介质航行器是一种既可以在空中飞行又可以在水下潜航的新概念航行器,基于仿生学原理,提出一种通过改变外形实现水空介质跨越的航行器模型,通过入水试验装置和计算流体动力学方法,对航行器带攻角从空气到水的介质跨越过程进行了试验和数值仿真研究,得到了跨介质入水过程航行器的运动姿态和入水空泡形态,并通过数值仿真得到了航行器的升力、阻力、速度和加速度演化规律。同时基于数值模拟方法对有波浪情况和静水情况下航行器入水过程空泡演变以及运动特性进行对比。结果表明:提出的航行器构型在水中具有较好的姿态调整能力,波浪的有无和波高的不同都会对航行体入水运动特性造成影响。     

运载火箭推进剂复杂流动传热问题数值模拟中的模型简化方法

在运载火箭改进优化或新型运载火箭设计过程中,会遇到各种复杂的流动与传热问题。按照真实产品和物理过程直接建立的数值计算模型过于复杂,不同尺度的结构、流动与传热、长时间的飞行过程相互耦合,出现计算时间太长(数年)、收敛困难、程序调试困难等问题。针对初始条件复杂、飞行过程复杂、边界条件复杂3类具体问题,总结了3种相应的模型简化方法,分别是工程经验法、极限参数法、低维近似法,为推进剂复杂流动与传热的数值模拟提供参考。     

多机并联火箭羽流流场及其底部热环境分析

For the problem that the plume flow field structure of a multi engine parallel rocket is complicated and the bottom thermal environment is extremely harsh, which may cause the failure of the engine structural components, the plume flow field and thermal environment at different altitudes are studied through numerical simulation. The result is compared with the measured results in flight which shows that when the rocket is flying at a low altitude, the plume of the engines do not interfere with each other. As the flight altitude increases, the plumes gradually expand and begin to interfere with each other, and finally there is an obvious backflow at the bottom of the rocket. The maximum heat flux at the moment of take off is basically the same as the measured value in flight. Before the backflow occurs, the heat flux mainly consists of radiant heat, the convective heat flow increases as the flight altitude grows, but it is also much smaller than the peak heat flow at takeoff. The result has certain guiding significance for the optimal design of engine structure thermal protection.     

Near-wall topological patterns and flow structures over a simplified Danaus plexippus model

Studies on the high-lift mechanisms of butterfly gliding flights shed light on the design of the micro air vehicles. The flow field around a simplified Danaus plexippus model is investigated using the hydrogen bubble visualization and the Particle Image Velocimetry (PIV) techniques. There are three near-wall topological patterns with different Angles of Attack (AoAs): the separation bubble, the Leading-Edge Vortex (LEV) and the high AoAs flow. For the separation bubble pattern, two saddles and two foci form in the middle of the model. The features of the LEV pattern are the leading-edge separation lines. The topological characteristics of the separation lines are changed by the interaction between the LEV and the Wing-Tip Vortex (WTV). For the high AoAs flow pattern, four unstable foci are found at the forewing and the hindwing respectively. The angle between the trajectory of the WTV and the model increases with increasing AoA even though the slope of the WTV angle versus AoA curve declines at the moderate AoAs.     

Aerodynamic performance enhancement for flapping airfoils by co-flow jet

Introducing active flow control into the design of flapping wing is an effective way to enhance its aerodynamic performance. In this paper, a novel active flow control technology called Co-Flow Jet (CFJ) is applied to flapping airfoils. The effect of CFJ on aerodynamic performance of flapping airfoils at low Reynolds number is numerically investigated using Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation with Spalart-Allmaras (SA) turbulence model. Numerical methods are validated by a NACA6415-based CFJ airfoil case and a S809 pitching airfoil case. Then NACA6415 baseline airfoil and NACA6415-based CFJ airfoil with jet-off and jet-on are simulated in flapping motion, with Reynolds number 70,000 and reduced frequency 0.2. As a result, CFJ airfoils with jet-on generally have better lift and thrust characteristics than baseline airfoils and jet-off airfoil when C_μ is greater than 0.04, which results from the CFJ effect of reducing flow separation by injecting high-energy fluid into boundary layer. Besides, typical kinematic and geometric parameters, including the reduced frequency and the positions of the suction and injection slot, are systematically studied to figure out their influence on aerodynamic performance of the CFJ airfoil. And a variable C_μ jet control strategy is proposed to further improve effective propulsive efficiency. Compared with using constant C_μ, an increase of effective propulsive efficiency by 22.6% has been achieved by using prescribed variable C_μ for NACA6415-based CFJ airfoil at frequency 0.2. This study may provide some guidance to performance enhancement for Flapping wing Micro Air Vehicles (FMAV).     

Experimental study on plasma jet deflection and energy extraction with MHD control

This paper presents an integrated research scheme for vector deflection and energy extraction in a gas plasma jet under Magneto-Hydrodynamic (MHD) control. A MHD-controlled thrust-vector test rig was used to conduct the experimental research. A gas plasma was obtained by injecting ionization seeds of Cs₂CO₃ into the combustion chamber via artificially forced ionization. The effects of the gas temperature and ionization seed mass fraction on the plasma jet deflection and energy extraction were experimentally verified under an applied magnetic field. The experimental results were analyzed theoretically. The results showed that the deflection amplitude of the gas plasma jet and the extracted voltage signal intensity increased with increasing gas temperature and the ionization seed mass fraction. The extracted dynamic voltage signals proved that the ionization seeds of Cs₂CO₃ induced gas ionization at 1173 K. The experiment verified that it is feasible to simultaneously achieve jet deflection and extract energy under the action of an external magnetic field.     

Sensitivity analysis of flowfield modeling parameters upon the flow structure and aerodynamics of an opposing jet over a hypersonic blunt body

Implementation of an opposing jet in design of a hypersonic blunt body significantly modifies the external flowfield and yields a considerable reduction in the aerodynamic drag. This study aims to investigate the effects of flowfield modeling parameters of injection and freestream on the flow structure and aerodynamics of a blunt body with an opposing jet in hypersonic flow. Reynolds-Averaged Navier-Stokes (RANS) equations with a Shear Stress Transport (SST) turbulence model are employed to simulate the intricate jet flow interaction. Through utilizing a Non-Intrusive Polynomial Chaos (NIPC) method to construct surrogates, a functional relation is established between input modeling parameters and output flowfield and aerodynamic quantities in concern. Sobol indices in sensitivity analysis are introduced to represent the relative contribution of each parameter. It is found that variations in modeling parameters produce large variations in the flow structure and aerodynamics. The jet-to-freestream total-pressure ratio, jet Mach number, and freestream Mach number are the major contributors to variation in surface pressure, demonstrating an evident location-dependent behavior. The penetration length of injection, reattachment angle of the shear layer, and aerodynamic drag are also most sensitive to the three crucial parameters above. In comparison, the contributions of freestream temperature, freestream density, and jet total temperature are nearly negligible.     

扫频式射流对涡轮叶栅间隙泄漏流动影响的数值研究

为控制涡轮叶栅中叶顶间隙泄漏流动和改善涡轮气动性能,将扫频式射流器(SJA)作为一种主动流动控制方法应用在涡轮叶栅的研究中。通过非定常数值计算,分析了SJA对涡轮叶栅叶顶间隙流动的作用过程以及作用机理,并且研究了不同工况下SJA对涡轮叶顶流场改善效果以及不同频率的SJA对叶顶流场的影响。结果表明:通过在涡轮叶栅上端壁增加单个SJA装置,可以有效地延迟上端壁的流动分离,其中最佳方案射流流量仅为进口总流量的0.35%,涡轮叶栅出口截面总压损失系数减少了11.48%。存在着最佳的频率284Hz,使SJA装置对流场的作用效果最佳,有效地改善了涡轮叶栅内的间隙流动。     

下缘板孔对涡轮叶片尾缘内冷通道流动换热影响的数值研究

涡轮叶片下缘板出气孔对内冷通道的流动和换热性能有较大影响。通过数值模拟方法研究下缘板出 气孔对尾缘和下缘板双路出气涡轮叶片尾缘内冷通道内的流动和换热特性,对比分析孔径、孔形和孔位置对尾 缘溢流孔流量系数、尾缘出流比、尾缘通道内总压系数和尾缘内冷腔壁面换热特性的影响。结果表明:下缘板 出气孔孔径对流量系数分布的影响显著,孔径增大,尾缘溢流孔流量系数下降,尾缘出流比减小,尾缘内冷通道 内压力损失降低,内冷腔平均换热系数增大;孔形对上游内冷通道内流动和换热几乎没有影响;孔位置变化对 内冷通道壁面整体的换热系数影响很小,对局部影响较大。     

导风轮叶片偏置对高压比串列离心压气机性能的影响

为提升高压比串列离心压气机的性能,借鉴常规一体化离心叶轮中偏置分流叶片的方法,针对某高压比串列离心压气机,应用数值仿真手段分析了串列叶轮中导风轮叶片周向偏置对压气机流场和性能的影响。通过对不同导风轮叶片偏置方案下压气机流场的分析,建立了压气机流动损失与偏置参数的关联性。研究表明:采用较大的偏置参数γ可降低导风轮叶片1前缘的激波强度,改善激波作用导致的泄漏涡破碎和流动分离,但过大或过小的γ方案中导风轮叶顶会出现二次泄漏致使低能流体的掺混损失增加;γ=65%方案压气机综合性能最佳,其压比和效率较γ=50%分别提高了1.5%和1.4%;对于串列离心压气机导风轮叶片周向位置的优化,在避免导风轮叶顶形成较强二次泄漏的前提下,应考虑采用较大的偏置参数γ,同时应防止诱导轮尾迹流体参与导风轮的叶顶泄漏。