Computational study of flow in a rocket-based combined cycle (RBCC) engine inlet

A two-dimensional axisymmetric viscous flow analysis of a rocket-based combined cycle (RBCC) engine inlet is performed using a time-marching numerical scheme to solve the Reynolds-averaged Navier-Stokes equations. The flow configuration is a subscale model of the engine inlet which was previously tested in the 1X1 supersonic wind tunnel at NASA Glenn Research Center. The computed results are compared with the experimental data which include static pressure profiles along the centerbody and the cowl surface of the inlet. The computational results show a reasonably good agreement with the experimental data.     

Measurement-integrated simulation of wall pressure measurements using a turbulent model for analyzing oscillating orifice flow in a circular pipe

The development of a highly functionalized orifice flowmeter with high accuracy under realistic conditions is desired. This paper presents a method for analyzing oscillating air flow through an orifice in a circular pipe. A measurement-integrated (MI) simulation using a standard k–ε model was used to reduce the computation time. In a previous study, the feedback law of the MI simulation was determined by considering the effect of the computational fluid dynamics (CFD) grid on contracted flow. However, the previous method required the measurement of inlet flow rate, which is not feasible in many applications. Therefore, an MI simulation was proposed that only requires wall pressures, which are much simpler to measure than flow rate. In this MI simulation, the wall pressure downstream of an orifice was measured, and a new proportional–integral controller feedback algorithm was developed to control the inlet flow rate in the computed flow field. The proposed MI simulations were performed for steady and oscillatory flow rates up to 10 Hz. It was found that this MI simulation provides accurate solutions at a significantly shorter computation time than conventional CFD analysis.     

超音速进气道三维流场数值仿真与性能分析

减小冲压增程炮弹特别是进气道的阻力,可以提高炮弹的射程。建立了冲压增程炮弹超音速进气道的物理模型,采用N-S方程,并运用有限体积法对进气道进行了三维流场的数值仿真,得到了超音速进气道的流场结构图及性能参数,有利进行分析。重点研究了攻角对超音速进气道性能的影响。理论计算了进气道的总压恢复系数,并对进气道总压恢复系数的理论计算值与数值模拟值进行比较分析,基本一致。得到的结论为冲压增程炮弹提供了设计有效参考。     

Soot emission prediction of a waste-gated turbo-charged DI diesel engine using artificial neural network

This study is about soot emission prediction of a waste-gated turbo-charged DI diesel engine using artificial neural network (ANN). For training the ANN model, six ranges of experimental data in previous study were used, and one range of data was kept for testing the accuracy of ANN predictions. The input parameters for the ANN are inlet manifold pressure, inlet manifold temperature, inlet air mass flow rate, fuel consumption, engine torque, and speed. Output parameter is the density of soot in the exhaust. The results show the ANN approach can be used to accurately predict soot emission of a turbo-charged diesel engine in different opening ranges of waste-gate (ORWG). Root mean-squared error (RMSE), fraction of variance (R ²), and mean absolute percentage error (MAPE) for predictions were found to be 1.19 (mg/m³), 0.9998, and 6.4%, respectively.     

在超音速压缩角下的湍流模型数值比较

在超音速飞机进气道的湍流数值仿真中,选择合适的湍流模型可以比较准确的捕捉激波位置和流场分布,对飞机发动机的设计和控制都是很重要的。但是对湍流模型的选择,文献中并无记载。针对上述情况选择了进气道的简化结构,分别运用标准的高雷诺数k-epsilon模型,重整化群RNG模型,Realizable k-epsilon模型及ASM模型等进行了数值仿真和比较。基于SIMPLE算法,采用相同的分析壁面函数处理法,并且对流项都采用MINMOD格式。从比较发现:RNG模型比标准的k-epsilon模型和Realizable k-epsilon模型精度高,比ASM模型精度稍低,但是比ASM模型省时间,最后作者认为RNG模型在算例中是最经济实用的湍流模型。     

超声速进气道流场的CFD数值仿真

进气道是航空推进系统的一个重要组成部分,进气道内的流场品质会显著影响发动机的性能.由于进气道内部流动的复杂性和其广泛的应用前景,进气道内的流动特性引起了人们广泛的关注.采用计算流体力学(CFD)方法,空间离散采用Harten-Yee的二阶迎风TVD格式,时间迭代采用隐式LU-SGS方法,数值求解Navier-Stokes(N-S)方程,对进气道内部流场进行了数值模拟,并研究了进气道内部流场的流场结构以及激波/边界层干扰问题.数值计算结果反映出了流场的基本物理现象,说明了所采用的研究方法是可行的.同时数值模拟结果对进气道的设计有一定的参考价值.     

Computation of supersonic inviscid flow around wings with a detached shock wave

The numerical method for computing inviscid supersonic flow around the front part of the arbitrary planform wings was presented in [1,2]. The aim of this paper, which is the extension of the previous ones, is to present a numerical method for computing the flow field above the remaining parts of the wing—the wingtip section and the central section of the wing. Here the problems are formulated for a three-dimensional steady gas-dynamic system of equations written in special curvilinear coordinates. Complicated physical domains of the solution are mapped on simple computational domains. For approximation of the differential equations the finite-difference second-order implicit schemes are used. The approximation of the wing surfaces is made with the help of the local cubic splines. According to the obtained algorithms calculations were made for the wing with elliptical planform and thick airfoil at M_∞ = 2 and M_∞ = 3.5 with the angle of attack α = 5°.     

Non-uniform recursive Doo–Sabin surfaces

This paper presents a generalization of Catmull–Clark-variant Doo–Sabin surfaces and non-uniform biquadratic B-spline surfaces called Non-Uniform Recursive Doo–Sabin Surfaces (NURDSes). One step of NURDS refinement can be factored into one non-uniform linear subdivision step plus one dual step. Compared to the prior non-uniform Doo–Sabin surfaces (i.e., quadratic NURSSes), NURDSes are convergent for arbitrary n-sided faces. Closed form limit point rules, which are important for applications in adaptive rendering and NC machining, are given as well.     

CFD analysis of performance characteristics of S-shaped diffusers with combined horizontal and vertical offsets

This paper presents the effect of combined horizontal (90°/90° turn) and vertical offsets (inlet and outlet in two different horizontal planes) on S-shaped diffusers, one with rectangular inlet (AS = 2) and rectangular outlet and other with rectangular inlet (AS = 2) and semi-circular outlet with area ratio 2 in both the cases having downstream settling length of 50 mm at Reynolds number 1.37 × 10⁵. A computer program based on finite volume technique, using standard k–ε turbulence model, has been adopted and modified to predict the flow. The results obtained from this study indicate reduced outlet pressure recovery accompanied with increase in non-uniformity in flow at the exit contributed by the offset effect. The comparison of pressure recovery of the extreme vertical offset value i.e. 1B in case of rectangular outlet diffuser and 1D in case of semi-circular outlet diffuser with zero vertical offset indicates that there is significant drop of pressure of the order of 14% and 7.3% respectively. Non-uniformity in flow at the outlet also increases from 12% to 32% and 6.4% to 21% in case of S-shaped diffuser with rectangular outlet and S-shaped diffuser with semi-circular outlet respectively, starting from zero vertical offset to extreme vertical offsetting magnitude for the case. This paper also shows that increase in Reynolds number has marginal effect on the outlet pressure recovery for both types of diffuser with all cases of vertical offset. The S-diffuser with semi-circular outlet with 0.25D vertical offsetting only fulfills most of the requirements of the flow diffusion process efficiently and effectively. For other cases, redesigning of geometry is required.     

On the application of boundary conditions to time dependent computations for quasi one-dimensional fluid flows

A study is made of the influence of boundary and initial conditions on time-dependent finite-difference solutions of quasi-one-dimensional duct flows. Several questions are addressed: (1) Under what conditions will a time-dependent solution converge to a steady-state supersonic flow, (2) Under what conditions will it converge to subsonic flow and (3) What conditions are necessary to insure a particular unique solution for subsonic flows. The results provide an orientation, or way of thinking, about the role of such conditions in time-dependent solutions of steady-state flows. The results also show that supersonic solutions are readily obtained by holding only pressure and temperature fixed at the duct inlet, and allowing velocity to float. However, subsonic solutions require pressure, temperature and velocity to be fixed at both the duct inlet and exit. If no conditions are held fixed at the exit, the results always converge to the supersonic solution, even if the fixed inlet mass flow is less than critical. In such a case, the program appears to generate additional mass flow between the inlet and throat, sufficient to choke the flow. These results also have some impact on two- and three-dimensional time-dependent solutions where subsonic flow is present on some or all portions of the flow boundaries.     

Computing the velocity of a rotating flow

To compute the time-dependent flow of a rotating incompressible fluid we consider the velocity–vorticity formulation of the Navier–Stokes equations in cylindrical coordinates. In the numerical method employed the velocity field at each time-step is found as the least squares solution of an overdetermined system of linear equations, Ax=b. We consider how to compute x using the preconditioned conjugate gradient algorithm for least squares (PCGLS) on a distributed parallel computer. The various aspects of using a parallel computer are discussed, and results for a wide range of parallel computers are presented. The parallel speed-up depends on the architecture but is typically about 80% of the number of processors used.     

Attacks on state-of-the-art face recognition using attentional adversarial attack generative network

With the broad use of face recognition, its weakness gradually emerges that it is able to be attacked. Therefore, it is very important to study how face recognition networks are subject to attacks. Generating adversarial examples is an effective attack method, which misleads the face recognition system through obfuscation attack (rejecting a genuine subject) or impersonation attack (matching to an impostor). In this paper, we introduce a novel GAN, Attentional Adversarial Attack Generative Network (A³GN), to generate adversarial examples that mislead the network to identify someone as the target person not misclassify inconspicuously. For capturing the geometric and context information of the target person, this work adds a conditional variational autoencoder and attention modules to learn the instance-level correspondences between faces. Unlike traditional two-player GAN, this work introduces a face recognition network as the third player to participate in the competition between generator and discriminator which allows the attacker to impersonate the target person better. The generated faces which are hard to arouse the notice of onlookers can evade recognition by state-of-the-art networks and most of them are recognized as the target person.

     

小型聚焦纹影实验台的设计与搭建

聚焦纹影系统能对垂直于光轴的特定平面内的流场细节进行显示,避免了视场范围内其余非均匀流场对背景成像的干扰。为实现对收缩喷管出口超声速流场的观测,设计并搭建了小型聚焦纹影实验台和超声速流动实验台。超声速流动实验台由2组收缩喷管组成,相距15 cm,垂直摆放,利用高压气源供气,使喷管出口继续膨胀产生超声速气流。首次利用CAD设计胶片上格栅间距尺寸,验证了在胶片纸上进行源光栅、刀口栅改造的正确性。综合考虑超声速流动区域的大小,选取了锐利聚焦深度和非锐利聚焦深度等设计指标。完成了对小型超声速流动实验台流场的拍摄,获得了其流场结构图。通过对比聚焦纹影、计算流体力学(CFD)仿真和传统纹影的流场图,说明此聚焦纹影实验台原理正确,可以达到预期的显示效果。同时,针对聚焦纹影系统高速流场的显示,提出合理的建议。     

某型涡扇发动机进口空气流量测量参数敏感性分析

基于某型涡扇发动机飞行台试飞,以某型涡扇发动机进口空气流量的测量为研究对象;首先建立了发动机进口空气流量计算模型以及相应的误差计算模型,其次采用辨识方法确定了测量参数的敏感系数,最后针对测量参数的无量纲敏感系数随飞行高度和发动机状态的变化进行了分析;分析结果表明:发动机进口空气对测量截面半径最为敏感,其次为主流区总压、主流区总温和主流区总静压差,对附面层位移厚度敏感性较差,且在发动机设计状态附面层对空气流量的影响为3%。     

Simulation of unsteady compressible flow in a channel with vibrating walls - Influence of the frequency

This study deals with the numerical solution of a 2D unsteady flow of a compressible viscous fluid in a channel for low inlet airflow velocity. The unsteadiness of the flow is caused by a prescribed periodic motion of a part of the channel wall with large amplitudes, nearly closing the channel during oscillations. The channel is a simplified model of the glottal space in the human vocal tract and the flow can represent a model of airflow coming from the trachea, through the glottal region with periodically vibrating vocal folds to the human vocal tract.The flow is described by the system of Navier–Stokes equations for laminar flows. The numerical solution is implemented using the finite volume method (FVM) and the predictor–corrector MacCormack scheme with Jameson artificial viscosity using a grid of quadrilateral cells. Due to the motion of the grid, the basic system of conservation laws is considered in the Arbitrary Lagrangian–Eulerian (ALE) form.The authors present the numerical simulations of flow fields in the channel, acquired from a program developed exclusively for this purpose. The numerical results for unsteady flows in the channel are presented for inlet Mach number M_∞ = 0.012, Reynolds number Re_∞ = 4.5 × 10³ and the wall motion frequency 20 and 100 Hz.     

进气道流量调节阀控制装置设计

由于飞机发动机进气道试验中空气流量需精确控制,需要采用一种响应迅速,且较为容易实现计算机精确控制的机械电子系统。该气道流量调节控制装置采用C8051F021单片机与易于单片机控制的步进电机来实现系统控制功能,用5×4的键盘作为输人来对电机的状态进行控制,键盘输入的数据送至单片机,并用数码管显示输入的行程和整个系统的运行工况与位移传感器的测量值,单片机输出信号控制步进电机的运行;系统设置了串口通信,通过MAX232接口电路实现控制系统与上位机的数据通信,通过上位机实现对进气道流量的控制。     

Early induction of secondary vortices for micromixing enhancement

In the present work a new type of micromixer has been proposed and its mixing characteristic has been analyzed. The micromixer can be viewed as a U-tube with a side inlet. Here micromixing is enhanced by the secondary vortex generation induced by the curvature of the tube. The flow in the mixer geometry is investigated theoretically to understand micro-mixing using computational fluid dynamics (CFD). For this we use the Navier–Stokes equations coupled with species transport. Mixing is quantified using mixing quality which is a measure of the uniformity of the concentration in a given geometry. Special attention is paid to the occurrence of the secondary vortices close to the mid point of the outer wall and its role in mixing. Simulations are also done to study the flow in U shaped channels. The simulation results show that the new design leads to an early introduction of secondary vortices than a simple U tube. Thus in the new design the secondary vortices are induced at a Re = 120 as opposed to the classical value of Re = 400 (when there is no side inlet) reported in the literature. Mixing is studied for different diffusivities and combination of inlet velocities. We also compare the performance of our design with the classical T and Y mixers. The early induction implies that we can have good mixing at low Re. Consequently, when used as a micro-reactor we can combine good mixing with high residence times to obtain good conversions in our system.     

Numerical simulation of turbulent flow in the inlet region of a smooth pipe

An algebraic-Q4 turbulent eddy viscosity model expresses the eddy viscosity as a solution of a quartic (Q4) equation. The model is applied to numerical simulation of developing turbulent flow in the inlet region of a smooth pipe. Predictions of the flow characteristics, such as velocity profiles accross and along a pipe, pressure drop along a pipe are found in good agreement with experimental data.     

Normal shock boundary layer control with various vortex generator geometries

Various vortex generators which include ramp, split-ramp and a new hybrid concept “ramped-vane” are investigated under normal shock conditions with a diffuser at Mach number of 1.3. The dimensions of the computational domain were designed using Reynolds Average Navier–Stokes studies to be representative of the flow in an external-compression supersonic inlet. Using this flow geometry, various vortex generator concepts were studied with Implicit Large Eddy Simulation. In general, the ramped-vane provided increased vorticity compared to the other devices and reduced the separation length downstream of the device centerline. In addition, the size, edge gap and streamwise position respect to the shock were studied for the ramped-vane and it was found that a height of about half the boundary thickness and a large trailing edge gap yielded a fully attached flow downstream of the device. This ramped-vane also provided the largest reduction in the turbulent kinetic energy and pressure fluctuations. Additional benefits include negligible drag while the reductions in boundary layer displacement thickness and shape factor were seen compared to other devices.     

液体冲压发动机仿真模型研究

发展了一种弹用液体冲压发动机仿真模型.利用小偏离线性化理论和分段集总参数法,分别研究了超声速进气道、燃烧室和可调收敛喷管的建模方法.以正激波作为上边界,燃油流量的小变化作为扰动方程,喷管作为下边界,从而建立了液体冲压发动机仿真模型.根据所建立的仿真模型,对冲压发动机系统进行了仿真实验.仿真结果验证了仿真模型的有效性和合理性.该模型可用于液体冲压发动机控制系统的研究.