弹体级间分离流场特性的数值模拟研究

用数值方法模拟了超声速弹体级间分离流场,给出了不同级间距离时几种典型的流场结构,两类典型的流态与实验观察一致。级间距离较小时,级间区为死水区,前体阻力为负,出现典型的"后体效应"现象;距离较大时,内外流之间出现很复杂的干扰波系,一定的分离距离后,后体前缘分离流动区前出现正激波,后体的干扰基本被隔绝。     

多段翼型粘性绕流多块网格数值模拟

采用多块结构网格和分区求解技术对多段翼型绕流进行雷诺平均NS方程数值模拟,并将计算的压力分布与实验进行了比较.     

两类非对称涡流动所诱导的摇滚运动

文章详细讨论了两类非对称涡流动诱发的模型摇滚运动.第1类是针对旋成体机身组合体模型, 其摇滚运动是由前体非对称涡流动诱发的, 运功形态呈现不确定性, 由模型头尖部的扰动触发形成.文章提出了快速旋转头尖部扰动的控制技术, 以抑制该类模型的大攻角摇滚运动.第2类是针对非常规机身的组合体模型, 其摇滚运动的主控流动是非常规机身和机翼的前缘分离涡流动, 这些流动是由组合体模型的边界条件确定的, 从而运动形态具有很好的确定性.所以, 这类模型的自由摇滚运动必须通过改变边界条件来改变诱发摇滚运动的流动, 以达到抑制模型自由摇滚运动的目的.最后, 文章还讨论了这类运动是由非对称的机翼涡涡强主控的.      

A hybrid CFD/characteristics method for fast characterization of hypersonic blunt forebody/inlet flow

A hybrid CFD/characteristic method(CCM) was proposed for fast design and evaluation of hypersonic inlet flow with nose bluntness, which targets the combined advantages of CFD and method of characteristics. Both the accuracy and efficiency of the developed CCM were verified reliably, and it was well demonstrated for the external surfaces design of a hypersonic forebody/inlet with nose bluntness. With the help of CCM method, effects of nose bluntness on forebody shock shapes and the flowfield qualities which dominate inlet performance were examined and analyzed on the two-dimensional and axisymmetric configurations. The results showed that blunt effects of a wedge forebody are more substantial than that of related cone cases. For a conical forebody with a properly blunted nose, a recovery of the shock front back to that of corresponding sharp nose is exhibited, accompanied with a gradually fading out of entropy layer effects. Consequently a simplification is thought to be reasonable for an axisymmetric inlet with a proper compression angle, and a blunt nose of limited radius can be idealized as a sharp nose, as the spillage and flow variations at the entrance are negligible, even though the nose scale increases to 10% cowl lip radius. Whereas for two-dimensional inlets, the blunt effects are substantial since not only the inlet capturing/starting capabilities, but also the flow uniformities are obviously degraded.     

一种适用锥头体的嵌入式大气数据传感系统改进校准算法

嵌入式大气数据传感系统的空气动力学模型基于钝头体推导,该模型是否适用于锥头体尚未得到证实。对一典型锥头体进行了空气动力学模型适用性验证,验证结果表明该空气动力学模型用于锥头体时动静压相对误差超过了2.5%。对此,提出了一种适用于锥头体的改进校准算法,并且进行了仿真验证。仿真结果表明动静压相对误差小于0.5%,改进的校准算法有效消除了模型误差。     

底部加热空气层自然对流的可视化实验研究

本文对底部加热的水平空气层的自然对流换热进行了可视化实验研究,采用实时全息干涉照相技术获得了温度场的激光干涉图及流场的烟可视化图象。实验结果表明,这种水平空气层自然对流换热是非稳态的,并且在空间上是三维的。     

Measurement of temperature and velocity fields in a heater unit by liquid crystal thermometry and particle image velocimetry

Temperature and velocity fields in a heating unit for automobiles are measured through a model experiment in water tunnel using flow visualizations and image analysis to investigate the mixing mechanism of the flow that has passed through the heater and not passed through it inside the unit. The temperature fields are measured by the liquid crystal visualization technique combined with the field calibration using full color spline fitting technique, and the velocity fields are evaluated by a particle imaging technique with a cross-correlation algorithm. These results indicate an enhanced flow mixing at larger mix-door angles, which results in a shorter mixing distance of temperature and velocity downstream of the mix door. The enhanced flow mixing is caused by the high velocity fluctuations created by the flow separation over the mix door.     

边条翼后掠角对钝头细长旋成体非对称流动的影响

为探究大迎角时边条翼前缘后掠角对钝头细长旋成体导弹绕流特性的影响,开展了模型表面测压和粒子图像测速的风洞实验,研究了亚临界Reynolds数Re=150000、迎角α=50°条件下不同前缘后掠角固定边条翼的钝头细长旋成体非对称绕流特性.结果表明,在边条翼上游区,后掠角增大使边条涡涡位更靠近前体物面且对称性更好,导致前体...     

In-flight laser anemometry for aerodynamic investigations on an aircraft

A differential laser Doppler anemometer (LDA) of NLR and a laser-two-focus (L2F) anemometer of DLR were selected to be installed in a research aircraft to demonstrate their capabilities for in-flight flow investigations. A ground test with the anemometers was performed before the flight tests. Experience with parallel operation of the anemometers was gained in a free jet and properties of both systems were investigated. During the flight tests the aircraft was flown in different atmospheric conditions to investigate whether the atmospheric seeding conditions are sufficient for measurements. Measurement volumes were traversed through the flow perpendicular to the fuselage. In this manner, the anemometers measured mean velocities, turbulence levels and the flow direction. The experiment showed that laser anemometry can be applied successfully for in-flight flow measurements.     

Effect of the supersonic transport configuration on the sonic boom parameters

Results of numerical and experimental investigations of the sonic boom parameters for two configurations of civil supersonic transport are presented. Numerical modelling is performed by a combined method based on calculating the spatial flow in the near zone of the aircraft configuration and subsequent determination of disturbed flow parameters at large distances from the examined model. Numerical results are compared with experimental sonic boom parameters measured in the near zone and with results of their recalculation to large distances within the framework of the quasi-linear theory. This validation allows the degree of adequacy of the inviscid Euler model for solving the posed problem to be determined. Reasons for certain disagreement between the calculated and experimental data are discussed. The analysis confirms the possibility of attenuating the sonic boom generated by supersonic transport with an unconventional configuration based on a tandem arrangement of two wings on the fuselage.     

On the influence of a single roughness element on the flow in supersonic boundary layer on a blunted cone

The work presents the results of numerical modeling of a supersonic flow around a blunted cone with an isolated cylindrical roughness on the forebody surface in the three-dimensional formulation. The roughness element is shown to distort the mean flow and to give rise to small-amplitude disturbances with distinguished spectral peaks in the boundary layer.     

Filming the invisible – time-resolved visualization of compressible flows

Essentially all processes in gasdynamics are invisible to the naked eye as they occur in a transparent medium. The task to observe them is further complicated by the fact that most of these processes are also transient, often with characteristic times that are considerably below the threshold of human perception. Both difficulties can be overcome by combining visualization methods that reveal changes in the transparent medium, and high-speed photography techniques that “stop” the motion of the flow. The traditional approach is to reconstruct a transient process from a series of single images, each taken in a different experiment at a different instant. This approach, which is still widely used today, can only be expected to give reliable results when the process is reproducible. Truly time-resolved visualization, which yields a sequence of flow images in a single experiment, has been attempted for more than a century, but many of the developed camera systems were characterized by a high level of complexity and limited quality of the results. Recent advances in digital high-speed photography have changed this situation and have provided the tools to investigate, with relative ease and in sufficient detail, the true development of a transient flow with characteristic time scales down to one microsecond. This paper discusses the potential and the limitations one encounters when using density-sensitive visualization techniques in time-resolved mode. Several examples illustrate how this approach can reveal and explain a number of previously undetected phenomena in a variety of highly transient compressible flows. It is demonstrated that time-resolved visualization offers numerous advantages which normally outweigh its shortcomings, mainly the often-encountered loss in resolution. Apart from the capability to track the location and/or shape of flow features in space and time, adequate time-resolved visualization allows one to observe the development of deliberately introduced near-isentropic perturbation wavelets. This new diagnostic tool can be used to qualitatively and quantitatively determine otherwise inaccessible thermodynamic properties of a compressible flow.     

复合材料机身结构声学特性及影响参数分析

针对复合材料(以下简称"复材")结构进行声振分析,通过无限大障板理论和波动方程,分析复材平板和曲板结构的传声损失,并利用统计能量法分析壁板的隔声性能,与文献中的实验结果进行对比,验证建模的有效性。然后将复合材料机身结构等效成一个复材圆柱壳体结构,分析不同参数,包括压差、曲率半径、长度、铺层角度、纤维材料、加筋等对结构隔声性能的影响。最后与金属机身结构进行隔声性能对比,发现:在环频率与吻合效应频率之间,金属机身结构的传声损失明显大于复材机身结构,而在吻合效应频率以上频段,由于复材结构的吻合效应频率向低频移动,其传声损失好于金属机身结构。     

The vibro-acoustic response and analysis of a full-scale aircraft fuselage section for interior noise reduction

The surface and interior response of a Cessna Citation fuselage section under three different forcing functions (10-1000 Hz) is evaluated through spatially dense scanning measurements. Spatial Fourier analysis reveals that a point force applied to the stiffener grid provides a rich wavenumber response over a broad frequency range. The surface motion data show global structural modes (approximately < 150 Hz), superposition of global and local intrapanel responses (approximately 150-450 Hz), and intrapanel motion alone (approximately > 450 Hz). Some evidence of Bloch wave motion is observed, revealing classical stop/pass bands associated with stiffener periodicity. The interior response (approximately < 150 Hz) is dominated by global structural modes that force the interior cavity. Local intrapanel responses (approximately > 150 Hz) of the fuselage provide a broadband volume velocity source that strongly excites a high density of interior modes. Mode coupling between the structural response and the interior modes appears to be negligible due to a lack of frequency proximity and mismatches in the spatial distribution. A high degree-of-freedom finite element model of the fuselage section was developed as a predictive tool. The calculated response is in good agreement with the experimental result, yielding a general model development methodology for accurate prediction of structures with moderate to high complexity.     

Optical tools for aerodynamics

Visualization of flows has a special meaning in aerodynamics. Unsteady three dimensional flow fields need a visual display of experimental as well as theoretical results. Especially in experiments optical visualization techniques often lead to completely new insights into flow phenomena. Some discoveries which have been made this way are described in this review. Topics considered in detail are vortex obstacle interaction, Particle Image Velocimetry, long range Laser Doppler Velocimetry and Pressure Sensitive Paint.     

Wall-PIV as a near wall flow validation tool for CFD: Application in a pathologic vessel enlargement (aneurysm)

Flow visualization of a near wall flow is of great importance in the field of biofluid mechanics in general and for studies of pathologic vessel enlargements (aneurysms) particularly. Wall shear stress (WSS) is one of the important hemodynamic parameters implicated in aneurysm growth and rupture. The WSS distributions in anatomically realistic vessel models are normally investigated by computational fluid dynamics (CFD). However, the results of CFD flow studies should be validated. The recently proposed Wall-PIV method was first applied in an enlarged transparent model of a cerebri anterior artery terminal aneurysm made of silicon rubber. This new method, called Wall-PIV, allows the investigation of a flow adjacent to transparent surfaces with two finite radii of curvature (vaulted walls). Using an optical method which allows the observation of particles up to a predefined depth enables the visualization solely of the boundary layer flow. This is accomplished by adding a specific molecular dye to the fluid which absorbs the monochromatic light used to illuminate the region of observation. The results of the Wall-PIV flow visualization were qualitatively compared with the results of the CFD flow simulation under steady flow conditions. The CFD study was performed using the program FLUENT®. The results of the CFD simulation were visualized using the line integral convolution (LIC) method with a visualization tool from AMIRA®. The comparison found a very good agreement between experimental and numerical results.     

Simultaneous Shadow, Schlieren and Interferometric Visualization of Compressible Flows

For detailed investigations of processes and phenomena in the flow of compressible fluids, it is sometimes necessary to apply more than just one flow visualization technique as each method has its own characteristic strengths and weaknesses. In the case of flows with a low degree of repeatability, it may become mandatory to perform these multiple visualizations within the same experiment at identical or at least almost identical instants. This paper describes how two or more density-sensitive visualization techniques can be coupled in order to obtain simultaneously the distribution of density and its gradient and/or its second derivative in a flow field. The resulting optical systems are more complex than a conventional single visualization apparatus, but they can provide an unprecedented wealth of information about the flow field. By applying multiple visualization techniques, the inherent shortcomings of each individual method can be overcome and the risk of overlooking or misinterpreting certain flow features is reduced.     

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

Abstract

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.     

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic numbernehd) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).     

压气机叶片前缘楔形角对前缘分离泡的影响

在水洞内使用氢气泡流动显示方法研究环绕压气机叶片前缘的流动。实验结果表明,在前缘圆弧的下游有明显的分离泡存在。前缘圆弧半径相同而前缘楔形角不同的叶片,在同一工况下,前缘楔形角越大分离泡越小,流动分离越弱。