On the motion of particles in a fluid under the influence of a large velocity gradient

The motion of seeding particles as used in laser Doppler anemometry is investigated in the presence of a large velocity gradient across aerodynamic shocks under different flow conditions. Experimentally obtained results are presented and compared with theoretical predictions based upon the size distribution of the seeding particles used. It is found that the agreement of experimental and theoretical results depends on the flow conditions as well as on the particle material.     

高Bond数下黏性液滴的Rayleigh-Taylor不稳定性

给出了高Bond数下黏性液滴表面Rayleigh-Taylor线性不稳定性的分析解，这种不稳定性对于超音速气流作用下液滴破碎的早期阶段起着至关重要的作用．基于稳定性分析的结果，导出了用于估算稳定液滴的最大直径及液滴无量纲初始破碎时间的计算式，这些计算式与相关文献给出的实验和分析结果比较显示了良好的一致．     

Noise radiated by a non-isothermal,temporal mixing layer. Part I: Direct computation and prediction using compressible DNS

Direct numerical simulations (DNSs) are performed in order to study acoustic emissions generated during the transition of isothermal and non-isothermal mixing layers. The sound from temporally evolving mixing layers is computed directly using DNS for a computational domain, which includes both aerodynamic and acoustic fields. Good precision of the computed acoustic field is ensured by using a numerical code based on high-order finite difference schemes of quasi-spectral accuracy. Two- and three-dimensional simulations of mixing layers are performed for various Mach numbers and temperature ratios. For each case, the acoustic radiation of the mixing layer transition is investigated. Comparisons illustrate the importance of the combined effects of temperature and Mach number on the acoustic intensity. Qualitative agreement with existing experimental observations for hot jet flows is observed. It is also found that the appearance of three-dimensional motion leads to a substantial reduction of sound emissions. In the second part of this study, DNS data are used to perform acoustic analogy predictions. Excellent agreement between direct computations and predictions is obtained in all cases. Analysis of the source terms yields a new interpretation of temperature and Mach number effects, based on the predominance of one term over the other.     

Pulsed Air-jet Actuators for Flow Separation Control

The design and validation of a microfabricated pulsed air-jet actuator for practical application to flow-separation control at full-scale operating conditions on a medium/large air vehicle is presented. The actuator device is designed to generate streamwise vortices within the boundary layer and comprises a pitched and skewed orifice of 200 μm diameter through which a high velocity (200–300 m/s) jet of air can be modulated by operation of a piezoelectric microvalve. This paper describes the overall design and manufacture of the actuator device with particular reference to the impact of fluid dynamic effects on the design and operation. Key results obtained from both static and dynamic tests of a prototype device are also presented and compared with original predictions. It is shown that the device that was developed and tested fulfils all the original design requirements with regard to size, jet velocity and operating frequency. The developed device has dimensions of approximately 5 mm × 2 mm in the plane of the aerodynamic surface in which it is imbedded and a thickness of 1 mm. Peak jet velocities in excess of 300 m/s through a 200 μm diameter orifice at 500 Hz have been demonstrated with peak driving voltages of 90 V and a nominal electrical power consumption of 50 mW.     

Wind tunnel and full-scale study of wind effects on a super-tall building

This paper presents the analyzed results from a combined wind tunnel and full-scale study of the wind effects on a super-tall building with a height of 420 m in Hong Kong. In wind tunnel tests, mean and fluctuating forces and pressures on the building models for the cases of an isolated building and the building with the existing surrounding condition are measured by the high-frequency force balance technique and synchronous multi-pressure sensing system under two typical boundary layer wind flow fields. Global and local wind force coefficients and structural responses are presented and discussed. A detailed study is conducted to investigate the influences of incident wind direction, upstream terrain conditions and interferences from the surroundings on the wind loads and responses of the high-rise structure. On the other hand, full-scale measurements of the wind effects on the super-tall building have been performed under typhoon conditions. The field data, such as wind speed, wind direction, structural acceleration and displacement responses have been simultaneously and continuously recorded during the passage of 12 typhoons since 2008. Analysis of the field measured data is carried out to investigate the typhoon effects on the super-tall building. Finally, the model test results are compared with the full-scale measurements for verification of the wind tunnel test techniques. The comparative study shows that the wind tunnel testing can provide reasonable predictions of the structural resonant responses. The resonant displacement responses are comparable to the background displacement responses so that the contribution of the background responses to the total displacement responses should not be underestimated. The outcome of the combined wind tunnel and full-scale study is expected to be useful to engineers and researchers involved in the wind-resistant design of super-tall buildings.     

AN APPROXIMATE MODEL OF UNSTEADY AERODYNAMICS FOR HYPERSONIC PROBLEMS AT HIGH ALTITUDE

针对高空高马赫数飞行环境和强黏性干扰的物理特性, 在当地流活塞理论的基础上引入有效外形修正, 发展了黏性修正当地流活塞理论, 结合定常N-S方程解给出了高空高马赫数下针对该方法的有效外形的判据, 并通过数值算例对该判据进行了验证.通过对典型尖头薄翼和典型钝头翼的一系列二维非定常算例, 将该方法与一阶活塞理论、基于欧拉(Euler)方程的当地流活塞理论和非定常N-S方程数值解进行了对比. 结果显示在高度为40~70 km、马赫数为10~20范围内, 通过该方法计算得到的非定常气动力与非定常N-S方程数值解吻合较好, 明显优于活塞理论和基于Euler方程的当地流活塞理论.该方法克服了传统的活塞理论和当地流活塞理论不能用于高空高马赫数这类强黏性效应情况的弊端, 在较宽的马赫数、攻角、飞行高度范围内都有良好的适用性, 同时其计算效率远高于非定常N-S方程.     

BOUNDARY TREATMENT AND AN EFFICIENT PRESSURE ALGORITHM FOR INTERNAL TURBULENT FLOWS USING THE PDF METHOD

The generalized Langevin model, which is used to model the motion of stochastic particles in the velocity–composition joint probability density function (PDF) method for reacting turbulent flows, has been extended to incorporate solid wall effects. Anisotropy of Reynolds stresses in the near-wall region has been addressed. Numerical experiments have been performed to demonstrate that the forces in the near-wall region of a turbulent flow cause the stochastic particles approachi ng a solid wall to reverse their direction of motion normal to the wall and thereby, leave the near-wall layer. This new boundary treatment has subsequently been implemented in a full-scale problem to prove its validity. The test problem considered here is that of an isothermal, non-reacting turbulent flow in a two-dimensional channel with plug inflow and a fixed back-pressure. An efficient pressure correction method, developed in the spirit of the PISO algorithm, has been implemented. The pressure correction strategy is easy to implement and is completely consistent with the time- marching scheme used for the solution of the Lagrangian momentum equations. The results show remarkable agreement with both k–ϵ and algebraic Reynolds stress model calculations for the primary velocity. The secondary flow velocity and the turbulent moments are in better agreement with the algebraic Reynolds stress model predictions than the k– ϵ predictions. © 1997 by John Wiley & Sons, Ltd.     

AERODYNAMIC STIFFNESS AND DAMPING EFFECTS IN THE ROTATIONAL GALLOPING OF A RECTANGULAR CROSS-SECTION

The galloping oscillation with a single rotational degree of freedom is investigated for a prismatic beam with a rectangular cross-section. For this particular geometric configuration, the effect of the unsteady aerodynamic forces can be understood as a combination of aerodynamic stiffness and damping terms, with the latter being the most relevant for the instability behaviour. A theoretical prediction of both these effects is made for this type of galloping behaviour, based on a quasi-steady aerodynamic approach. Comparison to experimental results obtained in wind tunnel experiments reveals significant shortcomings of the quasi-steady approach in predicting, in particular, the damping effect. For the stiffness effect, the agreement is much better.     

Distinctive features of the flow past high-speed flight vehicles with air-breathing engines on subsonic,transonic, and low supersonic velocity ranges

The aerodynamic characteristics and distinctive features of the flow past hypersonic integral-layout flight vehicles with air-breathing engines intended for cruise flight in the atmosphere are experimentally investigated. The experiments were conducted on a simplified model designed with regard for the general principles of integration of vehicles of the class considered. The tests were performed in a wind tunnel over the Mach and Reynolds number ranges 0.6 ≤ M ≤ 4 and 6.3 × 10⁶ ≤ Re ≤ 16 × 10⁶, respectively. Balance testing was carried out, the pressure distributions over the vehicle surface were measured, and the flowfields on the model surface were photographed. The effects of mounting a nacelle and contouring the internal duct are considered. The effect of the corrections on the duct flow in the absence of jet modeling is estimated. The results obtained can be used as a basis for developing the aerodynamic configurations of integral-layout flight vehicles, for forming their thrust and aerodynamic parameters under full-scale flight conditions, and for testing computation methods.     

Calculation of nonlinear aerodynamic characteristics of a wing using a 3‐D panel method

The nonlinear aerodynamic characteristic of a wing is investigated using the frequency‐domain panel method. To calculate the nonlinear aerodynamic characteristics of a three‐dimensional wing, the iterative decambering approach is introduced into the frequency‐domain panel method. The decambering approach uses the known nonlinear aerodynamic characteristic of airfoil and calculates two‐variable decambering function to take into consideration the boundary‐layer separation effects for the each section of the wing. The multidimensional Newton iteration is used to account for the coupling between the different sections of wing. The nonlinear aerodynamic analyses for a rectangular wing, a tapered wing, and a wing with the control surface are performed. Present results are given with experiments and other numerical results. Computed results are in good agreement with other data. This method can be used for any wing having different nonlinear aerodynamic characteristics of airfoil. The present method will contribute to the analysis of aircraft in the conceptual design because the present method can predict the nonlinear aerodynamic characteristics of a wing with a few computing resources and significant time. Copyright © 2007 John Wiley & Sons, Ltd.     

Sail aerodynamics: Understanding pressure distributions on upwind sails

The pressure distributions on upwind sails is discussed and related to the flow field around the headsail and the mast/mainsail. Pressures measured on several horizontal sections of model-scale and full-scale sails are used to provide examples. On the leeward side of the sails, leading edge separation and turbulent reattachment occurs, sometimes followed by trailing edge separation. On the windward side, leading edge separation occurs on the mast/mainsail and, at low angles of attack, it can also occur on the headsail. Differences were found between the leading edge bubbles on the two sails. Pressure trends for different angles of attack are presented, and these can be explained in terms of standard aerodynamic theory, particularly in terms of short and long leading edge separation bubble types. It was found that the pressure distributions measured on mainsails at full- and model-scale showed good agreement on both the windward and leeward sides.     

Aerodynamic design on high-speed trains

Compared with the traditional train, the operational speed of the high-speed train has largely improved, and the dynamic environment of the train has changed from one of mechanical domination to one of aerodynamic domination. The aerodynamic problem has become the key technological challenge of high-speed trains and significantly affects the economy, environment, safety, and comfort. In this paper, the relationships among the aerodynamic design principle, aerodynamic performance indexes, and design variables are first studied, and the research methods of train aerodynamics are proposed, including numerical simulation, a reduced-scale test, and a full-scale test. Technological schemes of train aerodynamics involve the optimization design of the streamlined head and the smooth design of the body surface. Optimization design of the streamlined head includes conception design, project design, numerical simulation, and a reduced-scale test. Smooth design of the body surface is mainly used for the key parts, such as electric-current collecting system, wheel truck compartment, and windshield. The aerodynamic design method established in this paper has been successfully applied to various high-speed trains (CRH380A, CRH380AM, CRH6, CRH2G, and the Standard electric multiple unit (EMU)) that have met expected design objectives. The research results can provide an effective guideline for the aerodynamic design of high-speed trains.     

Aeroelastic analysis of swept pre-twisted wings

In this paper, aeroelastic modeling of aircraft wings with variations in sweep angle, taper ratio, and variable pre-twist angle along the span is considered. The wing structure is modeled as a classical beam with torsion and bending flexibility. The governing equations are derived based on Hamilton’s principle. Moreover, Peters’ finite state aerodynamic model which is modified to take into account the effects of the wing finite-span, the wing sweep angle, and the wing pre-twist angle, is used to simulate the aerodynamic loads on the wing. The coupled partially differential equations are discretized to a set of ordinary differential equations using Galerkin’s approach. By solving these equations the aeroelastic instability conditions are derived. The results are compared with some experimental and analytical results of previous published papers and good agreement is attained. Effects of the wing sweep angle, taper ratio, bending to torsional rigidity, and pre-twist angle on the flutter boundary in several cases are studied. Results show that these geometrical and physical parameters have considerable effects on the wing flutter boundary.     

Hypersonic flow in an MHD-acceleration facility and under full-scale conditions

A formulation of the problem, a method of solution, and calculated results are considered for flow past the windward side of a sphere under conditions of an aerodynamic facility (a wind tunnel) with magnetohydrodynamic (MHD) acceleration of the air flow and the corresponding full-scale flight conditions in the Earth's atmosphere. Calculations were performed on the basis of numerical solution of Navier-Stokes equations taking into account thermochemical nonequilibrium of air and catalytic properties of the body surface. Results of the mathematical simulation of flow around a sphere in an MHD-acceleration wind tunnel are compared with experimental data obtained at the Central Aerohydrodynamic Institute (TsAGI). The problem of recalculation of experimental data for full-scale conditions is analyzed. Central Aerohydrodynamic Institute, Zhukovskii 140160. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 2, pp. 91–102, March–April, 1998.     

Computation of the stability derivatives via CFD and the sensitivity equations

The method to calculate the aerodynamic stability derivates of aircrafts by using the sensitivity equations is ex- tended to flows with shock waves in this paper. Using the newly developed second-order cell-centered finite volume scheme on the unstructured-grid, the unsteady Euler equations and sensitivity equations are solved simultaneously in a non-inertial frame of reference, so that the aerodynamic stability derivatives can be calculated for aircrafts with complex geometries. Based on the numerical results, behavior of the aerodynamic sensitivity parameters near the shock wave is discussed. Furthermore, the stability derivatives are analyzed for supersonic and hypersonic flows. The numerical results of the stability derivatives are found in good agree- ment with theoretical results for supersonic flows, and variations of the aerodynamic force and moment predicted by the stability derivatives are very close to those obtained by CFD simulation for both supersonic and hypersonic flows.     

A model for the aerodynamic coefficients of rock-like debris

This study presents a model to determine the aerodynamic forces on an irregularly shaped object for prediction of trajectories of debris. We classify the debris according to shape and use wind tunnel tests to create a database of aerodynamic forces of similarly sized objects. We have then validated the database results against wind tunnel measurements of aerodynamic coefficients of rock-like debris objects. Although the individual errors may be significant, the results show a generally good agreement with our model prediction and a small average error.     

Numerical Study of the Aerodynamic Performance of a Train with a Crosswind for Different Embankment Heights

A numerical study using improved delayed detached eddy simulation (IDDES) was used to investigate the influence of the embankment height on the aerodynamic performance of a high-speed train travelling under the influence of a crosswind. The results of the flow predictions were used to explore both the instantaneous and the time-averaged flows and the resulting aerodynamic forces, moments and slipstreams. An increase of the aerodynamic drag and side forces as well as the lift force of the head and middle cars were observed with rising embankment height. While the lift force of the tail car decreased with the increasing embankment height. Furthermore, the height of the embankment was found to have a strong influence on the slipstream on the leeward side of the train. The correlation between the embankment height and the slipstream velocity on the windward side, was rather small. The flow structures in the near-wake of the leeward side of the train, responsible for the aerodynamic properties of the train were analyzed, showing strong dependency on the embankment height.

     

Spacecraft aerodynamics and trajectory simulation during aerobraking

This paper uses a direct simulation Monte Carlo （DSMC） approach to simulate rarefied aerodynamic characteristics during the aerobraking process of the NASA Mars Global Surveyor （MGS） spacecraft. The research focuses on the flowfield and aerodynamic characteristics distribution under various free stream densities. The vari- ation regularity of aerodynamic coefficients is analyzed. The paper also develops an aerodynamics-aeroheating-trajectory integrative simulation model to preliminarily calculate the aerobraking orbit transfer by combining the DSMC technique and the classical kinematics theory. The results show that the effect of the planetary atmospheric density, the spacecraft yaw, and the pitch attitudes on the spacecraft aerodynamics is significant. The numerical results are in good agreement with the existing results reported in the literature. The aerodynamics-aeroheating-trajectory integrative simulation model can simulate the orbit transfer in the complete aerobraking mission. The current results of the spacecraft trajectory show that the aerobraking maneuvers have good performance of attitude control.     

“Models” of the fatigue process

Some of the instabilities in behavior of material and the structural readjustments that occur to alter the relationships between load and peak stress are discussed by visualizing each level of observation as a type of idealized “model” representing the structural behavior. Nonlinearities and chance effects occur in making predictions from models on the atomistic or crystalline scale to those of specimens, structural components, or full-scale structures. These nonlinear effects, together with the statistical aspects of material behavior, load history, and manufacturing variables, are emphasized to illustrate the difficulty in present-day practice of making any quantitative prediction of fatigue life or fatigue strength of the final product.     

Aerodynamic interference between high-speed slender bodies

Under supersonic flow conditions, slender bodies in close proximity induce aerodynamic interference effects. This paper aims to quantify the magnitude of the resulting interference loads and to understand the underlying flow-physics mechanisms that cause them. A pair of identical slender bodies are investigated through a series of wind-tunnel experiments and supporting computational fluid dynamics (CFD) predictions. The bodies induce a complex interference flowfield, which tends to be bespoke to each configuration. The flow features include impinging shock and expansion waves, conical shock reflections, strong skewing of the boundary-layer flows and shock diffraction. The effects of axial stagger, lateral separation and the strength of the primary disturbance flow field are evaluated. The interference loads are found to be most sensitive to the initial location of the primary disturbance but are also affected by its strength. In addition, maximum interference loads which equate to an effective incidence of up to 6° are observed. Finally, very good agreement is found between the measurements and the CFD predicted normal force and pitching moment.