微槽-吸气组合控制平板边界层多模态稳定性研究

边界层转捩会使高超声速飞行器壁面摩阻和热流显著增加,因此在高超声速飞行器设计过程中往往占据重要地位.针对高超声速飞行器多模态转捩控制问题,提出了微槽道(1 mm)与边界层吸气的组合控制方法,并通过直接数值模拟和线性稳定性理论研究了Ma=4.5平板边界层的稳定性及组合控制效果.边界层在无控状态时,同时存在失稳的第一、二模态波,且二维第二模态波最不稳定;单纯施加微槽道控制时,边界层第二模态波会被抑制但第一模态波会被略微激发.对比而言,采用“微槽-吸气”组合控制后,不仅增强了对第二模态波的抑制效果,而且减弱了第一模态波的激发程度;同时随着吸气强度的增加,第二模态波不稳定区域明显收缩、频率显著增高,而第一模态波则变化不明显.相较于单纯的微槽道,吸气增强了“微槽吸收”与“声波散射”作用,因此中等吸气强度下该组合控制方法对第一和第二模态波的增长率分别实现了12.63%和28.02%的抑制效果.以上结果表明“微槽-吸气”组合控制手段具有适用宽频、布置区域灵活的优点,展现出了一定的多模态控制效果.     

等离子体减阻技术的研究进展

利用等离子体的特性减小飞行器的气动阻力是一种新概念的减阻思路.目前国外的一些风洞实验和计算结果已证明了等离子体减阻效果的显著性.本文介绍了国外等离子体减阻技术的研究进展,包括:逆向等离子体喷流减阻、基于等离子体边界层控制减阻和局部施加能量点源减阻.分析了等离子体减阻的基本原理及所涉及的一些关键技术点.最后针对我国的减阻研究情况,提出了等离子体减阻技术的研究方向.      

Drag prediction,decomposition and visualization in unstructured mesh CFD solver of TAS‐code

The accuracy of drag prediction in unstructured mesh CFD solver of TAS (Tohoku University Aerodynamic Simulation) code is discussed using a drag decomposition method. The drag decomposition method decomposes total drag into wave, profile, induced and spurious drag components, the latter resulting from numerical diffusion and errors. The mesh resolution analysis is conducted by the drag decomposition method. The effect of an advanced unstructured mesh scheme of U‐MUSCL reconstruction is also investigated by the drag decomposition method. The computational results show that the drag decomposition method reliably predicts drag and is capable of meaningful drag decomposition. The accuracy of drag prediction is increased by eliminating the spurious drag component from the total drag. It is also confirmed that the physical drag components are almost independent of the mesh resolution and scheme modification. Copyright © 2007 John Wiley & Sons, Ltd.     

DRAG REDUCTION TECHNIQUE FOR LARGE TRANSPORT AIRCRAFT

回顾了到目前为止可能用在大型飞机上的相关减阻技术,从三个方面(减小摩擦阻力、降低诱导阻力、减小激波阻力)详细介绍了多种减阻技术的方法和原理. 其中减小摩擦阻力分成层流减阻和湍流减阻两个部分. 层流减阻中吸气控制方法、湍流减阻中沟槽壁面控制方法、翼梢小翼技术、激波鼓包技术等都有可能成为改进新一代大型运输机阻力特性的实用技术. 粗糙阵列、等离子体激励器、动态控制三维鼓包等新技术也为减阻技术的实际应用提供了新的可能性.     

添加剂湍流减阻流动与换热研究综述

添加剂湍流减阻是指在液体的管道湍流中添加少量的高分子聚合物或某种表面活性剂从而使湍流阻力大大降低的现象.从其被发现至今,经过近半个世纪的研究(实验研究、理论分析、数值模拟和实际系统的应用研究),尽管对这一现象及其实际应用价值已有了较为深入的认识,但仍有许多方面尚有欠缺,例如对湍流减阻的机理仍然在探索中.本文归纳评述了高分子聚合物或表面活性剂添加剂湍流减阻流动与换热现象的研究现状,从湍流减阻剂的特性、减阻剂的湍流减阻机理、湍流减阻发生时的换热机理、减阻流动速度场分布和换热控制等几个方面综述了添加剂湍流减阻流动与换热特性,并综述了湍流减阻剂在实际工业系统中的应用情况,在对添加剂湍流减阻机理、有湍流减阻发生时的对流换热机理等的理解方面进行了新的总结.      

Friction Drag Variation via Spanwise Transversal Surface Waves

The introduction of spanwise velocity is a promising technique to effect the near-wall turbulent flow field to influence friction drag. However, the essential physical mechanism which significantly reduces friction drag has not been understood, yet. It is the objective of this numerical study to improve the fundamental knowledge on the drag reduction mechanism. The investigation is based on spanwise traveling transversal surface waves which are applied to modify the near-wall flow field and to influence friction drag. Two actuation configurations are analyzed in detail. Compared with an unactuated flat plate boundary layer simulation the first wave setup, which represents a low frequency wave at an amplitude larger than the viscous sublayer, leads to a reduced wall-shear stress resulting in friction drag reduction of up to 9%. The second wave setup, which possesses a higher frequency and an amplitude in the range of the viscous sublayer, yields an increase of friction drag of about 8%. Unlike previous investigations which focus on excitation setups to lower friction drag, the comparison of the two wave setups in this study allows to identify the effects which on the one hand, lead to drag reduction and on the other hand, result in drag increase. That is, due to the pronounced differences the major effects determining the friction distribution are more evident. The two key features for drag reduction are the damping of the wall-normal vorticity fluctuations above the entire surface and the decrease of turbulence production. Furthermore, the effect of rearranging streamwise vorticity, which has been stated to be responsible for drag reduction, is found to occur at increasing and decreasing drag, i.e., it is not the effect that lowers the friction drag.     

水下减阻技术研究综述

在简要回顾早期减阻研究的基础上,对现有典型水下湍流减阻技术进行了较深入地分析.重点介绍了脊状表面减阻、微气泡减阻和疏水/超疏水表面减阻的研究现状.分别从实验研究和理论研究两方面对其进行了阐述,并着重强调了各自的减阻机理. 此外,还简要介绍了柔顺壁面减阻、壁面振动减阻等其它减阻技术.展望了水下减阻技术今后的研究重点及其应用前景.      

The effect of an unsteady drag force on the structure of a non-equilibrium region behind a shock wave in a gas-particle mixture

For numerical analysis of shock wave propagation in gas-particle mixtures, drag coefficients of a sphere in steady flows are generally used. However, it is shown both experimentally and numerically that a shock loaded solid sphere experiences unsteady drag forces. The paper describes a model of unsteady drag force and its effect on the structure of the non-equilibrium region behind a shock front traveling in a dusty gas. The results are compared with those obtained by using a steady drag coefficient and are discussed. It is demonstrated that the large drag force at the early stage of the interaction between shock-wave induced flow and a solid particle affects the flow structure that is obtained with a steady drag force.      

大型飞机阻力预示与减阻研究

简要说明了大型飞机减阻的重要意义，对国内外大型飞机计算流体动力学(CFD)和风洞实 验等阻力预示技术进行了初步的评估和分析，重点论述了大型飞机减阻气动布局及装置、减 低诱导阻力方法和减低摩阻方法的研究现状和发展趋势，指出中国需要进一步提高CFD和风 洞实验阻力预示技术水平，同时加强对气动布局、优化设计以及流动控制等基础科学技术问 题的研究. 对中国发展大型飞机能起到借鉴和参考的作用.     

Numerical investigations on gas–solid flow in circulating fluidized bed risers using a new cluster-based drag model

A cluster-based drag model is proposed for the gas–solid circulating fluidized bed (CFB) riser by including the cluster information collected from image processing and wavelet analysis into the calculation of system drag. The performance of the proposed drag model is compared with some commonly used drag models. A good agreement with the experimental data is achieved by the proposed cluster-based drag model. Error analysis of the proposed cluster-based drag model based on the local distributions of solids holdup and particle velocity is conducted. The clustering phenomenon in the low-density and high-density CFB risers and the effect of the cluster size on the simulation accuracy are also numerically studied by the proposed drag model.     

Calculation of all drag components for 3-D bodies in wind tunnel flows. A boundary element-wake approach

A boundary element-wake numerical approach is developed and used to determine all drag components of a three dimensional body in a wind tunnel flow. The approach decomposes the total drag into three components; the profile drag, the cross flow drag (induced drag), and the tunnel-wall effect component, each with its own physical significance. Additionally, the cross flow drag component is divided into two components, the vortex component and the source (dilatation) component. In the present approach, the transverse kinematics relations are expressed as integral representations of the axial vorticity and the transverse dilatation (source strength). This advantage permits the vortex and the source drag computations to be performed only in the vortical area of the transverse wake and hence avoids excessive computations. Also, the procedures distinguish the contribution of the transverse dilatation to the cross flow drag. The validity of the present procedure is examined by comparing the present results against the experimental data of reference [1] for a car and wing models. The comparison shows that the present computed total drag, for the wing and the car models, agrees very well with the experimental data, provided that the wake data are measured at survey planes moderately distant from the body.     

Variation of drag coefficients in an interacting drop stream

Variation of the drag coefficient of closely spaced drops in a stream injected into a turbulent flow is studied experimentally. Three different regions are identified. In the first region, close to the injector, drops flow in the wake of each other. The drag coefficient in this region is much smaller (by a factor of 4 to 5) than the standard drag coefficient, and its magnitude is dependent on the drop initial spacing. Shortly downstream of the injection point, the transition region starts, where the drag coefficient increases rapidly approaching the drag coefficient of a single isolated drop. And further down-stream when drops are dispersed significantly, the drag coefficient will behave the same way as a single isolated drop.     

飞行器船尾设计对亚声速底阻影响及风洞试验验证

减小阻力尤其是底部阻力,是飞行器增加航程的重要手段,而船尾布局则是减小底部阻力的有效措施.为研究船尾修型设计在亚声速段对飞行器阻力的影响规律,对某飞行器外形开展船尾修型设计并进行数值模拟,分析了不同船尾形状和船尾角度对飞行器阻力的影响情况,并开展了风洞试验验证.结果表明,船尾修型设计可以有效减小底部阻力;船尾采用曲线过...     

Computational fluid dynamics simulation of hydrodynamics in the riser of an external loop airlift reactor

Local hydrodynamics in the riser of an external loop airlift reactor (EL-ALR) are identified and the performances of three drag models are evaluated in computational fluid dynamics simulation. The simulation results show that the Schiller–Naumann drag model underestimated the local gas holdup at lower superficial gas velocity whereas the Tomiyama drag model overestimated that at higher superficial gas velocity. By contrast, the dual-bubble-size (DBS)-local drag model gave more reasonable radial and axial distributions of gas holdup in all cases. The reason is that the DBS-local drag model gave correct values of the lumped parameter, i.e., the ratio of the drag coefficient to bubble diameter, for varying operating conditions and radial positions. This ratio is reasonably expected to decrease with increasing superficial gas velocity and be smaller in the center and larger near the wall. Only the DBS-local drag model correctly reproduced these trends. The radial profiles of the axial velocity of the liquid and gas predicted by the DBS-local model also agreed well with experimental data.     

Evaluation of the drag force by integrating the energy dissipation rate in stokes flow for 2D domains using the FEM

The total drag force on the surface of a body, which is the sum of the form drag and the skin friction drag in a 2D domain, is numerically evaluated by integrating the energy dissipation rate in the whole domain for an incompressible Stokes fluid. The finite element method is used to calculate both the energy dissipation rate in the whole domain as well as the drag on the boundary of the body. The evaluation of the drag and the energy dissipation rate are post-processing operations which are carried out after the velocity field and the pressure field for the flow over a particular profile have been obtained. The results obtained for the flow over three different but constant area profiles—a circle, an ellipse and a cross-section of a prolate spheroid—with uniform inlet velocity are presented and it is shown that the total drag force times the velocity is equal to the total energy dissipation rate in the entire finite flow domain. Hence, by calculating the energy dissipation rate in the domain with unit velocity specified at the far-field boundary enclosing the domain, the drag force on the boundary of the body can be obtained.     

A NUMERICAL STUDY OF OPTIMAL DRAG REDUCTION FOR TURBULENT TRANSONIC PROJECTILES USING A PASSIVE CONTROL

Abstract

The purpose of this research is to numerically study a drag reduction method—passive control of shock/boundary layer interaction, which is applied to the boattail portion of a secant-ogive-cylinder-boattail projectile in turbulent transonic flows. The flow pattern and the components of aerodynamic drag computed from numerical data are analyzed. The effectiveness of this method is studied by varying the values of parameters such as porosity distribution, maximum porosity factor and size of porous region. The conditions for optimal drag reduction are investigated and reported. The present results show that the use of this passive control method can not only reduce the boattail drag but also the base drag, and results in an additional 8% total drag reduction compared to that without the passive control technique. This passive control method can be an effective approach for the design of high-performance projectiles in the transonic regime.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.     

Robust design and numerical simulation on drag reduction by a mixture film for liquid turbulence in a channel

An important way of increasing the speed and lowering the fuel consumption of ships is by decreasing the frictional drag. One of the most promising techniques for reducing drag is the use of air bubbles. The goal of this investigation is to establish a set of optimum robust parametric levels for drag reduction by a mixture (air–water) film in turbulent channel flow. Based on the conditions laid out by the Taguchi orthogonal array method, turbulent flows, with air bubbles injected into a channel, are simulated using commercial computational fluid dynamics software. The local shear stress on the upper wall is computed to evaluate the efficiency of drag reduction. Many factors can affect drag reduction. The factors investigated in this study are the rate of air injection, bubble size, area of air injection, flow speed, and measured position of the shear stress. These factors have been investigated through the analysis of variance, which has revealed that the rate of air injection and water flow speed dominate the efficiency of drag reduction by a mixture film. According to the results, the drag can be reduced by an average of 83.4%; and when the configuration of the parametric levels is optimum the maximum drag reduction of 88.5% is achieved. Copyright © 2008 John Wiley & Sons, Ltd.     

上海陆域古河道分布及对工程建设影响研究

采用直接数值模拟方法, 对槽道湍流中展向振动流向传播的波动壁面的流动 控制和减阻问题进行了研究, 讨论了流向参数k_{x}对Stokes层、湍流拟序结构、湍流猝 发事件以及壁面阻力的影响, 并对此类波动壁面的湍流控制和减阻机理进行了讨论. 结果表 明, 当此类波动壁面被用来调制近壁流动时, 仅低频波对湍流流场具有显著影响, 可导致湍 流猝发事件的频率和强度的显著变化; 波数k_{x}的增大对于湍流猝发事件的频率和强度增 减的影响并不同步, 存在一个最优的波数k_{x}, 在其调制下, 固有流场对诱导流场的影响 最弱, 而诱导流场对固有流场的影响显著, 减阻效果最好.     

鼓泡流化床中流动特性的多尺度数值模拟

鼓泡流化床因其较高的传热特性以及较好的相间接触已经被广泛应用于工业生产中,而对鼓泡流态化气固流动特性的充分认知是鼓泡流化床设计的关键.在鼓泡流化床中,气泡相和乳化相的同时存在使得床中呈现非均匀流动结构,而这种非均匀结构给鼓泡流化床的数值模拟造成了很大的误差.基于此,以气泡作为介尺度结构,建立了多尺度曳力消耗能量最小的稳定性条件,构建了适用于鼓泡流化床的多尺度气固相间曳力模型.结合双流体模型,对A类和B类颗粒的鼓泡流化床中气固流动特性进行了模拟研究,分析了气泡速度、气泡直径等参数的变化规律.研究表明,与传统的曳力模型相比,考虑气泡影响的多尺度气固相间曳力模型给出的曳力系数与颗粒浓度的关系是一条分布带,建立了控制体内曳力系数与局部结构参数之间的关系.通过模拟得到的颗粒浓度和速度与实验的比较可以发现,考虑气泡影响的多尺度曳力模型可以更好地再现实验结果.通过A类和B类颗粒的鼓泡床模拟研究发现,A类颗粒的鼓泡床模拟受多尺度曳力模型的影响更为显著.