圆柱绕流的电磁控制

流体绕过非流线物体时，在物体尾部形成涡街，使其表面周期性变化的阻力和升力增加，从而导致物体振荡，产生噪音。本文通过实验和计算，研究圆柱绕流的电磁控制，阐述浸于弱电介质溶液中，表面包覆电磁激活板的圆柱，在电磁力作用下的流体控制原理，讨论电磁力的消涡、减阻和减振过程。     

Supercritical and subcritical dynamic flow-induced instabilities of a small-scale wind turbine blade placed in uniform flow

There is a growing interest in extracting more power per turbine by increasing the rotor size in offshore wind turbines. As a result, the turbine blades will become longer and therefore more flexible, and a flexible blade is susceptible to flow-induced instabilities. In order to design and build stable large wind turbine blades, the onset of possible flow-induced instabilities should be considered in the design process. Currently, there is a lack of experimental work on flow-induced instabilities of wind turbine blades. In the present study, a series of experiments were conducted and flow-induced instabilities were observed in wind turbine blades. A small-scale flexible blade based on the NREL 5 MW reference wind turbine blade was built using three-dimensional printing technique. The blade was placed in the test section of a wind tunnel and was subjected to uniform oncoming flow, representing the case of a parked wind turbine blade. The blade׳s tip displacement was measured using a non-contacting displacement measurement device as the oncoming wind speed was increased. At a critical wind speed, the blade became unstable and experienced limit cycle oscillations. The amplitude of these oscillations increased with increasing wind speed. Both supercritical and subcritical dynamic instabilities were observed. The instabilities were observed at different angles of attack and for blades both with and without a geometric twist. It was found that the blade twist had a significant influence on the observed instability: a blade without a twist experienced a strong subcritical instability.     

Design approach for cavitation tolerant hydrofoils and blades

Cavitation inception and growth on conventional shape hydrofoils and blades leads initially to a jump of their flow-induced noise, further to an amplification of flow-induced vibration with frequently assisted erosion and finally, to a lift/thrust decrease combined with the drag increase. These undesirable cavitation effects can be mitigated or even suppressed for stable partial cavities experiencing no tail pulsations. A design approach enhancing performance of cavitating hydrofoils/blades by maintaining stable partial cavities is described. Experimental data manifesting an increase of hydrofoil lift with reduction of its drag and of force pulsations by such design are provided. Application of this design approach to propeller/turbine blades and advantages of its employment for blades operating in non-uniform incoming flows are analyzed. The possibility of an increase of the lift to drag ratio and of a reduction of the cavity volume oscillation in gust flows for blade sections is numerically manifested.     

Large Eddy Simulation of Flow Control Around a Cube Subjected to Momentum Injection

The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7×10⁴. Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0° and 30°, respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0° yaw angle case and about 44.1 % reduction in the 30° yaw angle case. In the case of 0° yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30° yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.     

On the relationship between drag and vertical velocity fluctuations in flow over riblets and liquid infused surfaces

Direct numerical simulations (DNS) of flow over triangular and rectangular riblets in a wide range of size and Reynolds number have been carried out. The flow within the grooves is directly resolved by exploiting the immersed-boundary method. It is found that the drag reduction property is primarily associated with the capability of inhibiting vertical velocity fluctuations at the plane of the crests, as in liquid-infused surfaces (LIS) devices. This is mimicked in DNS through artificial suppression of the vertical velocity component, which yields large drag decrease, proportionate to the riblets size. A parametrization of the drag reduction effect in terms of the vertical velocity variance is found to be quite successful in accounting for variation of the controlling parameters. A Moody-like friction diagram is thus introduced which incorporates the effect of slip velocity and a single, geometry-dependent parameter. Reduced drag-reduction efficiency of LIS-like riblets is found as compared to cases with artificially imposed slip velocity. Last, we find that simple wall models of riblets and LIS-like devices are unlikely to provide accurate prediction of the flow phenomenon, and direct resolution of flow within the grooves in necessary.     

Investigation of vortical flow over bluff bodies with base cavities

Based on our previous research about drag reduction in term of the base cavity length using two dimensional simulations, this paper describes a numerical study of a bluff body of which the number of base cavities is successively increased and the cavity geometries are also modified to assume different shapes. Here we attempt to find an effective configuration to reduce the drag by increasing the number of base cavities. The numerical simulations examining varied number of base cavities reveal the presence of different strength of vortices in the wake zone which is the reason why the drag coefficients are distinctly different for different cases. In the case with double and triple rectangular cavities, we use the pressure contours snapshots at successive time instants to describe the wake evolution. We further investigate the effect of variable base cavity shapes for a constant cavity length at an identical time instant. A total of two different geometries of base cavities are discussed here: the rectangular and the sinusoidal cavities with sharp and rounded trailing edges, respectively. The numerical results reveal that the former is an effective drag reduction configuration which can produce a significant base pressure recovery corresponding to the strength of the vortices shown in the pressure contour figures. While the latter shows no obvious reduction in drag coefficient and a similar intensity of vortex in the wake zone compared with the unmodified case. Reductions in drag are observed for all the investigated cavity configurations, and additionally it is found that the magnitude of the reduction bears a direct relationship with the number of the cavities up to a certain minimum value.     

超声速钝体逆向喷流减阻的数值模拟研究

为研究逆向喷流技术对超声速钝体减阻的影响,采用标准k-ε湍流模型,通过求解二维Navier-Stokes方程对超声速球头体逆向冷喷流流场进行了数值模拟,并着重分析了喷口总压、喷口尺寸对流场模态和减阻效果的影响。计算结果显示:随着喷流总压的变化,流场可出现两种流动模态,即长射流穿透模态和短射流穿透模态;喷流能使球头体受到的阻力明显减小;存在最大减阻临界喷流总压值(在所研究参数范围内最大减阻可达51.1%);在其它喷流物理参数不变时,随着喷口尺寸的增大,同一流动模态下的减阻效果下降。本文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

Methods of near-wall pressure-fluctuation measurements in the presence of vibration

Near-wall pressure fluctuations in turbulent flows are of considerable interest in many engineering applications. We shall concentrate on a number of specific questions related to the resolution of components of wall pressure spectra. Our emphasis shall be on outstanding problems of turbulent pressure fluctuations in the presence of vibration. A study on the interaction of a transducer with wall vibration resulting from near-wall turbulent flows has been performed. Three methods are described for the study of spectral components of turbulent surface pressure in conditions of flow-induced vibration: the method of separation of turbulent and vibration signals; the method of a vibration-proof turbulent pressure transducer; and a modified method of vibration suppression. A method of low-frequency acoustic-noise suppression is also suggested.     

流致振动能量收集的钝头体几何设计研究

流致振动蕴含着可观的能量, 通过能量收集技术可将其转化为电能. 为提高低速流场中能量转化效率, 本文实验研究了不同截面下钝头体以及它们的宽厚比(W/T)对流致振动能量收集特性的影响, 并通过计算流体动力学(computational fluid dynamic, CFD)仿真分析了尾流特性. 流致振动能量收集装置由压电悬臂梁和不同截面的钝头体构成. 首先搭建了流致振动能量收集风洞实验平台, 钝头体的截面分别设置为矩形、三角形和D形, 宽厚比分别设定为1, 1.3, 1.8和2.5. 然后利用实验方法分析不同形状钝头体的宽厚比(W/T)对位移响应和电压响应的影响规律. 最后通过计算流体动力学模拟揭示实验结果的内在力学机理. 实验结果表明, 当钝头体截面为矩形时, 增大宽厚比可以显著提高电压输出峰值; 当钝头体为三角形和D形时, 增加宽厚比将使系统呈现“驰振”→“驰振 + 涡激振动”→“涡激振动”响应特性变化趋势, 提高了低风速时的能量收集效果. CFD结果解释了实验现象, 即随着宽厚比增加, 钝头体尾流会产生更加强劲的涡街, 显著提高流致振动能量收集效果. 相关结果可优化流致振动能量收集装置结构, 为提高低速流场的能量收集效果提供理论和实验依据.      

Numerical investigation of fluid flow past circular cylinder with multiple control rods at low Reynolds number

Laminar flow past a circular cylinder with multiple small-diameter control rods is numerically investigated in this study. The effects of rod-to-cylinder spacing ratio, rod and cylinder diameter ratio, cylinder Reynolds number, number of control rods and angle of attack on the hydrodynamics of the main circular cylinder are investigated. Four different flow regimes are identified based on the mechanism of lift and drag reduction. The range of rod-to-cylinder spacing ratio where significant force suppression can be achieved is found to become narrower as the Reynolds number increases in the laminar regime, but is insensitive to the diameter ratio. The numerical results for the case with six identical small control rods at Re=200 show that the lift fluctuation on the main cylinder can be suppressed significantly for a large range of spacing ratio and various diameter ratios, while the drag reduction on the main cylinder is also achieved simultaneously. The six-control-rod arrangement has shown better performance in flow control than the arrangements with less control rods, especially in terms of force reduction at various angles of attack.     

NUMERICAL SIMULATION OF LINEAR AND NONLINEAR DISTURBANCE EVOLUTION IN A BOUNDARY LAYER WITH COMPLIANT WALLS

A computational method capable of simulating the spatial evolution of disturbances in a boundary-layer flow with compliant coatings has been developed. The flow geometry being an unknown of the problem, this difficulty is overcome by use of a mapping, the domain being fixed in the computational coordinates. The model takes into account the nonlinear fluid-structure interaction all over the flow field, as well as nonparallel effects due to the wall displacement and to the boundary-layer growth. First, the numerical solution procedure is tested by focusing on the linear and nonlinear spatial disturbance evolution for a spring-backed elastic plate which is only unstable with respect to Tollmien–Schlichting-type travelling waves. The numerical procedure is then used to study the influence of the initial disturbance amplitude on the disturbance development for a tensioned membrane. Finally, to simulate a true physical experiment, a spring-backed elastic plate of finite length is considered. It is shown that the numerical model is capable of detecting the interaction between Tollmien–Schlichting instabilities and flow-induced surface instabilities at the interface.     

Modification of static-wing tip vortex via a slender half-delta wing

The modification of the tip vortex generated by a rectangular NACA 0012 wing via a tip-mounted slender half-delta wing (HDW) was attempted experimentally at Re=2.81×10⁵. In addition to the increase in lift with increasing HDW deflection, compared to the baseline wing, the roll-up process of the tip vortex was also found to be significantly modified, as a result of the breakdown of the HDW vortex. The addition of the HDW also caused an increased total drag. Fortunately, the lift-induced drag was found to be reduced compared to its baseline counterpart for 0° and 5° HDW deflections. The change in the lift-induced drag also translates into a virtually unchanged profile drag, regardless of HDW deflection. In short, the largest lift-induced drag reduction achieved by the zero-deflection HDW resulted in an improved lift-to-drag ratio, at high angle-of-attack range, compared to the baseline wing.     

Wingtip vortex control via the use of a reverse half-delta wing

The effect of a 65° sweep reverse half-delta wing (RHDW), mounted at the squared tip of a rectangular NACA 0012 wing, on the tip vortex was investigated experimentally at Re?=?2.45?×?10⁵. The RHDW was found to produce a weaker tip vortex with a lower vorticity level and, more importantly, a reduced lift-induced drag compared to the baseline wing. In addition to the lift increment, the RHDW also produced a large separated wake flow and subsequently an increased profile drag. The reduction in lift-induced drag, however, outperformed the increase in profile drag and resulted in a virtually unchanged total drag in comparison with the baseline wing. Physical mechanisms responsible for the RHDW-induced appealing aerodynamics and vortex flow modifications were discussed.     

An experimental investigation of single-phase natural circulation behavior in a rectangular loop with Al2O3 nanofluids

In this paper, the natural circulation behavior in a rectangular loop was investigated experimentally with water and different concentration of Al₂O₃ nanofluids (0.3–2% by wt. and particle size 40–80 nm). It was demonstrated that, not only the flow instabilities are suppressed but also the natural circulation flow rates are enhanced with nanofluids. The enhancement in natural circulation flow rate and suppression of instabilities were found to be dependent on the concentration of nanoparticles in water.     

Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

This paper highlights steady and unsteady measurements and flow control results obtained on an Ahmed model with slant angle of 25° in wind tunnel. On this high-drag configuration characterized by a large separation bubble along with energetic streamwise vortices, time-averaged and time-dependent results without control are first presented. The influence of rear-end periodic forcing on the drag coefficient is then investigated using electrically operated magnetic valves in an open-loop control scheme. Four distinct configurations of flow control have been tested: rectangular pulsed jets aligned with the spanwise direction or in winglets configuration on the roof end and rectangular jets or a large open slot at the top of the rear slant. For each configuration, the influence of the forcing parameters (non-dimensional frequency, injected momentum) on the drag coefficient has been studied, along with their impact on the static pressure on both the rear slant and vertical base of the model. Depending on the type and location of pulsed jets actuation, the maximum drag reduction is obtained for increasing injected momentum or well-defined optimal pulsation frequencies.     

Frequency effects on lift and drag for flow past an oscillating cylinder

A transversely oscillating cylinder in a uniform flow is modeled to investigate frequency effects of flow-induced wake on lift and drag of the cylinder. Specifically, verified unsteady fluid dynamic simulations using an immersed-boundary method in a fixed Cartesian grid predict the flow structure around the cylinder and reveal how the integration of surface pressure and shear distributions provides lift and drag on the oscillating cylinder. In this study, frequency ranges to be considered are both near and away from the natural frequency of wake vortex shedding. Subsequently, the effects of frequency lock-in, superposition and demultiplication on lift and drag are discussed based on the spectral analysis of time histories of lift and drag.     

Study of active flow control for a simplified vehicle model using the PANS method

Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present numerical study deals with active flow control for a quasi-2D simplified vehicle model using a synthetic jet (zero net mass flux jet). Recently developed near-wall Partially-Averaged Navier–Stokes (PANS) method, based on the ζ–f RANS turbulence model, is used. The aim is to validate the performance of this new method for the complex flow control problem. Results are compared with previous studies using LES and experiments, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS method predicts a drag reduction of approximately 15%, which is closer to the experimental data than the previous LES results. The velocity profiles predicted by the PANS method agree well with LES results and experimental data for both natural and controlled cases. The PANS prediction showed that the near-wake region is locked-on due to the synthetic jet, and the shear layer instabilities are thus depressed which resulted in an elongated wake region and reduced drag. It demonstrates that the PANS method is able to predict the flow control problem well and is thus appropriate for flow control studies.     

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.     

Magnetic Field Effects on Axis-Switching and Instabilities in Rectangular Plasma Jets

The effect of an external magnetic field on the evolution of rectangular plasma jets is examined. Specifically investigated is the influence of a primarily axial magnetic field on the uniquely characteristic axis-switching phenomenon of rectangular jets and flow instabilities. The results indicate that the magnetic field decelerates the jet (more rapid spreading), prevents axis-switching and inhibits instabilities. The key physical mechanisms underlying the changes are (1) the ability of the magnetic field to reverse the direction of vorticity and (2) transfer of energy from kinetic to magnetic forms. This study has important implications for magneto-hydro-dynamic flow control and propulsion applications.     

壁面展向周期振动的槽道湍流减阻机理的研究

利用直接数值模拟研究了带有壁面展向周期振动的槽道湍流.壁面在展向的周期运动使湍流受到抑制,并使壁面摩擦阻力减小.通过对雷诺应力输运方程的分析研究了壁面展向周期振动的减阻机理,进一步揭示了压力变形项在湍流抑制中的关键作用.