Plasma Streamwise Vortex Generators for Flow Separation Control on Trucks

An experimental study of the effect of Dielectric Barrier Discharge plasma actuators on the flow separation on the A-pillar of a modern truck under cross-wind conditions has been carried out. The experiments were done in a wind tunnel with a 1:6 scale model of a tractor-trailer combination. The actuators were used as vortex generators positioned on the A-pillar on the leeward side of the tractor and the drag force was measured with a wind-tunnel balance. The results show that the effect at the largest yaw angle (9 degrees) can give a drag reduction of about 20% and that it results in a net power reduction. At lower yaw angles the reduction was smaller. The present results were obtained at a lower Reynolds number and a lower speed than for real driving conditions so it is still not yet confirmed if a similar positive result can be obtained in full scale.     

Shock-Induced Borehole Waves and Fracture Effects

We perform wave experiments using a vertical shock tube setup. Shock waves are generated by the rupture of a thin membrane. In the test section the incident pressure waves generate borehole-guided waves along water-saturated samples. The tube is equipped with side wall gages and a mobile pressure probe, so that the attenuation and reflection of the wave can be measured. The computation for a single horizontal fracture intersecting a vertical borehole gives a quantitative prediction of reflection and transmission of borehole-guided waves. Three different fracture apertures are used for the calculation. Fracture aperture significantly affects both reflection and transmission coefficients. Large fractures increase reflectivity and decrease transmissivity. In the experiment, we found that both pressures above and below the fracture are influenced by the fracture aperture indeed, thus indicating the potential for fracture detection by borehole waves.     

Shock-Induced Borehole Waves and Fracture Effects

We perform wave experiments using a vertical shock tube setup. Shock waves are generated by the rupture of a thin membrane. In the test section, the incident pressure waves generate borehole-guided waves along water-saturated samples. The tube is equipped with side wall gages and a mobile pressure probe, so that the attenuation and reflection of the wave can be measured. The computation for a single horizontal fracture intersecting a vertical borehole gives a quantitative prediction of reflection and transmission of borehole-guided waves. Three different fracture apertures are used for the calculation. Fracture aperture significantly affects both reflection and transmission coefficients. Large fractures increase reflectivity and decrease transmissivity. In the experiment, we found that both pressures above and below the fracture are influenced by the fracture aperture indeed, thus indicating the potential for fracture detection by borehole waves.     

Experimental Investigation of Three-Dimensional Vortex Structures Downstream of Vortex Generators Over a Backward-Facing Step

An experimental investigation of vortex generators has been carried out in turbulent backward-facing step (BFS) flow. The Reynolds number, based on a freestream velocity U₀ = 10 m/s and a step height h = 30 mm, was Re_h = 2.0 × 10⁴. Low-profile wedge-type vortex generators (VGs) were implemented on the horizontal surface upstream of the step. High-resolution planar particle image velocimetry (2D-2C PIV) was used to measure the separated shear layer, recirculation region and reattachment area downstream of the BFS in a single field of view. Besides, time-resolved tomographic particle image velocimetry (TR-Tomo-PIV) was also employed to measure the flow flied of the turbulent shear layer downstream of the BFS within a three-dimensional volume of 50 × 50 × 10 mm³ at a sampling frequency of 1 kHz. The flow control result shows that time-averaged reattachment length downstream of the BFS is reduced by 29.1 % due to the application of the VGs. Meanwhile, the Reynolds shear stress downstream of the VGs is considerably increased. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) have been applied to the 3D velocity vector fields to analyze the complex vortex structures in the spatial and temporal approaches, respectively. A coherent bandwidth of Strouhal number 0.3 < St_h < 0.6 is found in the VG-induced vortices, and moreover, Λ-shaped three-dimensional vortex structures at St_h = 0.37 are revealed in the energy and dynamic approaches complementarily.     

Effect of Vortex Generators on the Aerodynamic Performance of High-Speed Trains

Flow, Turbulence and Combustion - A vortex generator (VG) can effectively prevent the airflow from separating prematurely and transfer the energy to the low-energy airflow at the bottom of the...     

纵向涡强化换热的实验研究

在流道内安装三角翼对涡发生器可以产生纵向涡旋,本文研究了Re＝800－8000范围内,以空气作为介质,涡发生器对加热片的强化换热效果,并通过红外热像仪测量加热片面的温度场,结果表明：与光滑加热片换热面比较,加装三角翼对涡发生器后,强化换热效果明显。本文条件下,在雷诺数为6000时,加装涡发生器整体强化换热效果达到50．8％。     

The Flow Structure Produced by Pulsed-jet Vortex Generators in a Turbulent Boundary Layer in an Adverse Pressure Gradient

The effect of pulsed jet vortex generators on the structure of an adverse pressure gradient turbulent boundary layer flow was investigated. Two geometrically optimised vortex generator configurations were used, co-rotating and counter-rotating. The duty cycle and pulse frequency were both varied and measurements of the skin friction (using hot films) and flow structure (using stereo PIV) were performed downstream of the actuators. The augmentation of the mean wall shear stress was found to be dependent on the net mass flow injected by the actuators. A quasi steady flow structure was found to develop far downstream of the injection location for the highest pulse frequency tested. The actuator near field flow structure was observed to respond very quickly to variations in the jet exit velocity.     

冲击破碎高能推进剂的燃烧异常

为研究高能推进剂的冲击损伤状态对其使用安全性的影响,以爆轰波为加载手段,对NEPE推进剂试样进行冲击损伤。经扫描电镜观察和密度测量,冲击试样的基体材料和固体颗粒破坏严重,密度降低。热分析实验表明,损伤有利于试样中硝酸酯热分解的进行。密闭爆发器实验显示,损伤使燃烧速度加快。     

动失速型非定常分离涡结构的控制

本文对动失速型非定常分离涡结构的控制方法，在低速风洞中应用相平均测压技术进行了实验研究。在二元平板模型中部安装一作俯仰振荡的扰流板产生动失速型分离涡，在其上游安装另一用作控制的小扰流板。实验结果表明，应用前置的振荡小扰流板可影响并改变动失速分离涡的强度和对流特性。在最有利的控制相位下，涡吸力峰可降低４８％，涡对流时间可以推迟０．１９周期。对于间歇式振荡扰流板，采用相位提前控制方式比相位滞后控制方式更有效。     

On the Control of the Fastest Growing Görtler Vortex

The response of the fastest growing linear G?rtler vortex to wall suction or blowing is investigated. The fastest growing mode occurs in flows of high local curvature and we show how its growth may be inhibited using very small amounts of suction or blowing. We show how the suction or blowing should be distributed in order to minimize the growth of a disturbance over a specified interval. Received 3 July 1996 and accepted 10 November 1998     

微型射流涡流器对湍流边界层控制的数值研究

运用数值方法,模拟出展向分布的同向倾斜微型射流列与平板湍流边界层相互作用形成流向涡列的流场结构,验证了利用其来对湍流边界层进行控制的可能性.随射流间距减小,流向涡列控制作用流向渗透能力增强,但作用区域减小;随射流速度提高,流向涡列控制作用增强,但过大的射流速度反而会导致流向涡列在局部区域内控制作用的下降;随射流俯仰角减小、倾斜角增大,流向涡列初始控制作用增强,但过小的俯仰角、过大的倾斜角会导致流向涡列流向控制区域明显缩小.要保证流向涡列具有较强的湍流边界层控制作用,必须通过合理配置射流列各主要参数,在保证各流向涡具有一定强度的同时,还要确保各流向涡在形成时部分嵌入边界层内部.     

Optimal Control of a Vortex Trapped by an Airfoil with a Cavity

An airfoil with a cavity traps a vortex; the lift increases but the vortex shows great receptivity to upstream disturbances. A simple potential flow model confirms that the vortex stability basin is of a reduced extent. In this paper we present a control technique stabilizing the vortex position based on a potential flow model. The actuators are sources/sinks at the wall and the suction/blowing law is obtained by the adjoint optimization method. This revised version was published online in July 2006 with corrections to the Cover Date.     

多障碍物通道中激波诱导气相爆轰的数值研究

应用数值模拟方法,研究了直通道中激波经过多块矩形障碍物时诱导 H2/O2混合气体起爆的物理机制. 研究表明：在前导激波强度不足以诱导波后气 体直接起爆的条件下,经过激波压缩的可燃气体也可能在远离激波的障碍物之间的凹槽部位 起爆;障碍物表面产生的压缩波、膨胀波和气流滑移面对可燃气体的起爆、爆轰波的形成和 传播过程有重要的影响;添加不同稀释比的氮气可以影响爆轰波后流场的温度分布;增加障 碍物的间距可以改变可燃气起爆位置.     

Large-Eddy and Direct Numerical Simulations of the Bachalo-Johnson Flow with Shock-Induced Separation

The Bachalo-Johnson experiment on an axisymmetric bump has been a primary validation case for turbulence models in shock-boundary-layer interactions since the 1980’s. In the present work, Wall-Modelled Large-Eddy Simulations (WMLES) of this flow were conducted using Improved Delayed Detached-Eddy Simulation (IDDES) as the sub-grid-scale (SGS) and wall model, with a synthetic turbulence generator, expecting close enough agreement with experiment. However, the WMLES results are disappointing, even in terms of the shock position, even though the results from two grids with 4.7 × 10⁸ and 1.6 × 10⁹ cells respectively agree well with each other. This failure of grid refinement to warn of an inaccurate simulation is of great concern, and the reasons for it are explored. We then conducted a Direct Numerical Simulation (DNS) embedded in the LES over a reduced domain, with 8 × 10⁹ grid cells. The DNS has a far more accurate shock position and overall pressure distribution. The skin friction in the favourable pressure gradient is also much higher than in the LES; thus, wide differences appear upstream of the shock wave, most probably caused by the rapid acceleration which leads to atypical shear-stress profiles. Other SGS models were tried, and performed worse than IDDES. The DNS essentially fulfils the initial expectations although in a reduced domain and provides data for turbulence-modelling studies, for instance by extracting an effective eddy viscosity from it. The most noticeable remaining disagreement with experiment is over the Reynolds shear stress.     

Steady Vortex Ring with Isochronous Flow in the Vortex Core

Steady-state solutions to the problem of a thin vortex ring in an inviscid incompressible fluid in infinite space are investigated. The Fraenkel procedure is used to construct the steady-state solutions. In this procedure a given vorticity distribution in plane flow with circular streamlines is transformed into a steady vortex ring using an expansion in the ring thinness parameter. For example, a two-dimensional vortex of constant vorticity is transformed into a steady vortex ring with the uniform distribution in which the absolute value of vorticity is proportional to the distance from the axis of symmetry. The principal aim of our study is to construct the algorithm of finding the flow for an isochronous vortex ring in which the periods of revolution are the same for all the liquid particles in the vortex core. The problem is that the two-dimensional distribution which goes over in the isochronous ring in accordance with the Fraenkel procedure is unknown in advance. In particular, the ring with the uniform distribution is not isochronous despite the isochronism of the initial two-dimensional flow. In this connection the Fraenkel procedure is significantly modified so that the initial two-dimensional vorticity distribution is determined in each of the steps of the iteration procedure. The solution for the vortex ring with the uniform distribution obtained in the present study is significantly used to construct the isochronous solution. The necessary corrections to the former solution are calculated in each step. Obtaining of the isochronous flow is the key step for the investigation of stability of three-dimensional oscillations of the vortex ring since the oscillation spectrum of this flow is discrete.     

Passive Control of the Vortex Wake Past a Flat Plate at Incidence

A passive control, based on wall suction acting at the leading edge, is proposed to stabilize the vortex shedding from a flat plate at incidence. The correct suction amount is determined by a potential flow model where the large-scale vortical structures formed near the plate edges are represented by point vortices of variable intensity, and the wall suction by an adequately placed sink. We concentrate on the case of a plate that is broadside to the flow and show that the stabilization of the vortex wake can be obtained by simple passive backside suction. In such a case geometric shaping and passive suction have similar effects on the vortex Hamiltonian. The model predictions compare well with the results obtained by blob-vortex simulations, thus confirming the stabilization of the unsteady wake past the plate. Received 5 April 2002 and accepted 6 August 2002 Published online 3 December 2002 Communicated by M.Y. Hussaini     

涡旋波流场的涡量测试与计算及特征参数的影响

从实验和模拟计算两个方面,对涡旋波流场的速度矢量图和涡量进行了定量测试和综合分析,并对涡旋波流动与其特征影响参数之间的关系进行了研究。结果表明:涡旋波流动只在二维层流状态下产生,涡旋波形成后,Re数对旋涡尺寸的影响较小,涡量随Re数的增大而增大,但涡量不随时间的增加而单调增加。随着St数的减小,旋涡尺寸明显增大,涡量却随之减小。本文也对不同槽道内的涡旋波流场进行了数值模拟计算,进而确定了涡旋波流场的形成条件。     

Vortex structures in geophysical convection

An intriguing variety of vortex structures arise during buoyant convection, especially in the presence of background stratification and rotation. These vortices play an important role in environmental fluid motions, bearing upon small-scale turbulence to planetary-scale circulation. A brief review of vortex motions associated with buoyant convection is presented in this paper, emphasizing the sources of vorticity, evolution of vortex structures and their role in oceanic and atmospheric dynamics. The genesis of a variety of vortices, for example, mushroom vortices, geostrophic and ageostrophic vortices, dipolar structures and hetons in buoyant convection flows is described, and parameterizations to represent their properties are discussed. New laboratory and numerical simulation results on vortex-related phenomena in stratified and rotating fluids and their implications in geophysical convective flows are also presented.     

Reynolds Number Effects on the Coherent Dynamics of the Turbulent Horseshoe Vortex System

The adverse pressure gradient induced by a surface-mounted obstacle in a turbulent boundary layer causes the approaching flow to separate and form a dynamically rich horseshoe vortex system (HSV) in the junction of the obstacle with the wall. The Reynolds number of the flow (Re) is one of the important parameters that control the rich coherent dynamics of the vortex, which are known to give rise to low-frequency, bimodal fluctuations of the velocity field (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). We carry out detached eddy simulations (DES) of the flow past a circular cylinder mounted on a rectangular channel for Re = 2.0 × 10⁴ and 3.9 × 10⁴ (Dargahi, Exp Fluids 8:1–12, 1989) in order to systematically investigate the effect of the Reynolds number on the HSV dynamics. The computed results are compared with each other and with previous experimental and computational results for a related junction flow at a much higher Reynolds number (Re = 1.15 × 10⁵) (Devenport and Simpson, J Fluid Mech 210:23–55, 1990; Paik et al., Phys Fluids 19:045107, 2007). The computed results reveal significant variations with Re in terms of the mean-flow quantities, turbulence statistics, and the coherent dynamics of the turbulent HSV. For Re = 2.0 × 10⁴ the HSV system consists of a large number of necklace-type vortices that are shed periodically at higher frequencies than those observed in the Re = 3.9 × 10⁴ case. For this latter case the number of large-scale vortical structures that comprise the instantaneous HSV system is reduced significantly and the flow dynamics becomes quasi-periodic. For both cases, we show that the instantaneous flowfields are dominated by eruptions of wall-generated vorticity associated with the growth of hairpin vortices that wrap around and disorganize the primary HSV system. The intensity and frequency of these eruptions, however, appears to diminish rapidly with decreasing Re. In the high Re case the HSV system consists of a single, highly energetic, large-scale necklace vortex that is aperiodically disorganized by the growth of the hairpin mode. Regardless of the Re, we find pockets in the junction region within which the histograms of velocity fluctuations are bimodal as has also been observed in several previous experimental studies.     

Vortex structures in shear flows

Large-scale vortex structures in shear flows are investigated. An effective method of describing these objects, which makes it possible to go beyond the framework of weak nonlinearity, is proposed. This is especially important in investigating spatially isolated structures. Evolution-type equations describing the shape of the vortex structures are obtained and their steady-state solutions are examined. A detailed classification of the structures in two-dimensional and cylindrical channels is given. Attention is drawn to the qualitative similarity of some of these structures to the well-known structures in real turbulent flows (wall eddies, turbulent slugs). It is established that isolated vortex structures in a pipe whose radius is fairly large as compared with their transverse dimension have kinematic characteristics similar to those of Hill vortices. The prospects of the method are discussed.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 65–75, March–April, 1989.The author wishes to thank G. S. Golitsyn for his interest and G. I. Barenblatt and V. M. Gryanik for discussing the results and offering useful advice.