Aerodynamic jet steering using steady blowing and suction

An aerodynamic jet steering scheme using a combination of blowing and suction near the exit plane of the primary jet is demonstrated. Previous studies involving synthetic jet actuators have shown that jet steering or vectoring is achieved when primary jet fluid is drawn into the suction slot, and that the vectoring force increases with primary jet speed. These studies were limited by the high-actuation frequencies required to maintain vectoring at high primary jet speeds. The present steady technique does not suffer from this limitation, and requires suction and blowing flow rates which are a small fraction of that of the primary jet. This arrangement is studied experimentally and numerically. The results are presented primarily in terms of turning angle. It is found that for sufficient blowing flow rates (similar to the suction flow rate) the resultant turning angle increases linearly with the suction flow rate regardless of Reynolds number (up to 21,000). For insufficient blowing, the jet may be turned in the opposite direction.The authors would like to thank Ari Glezer for the loan of the primary jet facility, and Terry Zollinger for fabricating of the actuator and butterfly valve.     

定常吹气对倾转旋翼机向下载荷的影响

为进一步提高倾转旋翼机悬停状态下的有效载重,开展了定常吹气流动控制对向下载荷的影响研究。首先应用延迟脱体涡模拟(DDES)方法对翼型-90°迎角下非定常大范围分离流动结构进行了数值分析;然后分别开展了前缘吹气、后缘吹气降载措施研究,揭示了吹气降载的机理,并对不同吹气口位置和吹气动量系数的影响进行了定量分析,最后开展了前、后缘同时吹气作用下降载数值模拟研究。计算结果表明:前缘最佳吹气位置在翼型的前缘点,而后缘吹气最佳位置位于襟翼弦长的15%处;前缘吹气的降载效果要优于后缘吹气,而且吹气动量系数对向下载荷的影响较小;相对于初始未施加流动控制构型,阻力系数减小量可达到32.72%。     

Aerodynamic drag reduction by vertical splitter plates

The capacity of vertical splitter plates placed at the front or the rear of a simplified car geometry to reduce drag, with and without skew angle, is investigated for Reynolds numbers between 1.0 × 10⁶ and 1.6 × 10⁶. The geometry used is a simplified geometry to represent estate-type vehicles, for the rear section, and MPV-type vehicle. Drag reductions of nearly 28% were obtained for a zero skew angle with splitter plates placed at the front of models of MPV or utility vehicles. The results demonstrate the advantage of adapting the position and orientation of the splitter plates in the presence of a lateral wind. All these results confirm the advantage of this type of solution, and suggest that this expertise should be used in the automotive field to reduce consumption and improve dynamic stability of road vehicles.     

Aerodynamic drag improvements on a square-back vehicle at yaw using a tapered cavity and asymmetric flaps

Emissions of greenhouse gasses from passenger vehicles is a concern globally. One of the factors that influence the vehicles energy consumption is the aerodynamic drag, continuing to be an active topic of interest. This work investigates the vehicle wake in relation to aerodynamic drag in steady crosswind conditions.The vehicle used is a modified version of the generic Windsor geometry with wheels and a rearward-facing base cavity with nine angled surfaces, or flaps, distributed at the trailing edge of the cavity along the roof and sides. A surrogate model-based optimisation algorithm was used to minimise the drag coefficient by optimising the angle of each flap individually. The experiments were performed in the Loughborough University Large Wind Tunnel. The time-averaged and unsteady results of both the base pressures and tomographic Particle Image Velocimetry indicate that the optimised flap angles improve drag primarily by altering the wake balance. This is achieved by reducing the strength of a large leeward side vortex, reducing the crossflow within the wake.     

Bluff-body drag reduction using a deflector

A passive flow control on a generic car model was experimentally studied. This control consists of a deflector placed on the upper edge of the model rear window. The study was carried out in a wind tunnel at Reynolds numbers based on the model height of 3.1 × 10⁵ and 7.7 × 10⁵. The flow was investigated via standard and stereoscopic particle image velocimetry, Kiel pressure probes and surface flow visualization. The aerodynamic drag was measured using an external balance and calculated using a wake survey method. Drag reductions up to 9% were obtained depending on the deflector angle. The deflector increases the separated region on the rear window. The results show that when this separated region is wide enough, it disrupts the development of the counter-rotating longitudinal vortices appearing on the lateral edges of the rear window. The current study suggests that flow control on such geometries should consider all the flow structures that contribute to the model wake flow.     

Pathline analysis of traveling wavy blowing and suction control in turbulent pipe flow for drag reduction

Skin friction drag is much greater in turbulent flows as compared with that in laminar flows. It is well known that traveling wave control can be used to achieve a large drag reduction. In the present study, a direct numerical simulation of a turbulent pipe flow was performed to clarify the mechanism of the drag reduction caused by the traveling wave control. The flow induced by the control was evaluated using pathline analysis. Near the wall, a “closed flow” was formed, wherein the injected particles return to the wall owing to the suction flow. The random component of Reynolds shear stress was perfectly suppressed in the closed flow, which suggests that there was no turbulence. The controlled flow was categorized into four patterns, and each flow characteristic and drag reduction effect was discussed. When the closing rate is high, the drag decreases, while when the closing rate is low, i.e., when the injected particles are released into the main flow, the turbulence is maintained. If the thickness of the layer suppressing turbulence is insufficient, a significant effect in terms of the drag reduction cannot be expected. The large drag reduction owing to the traveling wave control can be attributed to the elimination of turbulence in the region near the wall.     

Research on optimal design method for drag reduction of vacuum pipeline vehicle body

The vacuum pipeline vehicle system is a novel conceptual system aimed to provide a high-speed public transportation service in the future, featured with a reduced-pressure tube in which pressurised capsules ride on a cushion of air that is driven by a combination of linear induction motors and air compressor. The capsule body has a great influence on the aerodynamic performance of vehicles. This work involves numerical simulations based on finite volume method to study the optimisation method for the parameter design of the vehicle body. The results show that the aerodynamic resistance of the optimised vehicle is further reduced, the drag reduction rate reaches 5.52%, and the speed of the Kantrowitz limit phenomenon is delayed from 760 to 860?km/h. The drag reduction effect is obvious.     

Separation flow control on a generic ground vehicle using steady microjet arrays

A model of a generic vehicle shape, the Ahmed body with a 25° slant, is equipped with an array of blowing steady microjets 6 mm downstream of the separation line between the roof and the slanted rear window. The goal of the present study is to evaluate the effectiveness of this actuation method in reducing the aerodynamic drag, by reducing or suppressing the 3D closed separation bubble located on the slanted surface. The efficiency of this control approach is quantified with the help of aerodynamic load measurements. The changes in the flow field when control is applied are examined using PIV and wall pressure measurements and skin friction visualisations. By activating the steady microjet array, the drag coefficient was reduced by 9–14% and the lift coefficient up to 42%, depending on the Reynolds number. The strong modification of the flow topology under progressive flow control is particularly studied.     

逆向喷流流场模态分析及减阻特性研究

逆向喷流减阻的基本原理是利用逆向高速喷流与飞行器绕流的相互作用，使飞行器周围的流场结构发生变化，致使飞行器的气动特性发生改变，从而改善飞行器的气动性能。利用数值模拟方法对轴对称球头、截锥的逆向喷流流场开展了研究，考虑了高温非平衡化学反应对流场的影响。模拟了球头和截锥在不同总压比时流场不同的模态：长穿透流模态（LPM）和短穿透流模态（SPM），得到了不同模态下钝体表面压力、气动力系数和不同模态之间转换的瞬态效应．简单分析了喷流在减阻方面的应用，给出了几个喷口参数与减阻效率之间的关系，提出了喷流减阻工程应用时应考虑的主要因素。     

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.     

Aerodynamic drag of a pair of bodies in transonic and supersonic flow

The aerodynamic characteristics of models of pairs of bodies on the flow acceleration and deceleration intervals are investigated experimentally at transonic and supersonic flow velocities. The dependence of the drag coefficient of the pair model on the relative drag of the leading body is determined for supersonic velocities.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 152–156, May–June, 1990.     

Active control of the turbulent drag by a localized periodical blowing dissymmetric in time

Experimental results on the reaction of the near wall turbulence and drag to a localized time periodical blowing are reported. The injection velocity is periodical and dissymmetric in time, with a rapid acceleration phase followed by a slow deceleration one. The flow is relaminarized during 70% of the oscillation period mainly during the deceleration phase. The latter maintains stable the vorticity layer induced by the blowing and prevents its roll-up contrarily to a sinusoidal time periodical blowing. Thus, a time mean drag reduction of 50% is obtained in the region recovering 200 wall units downstream of the blowing slot and this is 40% larger than the drag reduction obtained by a steady blowing with the same time mean severity parameter. The dissymmetric blowing annihilates considerably the wall turbulence activity and gives better results in terms of drag reduction compared with steady and sinusoidal blowing.

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Heterogeneous drag reduction in turbulent pipe flows using various injection techniques

The results of an experimental study of the injection of concentrated polymer solutions into the near-wall region of a turbulent pipe flow are reported. The injection experiments described here show drag reduction that was significantly larger than that obtained for homogeneous polymer solutions of the same average concentration. Local drag reduction and friction behavior was obtained by measuring pressure differences over a test section of 13 m in length. Furthermore the flow behaviour of the injected polymer solution was investigated by flow visualization experiments. Velocity profile measurements elucidate in case of near-wall injection that the turbulent structure could be altered in the near-wall and also in the core region of the pipe flow, indicating that the polymer lumps and threads created by the near-wall injection are able to influence a much wider spectrum of turbulent eddies in comparison to centreline injection or, all the more, to homogeneous drag reduction.     

Aerodynamic shape and drag scaling law of a flexible fibre in a flowing medium

The study of a flexible body immersed in a flowing medium is one of the best way to find its aerodynamic shape. This Letter revisited the problem that was first studied by Alben et al. (Nature 420, 479–481, 2002). To determine the aerodynamic shape of the fibre, a simpler approach is proposed. A universal drag scaling law is obtained and the universality of the Alben-Shelley-Zhang scaling law is confirmed by using dimensional analysis. A complete Maple code is provided for finding aerodynamic shape of the fibre in the flowing medium.     

Possibility of drag reduction usingd-type roughness

An experiment was carried out in a low-speed wind tunnel to study the turbulence structure of the boundary layer over a two-dimensional square cavity on a flat plate. The main purpose of this investigation is to examine the way a square cavity modifies the near-wall structure of the turbulent boundary layer leading to a possible drag reduction overd-type roughness. The experimental results on pressure coefficient and friction coefficient indicated a small reduction in total drag in this configuration. This seems to be due to the stable vortex flow observed within the cavity which absorbs and reorganizes the incoming turbulence in the cavity, thereby modifying the near-wall turbulence structure of the boundary layer. The resultant turbulence structure was very similar to that over drag-reducing riblets surface.