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.     

Control of Tollmien�CSchlichting instabilities by finite distributed wall actuation

Tollmien?CSchlichting waves are one of the key mechanisms triggering the laminar-turbulent transition in a flat-plate boundary-layer flow. By damping these waves and thus delaying transition, skin friction drag can be significantly decreased. In this simulation study, a wall segment is actuated according to a control scheme based on a POD-Galerkin model driven extended Kalman filter for state estimation and a model predictive controller to dampen TS waves by negative superposition based on this information. The setup of the simulation is chosen to resemble actuation with a driven compliant wall, such as a membrane actuator. Most importantly, a method is proposed to integrate such a localized wall actuation into a Galerkin model.     

The reduction of skin friction by riblets under the influence of an adverse pressure gradient

In this paper we address the effectiveness of riblets on skin friction reduction under the influence of an adverse pressure gradient. The measurements were taken in a wind tunnel. Skin friction was observed with a drag balance which has a reproducibility of better than 1%. The accuracy of the balance is estimated to be less than 1% for the case of zero-pressure gradient and at most 3% for a pressure gradient. The data on skin friction reduction at zero pressure gradient were consistent with previous results and amount to 5% at dimensionless riblet width of s ⁺ = 13. We find that at all adverse pressure gradients the skin friction reduction by riblets persists. At moderate pressure gradients the reduction increases somewhat to 7%. The velocity profile which is also measured, exhibits the characteristic shape for a boundary layer with an adverse pressure gradient and agrees well with theory. From the velocity profiles measured at two stations we estimated with the help of a momentum balance the skin friction and skin friction reduction. The results differ from the drag-balance data. Due to the poor accuracy of the momentum balance method which we estimate in our case, we conclude that the results obtained with this method are less reliable than those obtained with the drag balance. This throws some doubt on previous results on drag reduction under the influence of a pressure gradient which were based on the momentum balance method.     

Drag reduction by coupled systems: microbubble injection with homogeneous polymer and surfactant solutions

 The influence of homogeneous surfactant and homogeneous polymer solutions on the performance of microbubble skin friction reduction was investigated on an axisymmetric body. Carbon dioxide was injected into water, homogeneous surfactant (Aerosol OT) solutions, and homogeneous dilute polymer (Polyethylene oxide) solutions. Integrated skin friction measurements were obtained at two freestream velocities as a function of gas injection rate and polyethylene-oxide concentration. A moderate (50%) decrease in surface tension had little to no effect on the drag reducing characteristics of microbubble injection. At similar gas injection rates, microbubble injection exhibited more drag reduction in the polymer solutions than obtained with microbubble injection into water. However, the increased drag reduction obtained with polymer additives was no more than a multiplicative factor related to the baseline levels of drag reduction achieved by the individual methods, and suggests the mechanism for microbubble skin friction reduction acts independently of the polymer drag reduction. Received: 17 April 1998 / Accepted: 12 October 1998     

Interpretation of gas-film cooling against aero-thermal heating for high-speed vehicles

The possible application of the film-cooling technique against aero-thermal heating for surfaces of high-speed flying vehicles is discussed. The technique has been widely used in the heat protection of gas turbine blades. It is shown in this paper that, by applying this technique to high-speed flying vehicles, the working principle is fundamentally different. Numerical simulations for two model problems are performed to support the argument. Besides the heat protection, the appreciable drag reduction is found to be another favorable effect. For the second model problem, i.e., the gas cooling for an optical window on a sphere cone, the hydrodynamic instability of the film is studied by the linear stability analysis to observe possible occurrence of laminar-turbulent transition.     

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.     

Optimization of laminar-turbulent transition delay by means of local heating of the surface

The stability and position of laminar-turbulent transition in the boundary layer on a body heated near the leading edge are analyzed. The point of transition is found using the linear theory of the stability of plane-parallel flow and thee ^N -method. It is shown that by heating a tiny area near the leading edge to a temperature exceeding that of the oncoming flow by a factor of two to four, transition may be delayed, even on a thermally insulated surface. For highly radiating surfaces the energy saved by reducing the friction drag may exceed the heating energy by a factor of three. It is shown that by varying the pressure distribution and surface heating it is possible either to increase the airfoil lift for a fixed transition point or delay transition for a fixed lift.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 90–99, July–August, 1995.     

壁面温度在多参数下对平板边界层的影响研究

为了得到壁面温度在不同来流速度、不同湍流强度条件下对边界层转捩与减阻的影响规律,本文采用Transitionk-kl-ω模型对低来流速度下无压力梯度的光滑平板进行了数值模拟。结果表明,随着来流速度的升高,壁温升高所起到的减阻效果更好,即高来流速度对壁面温度更为敏感。当来流处于中高湍流强度下时,壁温升高能起到推迟转捩的作用,且随着湍流强度的升高,转捩推迟的效果越好,但减阻效果正好相反;当来流处于低湍流强度下时,壁温升高会使得转捩提前发生。壁温升高抑制了边界层内流体的脉动程度,使得层流的稳态不易被破坏,流动更加稳定;同时,壁温升高使得边界层内流体的速度梯度减小,从而降低了壁面摩擦系数,故壁温升高能起到推迟边界层转捩与减阻的作用。     

Effect of a dielectric barrier discharge on laminar-turbulent transition on a flat plate in a disturbed external flow

Numerical simulation is applied to study the distinctive features of the dielectric barrier discharge (DBD) effect on laminar-turbulent transition on a flat plate in the presence of disturbances in the external flow. The density distributions of the electric force and the discharge power acting on the gas are assumed to be uniform within the given discharge volume. To model the external disturbances the value of the turbulent viscosity in the differential model used is assumed to be nonzero at the boundary layer edge.     

Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow

Experimental results from a study of surface roughness effects on polymer drag reduction in a zero-pressure gradient flat-plate turbulent boundary layer are presented. Both slot-injected polymer and homogeneous polymer ocean cases were considered over a range of flow conditions and surface roughness. Balance measurements of skin friction drag reduction are presented. Drag reductions over 60% were measured for both the injected and homogeneous polymer cases even with fully rough surfaces. As the roughness increased, higher polymer concentration was required to achieve a given level of drag reduction for the homogeneous case. With polymer injection, increasing surface roughness caused the drag reduction to decrease to low levels more quickly when the polymer expenditure was decreased or the freestream velocity was increased. However, the percent drag reductions on the rough surfaces with polymer injection were often substantially larger than on the smooth surface. Remarkably, in some cases, the skin friction drag force on a rough surface with polymer injection was less than the drag force observed on a smooth surface at comparable conditions. An erratum to this article can be found at     

On the friction drag reduction mechanism of streamwise wall fluctuations

Understanding how to decrease the friction drag exerted by a fluid on a solid surface is becoming increasingly important to address key societal challenges, such as decreasing the carbon footprint of transport. Well-established techniques are not yet available for friction drag reduction. Direct numerical simulation results obtained by Józsa et al. (2019) previously indicated that a passive compliant wall can decrease friction drag by sustaining the drag reduction mechanism of an active control strategy. The proposed compliant wall is driven by wall shear stress fluctuations and responds with streamwise wall velocity fluctuations. The present study aims to clarify the underlying physical mechanism enabling the drag reduction of these active and passive control techniques. Analysis of turbulence statistics and flow fields reveals that both compliant wall and active control amplify streamwise velocity streaks in the viscous sublayer. By doing so, these control methods counteract dominant spanwise vorticity fluctuations in the near-wall region. The lowered vorticity fluctuations lead to an overall weakening of vortical structures which then mitigates momentum transfer and results in lower friction drag. These results might underpin the further development and practical implementation of these control strategies.     

Effect of heated underwater boundary layer of a prolate spheroid on viscous drag

 A prolate spheroid submerged in water can be heated for decreasing the viscous drag because of a decrease in viscosity with increasing temperature. The heated boundary layer experiment, based on this principle, was carried out in a circulating water channel of NCKU, and the viscous drag of a spheroid, with a five-to-one ratio of length to mid-diameter, was also measured by means of wake surveys. The difference of total-head between wake and undisturbed region was measured by two total-head tubes, and the water speed in the wake area was measured by an electromagnetic flowmeter. The results indicate that the viscous drag of the model decreases with surface heating, and the decrease in viscous drag of the model is 13% when the surface temperature is 17 °C above the ambient water temperature. The velocity gradient in the wake region and the total-heating differential readings between undisturbed and wake region also decrease with surface heating due to the delayed laminar-turbulent transition in the boundary layer. The values of the coefficient of viscous drag obtained without surface overheating are found to be in agreement, for the range of Froude numbers investigated, with the results obtained from previous experiments. Received: 18 June 1996/Accepted: 29 April 1997     

柔管自激振动特性与减阻效果的实验研究

为弄清柔管自激振动的湍流减阻效果的初步机理,在通过实验确认柔管确有湍流减阻效果的基础上,采用双重管结构和激光测位仪,对柔管的自激振动特性及湍流减阻效果及其两者的关系进行了实验研究．结果表明:管的壁厚越小,管外壁的自激振动的脉动位移有效值越大,湍流减阻效果越好;管壁外为压力平衡空气且雷诺数约为17500时,壁厚为2mm,3mm及4mm柔管的自激振动减阻率依次约为12％,10％,9％．这将为开发有效的管道流体输送技术提供参考．     

Numerical modeling of the laminar-turbulent transition control using a dielectric barrier discharge

The control of laminar-turbulent transition driven by Tollmien–Schlichting waves is studied. The control is realized by means of accelerating the boundary layer flow using a dielectric barrier discharge. As distinct from the previous studies based on the solution of the boundary layer equations, the discharge effect on the main flow and unstable disturbances are described by the Navier–Stokes equations.     

Analysis of polymer drag reduction mechanisms from energy budgets

The transfer of energy in drag reducing viscoelastic flows is analyzed through a sequence of energetic budgets that include the mean and turbulent kinetic energy, and the mean polymeric energy and mean elastic potential energy. Within the context of single-point statistics, this provides a complete picture of the energy exchange between the mean, turbulent and polymeric fields. The analysis utilizes direct simulation data of a fully developed channel flow at a moderately high friction Reynolds number of 1000 and at medium (30%) and high (58%) drag reduction levels using a FENE-P polymeric model.Results show that the primary effect of the interaction between the turbulent and polymeric fields is to transfer energy from the turbulence to the polymer, and that the magnitude of this transfer does not change between the low and high drag reduction flows. This one-way transfer, with an amplitude independent of the drag reduction regime, comes in contradiction with the purely elastic coupling which is implicit within the elastic theory of the polymer drag reduction phenomenon by Tabor and De Gennes (Europhys. Lett. 2, pp. 519–522, 1986).     

Drag reduction in the turbulent pipe flow of polymers

The paper concerns an experimental study of the fully developed turbulent pipe flow of several different aqueous polymer solutions: 0.25%, 0.3% and 0.4% carboxymethylcellulose (CMC), 0.2% xanthan gum (XG), a 0.09%/0.09% CMC/XG blend, 0.125% and 0.2% polyacrylamide (PAA). The flow data include friction factor vs. Reynolds number, mean velocity and near-wall shear rate distributions, and axial velocity fluctuation intensity u′ at a fixed radial location as a laminar/turbulent transition indicator. For each fluid we also include measurements of shear viscosity, first normal-stress difference and extensional viscosity. At high shear rates we find that the degree of viscoelasticity increases with concentration (0.3% CMC is an exception) for a given polymer, and in the sequence XG, CMC/XG, CMC, PAA, whilst at low shear rates the ranking changes to CMC, CMC/XG, XG, PAA. The extensional viscosity ranking is XG/CMC, XG, CMC, PAA at high strain rates and the same as that for the viscoelasticity at low shear rates. We find that the observed drag-reduction behaviour is consistent for most part with the viscoelastic and extensional-viscosity behaviour at the low shear and strain rates typical of those occurring in the outer zone of the buffer region.Although laminar/turbulent transition is practically indiscernible from the friction factor vs. Reynolds number plots, particularly for PAA and XG, the u′ level provides a very clear indicator and it is found that the transition delay follows much the same trend with elasticity/extensional viscosity as the drag reduction.     

二维平板湍流边界层减阻研究

利用一套以位移传感器为主的弹性平衡装置,测量几种不同表面形状设计的平板在充分发展的二维湍流边界层中的减阻效果,并对其减阻机理进行研究。结果表明,大涡破碎器 LEBU(Large Eddy Break-up)和其它减阻装置的形状和布置对表面摩擦阻力有较大影响,在有些设计状态下的平板得到了净减阻。     

Drag reduction of turbulent channel flows over an anisotropic porous wall with reduced spanwise permeability

The direct numerical simulation(DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.     

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.     

Amplitude method of prediction of laminar-turbulent transition on a swept-wing

The amplitudemethod of prediction of laminar-turbulent transition on a swept-wing initiated by the simultaneous action of free-stream turbulence and surface roughness is developed. Generation of growing intrinsic perturbations in the boundary layer is described by the mechanism of distributed generation, i.e., an external perturbation generates an instability mode with the same wavelength and frequency. Generation occurs in a small neighborhood of the neutral point at which the parameters of an external perturbation and the instability modes coincide. The development of proper perturbations in the boundary layer is described by the nonlinear method of parabolized stability equations (PSE). The criterion of laminar-turbulent transition is the combined amplitude. i.e., transition begins at a point at which the sum of amplitudes of steady and traveling modes reaches a critical value.