Riblets Surface减阻

总结riblets减阻研究。结合理论分析与技术验证两方面,探讨了riblets减阻技术应用于流体机械减阻的可行性,提出了流体机械riblets减阻研究建议。

具有Riblets边界面的粘弹性流体速度分布的实验研究

利用激光测速手段研究了明槽中Riblets边界上粘弹性流体流动的速度分布。实验证实 ,粘弹性流动在具有三角形的Riblets底边界时 ,其粘性底层与紊流核心区内的时均速度分布与窦国仁[1 ] 提出的粘弹性流体速度分布的理论公式很接近。在过渡区 ,时均速度的理论值与实测值存在着一定程度的差别 ,这反映了Riblets的影响。根据试验值 ,文中用试错法对窦氏公式予以修正 ,用其表述在具有Riblets底边界时的粘弹性流动的时均速度分布。对于脉动速度 ,实验表明其最大强度出现在过渡区 ,且该区域较之具有光滑壁面的清水流动的范围为宽。实验还显示 ,Riblets底边界存在的情况下 ,脉动速度与窦氏理论值是很接近的。

Characteristics and mechanism investigation on drag reduction of oblique riblets

This work primarily focuses on the drag reduction characteristics and mechanism investigation of oblique riblets. First, a calculation model of the oblique riblets surface is established, and Reynolds stress model(RSM) turbulence model is used for numerical simulation of the oblique riblets flow field. Subsequently, the influence of inclination angle between flow direction and arrangement direction of riblets on friction resistance and drag reduction rate is further analyzed. Through the investigation of the distribution of shear stress, pressure stress and velocity in oblique riblets boundary layer, the oblique riblets drag reduction mechanism is finally revealed. Results show that, with increase of velocity and inclination angle, the pressure resistance increases obviously, along with the decreasing of the viscous resistance distinctly. The maximum drag reduction rate of the oblique riblets is 7.33%. Moreover, when the inclination angle increases, the wall shear stress reduces on oblique riblets surface; while differential pressure increases at both sides of oblique riblets tips. In addition, when inclination angle is small, the secondary vortex at oblique riblets tips will disappear soon. New vortices will be formed inside the oblique riblets and cause the decrease of viscosity resistance. Thus, oblique riblets show a better drag reduction effect and have an effective control on boundary layer.

Coherent structures over riblets in turbulent boundary layer studied by combining time-resolved particle image velocimetry(TRPIV),proper orthogonal decomposition(POD),and finite-time Lyapunov exponent(FTLE)

Time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures（CSs） over a smooth surface and riblets, the proper orthogonal decomposition（POD） and finite-time Lyapunov exponent（FTLE） are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.

EFFECT OF THE RIBLETS SURFACE ON THE BOUNDARY LAYER DEVELOPMENT

The effect of riblets surface on flat plate boundary layer development hasbeen measured with laser Doppler velocimeter (LDV). In the present experiment, although the development of boundary layer can be divided into three regions, the laminarboundary layer region is increased, and the transition occurs at a larger Reynolds number.The Reynolds number of transition to turbulence Rextr is 4. 3 tinies as great as that in thesmooth flat plate case. In the turbulent boundary layer region, greater value of integralconstant C is obtained in log-law for velocity distribution. This indicates that the ribletssurface has obviously drag reduction characteristics. The maximum value of turbulence intensity and its position are less than the smooth surface results in the turbulent boundarylayer region.

Tomographic PIV investigation on coherent vortex structures over shark-skin-inspired drag-reducing riblets

Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer（TBL） is investigated.This is done by means of tomographic particle image velocimetry（TPIV） measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry（TRPIV） system initially,and then the TPIV measurements are performed.Two-dimensional（2D） experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s＋of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection（Q2） and sweep（Q4） events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events（Q2 and Q4）,thereby reducing the skin friction drag.

Coherent Spanwise Structures in Turbulent Boundary Layer over Drag-Reducing Riblets

The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peakto-peak spacing being approximately s?≈21)and 0.19 m/s( s?≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified KelvinHelmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been discussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y?≈20 in both cases of s?≈21 and s?≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.

On Aircraft Lift and Drag Reduction Using V Shaped Riblets

Reducing drag during take-off and nominal(cruise)conditions is a problem of fundamental importance in aeronautical engineering.Existing studies have demonstrated that v-shaped symmetrical riblets can effectively be used for turbulence control,with those with dimensionless depth h+=15 and dimensionless width s+=15 having the best drag reduction effect.In the present study,experimental tests have been conducted considering two models of the same size,one with smooth surface,the other with v-shaped riblets of the h+=15 and s+=15 type.The results show that for an angle of attack in the 8°~20°range(take-off stage),the maximum lift coefficient can be increased by 22%.For angle of attack between 8°and 14°,a drag reduction effect can be produced using riblets,which increases with the Reynolds number,leading to a decrease in the drag coefficient maximum of 36%.Flow visualization experiments have been carried out by means of Laser Induced fluorescence.

Flow-Induced Stream-Wise Vibration of Circular Cylinders

Results from a series of studies on the stream-wise vibration of a circular cylinder verifying Japan Society of Mechanical Engineers Standard S012-1998, Guideline for Evaluation of Flow-induced Vibration of a Cylindrical Structure in a Pipe, are summarized and discussed in this paper. Experiments were carried out in a water tunnel and in a wind tunnel using a two-dimensional cylinder model elastically supported at both ends of the cylinder and a cantilevered cylinder model with a finite span length that was elastically supported at one end. These cylinder models were allowed to vibrate with one degree of freedom in the stream-wise direction. In addition, we adopted a cantilevered cylinder model that vibrated with two degrees of freedom in both the stream-wise and cross-flow directions under the same vibration conditions as an actual thermocouple well. The value of the Scruton number (structural damping parameter) was changed over a wide range, so as to evaluate the value of the critical Scruton number that suppressed vibration of the cylinder. For the two-dimensional cylinder, two different types of stream-wise excitations appeared in the reduced velocity range of approximately half of the resonance-reduced velocity. For the stream-wise vibration in the first excitation region, due to a symmetric vortex flow, the response amplitudes were sensitive to the Scruton number, while the shedding frequency of alternating vortex flow was locked-in to half of the Strouhal number of vibrating frequency of a cylinder in the second excitation region. In addition, the effects of the aspect ratio of a cantilevered cylinder on the flow-induced vibration characteristics were clarified and compared with the results of a two-dimensional cylinder. When a cantilevered circular cylinder with a finite length vibrates with one degree of freedom in the stream-wise di-rection, it is found that acylinder with a small aspect ratio has a single excitation region, whereas a cylinder with a large aspect ratio has two excitation regions. Furthermore, the vibration mechanism of a symmetric vortex flow was investigated by installing a splitter plate in the wake to prevent shedding of alternating vortices. The vibration amplitude of acylinder with a splitter plate increased surprisingly more than the amplitude of a cylinder without a splitter plate. For a cantilevered cylinder vibrating with two degrees of freedom, the Lissajous figure of vibration of the first excitation region shows the trajectories of elongated elliptical shapes, and in the second excitation region, the Lissajous trajectories draw a figure “8”. The results and information from these experimental studies proved that Standard S012-1998 provides sufficient design methods for suppressing hazardous vibrations of cylinders in liquid flows.

Evaluation of Turbulence Models for the Design of Continuously Adapted Riblets

The extensive investigations during the last decades have verified the potential of ideal shaped riblet geometries for viscous drag reduction in the turbulent boundary layer.Basic requirement is an adaption of the riblet dimensions in terms of the local flow conditions.However,due to the complex manufacturing process of micro-scale surface structures most experimental measurements were conducted using riblets with constant dimensions.Therefore,the drag reducing potential of riblets could not be exploited.Thanks to the rapid development in micro structuring technologies it is now possible to manufacture continuously adapted riblets in almost industrial processing scales.To determine an appropriated area for the riblet application and continuously adapted riblet dimensions for the National Renewable Energy Laboratory(NREL)airfoil S809 information on the location of the laminar-turbulent transition and the wall shear stress distribution are required.For this purpose,numerical simulations were conducted using the transitional kkL-ωandγ-ReΘturbulence models as well as the fully turbulent SST implementation.Both transitional models show a sufficient precise prediction of the transition onset location in comparison with experimental data.Depending on the applied turbulence model computed riblet dimensions exhibit a certain deviation whereby a significant effect on the drag reduction can be expected.

基于涡识别技术的微结构壁面湍流减阻机理研究

为探究微结构壁面湍流减阻机理,采用粒子图像测速技术(PIV)观测微结构壁面的水流特性,分别对超疏水壁面和微沟槽壁面的流速发展、雷诺切应力变化及涡的分布规律进行分析。引入流速的概念,对Omega(Ω)涡识别方法进行改进,建立新的Ω_(M)涡识别方法,并采用该方法对近壁面涡结构进行识别,提出液流涡密度定义,定量分析近壁处不同强度涡的分布,为研究微结构壁面减阻机理奠定基础。研究结果表明:在相同雷诺数下,边界层内流速由大到小分别为超疏水壁面、微沟槽壁面和光滑壁面,两种微结构壁面对数律层流速分布与光滑壁面相比明显外移,说明微结构壁面具有减阻效果;雷诺切应力随法向无量纲距离y+的增大而先增大后减小,其中,超疏水壁面雷诺切应力的最大值与其他壁面相比最小;采用Ω_(M)方法对近壁处涡进行识别,发现近壁0.1倍水深内,两种微结构壁面上水流中涡结构以弱涡为主,在0.2倍水深内开始出现强涡;与光滑壁面相比,微结构壁面近壁处水流弱涡的涡密度增加,强涡的涡密度减小,说明微结构壁面抑制了水流中强涡的发展;在3种壁面中,微沟槽壁面的减阻率大于7.60%;超疏水壁面的弱涡涡密度大,强涡涡密度小,减阻效果最好,减阻率大于9.48%。微结构壁面可有效降低水流输运能耗、提高其输运效率。

有序化砂轮磨削筋条表面中力对表面的效应

为了提高筋条减阻表面的加工质量,对磨粒有序化砂轮的磨削力对工件表面几何形状的影响规律进行研究。首先,根据筋条减阻表面的参数,设计能够实现筋条减阻表面加工的错位排布砂轮;其次,建立了筋条减阻表面磨削过程的简化模型,并基于有限元对工件表面形貌进行了力学仿真;最后,研究磨削速度、磨削深度、磨粒角度、排布参数等对筋条表面几何形状的影响规律,综合探讨磨削力和表面形貌之间的关系。结果表明,在磨削速度增大时,工件表面材料的隆起变形高度随着磨削力的减小而增大;在磨削深度增大时,材料的隆起变形高度随着磨削力的增大而增大;在磨粒角度增大时,材料隆起变形高度随着磨削力的增大,先增大后减小;在磨粒排布参数增大的过程中,材料隆起变形高度随着磨削力的增大而增大。

Two-point statistics of coherent structures in turbulent flow over riblet-mounted surfaces

Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.

Drag Reducing and Increasing Mechanism on Triangular Riblet Surface

Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow structure inside riblet grooves.Force and flow structure on riblet surface are analyzed and compared with those of smooth surface based on the k-εturbulence model.Drag reducing and increasing mechanism is proved to be related to microvortexes induced inside riblets which lead to Reynolds shear stress reduction significantly and is considered to be the dominant factor resulting in wall friction reduction.Simulation results also show that the pressure drag generating from the deviation of static pressure on the front and rear ends of riblets occurs and grows exponentially with Mach number,which can cause drag increasing.Furthermore,near-wall vortical structures,Reynolds shear stress and static pressure on riblet surfaces are also analyzed in detail.

边界层变化对聚合物湍流减阻流动特性的影响

在流体中添加高分子聚合物不仅会产生减阻效应,还会降低传热性能。若将聚合物减阻应用于供冷、供热系统,可降低传输过程中的流动阻力和热量损失,从而实现能耗的节约。文章以壁湍流理论为依据,用模拟的方法研究了在圆管中设置不同高度的V形肋条对高聚物减阻效果的影响。结果表明:在管道内设置肋条会改变原有的边界层结构,肋高的增加会加大对边界层的破坏;不同肋高及减阻参数对湍动能和雷诺应力的影响趋势一致;当肋条高度增加到大于黏性底层厚度时,聚合物的减阻率降低,减阻效果减弱,且肋条高度越大,减阻效果所受影响越大;线性底层内的边界层变化对聚合物湍流减阻无影响,当缓冲层没有受到扰动和破坏时,聚合物减阻作用不受影响,而当缓冲层受到影响时,聚合物减弱了减阻作用。

基于非稳态模拟的SAE车模气动减阻降噪研究

以20°后背倾角阶梯背式SAE(Society of Automotive Engineers)模型为参考模型,基于改进延迟分离涡模拟(Improved Detached Eddy Simulation, IDDES)湍流模型,研究了分别在车模后倾斜面顶部和尾部添加涡发生器(Vortex Generators, VGS)和沟槽(Riblets, RTS)时的非稳态流场结构,分析了两种被动减阻装置的减阻机制.同时通过提取流场声源信息,利用计算气动声学方法(Computational Aeroacoustics, CAA)和Ffowcs Williams-Hawkings(FW-H)方程对汽车噪声进行分析,研究了两种附加装置的声学特性.结果表明,两种附加装置减小了尾部涡结构强度,降低了气动阻力,同时降低了脉动压力,不仅实现了减阻还具有降噪的效果.减阻率分别为2.41%、2.76%,总声压级最大降低值分别为9.55 dB、5.46 dB.

磨粒有序化砂轮滚磨外圆筋条表面的研究

为了在外圆零件表面加工出用于表面减阻的结构化筋条,基于滚齿加工原理,提出利用磨粒有序化(工程化)砂轮滚磨外圆筋条表面的加工方法。根据筋条表面的特征,对外圆筋条表面进行设计;利用筋条表面的特征参数设计了磨粒有序化砂轮,并给出实现滚磨外圆筋条表面的条件;对滚磨过程进行仿真分析,获得了磨削相关参数对筋条表面形貌的影响规律。研究结果表明:磨粒有序化砂轮能够实现对外圆筋条表面的磨削,筋条表面的主要特征为深宽比,选用合适的磨削用量和排布参数可以满足减阻功能深宽比筋条表面的滚磨。

EXPERIMENTAL STUDY ON TURBULENT BOUNDARY LAYER CHARACTERISTICS OVER STREAMWISE RIBLETS

Measurements of characteristics by means of a two-component Laser DopplerVelocimeter (LDV) were carried out in turbulent boundary layers over both a symmetric V-shapedribbed plate and a smooth one in a low speed wind tunnel. The present results clearly indicate thatthe logarithmic velocity profile over the riblets surface is shifted upward with a 30. 9% increasein the thickness of the viscous sublayer. Also a change in the log-law region is found. And themaximum value of streamwise velocity fluctuations is reduced by approximately 17%. The skewness andflatness factors do not show any change besides those in the region of y^+ 【 0. 6 . It is evidentthat the Reynolds shear stress over the riblets is reduced. Further more, in log-law region, theReynolds shear stress has a larger reduction of up to 18%.

鲨鱼沟槽表皮减阻机理的仿真研究

通过对鲨鱼沟槽表面特殊流场的数值仿真研究,探讨了沟槽表面存在减阻效果的内在机理。针对沟槽表面流场的特点,在数值计算过程中对其计算域、计算网格及其流动参数进行了合理化的处理,并尝试将力学相似原理运用于沟槽表面流场的数值仿真。仿真结果表明,沟槽表面与顺流的"反向旋转涡对"相互作用,产生"二次涡",削弱了"反向旋转涡对"的强度,进而抑制了低速条带的形成和发展,从而降低湍流猝发强度,实现湍流减阻。

不同间隔脊状表面的减阻数值仿真研究

对具有不同间隔的脊状表面在多个速度下进行了数值仿真计算,初步发现了间隔对于脊状表面减阻效果的影响规律。针对脊状表面流场的特点,在数值计算过程中对其计算域、计算网格及流动参数进行了合理的处理。仿真结果表明:有间隔的脊状表面减阻效果优于无间隔的脊状表面;间隔大小与脊状结构尺寸相当时,减阻效果最佳;脊状表面的间隔能够影响减阻效果的原因在于它能够抑制压差阻力的产生。