Natural Low-Frequency Riblets

A turbulent flow at modest Reynolds numbers (26,000–32,000, as defined in the text) over flat erodible sandy beds was investigated. The beds were composed of sand grains of uniform size with a diameter smaller than the viscous sublayer of the flow. When a modest sand transport (1.2 × 10-3–8.5 × 10-3 g/s·cm) was present, the near-wall coherent structures of the flow produced a bed form with a streaky longitudinal pattern. This pattern had a lateral spacing of about 100 viscous units and had less drag than a flat wall.The mechanism causing the drag reduction is due to a low-frequency riblet pattern which allows the flow to sustain higher wall shear stresses without additional erosion. That means during the riblet regime the sediment transport remains quasi-constant.The formation of the sand streaks as well as the drag reducing mechanism are discussed.     

脊状表面减阻特性的风洞试验研究

利用热线风速仪,对光滑表面和多个脊状表面在低速风洞中进行了表面流场测试。基于测得的边界层速度分布数据,利用对数律区速度分布公式,编程分别计算出光滑表面和脊状表面的壁面摩擦速度和虚拟原点。研究发现,脊状表面最大减阻量达13．5％;有减阻效果的脊状表面使边界层速度曲线上移、湍流强度下降;与光滑表面相比,脊状表面的位移厚度和动量损失厚度明显减小,也表明脊状表面具有减阻效果;位移厚度和动量损失厚度减少量随槽间距s^＋的增加呈现先变大后变小的趋势,在S^＋=12时达到最大。     

A numerical study of riblet effects on laminar flow through a plane channel

Numerical computations are reported of fully-developed flow through plane channels with knife-edge riblets. The study has covered riblet height: spacing ratios from 0.5 to 3 and riblet heights ranging from 0.1%–4.0% of the distance between the channel walls. In no case did results indicate a reduction in drag compared with the case of a smooth channel, a result that is in contrast with earlier studies. It is suggested that apparent drag reductions reported earlier in laminar flow may have arisen from the use of an insufficiently fine grid.     

Numerical and experimental investigation of the laminar boundary layer over riblets

One of the main aims of this work is to show to what extent drag reduction in a turbulent boundary layer can be ascribed to a purely viscous effect. A numerical and experimental study is performed in a laminar boundary layer over triangular riblets. The 2-D parabolic equations of motion are integrated using an x marching method and the discretised system is solved with the MSI algorithm. The influence of the riblet geometrical parameters and of the number of grid points is studied. Measurements are carried out in a water tunnel with forward scatter and backscatter laser-Doppler velocimetry extending within the riblets. The longitudinal velocity component measurements and computations are practically identical. Numerical results presented herein show that a slight drag reduction is obtained for s/h=1.2. It appears that, as far as friction is concerned, the wetted area is not the surface to be considered. Thus, the boundary layer over riblets would behave like a boundary layer on an equivalent smooth plate located beneath the crest plane. The numerical study in terms of the riblet height h shows best results are for h tending to zero, with the ratio s/h being equal to 1.2.     

固体颗粒对沟槽湍流边界层影响的实验研究

本文采用粒子图像测速技术(particles image velocimetry, PIV)研究固体颗粒对放置在平板湍流边界层中的平壁和沟槽壁面减阻效果的影响. 实验对清水和加入粒径为155 μm聚苯乙烯颗粒的流法向二维速度场信息进行采集, 对不同工况下的平均速度剖面、雷诺应力和湍流度等统计量进行对比, 分析流体在边界层中的行为. 运用空间局部平均结构函数提取了不同工况湍流边界层喷射?扫掠行为的空间拓扑结构并进行比较. 结果发现, 在不同的壁面条件下, 粒子加入后的对数律区中无量纲速度均略大于清水组, 雷诺切应力有所降低, 湍流度有所减弱. 对于不同流场速度下的沟槽而言, 颗粒的加入均降低了壁面附近的阻力, 而颗粒单独作用于光滑壁面的减阻效果并不明显. 加入粒子后的相干结构数目有所增加, 法向脉动速度下降. 沟槽壁面附近的相干结构数目有所增加, 法向脉动速度在自由来流速度较大时有所上升, 在速度较小时有所下降. 这表明不同减阻状况下的沟槽均能将大涡破碎成更多的涡, 并且粒子的加入强化了这种破碎作用.      

脊状表面减阻机理研究

针对脊状表面流场的特点,通过实验测量和数值模拟的方法对脊状表面微观流场进行了深入研究,获得了脊状表面湍流边界层的时均速度分布曲线、湍流度分布曲线和微观流场结构.为了得到脊状结构对壁面物性的影响,对脊状表面进行了疏水性测试,获得了液滴在脊状表面上的表观接触角,并通过水洞试验验证了脊状表面的减阻效果.研究表明,与光滑表面相比,脊状表面微观流场结构中存在"二次涡",近壁区的黏性底层厚度比平板的要厚得多,湍流度显著降低,且脊状表面表现出明显的疏水性.由此提出了基于壁面隔离效应、增大湍流阻尼效应和改变壁面物性效应的减阻机理.     

Heat exchanger design based on chaotic advection

Experiments on hydrodynamic and heat transfer behavior of the flow in a twisted curved channel were conducted in a water tunnel and also in two heat exchanger coils tested in a heat exchanger test facility. The flow regime, designated “chaotic advection,” is a subclass of laminar flow with high mixing properties. Preliminary results show that heat transfer is enhanced due to the chaotic trajectories generated in the flow.     

On near-wall turbulence-generating events in a turbulent boundary layer on a riblet surface

The boundary layer over a drag reducing riblet surface is investigated using hot-wire anemometry and flow visualisation. The concept of a riblet sublayer is introduced, and a definition is proposed in terms of a region of reduced turbulence energy production formed near the wall by the addition of riblets. The hot wire records are examined using a modified form of quadrant analysis, and results obtained over plain and riblet surfaces are compared. Close to the wall, the addition of riblets produces a marked reduction in the occurrence of ejection (2nd quadrant) events. A corresponding increase in the incidence of sweep (4th quadrant) events is accompanied by the development of a strong tendency toward a preferred event duration, and a preferred interval between events. These changes diminish rapidly with distance from the surface, becoming almost undetectable beyondy ⁺=40. They are discussed in the light of flow visualisation results, and interpreted in terms of mechanisms associated with the interaction between the riblets and the inner boundary layer flow structures. A conceptual model of the flow mechanisms in the riblet sublayer is proposed.     

Riblet drag reduction in mild adverse pressure gradients: A numerical investigation

Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s⁺ (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.     

湍流流动中鲨鱼皮表面流体减阻研究进展

快速游动的鲨鱼, 其皮肤表面沿流动方向有序地排列着沟槽状结构, 人们认为这种结构能在湍流流动 中减小表面摩擦阻力. 人们仿真这种生物结构来进行实验研究和应用, 通过复制和改善鲨鱼皮肤表面沟槽状 结构, 使得摩擦阻力最大减小了近10%. 在实验和模拟仿真中, 人们不断讨论和研究湍流流动阻力的形成机制 和沟槽减阻的理论特性. 本文综述了沟槽减阻理论特性的一些研究方法, 并且归纳定义了沟槽减阻最优几何 形状及其尺寸; 详细考虑流体流动的特点, 给出了一种用来选择最优沟槽形状及其尺寸的方法; 综述了目前的 沟槽加工制造技术. 由于鲨鱼皮肤表面存在少量黏液, 从仿生学的角度, 文章最后综述并展望了通过局部应用 疏水性材料来改变沟槽附近流场属性, 从而达到更大程度上减小阻力的目标.      

Parametric Study on a Sinusoidal Riblet for Drag Reduction by Direct Numerical Simulation

The direct numerical simulation of fully developed turbulent channel flow with a sinusoidal riblet surface has been carried out at the friction Reynolds number of 110. Lateral spacing of adjacent walls in a sinusoidal riblet is varied sinusoidally in the streamwise direction. The average lateral spacing of a sinusoidal riblet is larger than the diameter of a quasi-streamwise vortex and its wetted area is smaller than that of ordinary straight-type riblets. We investigate the effect of sinusoidal riblet design parameters on the drag reduction rate and flow statistics in this paper. The parametric study shows that the maximum total drag reduction rate is approximately 9.8% at a friction Reynolds number of 110. The riblet induces downward and upward flows in the expanded and contracted regions, respectively, which contribute to periodic Reynolds shear stress. However, the random Reynolds shear stress decreases drastically as compared with the flat surface case, resulting in the reduction of total drag owing to the sinusoidal riblet. We also performed vortex tracking to discuss the motion of the vortical structure traveling over the sinusoidal riblet surface. Vortex tracking and probability analysis for the core of the vortical structure show that the vortical structure is attenuated owing to the sinusoidal riblet and follows the characteristic flow. These results show that the high skin-friction region on the channel wall is localized at the expanded region of the riblet walls. In consequence, the wetted area of the riblet decreases, resulting in the drag-reduction effect.     

Drag measurements on V-grooved surfaces on a body of revolution in axial flow

In water flows with velocities of up to 9 m/s the friction drag of a body of revolution in axial flow was investigated for dependence on the body surface structure. This was done for different types of riblet film fixed on the surface with the riblet direction aligned with the flow. The lateral spacing between the triangular shaped riblets varied between 0.033 mm and 0.152 mm. In all cases the riblet spacing was equal to the riblet height. For comparison a smooth reference film was used.Depending on the Reynolds number and the non-dimensional riblet spacings ⁺, a turbulent drag reduction of up to 9% could be achieved with riblets in comparison with the flow over a smooth surface.In the region of transition to turbulent flow and with non-dimensional riblet spacings ofs ⁺10–15 drag reductions of up to 13% were obtained. It is therefore conjectured, that in addition to hampering the near wall momentum exchange, the riblets can delay the development of initial turbulent structures in time and space.     

Flow over convergent and divergent wall riblets

Fast swimming sharks have small riblets on their skin, which are assumed to improve the swimming performance of the fish. Fluid dynamic experiments in water as well as in air confirm this assumption. With riblet surfaces as compared to smooth surfaces, drag reductions up to about 10% were measured. The overall riblet pattern on sharks shows parallel riblets directed from head to tail, but besides this overall pattern fast swimming sharks have also small areas with converging riblets and others with diverging riblets. In the present study the velocity field over convergent and divergent riblet patterns is investigated by hot-wire measurements in turbulent pipe flow. Significant changes in the near wall velocity field were found.     

Dryout in serpentine evaporators

Waterside on-load corrosion failures in power station evaporators have resulted from dryout along the upper surfaces of the tubes in serpentine arrangements.

It is shown that this type of dryout can occur at intermediate qualities over a wide range of flow rates and pressures for heat fluxes much less than in vertical tubes.

A type of flow which is here called “surge flow” has been identified as being particularly likely to lead to dryout, because the top surfaces are wetted only intermittently by waves or plugs of frothy liquid.

A method is being developed for assessing the likelihood of dryout under surge flow conditions. It correctly predicts the dryout boundaries in a pressurized steam/water facility.     

Experimental measurements of flow and flame spread in the development of a fire testing facility

A fire testing facility named the “MSU Fire Tunnel” has been developed. The intent was to devise a testing apparatus that controlled the flow of oxidizing gas in the tunnel to an extent not heretofore accomplished. A novel approach was developed for mounting the flame-spread samples flush with the surface of an “airfoil”. This method avoids previous complications of determining the exact position of the leading edge of the velocity boundary layer. Data were gathered for the flow field using hot-wire anemometry. These data indicated that a zero-pressure gradient Blasius boundary layer flow was established along the airfoil and fuel sample surfaces. Opposed-flow flame-spread tests were conducted and correlations were produced that support the predictive capacity of this apparatus. It was shown that the opposed flow flame-spread data allowed distinctions to be made between correlations of previous researchers. No such comparisons were formerly possible. A finite-chemistry correlation was shown to be consistent with, and similar to, correlations derived in the previous work.     

Modelling the time-dependent flow over riblets in the viscous wall region

The flow over drag reducing riblets is examined computationally using a time-dependent model of the viscous wall region. The flow at the upper bound of the computational domain (y ⁺?40) is described using a streamwise eddy model consisting of two scales. A control-volume finite-element method utilizing triangular meshes is used to exactly fit the riblet cross-sectional geometry. Observations of the transient flow conditions suggest that the riblets limit the lateral spread of fluid inrushes towards the wall and retain low momentum fluid in the riblet valleys effectively isolating much of the wall from such inrushes. The generation of intermittent secondary vortices within the riblet valleys also occurs; however, these appear to be quite weak and fairly short-lived.     

Observations of “granular jump” in the pneumatic conveying system

This paper presents a preliminary study of a previously unreported phenomenon of the “gas driven granular jump”, observed in the gas–solids flow within the pneumatic conveying system. From the phenomenological point of view, it resembles the already known processes such as hydraulic jumps in shallow water or granular jumps in granular flows in chutes or avalanches (although it seems most appropriate to explain it by analogy to a propagating granular bore). Clearly, unlike in classical phenomena of this type, the flow itself is driven by the aerodynamic forces related to the gas flow and the behaviour of the front of the “jump” is modified significantly by their presence. A series of high-speed camera visualisations are presented, which focus on this unusual behaviour of the flow on the border-line between cluster and stratified flow regimes in a horizontal pipe. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. The fluid dynamical aspects and possible mechanisms behind the new phenomenon are discussed and the results obtained are compared quantitatively with simple theoretical models.     

The Optimal Shape of Riblets in the Viscous Sublayer

Our aim is to find the optimal shape of periodically distributed microstructures on surfaces of swimming bodies in order to reduce their drag. The model describes the flow in the viscous sublayer of the boundary layer of a turbulent flow. The microscopic optimization problem is reduced applying homogenization. In the reduced so-called macroscopic optimization problem we minimize the Navier constant subject to the boundary layer equations which are solved in a very small part of the original domain. Under the assumptions that the microstructures can be represented as smooth functions the sensitivity can be determined analytically. The optimization problem is then solved by a sensitivity based method (steepest descent with optimal step size) and the state equations are solved in each iteration with an external software. Our reduced model is validated by comparing the results from the homogenized model with those obtained by simulating the whole rough channel. An improved shape is found and a drag reduction up to 10% can be shown.