Effects of the Reynolds number on the mean skin friction decomposition in turbulent channel flows

As the Reynolds number increases, the skin friction has been identified as the dominant drag in many practical applications. In the present paper, the effects of the Reynolds number on the mean skin friction decomposition in turbulent channel flows up to Reτ= 5 200 are investigated based on two different methods, i.e., the FukagataIwamoto-Kasagi(FIK) identity(FUKAGATA, K., IWAMOTO, K., and KASAGI, N.Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows.Physics of Fluids, 14(11), L73–L76(2002)) and the Renard-Deck(RD) identity(DECK,S., RENARD, N., LARAUFIE, R., and WEISS, P.′E. Large-scale contribution to mean wall shear stress in high-Reynolds-number flat-plate boundary layers up to Reθ= 13 650.Journal of Fluid Mechanics, 743, 202–248(2014)). The direct numerical simulation(DNS) data provided by Lee and Moser(LEE, M. and MOSER, R. D. Direct numerical simulation of turbulent channel flow up to Reτ≈ 5 200. Journal of Fluid Mechanics,774, 395–415(2015)) are used. For these two skin friction decomposition methods, their decomposed constituents are discussed and compared for different Reynolds numbers.The integrands of the decomposed constituents are locally analyzed across the boundary layer to assess the actions associated with the inhomogeneity and multi-scale nature of turbulent motion. The scaling of the decomposed constituents and their integrands are presented. In addition, the boundary layer is divided into three sub-regions to evaluate the contributive proportion of each sub-region with an increase in the Reynolds number.

Friction stir welding of AZ31 magnesium alloys processed by equal channel angular pressing

Equal channel angular pressing （ECAP） is an effective thermo-mechanical process to make ultrafine grains. An investigation was carried out on the friction stir welding （FSW） of ECAPed AZ31 magnesium alloys with a thickness of 15 mm. For different process parameters, the optimum FSW conditions of ECAPed AZ31 magnesium alloys were examined. The basic characterization of weld formation and the mechanical properties of the joints were discussed. The results show that the effect of welding parameters on welding quality was evident and welding quality was sensitive to welding speed. Sound joints could be obtained when the welding speed was 37.5 mm/min and the rotation speed of the stir tool was 750 r/min. The maximum tensile strength （270 MPa） of FSW was 91% that of the base materials. The value of microhardness varied between advancing side and retreating side because of the speed field near the pin of the stir tool, which weakened the deformed stress field. The value of microhardness of the welding zone was lower than that of the base materials. The maximum value was located near the heat-affected zone （HAZ）. Remarkable ductile character was observed from the fracture morphologies of welded joints.

Investigation of turbulence and skin friction modification in particle-laden channel flow using lattice Boltzmann method

Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation(DNS) with a lattice Boltzmann(LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow,the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction.

Effect of friction on equal channel angular extrusion process

The influence of the friction between deformed material and die wall during equal channel angular extrusion (ECAE) was studied. Because of the special design of the extrusion die, the direction of the frictional force has been altered and the distribution of the strain field has been uniformed. The results of the finite element analysis(FEA) indicate that the homogeneity of the finial deformation has been greatly enhanced with this method provided. The ratio of maximum strain to minimum strain along the width is reduced from 6.39 to 1.03. Along the billet length, the strain homogeneity region increases more obviously and the strain is uniformed more greatly than that of common extrusion. The lubrication before extrusion reduces greatly or even it is ignored, which has a great significance for the industrial application of ECAE.

Saint-Venant Equations and Friction Law for Modelling Self-Channeling Granular Flows: From Analogue to Numerical Simulation

Rock avalanches are catastrophic events involving important granular rock masses (>106 m3) and traveling long distances. In exceptional cases, the runout can reach up to tens of kilometers. Even if they are highly destructive and uncontrollable events, they give important insights to understand interactions between the displaced masses and landscape conditions. However, those events are not frequent. Therefore, the analogue and numerical modelling gives fundamental inputs to better understand their behavior. The objective of the research is to understand the propagation and spreading of granular mass released at the top of a simple geometry. The flow is unconfined, spreading freely along a 45° slope and deposit on a horizontal surface. The evolution of this analogue rock avalanche was measured from the initiation to its deposition with high speed camera. To simulate the analogue granular flow, a numerical model based on the continuum mechanics approach and the solving of the shallow water equations was used. In this model, the avalanche is described from a eulerian point of view within a continuum framework as single phase of incompressible granular material. The interaction of the flowing layer with the substratum follows a Mohr-Coulomb friction law. Within same initial conditions (slope, volume, basal friction, height of fall and initial velocity), results obtained with the numerical model are similar to those observed in the analogue. In both cases, the runout of the mass is comparable and the size of both deposits matches well. Moreover, both analogue and numerical modeling gave same magnitude of velocities. In this study, we highlighted the importance of the friction on a flowing mass and the influence of the numerical resolution on the propagation. The combination of the fluid dynamic equation with the frictional law enables the self-channelization and the stop of the granular mass.

A NEW METHOD OF CHANNEL FRICTION INVERSION BASED ON KALMAN FILTER WITH UNKNOWN PARAMETER VECTOR

Channel friction is an important parameter in hydraulic analysis. A channel friction parameter inversion method based on Kalman Filter with unknown parameter vector is proposed. Numerical simulations indicate that when the number of monitoring stations exceeds a critical value, the so lution is hardly affected. In addition, Kalman Filter with unknown parameter vector is effective only at unsteady state. For the nonlinear equations, computations of sensitivity matrices are time-costly. Two simplified measures can reduce computing time, but not influence the results. One is to reduce sensitivity matrix analysis time, the other is to substitute for sensitivity matrix.

电力液压系统中的密封摩擦特性分析与优化

随着电力液压系统中发动机的快速发展,活塞组件面临着越来越苛刻的工作环境。为了降低发动机活塞组的摩擦损失功,提高电力液压系统的工作效率,通过对电力液压系统中活塞环组密封摩擦特性影响因素进行分析,再通过正交试验对实验进行优化,分析不同环组配合下的曲轴偏置、活塞销偏置、配缸间隙和活塞中凸点高度等因素对密封和摩擦特性的影响规律。实验结果表明,曲轴偏置是对摩擦损失功最主要的影响因素,配缸间隙是对窜气量最主要的影响因素。优化后方案与原始方案相比,窜气量减少至39.53 L/min,摩擦损失功为0.45 kW。优化后的方案密封性得到了改善,同时摩擦损失功和敲击噪声均有所减少。通过对电力液压系统中的密封摩擦特性进行深入研究和优化,可以为提高液压系统的可靠性和效率提供理论依据和实践指导。

并联通道中超临界RP-3航空煤油流量分配特性数值研究

针对热流偏差下空-油换热器的应用,进行了三种类型并联通道中超临界压力RP-3航空煤油流量分配数值研究,探究了进口压力、进口温度、进口总流量、进出口布置对双通道流量分配的影响机制。通过摩擦压降和加速压降的演变机制揭示了流量非对称分配的原因,讨论了通道质量流速和温度的分布特征及其对换热的影响,进行了(火积)耗散效应分析。根据热流密度比值和运行参数提出了流量偏差的预测准则,预测偏差处于±15%范围内。结果表明:随着双通道热流差别增大,两者流量偏差增大,源于加速压降和摩擦压降的平衡机制。热流比值小于1时,摩擦压降主导流量分配;热流比值大于1时,加速压降主导流量分配。热流比值等于1时,双通道流量等量分配。提高进口压力、进口温度、进口总流量双通道流量偏差均受到抑制。高热流偏差下通道出现流量不连续和偏差管温度反向分层问题。相对流量偏差和(火积)耗散偏差的变化规律一致,I型布置相比U型和Z型布置更好。

错排圆台通道不可逆损失的数值分析

为了加强板式换热器的结构强度,提出了一种圆台结构,并利用热力学第二定律研究了通道参数对错排圆台通道不可逆损失的影响。采用数值方法研究了雷诺数、圆台角度、圆台纵向间距比和横向间距比对圆台通道的摩擦熵产、传热熵产及无量纲熵产等的影响规律。结果表明:在通道参数范围内,圆台通道的摩擦熵产随着雷诺数的增大而增大,传热熵产随着雷诺数的增大而减小,无量纲熵产随着雷诺数的增大而先减小后增大;当圆台角度、纵向间距比和横向间距比分别从0~30°、1~2和1~2增大时,圆台通道的摩擦熵产分别平均增大了41.36%,减小了10.93%和45.26%,传热熵产分别平均减小了27.67%,增大了117.69%和154.45%。

3003-H14铝合金微通道扁管波形冲压的摩擦边界条件优化

波形微通道扁管成为新一代热交换系统关键零部件,但是成形过程中截面畸变严重且受摩擦条件影响。本文建立了基于实验验证的3003-H14铝合金微通道薄壁扁管波形冲压成形-回弹有限元模型,模拟分析了动模-管坯摩擦系数μ_(1)和定模-管坯摩擦系数μ_(2)对截面畸变率分布和平均截面畸变率δh的影响。研究发现,筋的出现大大降低了扁管横截面的截面畸变量,但是对边缘孔的约束作用较弱。纵截面的畸变呈现为有规律的高低波动状态。对于横截面畸变,μ_(1)越大,δh越大;δh随着μ_(2)的增大,先减小后增大。在降低扁管整体的横截面畸变率时,应该考虑摩擦对之间的配合影响。对于纵截面畸变,μ_(1)和μ_(2)越大,δh越大。动、定模与管坯之间的润滑条件越好,越有利于降低纵截面畸变。μ_(1)=0.02、μ_(2)=0.02和μ_(1)=0.02、μ_(2)=0.17均为微通道扁管的最佳摩擦工艺条件。本研究能够为波形微通道薄壁扁管的精确成形提供借鉴。

矩形窄缝通道内单相水流动特性研究

通过理论分析,得到了计算矩形窄缝通道单相层流摩阻系数的公式。对小高宽比矩形窄缝通道内的流动特性进行了实验研究,结果表明:矩形窄缝通道内单相摩阻系数随Re变化的曲线和圆管有相同的趋势,但圆管流动摩阻公式不适用于矩形窄缝通道。矩形通道内摩阻系数与Re和通道截面高宽比有关,通道高宽比越小,摩阻系数越大。实验结果和理论推导结论一致。从截面湿周和切向应力两方面解释了高宽比对矩形窄缝通道内单相水层流摩阻特性的影响机理。

方形等通道角挤压力计算与分析

通过对等通道角挤压(ECAP)变形过程进行分析,并运用上限定理,推导出ECAP变形过程挤压力计算公式,并验证了公式的可行性。研究了模具结构和摩擦力对ECAP挤压力的影响,结果表明,随着模具内角和外角的增大,ECAP挤压力降低,模具内角对ECAP挤压力的影响更大;摩擦力对ECAP挤压力的影响显著,接触面粗糙时的挤压力是光滑时的19倍。为模具设计和模具材料、挤压设备的选择提供了理论依据。

M型截面流道对旋涡泵性能的影响分析

为了减少旋涡泵流道内部冲击损失，提升旋涡泵的性能，研究了基于面积不变理论改变旋涡泵流道截面形状的方法。采取流道形状贴合流动状态的策略，改变流道截面形状为M型，利用CFD技术对参考泵和M型截面流道的内部流动进行多工况三维数值模拟分析，对比两者之间的扬程、效率曲线，并分析流道截面的流线图。结果表明，相比矩形流道，M型截面流道在扬程和效率上均有所提升，流道内叶片出口处的乱流有所改善，M型截面流道中液流更加平滑，但是过流面积增加，摩擦损失也随之增大。

百米级超长预应力束张拉试验研究

以常州市S122省道长江路上跨桥为研究对象,通过现场试验,研究百米级超长预应力束的初始张拉力及持荷时间,根据孔道摩阻试验得出孔道摩阻系数对理论模型进行检算。结果表明,100 m级超长预应力束的初始张拉力宜取张拉控制力的30%,持荷时间宜取6~8 min,孔道摩阻系数较规范值大,造成结构挠度增大及混凝土应力储备减小,但仍在规范允许范围之内,实测孔道摩阻系数对超长束的设计及施工具有重要参考价值。

开孔折流板流动阻力特性

为了开发研制新型高效换热设备,针对新提出的一种开孔折流板强化传热方式,对流动和传热状况进行了实验研究。设计和建立了一套流动阻力特性试验装置,对流体流经交错布置多个开孔折流板的矩形通道时的阻力特性进行了测试。通过所引入的参数f/f0比较了折流板开孔与不开孔情况下阻力系数的变化特点。实验结果发现:折流板开孔后的阻力系数变化趋势基本相同;对于开孔密度分别为0.42%、1.41%和2.51%的折流板,在一定的Re数范围内其阻力系数将大于未开孔折流板。其余折流板的f/f0均小于1,且开孔密度越大,阻力系数减小的程度越高。Re数等于3000左右f/f0出现极大值。综合比较传热实验结果发现,设置开孔折流板是一种有效的传热强化措施。

明渠均匀流的摩阻流速及流速分布

在水槽系统上对明渠恒定均匀流进行了系列实验,利用实测流速资料,对明渠均匀流摩阻流速的各种方法(雷诺应力法、对数流速分布法、阻力平衡法等)的计算结果进行了对比,并分析了流速分布。结果表明:(1)利用阻力平衡法计算摩阻流速时应考虑边壁影响;(2)包含尾流函数的对数-尾流公式比单一对数公式可以更好的符合实测数据。用单一对数公式描述垂线流速分布时,应采用较小的Karman常数;(3)现有大量明渠均匀流实测资料中,用常用的Karman常数(κ=0.4)时单一对数公式仍可以较好的表示全水深流速分布是因为采用了较大的摩阻流速。

带扰流小槽道内单相流动阻力特性实验

采用水作为工质,实验研究入口或出口端加入圆柱扰流的不同高宽比多槽道散热器的压降特性;结果表明:单位长度压降随雷诺数成线性关系.提出了雷诺数、水力学直径和槽道高宽比的拟合准则;拟合公式在实验数据误差范围内很好反应类似系统的流阻特性.对于线切割小槽道,其摩阻系数和雷诺数成反比,且较圆管理论值偏大;对线切割槽道阻力特性分析发现,表面粗糙度是其最主要的影响因素,而扰流对其影响较小;在槽道前部设置扰流柱可以增强换热.

不同球窝/球凸结构的旋转矩形通道传热及阻力特性研究

采用数值模拟方法研究了旋转工况下球窝/球凸结构对通道传热特性和阻力特性的影响,利用SSTk-ω湍流模型求解了黏性Navier-Stokes方程,其中矩形通道的一侧布置球凸,另一侧布置球窝,球窝/球凸的结构分为圆形、竖椭圆和横椭圆.研究结果表明:球凸的迎风前缘传热能力大大增强,背风后缘及其之后的尾迹区域出现流动分离且传热性能有所减弱;球窝的前缘出现了流动分离且传热性能明显减弱,后半球窝处的分离流体再附使传热能力增强.通道旋转对传热阻力特性的影响很大,主要是旋转使得球窝/球凸表面的流动分离再附提前发生或延迟出现,后缘面流动分离再附频繁出现.比较3种通道发现:竖椭圆球窝/球凸通道和圆形球窝/球凸通道的平均Nu和摩擦系数几乎相等,横椭圆球窝/球凸通道的平均Nu和摩擦系数明显大于圆形、竖椭圆通道.

燃气轮机透平叶片冷却通道传热特性研究进展

燃气轮机透平叶片冷却通道的设计及开发对燃气轮机效率、运行安全性等具有重要意义,透平叶片冷却通道的传热特性研究是透平叶片中的重要领域。文中首先总结了透平叶片中典型U型通道中扰流冷却的研究现状:在U型通道中布置肋片、针状翅片、球窝/球凸及组合传热结构是重要的冷却方式,研究已深入至多种组合传热结构及旋转通道传热的层面;其次总结了冷却通道与气膜孔结构耦合传热的研究现状,气膜孔的布置改变了冷却通道内的原流场结构及传热特性,布置肋片、球窝等强化传热结构并与气膜孔相耦合时的流动及传热特性是目前研究的重点;最后总结了内部冷却与外部冷却相耦合时的传热特性,如叶片尾缘劈缝叶背面区域的气膜冷却与对流传热共同作用,指出该耦合传热过程是透平叶片中的重要现象。通过对透平叶片冷却通道传热特性的研究综述,指出在组合传热结构与气膜孔等相互耦合时的冷却通道传热特性还有较多的研究内容需要扩充,同时考虑适用于冷却通道的低阻高效强化传热结构也是目前关注的一大重点。

车门玻璃升降系统部件公差设计及优化

针对升降过程中车门玻璃与导槽密封条底部容易产生额外接触摩擦力的问题,运用VSA偏差分析软件,虚拟仿真分析了车门玻璃与导槽密封条装配间隙偏差。仿真偏差结果与实际车门玻璃升降系统三维模型装配间隙进行对比,发现该系统车门玻璃活动间隙过大,从而对玻璃升降系统中关键部件的公差进行了重新设计。重新设计公差后,再次对系统进行仿真,结果显示该系统车门装配间隙偏差值减小了,从而避免了车门玻璃与导槽密封底部额外接触摩擦力产生的情况。