The effects of surface roughness geometry of flow undergoing Poiseuille flow by molecular dynamics simulation

Numerical simulation of Poiseuille flow of liquid Argon in a rough nano-channel using the non-equilibrium molecular dynamics simulation is performed. Density and velocity profiles across the channel are investigated in which roughness is implemented only on the lower wall. The Lennard–Jones potential is used to model the interactions between all particles. The effects of surface roughness geometry, gap between roughness elements (or roughness periodicity), surface roughness height and surface attraction energy on the behavior of the flow undergoing Poiseuille flow are presented. Results show that surface shape and roughness height have a decisive role on the flow behaviors. In fact, by increasing the roughness ratio (height to base ratio), the slip velocity and the maximum velocity in the channel cross section are reduced, and the density fluctuations near the wall increases. Results also show that the maximum density near the wall for a rough surface is less than a smooth wall. Moreover, the simulation results show that the effect of triangle roughness surface on the flow behavior is more than the cylindrical ones.     

An intermittency model for predicting roughness induced transition

An intermittency transport equation for RANS modeling, formulated in local variables, is extended for roughness-induced transition. To predict roughness effects in the fully turbulent boundary layer, published boundary conditions for k and ω are used. They depend on the equivalent sand-grain roughness height, and account for the effective displacement of wall distance origin. Similarly in our approach, wall distance in the transition model for smooth surfaces is modified by an effective origin, which depends on equivalent sand-grain roughness. Flat plate test cases are computed to show that the proposed model is able to predict transition onset in agreement with a data correlation of transition location versus roughness height, Reynolds number, and inlet turbulence intensity. Experimental data for turbine cascades are compared to the predicted results to validate the proposed model.     

粗糙表面的分形特征与分形表达研究

得用触针轮廓仪和数据采集系统对磨削和车削表面的粗糙轮廓曲线进行了测量，并就粗糙表面的分形特征作了分析与讨论，同时还提出了粗糙表面的特征粗糙度概念及其定义，并将其用表面粗糙度水平的表述。     

Surface roughness effects in a short porous journal bearing with a couple stress fluid

In this paper, a theoretical study of the effect of surface roughness in hydrodynamic lubrication of a porous journal bearing with couplestress fluid as lubricant is made. The modified Reynolds equations accounting for the couple stresses and randomized surface roughness structure are mathematically derived. The Christensen stochastic theory of hydrodynamic lubrication of rough surfaces is used to study the effects of surface roughness on the static characteristics of a short porous journal bearing with couplestress fluid as lubricant. Further, it is assumed that, the roughness asperity heights are small compared to the film thickness. It is observed that, the effects of surface roughness on the bearing characteristics are more pronounced for couplestress fluids as compared to the Newtonian fluids.     

Calculations of stratified wavy two-phase flow in pipes

Calculations of fully developed, stratified wavy gas–liquid pipe flow is presented. The wavy interface is represented by an equivalent interfacial roughness obtained from experimental data, which is made non-dimensional following the Charnock formulation. The two-dimensional, steady-state axial momentum equation is solved together with a two-layer turbulence model, which is modified to account for the roughness introduced at the interface. The governing equations are discretized using a finite difference method on a composite, overlapping grid with local grid refinement near the interface and the wall. The immersed interface method is used to make the numerical scheme well-defined across the interface, and a level set function is used to represent the interface. Numerical calculations are found to compare satisfactorily with experimental data.     

一种描述结构面剖面线粗糙度的新方法

采用法向矢量单位圆描述结构面剖面线粗糙度,从各微分段的角度关系阐述粗糙度,进而将二维问题转化为一维问题处理并提出"角度粗糙度"的概念。考虑到各微分段的实际长度对粗糙度的贡献,采用加权均值与加权方差定量描述角度粗糙度; 角度粗糙度越大表明该剖面越粗糙。对规则剖面线与不规则剖面线采用"角度粗糙度"进行描述,所得结果跟已有的剖面线粗糙性表征方法相比,二者具有较好的一致性,表明新方法评价的正确性与可行性。     

岩体结构面粗糙度系数定量表征研究进展

1978年,Barton提出的节理粗糙度系数(joint roughness coefficient,JRC)被国际岩石力学学会作为评估节理粗糙度的标准方法.然而该方法存在人为估值的主观性缺陷.就此,国内外学者围绕岩体结构面粗糙度定量化表征开展了大量的研究工作.首先,从二维节理轮廓线到三维岩体结构面,系统地阐述了其粗糙度定量化表征方法研究进展,并总结了各方法参数与JRC的关系;评价了各表征参数的本质特性及其适用性;指出了各方法参数获取过程中存在的问题,主要有:采样间隔的影响,三角形单元划分的影响,如何确定综合参数法中各参数的权重;针对这些问题,给出了笔者的一些想法、建议.与此同时,对结构面粗糙度表征的两个热点问题,即各向异性和尺寸效应的研究也进行了详细总结分析.最后,笔者认为:(1)分形维数因是描述自然界复杂几何体的一种简洁有力的工具,其仍是结构面粗糙度定量描述的有效方法;(2)3D打印技术的应用,有望在开展结构面各向异性、尺寸效应研究方面取得突破性进展.     

Evolution of vortex structure in boundary layer transition induced by roughness elements

The particle image velocimetry (PIV) and hydrogen-bubble visualization technique are used to investigate the flat-plate boundary layer transition induced by an array of roughness elements. The streamwise evolutions of the mean and fluctuation velocity are analyzed, and the critical Reynolds number Re _k,c is determined between 339 and 443 under current experimental setup. The hairpin vortices shed from supercritical roughness elements are visualized by swirling strength, in which two pairs of counter-rotating vortices can be observed: one vortex pair is the manifestation of the neck of the hairpin vortices shed from the top of the roughness; the other vortex pair, which originates from the lower part of the roughness, comes from the streamwise vortices that are constantly perturbed by the hairpin vortex shedding.     

Small-Scale Roughness Effects on Laminar Separation

In this study an interacting boundary-layer (IBL) algorithm is used to investigate small-scale surface roughness effects on the laminar separation mechanism, where small-scale is intended to mean roughness fully contained within the boundary layer. Steady, laminar breakaway separation is computed for two-dimensional flow past a symmetric biconvex airfoil with small-scale roughness elements added to the surface. In this case the flow separation is generated at the trailing edge of a biconvex airfoil, but results are relevant to laminar separation points in general such as that occurring in the leading-edge region. The study is interested primarily in the laminar separation point, and not necessarily the entire bubble and downstream region. The use of the IBL method made it possible to achieve the required fine resolution in areas of interest. For some roughness geometries and flow conditions, up to 15000 grid points (over 4 million total grid points) were used in the streamwise direction to capture the resulting flow physics, which would still be time restrictive with a full Navier–Stokes algorithm. A number of different small-scale roughness configurations were evaluated including variations of roughness height, wavelength, distribution, and geometry. Results from this work show that small-scale roughness can alter the characteristics of the laminar separation point in low-speed flows.     

Heat transfer and bubble formation on horizontal copper tubes with different diameters and roughness structures

Heat transfer in flooded evaporators of the refrigeration, air conditioning or process industries is mainly enhanced by modifying the surface structure of evaporator tubes in the micro and/or macro range. To quantify the effect of such modifications, however, the influence of the basic roughness structure on the heated surface has to be separated. Starting from recent publications, experimental results of heat transfer and bubble formation from horizontal copper tubes with different outer diameters (8 or 25 mm) and roughness structures to various boiling liquids are analyzed in this paper to improve our knowledge of the specific events connected with the formation of bubbles at active nucleation sites and their effect on local heat transfer. It is shown that a single, standardized roughness parameter like the (integral) mean roughness height P _a is not sufficient to explain the effect of the heating surface structure on nucleate boiling heat transfer. Instead, detailed information on characteristic roughness parameters of the heated surfaces is necessary for the analysis, making it possible to define the size and form of cavities included in the roughness structure and their positions on the surface. An analysis that aims in this direction is given in a separate contribution to this special issue by A. Luke, who prepared the surfaces and provided the basic data on the set of standardized roughness parameters, the probability distributions of which are used in this paper.     

Wall roughness effects in laminar flows: an often ignored though significant issue

The effect of wall roughness, which is strong in turbulent flows often, is neglected in laminar flows, though without justification. With an experimental set-up which allows for changes in the relative roughness of a channel without requiring manipulation of the rough channel surface, it can be shown that there is a non-negligible influence of wall roughness even for laminar flows. Based on the consideration of entropy production in these flows, an increased dissipation rate in the vicinity of the roughness elements is identified as the physical mechanism that leads to an increased total head loss when the walls are no longer smooth.     

Quadrant analysis of persistent spatial velocity perturbations over square-bar roughness

We explore a new application of the quadrant method in the context of the double-averaged Navier–Stokes equations for studying open channel flow near rough beds. Quadrant analysis is applied to spatial disturbances of time-averaged velocity components, using the experimental data from flow over two-dimensional regular transverse square-bar roughness. The spatial velocity disturbances change periodically performing a full cycle over a single roughness element, so that the quadrant diagrams are regular closed orbits. A colour code is used to produce a quadrant map of the flow cross-section, which reveals contributions from each quadrant to the time-averaged momentum transfer.     

Hypersonic viscous flow over large roughness elements

Viscous flow over discrete or distributed surface roughness has great implications for hypersonic flight due to aerothermodynamic considerations related to laminar?Cturbulent transition. Current prediction capability is greatly hampered by the limited knowledge base for such flows. To help fill that gap, numerical computations are used to investigate the intricate flow physics involved. An unstructured mesh, compressible Navier?CStokes code based on the space?Ctime conservation element, solution element (CESE) method is used to perform time-accurate Navier?CStokes calculations for two roughness shapes investigated in wind tunnel experiments at NASA Langley Research Center. It was found through 2D parametric study that at subcritical Reynolds numbers, spontaneous absolute instability accompanying by sustained vortex shedding downstream of the roughness is likely to take place at subsonic free-stream conditions. On the other hand, convective instability may be the dominant mechanism for supersonic boundary layers. Three-dimensional calculations for both a rectangular and a cylindrical roughness element at post-shock Mach numbers of 4.1 and 6.5 also confirm that no self-sustained vortex generation from the top face of the roughness is observed, despite the presence of flow unsteadiness for the smaller post-shock Mach number case.     

Spectral simulation and shock absorber identification

In vehicle dynamics shock absorbers are used for the optimization of driving comfort and driving safety. Therefore, it is necessary to identify characteristics of shock absorbers under real conditions. This paper introduces the use of hardware-in-the-loop simulations for the identification of shock absorbers involving stochastic models of the road roughness. For this purpose a dynamic hydraulic test stand is used replacing the classical mechanical test stands which allow only sinusoidal excitation. For the Monte Carlo simulation with a real shock absorber in the loop, the random excitation of ground roughness is generated using a modified spectral representation method based on the famous contributions of Shinozuka. Motion and force of the shock absorber are measured and fed back to the Monte Carlo simulation of a car model in real time. The characteristic of the shock absorber is identified using the classical least squares method and a correlation-based method. A piecewise linear model for the characteristic relating the damping force and the velocity of the piston is applied for the shock absorber identification.     

Friction factors and roughness measurements of tubular mineral membranes

No direct measurement of the relative roughness is available for mineral porous media because of the low mechanical resistance of such materials. In this study a method for the experimental determination of the internal diameter and the equivalent roughness is proposed for different commercial membranes used in ultrafiltration and microfiltration processes. The use of classical friction factor correlations is also discussed. The main results are the estimation of the hydraulic diameter of tubular membranes and the use of a quadratic form in order to predict friction factors and the equivalent roughness with an accuracy better than 15%.     

Can crack front waves explain the roughness of cracks?

We review recent theoretical progress on the dynamics of brittle crack fronts and its relationship to the roughness of fracture surfaces. We discuss the possibility that the small scale roughness of cracks, which is characterized by a roughness exponent ?0.5, could be caused by the generation, during local instabilities by depinning, of diffusively broadened corrugation waves, which have recently been observed to propagate elastically along moving crack fronts. We find that the theory agrees plausibly with the orders of magnitude observed. Various consequences and limitations, as well as alternative explanations, are discussed. We argue that another mechanism, possibly related to damage cavity coalescence, is needed to account for the observed large scale roughness of cracks that is characterized by a roughness exponent ?0.8.     

引入特征粗糙度参数的Stribeck曲线试验研究

为探讨表面粗糙度对Stribeck曲线的影响,对不同初始表面的不锈钢销试件与45#钢盘试件在浸油润滑条件下进行摩擦磨损试验,研究摩擦系数的变化规律.结果表明:摩擦副表面越粗糙,对应Stribeck曲线上混合润滑区域面积越大,曲线斜率越小,使得不同表面粗糙度下的摩擦系数试验模型不具有唯一性.因此,将由分形参数导出的能客观表征粗糙表面的"特征粗糙度"参数引入Stribeck动压参数,从而提出新的动压参数.在新的动压参数下,具有不同表面粗糙度摩擦副的Stribeck曲线具有较好的一致性,继而可建立与粗糙度无关的摩擦系数试验模型.     

A unified theory of turbulent flow

Summary A general theory of turbulent flow is applied to incompressible flow in a circular pipe. The theoretical mean velocity distribution is found to be in good agreement with experiment, but there is some discrepancy in the normal stress distribution. The available pressure drop data are used to estimate the value of the apparent wall velocity as a function of Reynolds number and roughness. It is found that the results can be represented by simple expressions which in turn imply simple expressions for the pressure drop as a function of Reynolds number and roughness. However, it has not been possible to derive these results from fundamental considerations. The basis of Reynolds analogy and the application of the theory to channel flow are also discussed.     

Surface-roughness measurement using dry friction noise

A new technique for determining surface roughness is introduced. It is based upon measuring the generated noise due to dry friction of a metallic blade which travels over the surface under consideration. If the frictional force is made small enough to excite the blade, and not the entire system, then the noise will be proportional to surface roughness, and independent of the measured specimen size and material. A prototype acoustic device was designed and constructed and then used to substantiate the technique. Several specimens were machined by different machining processes to obtain a wide range of roughnesses. The specimen surface roughness was measured by a widely used commercial instrument (Talysurf 10), and the prototype transducer. A straight line correlation (on a logarithmic graph) between sound pressure levels (SPL), obtained from the transducer, and roughnesses, obtained from the Talysurf instrument, was found over the range from 0.025 to 100 m. The coefficient of correlation between the measured values of SPL and roughness was as high/as 98 percent. Therefore, the technique could be adopted for measuring roughness and the prototype may thus be considered as a new roughness measuring device.The prototype device has the advantages of being sensitive, accurate and compact. It also has the capability of measuring the roughness of almost any type of surface regardless of its shape, material and complexity. Hence it is considered adequate for most engineering applications.     

Effective macroscopic adhesive contact behavior induced by small surface roughness

In this paper we study a model contact problem involving adhesive elastic frictionless contact between rough surfaces. The problem's most notable feature is that it captures the phenomenon of depth-dependent-hysteresis (DDH) (e.g., see Kesari et al., 2010), which refers to the observation of different contact forces during the loading and unloading stages of a contact experiment. We specifically study contact between a rigid axi-symmetric punch and an elastic half-space. The roughness is represented as arbitrary periodic undulations in the punch's radial profile. These undulations induce multiple equilibrium contact regions between the bodies at each indentation-depth. Assuming that the system evolves so as to minimize its potential energy, we show that different equilibrium contact regions are selected during the loading and unloading stages at each indentation-depth, giving rise to DDH. When the period and amplitude of our model's roughness is reduced, we show that the evolution of the contact force and radius with the indentation-depth can be approximated with simpler curves, the effective macroscopic behavior, which we compute. Remarkably, the effective behavior depends solely on the amplitude and period of the model's roughness. The effective behavior is useful for estimating material properties from contact experiments displaying DDH. We show one such example here. Using the effective behavior for a particular roughness model (sinusoidal) we analyze the energy loss during a loading/unloading cycle, finding that roughness can toughen the interface. We also estimate the energy barriers between the different equilibrium contact regions at a fixed indentation-depth, which can be used to assess the importance of ambient energy fluctuations on DDH.