Rectangular Roughness Research Articles

Lattice Boltzmann simulation of two-phase flow involving non-Newtonian fluid in rough channels

In the present work, a lattice Boltzmann model for two-phase flow involving non-Newtonian fluid is proposed by incorporating the rheological equation into the LBM method for two-phase flow with large density ratio based on the phase field theory. In order to guarantee the continuity of velocity and stress at the interface, the viscosity variation of a power-law fluid at the interface is obtained by using an inverse linear interpolation scheme. The validation of current model is performed by modeling single bubble rising in power-law fluids. Then, the displacement of Newtonian fluid by power-law fluids in rough channel characterized by rectangular roughness element is simulated in the absence of gravity effects. The influences of power-law index and geometric parameters of roughness element including relative roughness and element spacing on the displacement process are analyzed. With the help of shape evolution plots, the differences in the behavior of displaced fluid are compared. By means of velocity profiles, the variations of displacement velocity are discussed. These results demonstrate the reliability and suitability of the current two-phase LBM model for non-Newtonian flow simulations.

Modeling different structures in perturbed Poiseuille flow in a nanochannel by using of molecular dynamics simulation: Study the equilibrium

In this study modeling different structures in perturbed Poiseuille flow in a nanochannel by using of molecular dynamics simulation was performed. Nanochannel limited by parallel Platinum plates containing an Argon fluid. We found that the wall roughness increases the velocity in the nanochannel by shrinking the cross-section. Also, the rectangular roughness and hemispherical roughness have the maximum and minimum effects on the flow properties, respectively. The other result is that the nanochannel roughness reduces the range of fluctuations near the walls. This is due to the fact that less fluid particles reside in the vicinity of the rough nanochannel walls compared the ideal nanochannel. Finally, changing the configuration of the nanochannel roughness from ellipsoid to hemi-spherical increases the number of Pt atoms in the structure and the simulated system; therefore, the total energy is also expected to increase.

Optimisation of flow resistance and turbulent mixing over bed forms

Previous work on the interplay between turbulent mixing and flow resistance for flows over periodic rib roughness elements is extended to consider the flow over idealized shapes representative of naturally occurring sedimentary bed forms. The primary motivation is to understand how bed form roughness affects the carrying capacity of sediment-bearing flows in environmental fluid dynamics applications, and in engineering applications involving the transport of particulate matter in pipelines. For all bed form shapes considered, it is found that flow resistance and turbulent mixing are strongly correlated, with maximum resistance coinciding with maximum mixing, as was previously found for the special case of rectangular roughness elements. Furthermore, it is found that the relation between flow resistance to eddy viscosity collapses to a single monotonically increasing linear function for all bed form shapes considered, indicating that the mixing characteristics of the flows are independent of the detailed morphology of individual roughness elements.

Transport of suspended sediments under the influence of bank macro‐roughness

AbstractRiver restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro‐roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier‐fed streams and very low‐gradient reaches of large catchment rivers, these lateral cavities may trap these sediments. Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. Herein, we analyse the influence of these macro‐roughness elements on the transport of fine sediments in the main channel. Laboratory tests with uniform flow charged with sediments in a channel with banks equipped with large‐scale rectangular roughness elements were carried out. The laboratory experiments covered a wide range of rectangular cavity geometrical configurations and shallowness ratios. The influence of key parameters such as flow shallowness, geometric ratios of the cavities and initial sediment concentration was tested. Surface particle image velocimetry, sediment samples and temporal turbidity records were collected during the experiments. The amount of sediments captured by the cavities, the temporal evolution of the concentration of sediments in suspension and the flow hydrodynamics are cross‐analysed and discussed. It is shown that the trapping efficiency of the macro‐roughness elements is a clear function of the channel geometry and the shallowness of the flow. Copyright © 2017 John Wiley & Sons, Ltd.

Performance of shape indices and classification schemes for characterising perceptual shape complexity of building footprints in GIS

ABSTRACTShape characterisation is important in many fields dealing with spatial data. For this purpose, numerous shape analysis and recognition methods with different degrees of complexity have so far been developed. Among them, relatively simple indices are widely used in spatial applications, but their performance has not been investigated sufficiently, particularly for building footprints (BFs). Therefore, this article focuses on BF shape characterisation with shape indices and classification schemes in a GIS environment. This study consists of four phases. In the first phase, the criteria for BF shape complexity were identified, and accordingly, benchmark data was constructed by human experts in three shape complexity categories. In the second phase, 18 shape indices were selected from the literature and automatically computed in GIS. The performance of these indices was then statistically assessed with histograms, correlation matrix and boxplots, and consequently four indices were found to be appropriate for further investigation. In the third phase, two new indices (Equivalent Rectangular index and Roughness index) were proposed with the objective to measure some BF shape characteristics more efficiently. The proposed indices also were found to be appropriate with the same statistical assessment procedures. In the final phase, BF shape complexity categories were created with the pairs of six appropriate indices and four choropleth mapping classification schemes (equal intervals, natural break, standard deviation, and custom) in GIS. The performance of the index–scheme pairs was assessed against the benchmark data. The findings demonstrated that both new indices and two of the selected indices (Convexity and Rectangularity) delivered higher performance. The custom classification scheme was found more ideal to reveal absolute shape complexity with the index value ranges derived from the boxplots while the other classification schemes were more appropriate to reveal relative shape complexity.

The effect of geometrical parameters, roughness and the number of nanoparticles on the self-diffusion coefficient in Couette flow in a nanochannel by using of molecular dynamics simulation

The effect of geometrical parameters, roughness and the number of nanoparticles on self-diffusion in Couette flow in a nanochannel is investigated by using of molecular dynamics simulation. Gold nanoparticles (50, 100 and 200) were also distributed randomly between the nanochannel upper and lower walls. The effect of geometrical parameters of rectangular roughness in a nanochannel and the number of Gold nanoparticles on global self-diffusion and local self-diffusion as well as the average amounts were investigated in Couette flow. The results show that by increasing the roughness height, the particles are entrapped within the roughness and hence reduce the local self-diffusion coefficient in the vicinity of the upper wall. Also, by increasing the roughness height, the roughness length has less effect on the local diffusion coefficient.

Influences of target surface small-scale rectangle roughness on impingement jet array heat transfer

The present investigation considers the effects of special roughness patterns on impingement target surfaces to improve the effectiveness and surface heat transfer augmentation levels of impingement jet array cooling. This investigation utilizes various sizes, distributions, shapes, and patterns of surface roughness elements for impingement cooling augmentation. The surface roughness shape considered here is rectangle, in combination with larger rectangular pins. Combinations of small rectangle roughness and large pins are considered together, along with arrays of small rectangular roughness alone. Tests are performed at impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Local and overall impingement cooling performance depends upon the pattern, distribution, arrangement, and height of the roughness elements, as well as upon the jet Reynolds number. Depending upon the magnitude of jet Reynolds number, different behavior and trends are observed for the small rectangle roughness and large pins together, compared with arrays of small rectangular roughness alone. Overall, results demonstrate the remarkable ability of target surface roughness to produce increased surface heat transfer augmentation levels of impingement jet array cooling, relative to target surfaces which are smooth.

Numerical analysis of surface roughness effects on the Poiseuille flow caused by a small pressure drop

The rarefied gas flow caused by a pressure drop in a planar microchannel of finite length is analysed numerically to investigate the effect of surface roughness on thermal and hydrodynamic flow characteristics. The kinetic approach based on the nonlinear S-model of the Boltzmann kinetic equation coupled with the Maxwell’s diffuse gas-surface interaction model imposed at the wall is used. An implicit scheme for the solution of the S-model kinetic equation is applied and the algorithm has been optimised for the use of massive parallelization in the velocity space. A surface roughness is configured with series of triangular, semicircular, trapezoidal and rectangular obstructions. A competition between rarefaction and effects of roughness geometry is estimated by varying a rarefaction regime of flow from continuum up to free–molecular regime. The results indicate that the global flow performances in terms of the Poiseuille number, mass and thermal energy flow rates are affected by the wall roughness geometry when compared to the flow performances for a smooth channel. The triangular roughness produces the smallest impact on a gas flow, while the rectangular roughness the largest one. Rarefaction plays a significant role enhancing the effect of the surface roughness. In the continuum flow regime the roughness elements induce strong recirculation and flow separation regions. Moreover, the weakly rarefied flow strongly depends on the shape and the spacing of roughness elements. On the other hand, for the highly rarefied flow the global parameters are almost independent of the shape of elements. The increase in the spacing of roughness elements results in the reduction of the Poiseuille number and an enhancement of mass and thermal energy flow rates for all shapes of roughness elements.

High-Speed Particle Image Velocimetry of the Flow around a Moving Train Model with Boundary Layer Control Elements

Trackside induced airflow velocities, also known as slipstream velocities, are an important criterion for the design of high-speed trains. The maximum permitted values are given by the Technical Specifications for Interoperability (TSI) and have to be checked in the approval process. For train manufactures it is of great interest to know in advance, how new train geometries would perform in TSI tests. The Reynolds number in moving model experiments is lower compared to full-scale. Especially the limited model length leads to a thinner boundary layer at the rear end. The hypothesis is, that the boundary layer rolls up to characteristic flow structures in the train wake, in which the maximum flow velocities can be observed. The idea is to enlarge the boundary layer using roughness elements at the train model head so that the ratio between the boundary layer thickness and the car width at the rear end is comparable to a full-scale train. This may lead to similar flow structures in the wake and better prediction accuracy for TSI tests. In this case, the design of the roughness elements is limited by the moving model rig. Small rectangular roughness shapes are used to get a sufficient effect on the boundary layer, while the elements are robust enough to withstand the high accelerating and decelerating forces during the test runs. For this investigation, High-Speed Particle Image Velocimetry (HS-PIV) measurements on an ICE3 train model have been realized in the moving model rig of the DLR in Gottingen, the so called tunnel simulation facility Gottingen (TSG). The flow velocities within the boundary layer are analysed in a plain parallel to the ground. The height of the plane corresponds to a test position in the EN standard (TSI). Three different shapes of roughness elements are tested. The boundary layer thickness and displacement thickness as well as the momentum thickness and the form factor are calculated along the train model. Conditional sampling is used to analyse the size and dynamics of the flow structures at the time of maximum velocity in the train wake behind the train. As expected, larger roughness elements increase the boundary layer thickness and lead to larger flow velocities in the boundary layer and in the wake flow structures. The boundary layer thickness, displacement thickness and momentum thickness are increased by using larger roughness especially when applied in the height close to the measuring plane. The roughness elements also cause high fluctuations in the form factors of the boundary layer. Behind the roughness elements, the form factors rapidly are approaching toward constant values. This indicates that the boundary layer, while growing slowly along the second half of the train model, has reached a state of equilibrium.

Aerodynamic Properties of Rough Surfaces with High Aspect-Ratio Roughness Elements: Effect of Aspect Ratio and Arrangements

We examine the effect of varying roughness-element aspect ratio on the mean velocity distributions of turbulent flow over arrays of rectangular-prism-shaped elements. Large-eddy simulations (LES) in conjunction with a sharp-interface immersed boundary method are used to simulate spatially-growing turbulent boundary layers over these rough surfaces. Arrays of aligned and staggered rectangular roughness elements with aspect ratio >1 are considered. First the temporally- and spatially-averaged velocity profiles are used to illustrate the aspect-ratio effects. For aligned prisms, the roughness length ( $$z_\mathrm{o}$$ ) and the friction velocity ( $$u_*$$ ) increase initially with an increase in the roughness-element aspect ratio, until the values reach a plateau at a particular aspect ratio. The exact value of this aspect ratio depends on the coverage density. Further increase in the aspect ratio changes neither $$z_\mathrm{o}$$ , $$u_*$$ nor the bulk flow above the roughness elements. For the staggered cases, $$z_\mathrm{o}$$ and $$u_*$$ continue to increase for the surface coverage density and the aspect ratios investigated. To model the flow response to variations in roughness aspect ratio, we turn to a previously developed phenomenological volumetric sheltering model (Yang et al., in J Fluid Mech 789:127–165, 2016), which was intended for low to moderate aspect-ratio roughness elements. Here, we extend this model to account for high aspect-ratio roughness elements. We find that for aligned cases, the model predicts strong mutual sheltering among the roughness elements, while the effect is much weaker for staggered cases. The model-predicted $$z_\mathrm{o}$$ and $$u_*$$ agree well with the LES results. Results show that the model, which takes explicit account of the mutual sheltering effects, provides a rapid and reliable prediction method of roughness effects in turbulent boundary-layer flows over arrays of rectangular-prism roughness elements.

Study of fluid flow behavior in smooth and rough nanochannels through oscillatory wall by molecular dynamics simulation

Abstract The method of molecular dynamics simulation is applied in order to study the behavior of liquid Argon flow within oscillatory Couette flows, in both smooth and rough nanochannels. To accomplish this study, the fluid velocity and the fluid slip in oscillatory Couette flows were used to assess the effects of: oscillatory velocity amplitude, speed frequency rate, channel height, wall density, and the amount of interaction between fluid and wall particles. Both smooth and rough walls were modelled in order to investigate the effect on the fluid patterns as well. Rectangular and triangular wall roughnesses in different dimensions were used to study this effect. The results indicate that an increase in the velocity amplitude increases the fluid slip, and decreases the fluid velocity fluctuations near the walls. Similar to the steady-state Couette flow, in oscillatory flow we observe a decrease in fluid slip by reducing the wall density. Moreover, by reducing the energy parameter between the fluid and wall, the fluid slip increases, and by reducing the length parameter the fluid slip decreases. Implementing the rectangular and triangular roughness to the bottom wall in the oscillatory flow results in a decrease in fluid slip, which is also similar to the usual non-oscillating flows.

On the Mean Flow Behaviour in the Presence of Regional-Scale Surface Roughness Heterogeneity

A suite of large-eddy simulations of the neutral atmospheric boundary layer is conducted to study the mean flow response to the presence of surface roughness heterogeneity at regional scales (surface roughness heterogeneity on the scale of several boundary-layer heights). The roughness heterogeneity is imposed using alternating rough wall patches with numerically resolved rectangular roughness elements of different packing densities. The flow near the surface is found to adjust rapidly, reaching equilibrium conditions at distances on the order of a single inter-roughness element spacing. Despite the regional heterogeneity in surface roughness, it is often desirable to parametrize the entire rough wall using one single effective roughness height. To develop such a parametrization the model of Bou-Zeid et al. [Water Resources Research 40(2):1, 2004] is extended to incorporate the displacement height, d. Predictions from this parametrization are compared with the simulations, with reasonably good agreement.

Ground effect due to rough and resonant surfaces

Particularly if the ground surface between noise source and receiver would otherwise be smooth and acoustically hard, structured low-rise ground roughness can be used as an alternative to conventional noise barriers. The techniques of periodic-spacing, absorptive covering, and local resonance can be used, as when broadening metamaterial stop bands, to achieve a broadband ground effect. This has been demonstrated both numerically and through laboratory experiments. Computations have employed multiple scattering theory, the Finite Element Method and the Boundary Element Method. The experiments have involved measurements over cylindrical and rectangular roughness elements and over their resonant counterparts created by incorporating slit-like openings. Resonant elements with slit openings have been found numerically and experimentally to add a destructive interference below the first roughness-induced destructive interference and thereby mitigate the adverse effects of the low-frequency surface waves generated by the presence of roughness elements. A nested configuration of slotted hollow roughness elements is predicted to produce multiple resonances and this idea has been validated through laboratory experiments.

An experimental study of the flow through and over two dimensional rectangular roughness elements: Deductions for urban boundary layer parameterizations and exchange processes

This paper investigates the flow through and over two-dimensional rectangular roughness elements, arranged in a building-street canyon geometry through a series of experiments. Geometries of different packing densities of the roughness elements (λp) were examined and the packing density values ranged from λp = 0.30 to 0.67. The purpose of the work is: (i) to investigate the flow physics observed both at the boundary layer scale as well as at the scale within the roughness elements for a range of packing densities, (ii) to deduce parameterizations of the adjusted rough boundary layer and their variation with a change in the packing density, and (iii) given a particular interest in and application to the urban atmosphere, a final aim at the roughness-element scale is to deduce the variation of the breathability with the packing density variation. Particle image velocimetery measurements of the velocity flow field as well as the turbulent kinetic energy and the Reynolds Stress (within and up to well-above the street canyons) were conducted. The results reveal qualitative flow features as well as features of the adjusted boundary layer structure—in particular the roughness and inertial sublayers, which can be associated with the surface roughness length, zero-plane displacement thickness, and the friction velocity. The lowest friction velocities are exhibited in the geometries with the highest- and lowest packing densities while the maximum friction velocities are observed in the medium-packed geometries. The exchange processes and breathability at the level of the roughness elements top were characterized and quantified by a mean exchange velocity. The results show that unlike friction velocity, the normalized exchange velocity (over the mean bulk velocity) for the most dense and sparse geometries differ by more than 80%, with the denser-packed geometries exhibiting lower exchange velocities; this is shown to be related with the thickness of the developed roughness sublayer.

The effect of tip size on the measured Ra of surface roughness specimens with rectangular profiles

When measuring rectangular and trapezoidal profile roughness specimens, the stylus tip increases the measured profile peak width and decreases the measured valley width. This can cause either an increase or a decrease in the apparent roughness average Ra, depending on the tip size and the ratio of peak width to valley width. Sometimes the change is larger than the combined measurement uncertainty from other sources. This raises the question as to whether measured surface parameters should be corrected for the effect of tip size.

Propagation of surge waves in channels with large-scale bank roughness

Surge waves from upstream have been studied systematically in a 40 m long channel. Its banks were equipped with large-scale rectangular roughness elements. In addition to a prismatic reference channel, 41 different configurations of rectangular bank cavities have been investigated. The purpose was to study the effect of large-scale bank roughness on steady and unsteady flows. The macro-roughness of the channel banks increases the flow resistance. Consequently, the absolute celerity of the surge wave front is also reduced by between 5 and 25%. Based on the concept of an effective channel width for the computation of the mean flow velocity, the relative celerity of the surge wave can be predicted for macro-rough channel configurations. The attenuation of the wave front height is increased by the macro-rough flow resistance as well as by the partial reflections of the surge waves in the cavities and passive retention of the latter.

Effect of Roughness of Confining Surface on Diffusive Motion of Fluid

We propose a model to study self-diffusion of fluid confined between two rough walls. The model is based on microscopic considerations wherein the configuration space of the body systems is divided into cells. Within the cell, it executes harmonic motion unless it finds a saddle point on potential energy hypersurface. Results are obtained for Lennard Jones fluid for different order of rectangular and sinusoidal roughness of confining walls. It is found that the roughness of wall significantly affects the motion of fluid and reduces the average diffusion of particles in agreement with simulation results. The reduction is more for rectangular than for sinusoidal roughness. The study has applicability for fluids flowing in biological systems.

Theoretical Determination of the Random-Incidence Scattering Coefficients of Infinite Rigid Surfaces with a Periodic Rectangular Roughness Profile

An exact solution is developed for the sound field reflected by an infinite rigid rough surface with a periodic rectangular profile. The random-incidence scattering coefficient of this surface is derived from the numerical calculation of this solution for several directions of the incident plane wave. Scattering coefficients' values are given in this paper for a great number of configurations by varying the geometrical parameters of the periodic profile, leading to a very detailed description of the scattering properties of this kind of profile. In particular, it is shown that, when the ratio H/L of the depth to the spatial period of the profile is small, the key parameter is H/λ, the ratio of the depth of the profile to the wavelength. The random-incidence scattering coefficient tends to increase with this ratio, as long as H/λ is less than 0.3. This evolution is similar to Gaussian and sine-shaped profiles analysed in previous studies. For H/λ > 0.3, the coefficient's value oscillates while converging to an asymptotic value which depends on the width of the wells. Resonance effects have also been highlighted for periodic rectangular profiles, in particular by comparing the computed scattering coefficients with the few measured values published in the scientific literature. The scattering coefficients' values published in this paper can be introduced in room acoustics' models to characterize periodic rectangular surfaces, provided that their dimensions are much greater than the wavelength. Additional measurements and BEM computations are required to deeply analyse the influence of the finite size and the acoustical absorption of real surfaces having this profile.

Flow Resistance Caused by Large-Scale Bank Roughness in a Channel

Systematic experimental investigations have been performed under steady flow conditions in a channel whose banks are equipped with large-scale rectangular roughness elements. The purpose was to determine the flow resistance owing to such macrorough banks. The practical motivation of the study is to see how morphological restoration of banks in channelized rivers, such as lateral cavities, influences the steady flow characteristics. The experiments performed in 40 different geometrical configurations revealed various two-dimensional flow characteristics in the bank cavities created by the roughness elements compared to the prismatic reference channel. The overall head loss of the flow is governed by the existence of different phenomena, such as vertical mixing layers, wake zones, recirculation gyres, coherent structures, and skin friction. The analysis of the experiments for steady flow conditions showed that the flow resistance is significantly increased in the macrorough configurations because of the disturbance of the bank geometry inducing large-scale depressions. The additional flow resistance attributable to macroroughness has been related to the forms of the banks. By separating the observed flow behavior into a normal recirculating, a reattachment and a square-grooved flow type, macrorough flow-resistance formulas according two different approaches could be developed that are in good agreement with the laboratory experiments

A dissipative particle dynamics study of flow in periodically grooved nanochannels

SUMMARY The effect of periodic rectangular wall roughness on planar nanochannel flow is investigated by dissipative particle dynamics simulation. The wall protrusion length is varied, and its effect on the flow is examined. Analysis of particle trajectories and average residence time reveals temporary trapping of fluid particles inside the rectangular cavities for a considerable amount of time. This trapping affects the density, velocity, pressure, and temperature distribution inside and close to the cavities. Inside the cavities, low-velocity regions and regions of high density related to high pressure and high temperature are observed. When compared with that of the channel with flat walls case, lower flow velocities, temperatures, and pressures are observed for grooved channels. The reduction of the above quantities is more pronounced as the protrusion length, that is, the roughness characteristic length, decreases. Finally, the relation of friction factor, f, with the flow Reynolds number is discussed. The model predicts = constant in the range . The results of this work are of direct relevance to the design of nanofluidic devices. Copyright © 2011 John Wiley & Sons, Ltd.