Lock-exchange release density currents over three-dimensional regular roughness elements

2017 ◽  
Vol 832 ◽  
pp. 793-824 ◽  
Author(s):  
Kiran Bhaganagar ◽  
Narasimha Rao Pillalamarri
Keyword(s):  
Surface Roughness ◽  
Volume Fraction ◽  
Rough Surfaces ◽  
Three Dimensional ◽  
Immersed Boundary ◽  
Channel Height ◽  
Density Currents ◽  
Mixing Processes ◽  
Fundamental Study ◽  
Roughness Elements

A fundamental study has been conducted to understand the front characteristics and the mixing in the flow of density currents over rough surfaces. A large-eddy simulation (LES) has been performed for lock-exchange release density currents over rough walls to shed light on the unsteady mixing processes. A volume-penalization method, which is a special case of the immersed-boundary method, has been implemented to realize the bottom-mounted rough topology. In this study, the LES has been conducted in a channel with a lower wall covered with three-dimensional cube- and pyramid-shaped roughness elements, such that all cases have the same base area, but differences in the roughness solidity and volume fraction of roughness. Both cases of identical roughness elements and those with randomness in height have been considered. The maximum roughness height for all cases is kept at a constant fraction (10 %) of the total channel height. The study focuses on the instantaneous mixing processes in lock-exchange release currents over rough surfaces. An important contribution of the work is that qualitative and quantitative analysis has been conducted to demonstrate additional mixing mechanisms due to the presence of surface roughness that enhances dilution of the current. Enhanced mixing due to roughness is related to the strength of the shear layer resulting from the roughness, and hence depends on friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}$). The combined role of current characteristics and $Re_{\unicode[STIX]{x1D70F}}$ together dictate the mixing processes and extent of dilution in density currents over surface roughness.

Influence of Three-Dimensional Roughness on Pressure-Driven Flow Through Microchannels

2003 ◽  
Vol 125 (5) ◽  
pp. 871-879 ◽  
Author(s):  
Yandong Hu ◽  
Carsten Werner ◽  
Dongqing Li
Keyword(s):  
Surface Roughness ◽  
Pressure Drop ◽  
Roughness Element ◽  
Three Dimensional ◽  
Channel Height ◽  
Roughness Effect ◽  
Microscale Flow ◽  
Roughness Elements ◽  
Pressure Driven Flow ◽  
Pressure Driven

Surface roughness is present in most of the microfluidic devices due to the microfabrication techniques or particle adhesion. It is highly desirable to understand the roughness effect on microscale flow. In this study, we developed a three-dimensional finite-volume-based numerical model to simulate pressure-driven liquid flow in microchannels with rectangular prism rough elements on the surfaces. Both symmetrical and asymmetric roughness element arrangements were considered, and the influence of the roughness on pressure drop was examined. The three-dimensional numerical solution shows significant effects of surface roughness in terms of the rough elements’ height, size, spacing, and the channel height on both the velocity distribution and the pressure drop. The compression-expansion flow around the three-dimensional roughness elements and the flow blockage caused by the roughness in the microchannel were discussed. An expression of the relative channel height reduction due to roughness effect was presented.

scholarly journals Optimal Perturbations in Boundary-Layer Flows Over Rough Surfaces

2013 ◽  
Vol 135 (12) ◽  
Author(s):  
S. Cherubini ◽  
M. D. de Tullio ◽  
P. De Palma ◽  
G. Pascazio
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Base Flow ◽  
Immersed Boundary ◽  
Boundary Layer Flows ◽  
Instability Theory ◽  
Roughness Elements ◽  
Tollmien Schlichting ◽  
Layer Flow ◽  
Lagrangian Optimization

This work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Four roughness elements with different heights have been studied, inducing amplification mechanisms that bypass the asymptotical growth of Tollmien–Schlichting waves. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation. This result demonstrates the relevance of varicose instabilities for such a flow and shows a different behavior with respect to the secondary instability theory of boundary layer streaks.

scholarly journals Optimal Perturbations in Boundary Layer Flows Over Rough Surfaces

2012 ◽  
Author(s):  
S. Cherubini ◽  
M. D. de Tullio ◽  
P. De Palma ◽  
G. Pascazio
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Base Flow ◽  
Immersed Boundary ◽  
Boundary Layer Flows ◽  
Instability Theory ◽  
Roughness Elements ◽  
Tollmien Schlichting ◽  
Layer Flow ◽  
Lagrangian Optimization

This work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. Amplification mechanisms are described which by-pass the asymptotical growth of Tollmien–Schlichting waves. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Two types of roughness elements have been studied, characterized by a different height. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation, pointing out the importance of varicose instabilities for such a flow and a different behavior with respect to the secondary instability theory of boundary layer streaks.

Contact Analysis of Multilayered Elastic/Plastic Solids With Rough Surfaces for Decreasing Friction and Wear

2005 ◽  
Author(s):  
Shaobiao Cai ◽  
Bharat Bhushan
Keyword(s):  
Surface Roughness ◽  
Contact Area ◽  
Yield Criterion ◽  
Rough Surfaces ◽  
Three Dimensional ◽  
Surface Displacement ◽  
Maximum Displacement ◽  
Elastic Plastic ◽  
Perfectly Plastic ◽  
Von Mises

A numerical three-dimensional contact model is presented to investigate the contact behavior of multilayered elastic-perfectly plastic solids with rough surfaces. The surface displacement and contact pressure distributions are obtained based on the variational principle with fast Fourier transform (FFT)-based scheme. Von Mises yield criterion is used to determine the onset of yield. The effective hardness is modeled and plays role when the local displacement meet the maximum displacement criterion. Simulations are performed to obtain the contact pressures, fractional total contact area, fractional plastic contact area, and surface/subsurface stresses. These contact statistics are analyzed to study the effects of the layer-to-substrate ratios of stiffness and hardness, surface roughness, and layers thickness of rough, two-layered elastic/plastic solids. The results yield insight into the effects of stiffness and hardness of layers and substrates, surface roughness, and applied load on the contact performance. The layer parameters leading to low friction, stiction, and wear are investigated and identified.

A lattice Boltzmann study on the drag force in bubble swarms

2011 ◽  
Vol 679 ◽  
pp. 101-121 ◽  
Author(s):  
J. J. J. GILLISSEN ◽  
S. SUNDARESAN ◽  
H. E. A. VAN DEN AKKER
Keyword(s):  
Reynolds Number ◽  
Liquid Phase ◽  
Lattice Boltzmann ◽  
Volume Fraction ◽  
Energy Levels ◽  
Bubble Radius ◽  
Three Dimensional ◽  
Immersed Boundary ◽  
Immersed Boundary Methods ◽  
Gas Phases

Lattice Boltzmann and immersed boundary methods are used to conduct direct numerical simulations of suspensions of massless, spherical gas bubbles driven by buoyancy in a three-dimensional periodic domain. The drag coefficient CD is computed as a function of the gas volume fraction φ and the Reynolds number Re = 2RUslip/ν for 0.03 φ 0.5 and 5 Re 2000. Here R, Uslip and ν denote the bubble radius, the slip velocity between the liquid and the gas phases and the kinematic viscosity of the liquid phase, respectively. The results are rationalized by assuming a similarity between the CD(Reeff)-relation of the suspension and the CD(Re)-relation of an individual bubble, where the effective Reynolds number Reeff = 2RUslip/νeff is based on the effective viscosity νeff which depends on the properties of the suspension. For Re ≲ 100, we find νeff ≈ ν/(1−0.6φ1/3), which is in qualitative agreement with previous proposed correlations for CD in bubble suspensions. For Re ≳ 100, on the other hand, we find νeff ≈ RUslipφ, which is explained by considering the turbulent kinetic energy levels in the liquid phase. Based on these findings, a correlation is constructed for CD(Re, φ). A modification of the drag correlation is proposed to account for effects of bubble deformation, by the inclusion of a correction factor based on the theory of Moore (J. Fluid Mech., vol. 23, 1995, p. 749).

scholarly journals Micro channels in macro thermal management solutions

2006 ◽  
Vol 10 (1) ◽  
pp. 81-98
Author(s):  
Boris Kosoy
Keyword(s):  
Surface Roughness ◽  
Thermal Management ◽  
High Performance ◽  
Electronic Device ◽  
Three Dimensional ◽  
Thermal Control ◽  
Consumer Products ◽  
Working Fluid ◽  
Channel Height ◽  
Micro Channels

Modern progress in electronics is associated with increase in computing ability and processing speed, as well as decrease in size. Future applications of electronic devices in aviation, aero space and high performance consumer products? industry demand on very stringent specifications concerning miniaturization, component density, power density and reliability. Excess heat produces stresses on internal components inside the electronic device, thus creating reliability problems. Thus, a problem of heat generation and its efficient removal arises and it has led to the development of advanced thermal control systems. Present research analyses a thermodynamic feasibility of micro capillary heat pumped net works in thermal management of electronic systems, considers basic technological constrains and de sign availability, and identifies perspective directions for the further studies. Computer Fluid Dynamics studies have been per formed on the laminar convective heat transfer and pressure drop of working fluid in silicon micro channels. Surface roughness is simulated via regular constructal, and stochastic models. Three-dimensional numerical solution shows significant effects of surface roughness in terms of the rough element geometry such as height, size, spacing and the channel height on the velocity and pressure fields.

Methods for the simulation of the pressure, stress, and temperature distribution in the contact of fractal generated rough surfaces

2016 ◽  
Vol 231 (4) ◽  
pp. 489-502 ◽  
Author(s):  
Balázs Magyar ◽  
Bernd Sauer
Keyword(s):  
Surface Roughness ◽  
Temperature Distribution ◽  
Plastic Material ◽  
Rough Surfaces ◽  
Three Dimensional ◽  
Sliding Contact ◽  
Linear Elastic ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic ◽  
Pressure Stress

In this paper, the influence of surface roughness on the local tribological load with a dry sliding contact is studied. First, three artificial rough surfaces with similar structure but different asperity heights are generated and projected on a smooth ball. After that, a contact pattern is determined between a rough ball and a smooth surface taking into account the elastic only as well as the linear elastic-perfectly plastic material description. On the basis of the calculated contact pressure distribution, the subsurface stresses and a three-dimensional temperature distribution in the sliding contact are calculated. The solutions show that a low surface roughness not necessarily results in low local tribological load of the surface.

scholarly journals Contrasts between momentum and scalar transport over very rough surfaces

2019 ◽  
Vol 880 ◽  
pp. 32-58 ◽  
Author(s):  
Qi Li ◽  
Elie Bou-Zeid
Keyword(s):  
Large Scale ◽  
Rough Surfaces ◽  
Three Dimensional ◽  
Turbulent Channel Flow ◽  
Passive Scalar ◽  
Scalar Transport ◽  
Roughness Sublayer ◽  
Quadrant Analysis ◽  
Roughness Elements ◽  
Dispersive Fluxes

Large-eddy simulations are conducted to contrast momentum and passive scalar transport over large, three-dimensional roughness elements in a turbulent channel flow. Special attention is given to the dispersive fluxes, which are shown to be a significant fraction of the total flux within the roughness sublayer. Based on pointwise quadrant analysis, the turbulent components of the transport of momentum and scalars are found to be similar in general, albeit with increasing dissimilarity for roughnesses with low frontal blockage. However, strong dissimilarity is noted between the dispersive momentum and scalar fluxes, especially below the top of the roughness elements. In general, turbulence is found to transport momentum more efficiently than scalars, while the reverse applies to the dispersive contributions. The effects of varying surface geometries, measured by the frontal density, are pronounced on turbulent fluxes and even more so on dispersive fluxes. Increasing frontal density induces a general transition in the flow from a wall bounded type to a mixing layer type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due to reduced contributions from large-scale motions in the roughness sublayer. This study highlights the need for distinct parameterizations of the turbulent and dispersive fluxes, as well as the importance of considering the contrasts between momentum and scalar transport for flows over very rough surfaces.

Density Currents Dynamics over Rough Beds

2015 ◽  
Vol 735 ◽  
pp. 159-162
Author(s):  
Reza Nasrollahpour ◽  
Mohamad Hidayat Jamal ◽  
Mehdi Ghomesi ◽  
Zulhilmi Ismail ◽  
Peiman Roushenas
Keyword(s):  
Surface Roughness ◽  
Temperature Gradient ◽  
Power Plants ◽  
Leading Edge ◽  
Solid Material ◽  
Reservoir Sedimentation ◽  
Density Currents ◽  
Roughness Elements ◽  
Rough Beds ◽  
Bottom Outlets

Density currents are flows driven by density differences caused by suspended fine solid material, dissolved contents, temperature gradient or a combination of them. Reservoir sedimentation is often related to sediment transport by density currents. This sedimentation can block bottom outlets, reduce the capacity of reservoir and harms the dam power plants. The head is the leading edge of density currents. In this paper, the influences of artificially roughened beds on dynamics of the frontal region of density currents are investigated experimentally. Three rough beds using conic roughness elements and a smooth bed were tested. The observed trend is that as the surface roughness increases the head concentration and velocity decreases.

Computation of Turbulent Flow Around Blade With Local Surface Roughness

2003 ◽  
Author(s):  
Yin-han Chang ◽  
Kazuyuki Toda ◽  
Makoto Yamamoto
Keyword(s):  
Surface Roughness ◽  
Turbulent Flow ◽  
Three Dimensional ◽  
Aerodynamic Performance ◽  
Operating Conditions ◽  
Local Surface ◽  
Three Dimensional Flow ◽  
Virtual Force ◽  
Roughness Elements ◽  
Aerodynamic Performances

To improve aerodynamic performance of a blade and to maintain its designed performance, many study have been focused on this point. The factors for aerodynamic performances of a blade are its geometry and operating conditions, which are almost fixed during its lifetime. On the other hand, the factor such as surface roughness generated by impure sand grain or oil droplet is variable one. The surface roughness on the blade affects the flow past it by its aerodynamic nature. The blade design can be improved by revealing this change in aerodynamic performance due to surface roughness. In this study, three-dimensional computations are carried out on the blade root with local surface roughness to investigate how the roughness will change the aerodynamic performance. The locality of the surface roughness distribution is simulated with particle tracks in three-dimensional flow. The model replacing the effect of roughness elements with virtual force is used to simulate turbulent flow around blade with local surface roughness.

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