Relative Roughness Increases Research Articles

INVESTIGATING THE CONTRAST OF SURFACE MARKING ON DIFFERENT COLOR CONNECTORS FOR TELECOMMUNICATIONS NEEDS

The development of marking technologies for telecommunications connectors requires increasingly detailed information about their applications. One solution is to use QR codes that contain sufficient information about the connectors' applications. Accurate marking of connectors requires a precise marking system, such as a laser marking system. In this study, a Rofin PowerLine F20 Varia fiber laser was used to mark four different types of ABS plastic connectors in various colors. To achieve optimal marking for all four colors, three experiments were conducted to achieve contrasting marking and high roughness. Easy QR code reading was achieved with an average marking power of 3 W to 4.2 W, a laser marking speed of 300 mm/s to 500 mm/s, and a scanning frequency of 50 kHz to 70 kHz at a constant pulse duration of 8 ns and raster spacing of 50 µm. The microstructural changes and change in roughness were determined using a laser scanning microscope (Olympus OLS5000), which revealed structural changes on the surface and an increase in relative roughness from 39% to 76%. This experiment shows that different parameters of the laser marking mode are required for each color.

Numerical simulation of flow boiling heat transfer in microchannel with surface roughness

In order to investigate the effect of surface roughness on the thermal-hydraulic performance of flow boiling in the microchannel, the surface with random roughness is generated by the Fourier series method, the VOF model and Lee model are used to simulate the flow boiling process. The results show that with the increase in relative roughness, Nu and fRe first increase and then decrease. The microchannel with a rough surface can achieve a better thermal-hydraulic performance than that with a smooth surface. Within the scope of this study, the thermal-hydraulic performance reaches the best when the relative roughness is 0.304%. Additionally, with the increase in the means width of the roughness elements, the surface undulation becomes gentler, the growth rate of Nu slows down, and the fRe increases monotonously. Besides, with the decrease in inlet velocity, the liquid film at the bottom of bubbles dries up, which results in drought spots at the bottom wall and deteriorates the heat transfer performance.

Linear stability of sand waves sheared by a turbulent flow

In this paper, the linear stability of sand waves sheared by a turbulent flow is analyzed. The velocity distribution in the streamwise direction is considered to follow the logarithmic law. The modified pressure distribution owing to the streamline curvature induced by the sand waves is employed in the formulation. The pressure distribution is derived using the Boussinesq approximation for the variation of streamline curvature over the flow depth. The flow model is coupled with the sediment transport model to study the stability of sand waves. Both the modes of sediment transport as bedload and suspended load are considered. The linear stability analysis reveals the favorable region for the formation of dunes and antidunes on the plane formed by the Froude number and the dimensionless wavenumber. The region of instability increases with an increase in particle parameter, while it decreases with an increase in relative roughness. The analysis reveals that the sand waves form beyond a limiting Froude number corresponding to the threshold Shields number. The theoretical results compare well with the available experimental data.

A FRACTAL MODEL FOR CAPILLARY FLOW THROUGH A SINGLE TORTUOUS CAPILLARY WITH ROUGHENED SURFACES IN FIBROUS POROUS MEDIA

In this paper, a fractal model for capillary flow through a single tortuous capillary with roughened surfaces in fibrous porous media is derived. The determined imbibition height and imbibition mass of capillary rise are in satisfying agreement with the existing models reported in the literature. It is found that the imbibition height and imbibition mass of capillary decreases with increasing relative roughness. Besides, it is observed that the equilibrium time in a single tortuous capillary with roughened surfaces decreases with an increase in relative roughness. In addition, it is seen that the imbibition height and imbibition mass of capillary rise increases with imbibition time. With the proposed fractal model, the physical mechanisms of capillary flow through a single tortuous capillary with roughened surfaces in fibrous porous media are better elucidated. One advantage of our fractal analytical model is that it contains no empirical constant, which is usually required in previous models.

EFFECTIVE THERMAL CONDUCTIVITY OF POROUS MEDIA WITH ROUGHENED SURFACES BY FRACTAL-MONTE CARLO SIMULATIONS

In this paper, the Fractal-Monte Carlo has been employed to simulate the effective thermal conductivity of porous media with roughened surfaces. The proposed probability model for the effective thermal conductivity of porous media with roughened surfaces can be expressed as a function of the relative roughness, porosity, minimum and maximum diameter of pores, fractal dimensions, and random number. The proposed model is validated by a satisfying agreement of our Fractal-Monte Carlo simulations and the experimental data. In our Fractal-Monte Carlo model, there is no extra empirical constant and each parameter has clear physical meaning. Then, the effects of micro-structural parameters of porous media on the effective thermal conductivity of porous media with roughened surfaces have been analyzed in detail. It can be found that the effective thermal conductivity of porous media with roughened surfaces decreases with the increase of relative roughness and tortuosity fractal dimension. Our results demonstrate that the proposed Fractal-Monte Carlo model can be used to characterize other transport properties such as mass transfer of porous media.

A FRACTAL MODEL FOR KOZENY–CARMAN CONSTANT AND DIMENSIONLESS PERMEABILITY OF FIBROUS POROUS MEDIA WITH ROUGHENED SURFACES

In this paper, fluid transport through fibrous porous media is studied by the fractal theory with a focus on the effect of surface roughness of capillaries. A fractal model for Kozeny–Carman (KC) constant and dimensionless permeability of fibrous porous media with roughened surfaces is derived. The determined KC constant and dimensionless permeability of fibrous porous media with roughened surfaces are in good agreement with available experimental data and existing models reported in the literature. It is found that the KC constant of fibrous porous media with roughened surfaces increases with the increase of relative roughness, porosity, area fractal dimension of pore and tortuosity fractal dimension, respectively. Besides, it is seen that the dimensionless permeability of fibrous porous media with roughened surfaces decreases with increasing relative roughness and tortuosity fractal dimension. However, it is observed that the dimensionless permeability of fibrous porous media with roughened surfaces increases with porosity. With the proposed fractal model, the physical mechanisms of fluids transport through fibrous porous media are better elucidated.

Relative Permeability Model Taking the Roughness and Actual Fluid Distributions into Consideration for Water Flooding Reservoirs

Reservoir relative permeability is greatly important to the development of water flooding reservoirs. Currently, most researches on relative permeability have not taken the roughness of pore surface and actual fluid distributions into consideration. In this paper, a novel relative permeability model for water flooding reservoirs taking the roughness and actual fluid distributions into consideration has been proposed by using the fractal theory. The novel model contains some key parameters, all of which have clear physical meanings, such as the immobile liquid film thickness, relative roughness, tortuosity fractal dimension $$ D_{\text{T}} $$ and pore fractal dimension $$ D_{\text{f}} $$ . The predicted results of the novel fractal relative permeability model are consistent with published experimental data. That verifies the correctness of the novel fractal relative permeability model. Finally, sensitive factor analysis of novel relative permeability model is conducted. We can find that the wetting fluid relative permeability decreases as the immobile wetting fluid film thickness or relative roughness increases. When the tortuosity fractal dimension or pore fractal dimension increases, the wetting relative permeability and non-wetting relative permeability will both decrease. An increase in maximum pore diameter or the decreasing of minimum pore diameter results in the reduction in fractal dimension of flow channel and discontinuous saturation. The increasing of maximum pore diameter results in an increase in the relative permeability of wetting fluid. The minimum pore diameter has tiny effect on the relative permeability.

Estimation and examination of linepack pressures in long liquid pipelines

In the past, many researchers have carried out water-hammer pressure analysis using Joukowsky equation. However, it has been observed that the computed pressure surge is no longer applicable based on the equation. The Joukowsky equation cannot be used even within the reflection time of the long pipeline. In such cases, the actual pressure rise due to the sudden closure of a quick acting valve will be several times more than that of the sudden increase in pressure as calculated by the Joukowsky equation. The phenomenon of rising pressure at the upstream of an instantaneously closed valve with the passage of time caused by the pipe friction is commonly called as linepacking. In this paper, various parameters affecting the linepack pressure have been thoroughly investigated. As the relative roughness increases, the resulting non-dimensional linepack pressure \( \left( {P_{LP} /P_{o} } \right) \) significantly increases and the proportionality constant was equal to 1.5. The linepack pressure was determined to be decreasing with increasing valve closure time. The dominant parameter that influences the linepack pressure is found to be the Reynolds number as compared to the Mach number, and the relative roughness. Furthermore, the linepack pressure is found to be proportional to frictional head loss \( \left( {h_{L} /D} \right) \), and inversely proportional to inlet pressure \( \left( {P_{o} /\left( {\gamma L_{o} } \right)} \right) \). Finally, a linear regression equation was developed in terms of non-dimensional variables to estimate the linepack pressure using hand calculations without undergoing numerical modeling procedures. The proposed equation was validated for sudden valve closure pressure histories available in the literature. The proposed method is applicable to long distance water supply pipelines where the linepack pressures are significant.

A fractal study of sound propagation characteristics in roughened porous materials

The influence of rough elements of porous materials on sound propagation characteristics in roughened porous materials is investigated. Rough surface topography on the roughened porous materials is simulated by the fractal geometry theory, in which relative roughness is defined clearly as a function of the fractal dimension, porosity, average diameter and diameter ratio of the hexagonal elements on pore wall of the porous materials. Based on the sound propagation model of the smooth porous materials, a sound propagation model of the roughened porous materials is built. The effective density and bulk modulus, acoustic impedance and propagation constant, flow resistivity and sound absorption coefficient of the roughened porous materials are derived and discussed as a function of the relative roughness. It is demonstrated that the sound absorption coefficient of the roughened porous materials is improved as the relative roughness increases. The model predictions for the sound absorption coefficient of the roughened porous materials are well agreed with that of the existing test results.

A Model for Water Flow Through Rock Fractures Based on Friction Factor

Rock fracture roughness and tortuosity caused by contact asperities produce extra resistance for fluid flow in comparison with the channel consisting of two smooth parallel plates. To characterise the role of roughness and tortuosity in water flow through rock fractures, the existing studies of the effect of fracture roughness and contact area (tortuosity) on fluid flow through rock fractures were firstly reviewed. Then, an explicit flow model was derived using the friction factor predictor previously proposed according to the flow data of sandstone fractures. Regarding the introduced relative roughness of rock fracture as the correction variable, the developed flow model can be considered as a corrected form of classic cubic law, where the relative roughness is defined as the ratio of the averaged peak asperity height to equivalent hydraulic aperture. Sensitivity analysis shows that the cubic law can overestimate the flow rate by 10 % when the relative roughness increases to 70.7. With further increase in relative roughness up to 300, which usually represents tight rock fractures, the flow rate is only approximately 64 % of that predicted by cubic law. The verification of this friction factor to granite and limestone fractures shows that the used friction factor predictor is in good accordance with the experimental data.

Experiments investigation of the effects of surface roughness on laminar flow in macro tubes

This paper experimentally investigated the effects of surface roughness on laminar flow in macro tubes (d=19mm). Head drop experiments were conducted in six organic glass tubes with relative roughness (Δ/d) ranging from 1/118.75 to 1/6.66. Water head drop data were used to characterize the friction factor over a Reynolds number ranging from 300 to 25000. The experimental results for tubes with relative roughness (Δ/d<5%) were in good agreement with Nikuradse’ study. The friction factor could be well predicted by theoretical correlation fRe in laminar flow. Obvious deviation of the friction factor from the classical theory was observed for tubes with large relative roughness (Δ/d>5%). The product of fRe was larger than 64 and increased with an increase in relative roughness. Earlier transition from laminar to turbulent flow was also confirmed. The critical Reynolds number linearly deceased as the relative roughness increased.

Effects of Surface Roughness on the Performance of Tangential Inlet Cyclone Separators

This study is carried out to investigate the effects of surface roughness on the flow field and cyclone performance. The flow inside the cyclone separator is modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow is predicted by solving the governing equations by using the Reynolds Stress turbulence model, and the modeling of the particle motions is based on a Lagrangian approach. The results of the numerical simulations are compared with experimental data as well as with the results of mathematical models. Analysis of computed results shows that increase of relative roughness due to corrosion, wear, or accumulation of particles on the inner walls considerably influences the tangential velocity, cyclone separation efficiency, and cyclone pressure drop especially for high inlet velocities. Decreases in cyclone collection efficiency and pressure drop with the increase in surface roughness are found to be more pronounced for high values ...

ROUGHNESS EFFECT OF DIFFERENT GEOMETRIES ON MICRO GAS FLOWS BY LATTICE BOLTZMANN SIMULATION

The roughness effects of the gas flows of nitrogen and helium in microchannels with various relative roughnesses and different geometries are studied and analyzed by a lattice Boltzmann model. The shape of surface roughness is simulated to be square, sinusoidal, triangular, and fractal. Numerical computations compared with theoretical and experimental studies show that the roughness geometry is an important factor besides the relative roughness in the study of the effects of surface roughness. The fractal boundary presents a higher influence on the velocity field and the resistance coefficient than other regular boundaries at the same Knudsen number and relative roughness. In addition, the effects of rarefaction, compressibility, and roughness are strongly coupled, and the roughness effect should not be ignored in studying rarefaction and compressibility of the microchannel as the relative roughness increases.

Direct simulation of roughness effects on rarefied and compressible flow at slip flow regime

A two dimensional numerical simulation is performed for incompressible and compressible fluid flows through microchannels in slip flow regime with consideration of slip and temperature jump boundary conditions. The wall roughness is simulated in two cases with triangular microelements and random shaped micro peaks distributed on wall surfaces to study the effects of roughness shape and distribution on the flow field. Various Mach and Knudsen numbers have been used to investigate the effects of rarefaction as well as compressibility. It is found that rarefaction has a more significant effect on the flow field in microchannels with higher relative roughness. It is also found that the effect of compressibility will be more noticeable when relative roughness increases. In addition a high influence of roughness distribution and shape can be seen for both compressible and incompressible flows. The numerical results have also been checked with available theoretical and experimental relations and a good agreement has been achieved.

Effect of Wall Roughness on Laminar Flow of Bingham Plastic Fluids through Microtubes

Miniaturization of traditional devices is in high demand formicro-electro-mechanical systems~MEMS!. Examples includebut are not limited to optics, communication and information sys-tems, fluidics, biotechnology, medicine, automotive, and aero-space. An area of interest in several engineering fields is the con-trol of fluid flow within microchannels and microtubes. Theapplications include, but are not limited to, fields such as vibrationcontrol of structures and systems using small devices, reactors formodification and separation of biological cells, energy systems asa mobile power supply, heat exchangers for micro and macro de-vices, and propulsion engines @1–3#.In recent years, there has been growing attention given to theliquid flow in microchannels in parallel with the development ofminiaturized devices and systems. The understanding of flowcharacteristics, such as velocity distribution and pressure loss isnecessary in design and process control of microfluidic devices.Peng, Peterson, and Wang @4# experimentally studied the flowcharacteristics of water flowing through rectangular microchan-nels having hydraulic diameters of 0.133 to 0.367 mm and heightto width ratios of 0.333 to 1. Their results indicated that the lami-nar flow transition occurred for the range of the Re number be-tween 200 and 700. They also claimed that friction behavior forboth laminar and turbulent flow depart from classical thermofluidcorrelations, and the friction factor is proportional to Re

Relatively Rough Flow Resistance Equations

The definitions of depth and hydraulic radius become ambiguous when bed roughness is large relative to flow depth. Various statistics are currently used to describe bed roughness and many different flow resistance formulas have been developed. The volumetric hydraulic radius \IR\dv\N and the standard deviation of bed surface elevations \Id\dz\N are rational and unambiguous measures suitable for large relative roughness conditions. Their influence on flow resistance is investigated using conceptual models and digital elevation models of natural alluvial beds. The results show that head-losses for large-scale relative roughness beds can be related to (\IR\dv/d\dz\N); the (\IR\dv/d\dz\N) exponent of power-law flow resistance equations increases from 1/6 to more than 1/2 as relative roughness increases, and flow velocity can be determined from boundary topography measures, water level and slope, without any calibrated coefficients. An overlooked form of the log law, using standard deviation \Id\dz\N, performs as well as power laws for predicting flow resistance with high relative roughness and it reverts to the conventional log law when relative roughness is low. A field technique for determining \IR\dv\N and \Id\dz\N is described.

Turbulent flow in open channels with smooth and rough flood plains

The effect of flood plain roughness in compound open channel flow is studied numerically with a low- Reynolds k-ω model, which is non-linear and therefore capable of predicting the turbulence anisotropy and the turbulence-driven secondary currents. The simulation of rough walls is achieved through a simple boundary condition for ω at the top of the roughness. Free surface modelling is also included, based on an empirical approach. For relative roughness greater than unity the lateral shear layer that develops at the main channelflood plain interface is intensified. The velocities follow the logarithmic law in the interaction region for high and low relative depths with smooth and rough flood plains. Secondary currents and turbulent shear stresses are reproduced well by the model when compared with available measurements and are found greater in the interaction region as relative roughness increases. Turbulent intensities are captured sufficiently with a slight underestimation of u' near the channel bed. Turbulence is enhanced at the interface when the flood plain bed is rough and remains unaffected in the rest of the flow. The free surface simulation is proved capable of reproducing the depression of the maximum velocity below the free surface which becomes more pronounced with increasing relative roughness.

Effect of roughness on the average and pulsating velocity characteristics

1. For all types of roughness the vertical distribution of the average velocities obeys the logarithmic law. 2. The dynamic roughness increases with increase in the geometric roughness. 3. With an increase of relative roughness of the bottom y0/H within limits from 0.5·10−3 to 6.4·10−3 the von Karman constant increases from 0.37 to 0.55 and then it decreases to 0.18. 4. The intensity of turbulence depends linearly on the relative roughness of the bottom. 5. The height of the near-bottom layer, determined from the position of the maximum value of the standard deviation of the longitudinal velocity component, increases with increase in the relative roughness of the bottom. 6. The intensity of the velocity fluctuations increases with increase in the diameter of the roughness projections. Inhomogeneity of the size of the roughness projections promotes an increase in the intensity of pulsations. In the case of a separated arrangement of the roughness projections the pulsation intensity also increases. 7. Channel flow has a layerwise eddy structure. We can assume that the axis of rotation of the eddies occurring near the bottom changes during their penetration into the mass of the flow.

Free Surface Flow In A Channel Of Large Relative Roughness

Characteristics of free surface flow over a bed consisting of hemispherical roughness elements have been studied experimentally. Theoretical bottom of the channel has been located at a distance of 35% of the diameter below the tops of hemispheres. Usual logarithmic laws are valid for velocity distribution and friction factor as long as flow depth is large relative to roughness height. Equivalent sand roughness of the bottom is equal to 2.5 times the diameter of hemispheres. At large values of relative roughness, on the other hand, lower velocities are observed than the logarithmic law predicts, and friction factor also increases by as much as 50%. Intensity of turbulence fluctuations decreases with the increase of relative roughness. Probability density function of velocity fluctuations near the rough bed tends away from the normal distribution.