Effects Of Relative Roughness Research Articles

Grain size effect on the anisotropic shear behavior of sand–textured geomembrane interface

A set of direct shear tests was conducted to investigate the effects of sand mean grain size and geomembrane surface roughness on the shear behaviour of the sand–geomembrane interface. Four types of sands with different mean grain sizes were used and the interfaces included a smooth geomembrane (SGM) and four textured ones with different asperity characteristics. The samples were prepared in a direct shear box at five sedimentation angles. The experimental results showed that the peak friction angle of the soil–textured geomembrane interface was dependent on the mean grain size, inherent anisotropy and surface roughness of the geomembrane. The shear strength of sand–textured geomembranes was two to three times larger than that of the sand–SGM interface. The peak and residual friction angles of sand-SGM interface occurred at sedimentation angle of 72° while it was 108° for sand-TGM interfaces. An increase in mean grain size increased the shear strength of the sand–textured geomembrane interface; while a reverse condition occurred for the sand–SGM interface. The shear strength occurred at larger horizontal displacement as mean particle size increased and the effect of relative roughness on peak friction angle increased with increase in mean grain size.

A NOVEL FRACTAL MODEL FOR THE INVASION DEPTH OF FLUID THROUGH THE TORTUOUS CAPILLARY BUNDLE WITH ROUGHENED SURFACES IN POROUS MEDIA

In this paper, a novel fractal model for the invasion depth of fluid through the tortuous capillary bundle with roughened surfaces in porous media is proposed. The capillary pressure effect is considered in the proposed model. The proposed model is expressed as a function of structure parameters of porous media, including the relative roughness, the fractal dimension for pore size distribution, the contact angle, the density, the gravitational acceleration, the tortuosity and porosity. The invasion depth can be quantitatively characterized by the proposed model. By using the fractal theory, the effect of relative roughness on the invasion depth is discussed. It is observed that the invasion depth decreases with increasing the relative roughness of the tortuous capillary bundle with roughened surfaces. In addition, it is found that the invasion depth increases with the increase of the invasion time. The proposed model predictions are in good agreement with the available experimental data. Each parameter in our model has clear a distinct physical meaning, which may contribute to comprehend the better understanding of seepage mechanisms.

An experimental investigations of the single phase fluid flow through mini pipes with circular cross section

The experimental investigation that has been conducted on the fluid flow in mini pipes with circular cross-sections are presented in this paper. The working fluid is water and its main physical-chemical analysis (pH, total hardness, electrical conductivity) were carried out. The liquid flow through mini pipes of 1, 2 and 3 mm diameter with simulated pressure drops from 1.01 to 61 bar is investigated and the experimental results are presented. The laminar and turbulent friction factor f at different pressure drop values, the transition from the laminar to turbulent flow, the effect of relative roughness, and the boundary-layer thickness, δ, are computed and studied. The experimental results are presented, discussed and analysed, according to the theoretical principles.

Effects of Relative Roughness and Particle Size on the Interface Behavior of Concrete Suction Caisson Foundation for Offshore Wind Turbines

The interface behavior between a caisson and the surrounding soil plays an important role in the installation of suction caissons as foundations for offshore wind turbines. A series of shear tests were carried out using a modified direct shear apparatus to study the interface shear behavior between sand and concrete. Sand samples with three particle size ranges (0.63–1.25 mm, 1.25–2.5 mm, 2.5–5.0 mm) and concrete plates with different relative roughness were used to explore the influence of the relative roughness parameter (Rn) and mean particle size (D50) on shear behavior. The responses from the pure sand shear test are also discussed for comparison. Test results show that the higher the relative roughness (Rn), the greater the maximum shear stress (τmax) appeared. The interface shear stress was weaker than that of the pure sand test. Furthermore, the interface friction angle (φ) of sand–concrete was closely related to the relative roughness of the concrete surface. Under the same conditions, the interface friction angle (φ) increased with relative roughness due to the effect of sand particles breakage and redistribution. By contrast, the effect of the mean particle size (D50) on the interface friction angle (φ) was less significant. However, for the pure sand shear test, the friction angle (φ′) obtained from the traditional shear test apparently increased with D50, indicating that the friction angle was more affected by D50 in the pure sand test than in the interface shear test.

A NOVEL FRACTAL SOLUTION FOR LAMINAR FLOW RESISTANCE IN ROUGHENED CYLINDRICAL MICROCHANNELS

In this work, a novel fractal model for the laminar flow in roughened cylindrical microchannels is proposed. The average height of rough elements is derived using the fractal theory. The effects of relative roughness on the friction factor and the Poiseuille number are discussed. It is found that the Darcy friction factor and the Poiseuille number increase with the increase in the relative roughness in the cylindrical microchannel. Besides, it is observed that the Darcy friction factor decreases with the increase in the Reynolds number. Each parameter of the proposed model has a clear physical meaning. The present model can properly reveal some mechanisms that affect the laminar flow in roughened cylindrical microchannels. The present model improves the understanding of the physical mechanisms of fluid flows through roughened cylindrical microchannels. Our model predictions are compared with the existing experimental data, and good agreement can be found.

FRACTAL ANALYSIS OF GAS FLOW THROUGH THE GAS DIFFUSION LAYER IN PROTON EXCHANGE MEMBRANE FUEL CELLS WITH ROUGHENED MICRO-CHANNELS

The effective permeability is one of the key parameters for porous fibrous gas diffusion layer (GDL), which plays a crucial role in the proton exchange membrane fuel cells (PEMFCs). However, the effect of the surface morphology of porous fibrous GDL on gas transport behaviors is so far been neglected. In order to take that into consideration, a new analytical model is presented for gas flow in porous fibrous GDL with roughened micro-channels based on the fractal scaling laws. Due to the existence of very small pores in the porous fibrous GDL, gas slippage effects through the small pores are also taken into account. The fractal gas permeability is expressed in terms of the relative roughness, other microstructural parameters as well as gas properties. The proposed fractal model is validated by comparing the model prediction with available experimental data. The effect of relative roughness and other micro-structural parameters of GDL on the permeability is analyzed in detail.

Effect of bed roughness on velocity profile and water entrainment in a sedimentary density current

In this study, the effect of bed roughness on velocity profile and water entrainment in a sedimentary density current for Richardson numbers of 1.2–7 (subcritical flow conditions) was investigated. Experiments were carried out in a tilting flume with four different bed slopes, four roughness heights, and two fluid densities of sedimentary density currents. The results showed that bed roughness significantly affects the general shapes of velocity profiles, especially in the wall region. Two empirical equations were developed as the functions of the relative roughness for the wall and jet regions of velocity profile using the measured velocities of density currents. Water entrainment was also affected by bed roughness and an empirical equation was developed describing the relationship of this phenomenon with the Richardson number and relative roughness. Sensitivity analysis of this equation by using elasticity coefficient method showed that the effectiveness of the Richardson number is 3.9 times more than the effect of relative roughness on water entrainment.

Investigation of surface roughness effects on fluid flow in passive micromixer

Surface roughness effects are dominant at microscale. In this study, microchannels are fabricated on Silicon substrate. The roughness morphology is modeled for the fabricated structure using Weierstrass-Mandelbrot function for self-similar fractals. A two dimensional model of hexagonal passive micromixer is analyzed with surface roughness present on inner walls of channels using parallel Lattice Boltzmann method, implemented on sixteen node cluster. The results are compared by simulating this micromixer structure using Navier---Stokes equations. The experimental results on the fabricated micromixers are also presented. The effects of relative roughness, fractal dimension and Reynolds number are discussed on laminar flow in hexagonal passive micromixers. The study concludes the importance of modeling surface roughness effect for better mixing efficiency.

Analysis of laminar-to-turbulent transition for isothermal gas flows in microchannels

In this work the laminar-to-turbulent transition in microchannels of circular cross-section is studied experimentally. In order to single out the effects of relative roughness, compressibility and channel length-to-diameter ratio on the Reynolds number at which transition occurs, experimental runs have been carried out on circular microchannels in fused silica—smooth for all purposes—and in stainless steel (which possess a high surface roughness), with a diameter between 125 and 180 μm and a length of 5–50 cm through which nitrogen flows. For each tube the friction factor has been computed. The values of the critical Reynolds number have been determined plotting the Poiseuille number (i.e., the product of the friction factor, f, times the Reynols number, Re) as a function of the average Mach number between inlet and outlet. The transitional regime was found to start no earlier than at values of the Reynolds number around 1,800–2,000. It has been observed that surface roughness has no effect on the hydraulic resistance in the laminar region for a relative roughness lower than 4.4%, and that friction factor obeys the Poiseuille law, if it is correctly computed taking compressibility into account. It is found that recent correlations for the prediction of the critical Reynolds number in microchannels that link the relative roughness of the microtubes to the critical Reynolds number do not agree with the present results.

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

Hydraulic Design of Stepped Spillways

An experimental study on a large model flume using fiber-optical instrumentation indicated that the onset of skimming flow is a function of critical depth, chute angle, and step height. Uniform mixture depths that determine the height of chute sidewalls and uniform equivalent clear water depths are described in terms of a roughness Froude number containing unit discharge, chute angle and step height. The spillway length needed to attain uniform flow is expressed as a function of critical depth and chute angle. The flow resistance of stepped spillways is significantly larger than for smooth chutes due to the macro roughness of the steps. The friction factor for uniform aerated flow is of the order of 0.1 for typical gravity dam and embankment dam slopes, whereas the effect of relative roughness is rather small. The energy dissipation characteristics of stepped spillways and the design of training walls are also discussed. The paper aims to focus on significant findings of a research program and develops design guidance to lessen the need for individual physical model studies. A design example is further presented.

Deposition of aerosol particles from turbulent flow onto rough pipe wall

Out of many different theoretical studies describing the deposition of small particles in different engineering situations, very little work has accounted for the surface roughness. An approach suitable for estimating deposition velocities of small particles on rough surfaces is presented in this paper. This approach has been applied to estimate the rate of deposition of particles onto a rough pipe wall during turbulent flow. The surface roughness is represented by the average height of pipe surface roughness, K. The friction velocity has been calculated by applying the empirical formula developed by Bennet and Myers in terms of roughness ratio ( K/ D) and the flow Reynolds number. Provisions have been taken to account for the particle eddy diffusivity as a contribution to fluid eddy diffusivity as proposed by Liu et al. The velocity of deposition for this model has been computed numerically by means of the three eighth roots method. The effect of relative roughness, particle size, particle density and flow Reynolds number on the deposition velocity has been examined. It is concluded that the wall roughness has a large effect on particle deposition velocity of a diameter smaller than 5 μm. This effect diminishes for particles of 10 μm. For a given relative roughness and Reynolds number, the particle density has nearly no effect on deposition velocity of particles of less than 0.1 μm. The results of the present approach are compared with the available experimental results as well as the theoretical predictions. Good agreement with experimental results is obtained.

A COMPARISON OF FLUVIAL AND COASTAL SIMILITUDE

A comparison of the conditions of similitude for movable bed scale models of rivers and estuaries on one hand, and beaches and shorelines on the other hand, is presented It is demonstrated that despite the fact that the knowledge in sediment transport by wave action is less advanced than in the case of steady current, the conditions of similitude for beaches are less stringent than for rivers In particular, the effect of relative roughness is comparatively unimportant in the case of beaches, while the necessity of similitude of head loss imposes an imperative condition in the case of scale models of rivers An introduction to a natural law of distortion for beaches is presented in analogy with the Lacey condition for rivers.

Effect of Roughness on the Friction Coefficient on a Closed Channel

THIS paper presents the results of one series of tests included in a comprehensive program of research on hydraulic friction and roughness being carried on at the University of California. An investigation was made of the friction coefficients of closed rectangular channels having the same absolute roughness. The walls of the channels were made of smooth brass artificially roughened by sixty degree V-grooves transverse to the direction of flow. The effect of relative roughness on skin friction is important in the extrapolation of experimental results on models to the prototype. At present Nikuradse's equation based on data of the pressure drop in pipes artificially roughened with sand is used for this purpose. The results of these tests provide data to justify this procedure. The friction coefficients of four channels having hydraulic radii ranging from 0.030 to 0.230 feet were determined for different rates of flow. The channels were constructed of two brass plates one-quarter inch by five and three-quarters inches by eight feet and spacers one-sixteenth, one-eighth, one-quarter, and one-half inch wide. Both plates and spacers were roughened their full length by the grooves machined 0.060 inches apart with an average depth of 0.0125 inches. Practically, the absolute roughness of the different channels was the same since the relative width of the plates, which were the same in all, was large compared to that of the spacers. The large aspect ratio of the channel also insured nearly two-dimensional flow. The plates were held in place by heavy iron castings running the full length of the channel. Fig. 1 is an end view of the assembled channel. Five piezometer holes were located nineteen and one-half inches apart along the center-line of one plate. Pressure drops were measured between adjacent holes with vertical or inclined, mercury or carbon-tetrachloride manometers. The results given are for the downstream pair of taps only, pressure drops farther upstream being appreciably affected by entrance conditions. Rates of flow were measured either by an orifice calibrated in place or by direct weighting. A second series of tests were made on the one-eighth, one-quarter, and onehalf inch set-ups after the original one had been dismantled, and the results checked with those of the first series. In the turbulent region where the friction coefficient for rough surfaces does not depend on Reynold's number, a function, containing two empirical constants, FIG. 1. End view of roughness channel showing brass roughness plates and cast iron supporting frame.