Effect Of Wall Roughness Research Articles

Prediction of Strength and Modulus of Jointed Rocks Using P-wave Velocity

Strength and deformation characteristics of jointed rocks are important parameters for the design of many civil and mining engineering structures. These parameters are difficult to obtain from direct testing of jointed rocks; hence it is usual practice to derive them through indirect approaches. In the present study, an attempt is made to obtain rough estimates of strength and modulus of jointed rocks through P wave velocity. Jointed specimens of a model rock were prepared for laboratory testing. Joints at different orientations and comprising of different joint roughness coefficients were used (JRC $$=$$ 2–4, 12–14 and 14–16). Ultrasonic P-wave velocity and UCS tests were conducted on the specimens. The effect of joint orientation and joint wall roughness on P-wave velocity was studied. An index, Joint Factor was used to quantify the effect of joint attributes on strength and modulus of jointed rock. It was observed that P-wave velocity is closely linked with Joint Factor. This study suggests a correlation to obtain Joint Factor Jf from P-wave velocity. P-wave velocity may be measured in the field and Jf may be computed through the suggested correlation. The computed Jf may then be used to get the strength and modulus values of jointed rocks. Charts are also suggested to roughly assess the shear strength parameters, cmass and ϕmass of the jointed rock.

Experimental and mechanistic study of dispersed micrometer-sized particle resuspension in a square straight duct with rough walls

The resuspension of graphite dust is an important phenomenon in the release of radioactivity and the safety of nuclear reactors during severe accidents. In this study, a visualization experimental platform is constructed to study effects of particle size, flow velocity, and wall roughness on the resuspension characteristics of graphite particles. A statistical model of particle resuspension applicable to monolayer dispersed particles is developed based on the moment equilibrium of the particles and the flow field characteristics, as calculated by the large-eddy simulation framework. The results show that particle resuspension can be divided into short- and long-term resuspension stages. Most particle resuspension occurs during the short-term stage. With increases in flow velocity and particle diameter, the aerodynamic or adhesion force acting on the particles increases, and corresponding particle resuspension fraction increases. The influence of rough walls on particle resuspension is related to both the force on the particles and the arm ratio between the wall morphology and the particle diameter. A comparison with the experimental results demonstrates that the particle resuspension model developed in this study accurately predicts the impact of flow velocity, particle size, and wall roughness on particle resuspension.

A wall stress model to predict high-Reynolds number flow over rough walls

Wall-modeled large eddy simulation of turbulent channel flow with rough walls was performed for Reynolds numbers of Reτr=615–2080. The eddy viscosity parameter in a wall stress model was modified to include the effects of wall roughness. Symmetric and asymmetric channel flows were studied for transitionally and fully rough states with ks+=62 and 112, respectively. The wall shear stress was estimated based on the velocity field within the wall layer using the total shear stress term. For the wall layer, the matching point was located well above the height of the virtual roughness elements. The numerical predictions for the mean velocity profile and Reynolds stresses compared well with the experimental data. The prediction for the mean velocity shift due to roughness agreed with the experimental measurements and empirical correlations. For the asymmetric channel flows with roughness on only one wall, the numerical simulations correctly reproduced the displacement of the mean flow away from the rough wall. Overall, the proposed wall-model allows for the prediction of the effects of wall roughness without resolving the geometry of the roughness elements on the wall. This is consistent with the goal of performing large eddy simulations of wall-bounded flows at high-Reynolds numbers with minimal resolution of the wall region, which results in a much reduced computational cost.

A mathematical model to study the wall roughness effects on the migration of inertial particles in a shear flow

Separation of particles in a fluid domain is relevant in various industrial applications. The effect due to the roughness is preponderant compared with that due to fluid inertia so that the Reynolds number is low and the creeping flow equations apply. The wall roughness is assumed to be rigid and periodic, varied in one direction. The trajectories of freely moving particles in a shear flow are calculated.

Study The Effect of Proppant Concentration of Screen-Out Phenomena While Hydraulic Fracture Job

screen out is a blocking of the fracture path caused by bridging the path or accumulation of the proppant inside the fracture, clumping or lodging of the (solid particles) proppant across the hydraulic fracture width that causes to restrict formation fluid to flow into the hydraulically fractured formation. To avoid the screen out happening needing to save fracturing fluids, hydraulic horsepower. Conditions that leading to screen out during the hydraulic fracturing job in a well, such as, the ratio between the fracture widths to particle diameter that called (β), proppant concentration,  and wall roughness. The effects of these factors are investigated experimentally in the present work by building an apparatus that meet the shape of the real hydraulic fracture. The plugging time were measured and monitored through glass windows on both sides of the apparatus to follow the behavior of the proppant inside the fracture during the flow of the fracturing fluid or when the proppant plugs the fracture. To study the effect of the fracture wall roughness on the screen out phenomena, apparatus was build to get two different fracture shapes, to meet the real fracture wall in reality. Through the relation between the different proppant concentration and the plugging time, one can indicate on the graph the plugging region and non plugging region for different values of β, proppant concentration and fracture shape, in order to avoid the screen out. During designing the hydraulic fracture, the values of β were very important factor that can lead to screen out, through the results found that when β=1, the screen out happened very fast even at low proppant concentration, but for β=2, 3 and 4 the screen out depends on the proppant concentration and fracture shape. For β=5 it was found that, there is no screen out occurring for wide range of proppant concentration. The effect of hydraulic fracture wall roughness is important because it effect the speed of the suspension when passing through the fracture.

Effect of crucible wall roughness on the laminar/turbulent flow transition of the Ga75In25 alloy stirred by a rotating magnetic field

The critical magnetic induction (Bcr) values of a melt flow produced by a rotating magnetic field (RMF), remaining laminar or turbulent, are essential in different solidification processes. In an earlier paper (Metall Res Technol 100: 1043–1061, 2003), we showed that Bcr depends on the crucible radius (R) and frequency of the magnetic field (f). The effect of wall roughness (WR) on Bcr was investigated in this study. Using ten different wall materials, we determined the angular frequency (ω) and Reynolds number (Re) as a function of the magnetic induction (B) and f using two different measuring methods (pressure compensation method, PCM; height measuring method, HMM). The experiments were performed at room temperature; therefore, the Ga75wt%In25wt% alloy was chosen for the experiments. Based on the measured and calculated results, a simple relationship was determined between Bcr and Re*, f, R, and WR, where the constants K1, K2, K3, and K4 depended on the physical properties of the melt and wall material:Bcrleft({Re}^{*},f,R,WRright)=frac{{Re}^{*}}{{R}^{2}}({K}_{1} {f}^{-K2}+{K}_{3} {f}^{-K4} WR)

The relevance of wall roughness modeling for simulation of powder flows in laser metal deposition nozzles

Point-particle large-eddy simulations and high-speed imaging are used to investigate effects of wall roughness and particle size on characteristics of a powder flow issuing from a vertical, round nozzle. Wall roughness effects on dynamics of particles can be characterized by the standard deviation of the roughness angle distribution Delta gamma, which is a hybrid roughness parameter that represents a combination of amplitude and spacing roughness parameters. We optically scan the inner nozzle surface to obtain the two-dimensional roughness profiles, using which Delta gamma is estimated. We adopt a stochastic approach in the numerical simulations to model the wall roughness. We find that this modeling is essential to obtain a good agreement between simulation and experimental results. The wall roughness is found to enhance the transverse dispersion of particles and to eliminate the preferential accumulation of particles in the near-wall region, giving rise to reduction of the mean particle velocity within the nozzle and to clustering of the particles in the nozzle core. Results also reveal that an increase in the particle size (characterized by Stokes number) and in the wall roughness leads to a reduction of the particle velocity and to an enhancement of the particle-stream divergence throughout the jet region. However, a saturation behaviour is observed in the particle-stream divergence with the Stokes number. All this dependence is rationalized by the fact that the Stokes number characterizes the particle response to the gas flow and the wall roughness determines the inelastic particle-wall collision frequency.

Wall roughness effects on the supersonic flow over a circular cylinder

Wall roughness effects on the supersonic flow over a circular cylinder

The effects of wall roughness on the flow dynamics behind a near-wall square cylinder

The effects of wall roughness on the flow dynamics behind a near-wall square cylinder

Onset of turbulence in channel flows with scale-invariant roughness

Using 3D direct numerical simulations of the Navier--Stokes equations, we study the effect of a self-affine wall roughness on the onset of turbulence in channel flow. We quantify the dependence of the turbulent intensity (proportional to the mean-squared velocity fluctuations) on the Reynolds number $\mathrm{Re}$ for different roughness amplitudes $A$. We find that for sufficiently high amplitudes, $A>{A}_{b}$, the transition changes its nature from being subcritical (as is known at $A=0$) to supercritical, i.e., the boundary roughness renders the flow unstable for $\mathrm{Re}>{\mathrm{Re}}_{l}$, where the critical ${\mathrm{Re}}_{l}$ decays nontrivially with increasing $A$. The dependence of the friction factor on $\mathrm{Re}$ is found to follow a generalized Forchheimer law, which interpolates between the laminar and inertial asymptotes. The transition between these two asymptotes occurs at a second critical ${\mathrm{Re}}_{\text{c}}$ which is comparable in magnitude to ${\mathrm{Re}}_{l}$. This implies that transitional flow is an integral part of flow in open fractures when $\mathrm{Re}$ is sufficiently high, and should be accounted for in effective modeling approaches.

Experimental study on the effect of wall roughness on heat transfer characteristics of supercritical carbon dioxide in vertical tubes

The heat transfer characteristics of supercritical carbon dioxide(S-CO2) are vital for S-CO2 power cycles. Previous studies primarily focused on the heat transfer of S-CO2 in smooth or ribbed tubes. Rough walls can affect the heat transfer in turbulent flows. Whereas roughness effects on S-CO2 heat transfer have rarely been studied. The heat transfer characteristics of S-CO2 in tubes (d=4.57 mm,16207≤Re≤91735) with various roughness (1.5 μm∼40 μm) are investigated experimentally in this study. The results reveal that rough wall helps alleviate heat transfer deterioration, which is embodied in: the critical heat load(q/G) for heat transfer deterioration increases from 0.11 kJ/kg to 0.17 kJ/kg as roughness increases. Heat transfer deteriorates in all tubes under high q/G, the temperature peak and deterioration length drop from 106.9 °C to 73.7 °C and 0.75 m to 0.25 m as roughness increases. No heat transfer deterioration in various tubes under low q/G, the heat transfer coefficient practically triples with increasing roughness. The effect of operation parameters on heat transfer performance of S-CO2 in various tubes is similar. A heat transfer correlation for rough tubes is proposed based on 5460 experimental data, which catches 90% of experimental data within ±20% error.

Investigation into Influence of Wall Roughness on the Hydraulic Characteristics of an Axial Flow Pump as Turbine

Pump as turbine (PAT) is a factual alternative for electricity generation in rural and remote areas where insufficient or inconsistent water flows pose a threat to local energy demand satisfaction. Recent studies on PAT hydrodynamics have shown that its continuous operations lead to a progressive deterioration of inner surface smoothness, serving the source of near-wall turbulence build-up, which itself depends on the level of roughness. The associated boundary layer flow incites significant friction losses that eventually deteriorate the performance. In order to study the influence of wall roughness on PAT hydraulic performance under different working conditions, CFD simulation of the water flow through an axial-flow PAT has been performed with a RNG k-ε turbulence model. Study results have shown that wall roughness gradually decreases PAT’s head, efficiency, and shaft power. Nevertheless, the least wall roughness effect on PAT hydraulic performance was experienced under best efficiency point conditions. Wall roughness increase resulted in the decrease of axial velocity distribution uniformity and the increase of velocity-weighted average swirl angle. This led to a disorderly distribution of streamlines and backflow zones formation at the conduit outlet. Furthermore, the wall roughness impact on energy losses is due to the static pressure drop on the blade pressure surface and the increase of turbulent kinetic energy near the blade. Further studies on the roughness influence over wider range of PAT operating conditions are recommended, as they will lead to quicker equipment refurbishment.

Reynolds number dependence of turbulent heat transfer over irregular rough surfaces

To study the scaling of turbulent heat transfer over a rough surface, we performed a series of direct numerical simulations on turbulent heat transfer over a three-dimensional irregular rough surface with varying the friction Reynolds numbers and relative roughness values. We considered rough surfaces with three different relative roughness values of 1/1.9, 1/4.3, and 1/9.0, and the simulations were performed at three friction Reynolds numbers of 115, 250, and 550. The temperature was treated as a passive scalar with a Prandtl number of unity. Regarding the scaling of the Reynolds analogy factor, which is defined as the ratio of the doubled Stanton number to the skin friction coefficient, a correlation function with the skin friction coefficient, equivalent roughness, and Prandtl number provides an accurate account of the effects of relative roughness, roughness Reynolds number, and friction Reynolds number. For scaling the turbulent momentum and energy fluxes, we introduced the decomposition of the turbulent fluxes into the smooth wall profiles at matched friction Reynolds numbers and their deviatoric components. The baseline smooth wall profile was found to account for the effect of the friction Reynolds number, while the deviatoric component incorporated the effect of the roughness Reynolds number. The dispersion fluxes, namely, the dispersive covariance and dispersion heat flux, were dominantly affected by the roughness Reynolds number rather than the friction Reynolds number. To obtain a better understanding of the effect of wall roughness on the momentum and heat transfer mechanisms, we analyzed the spatial and time-averaged momentum and energy equations and discussed the physical mechanisms that caused a decrease in the mean velocity and temperature from smooth wall profiles.

Influence of wall roughness on two-phase displacement

In the development of oil and gas fields, water flooding is still the most important development method. The influence of the existence of rough channel on two-phase flow cannot be ignored. To study the flow of oil-water two-phase in rough channel, the method of solving Navier-Stokes (N-S) equation directly is used to simulate the displacement process of two-phase fluid in a single rough channel, and the phase field method is used to trace the interface of two-phase fluid in real time. The effects of a series of factors, such as relative roughness height, horizontal roughness unit clearance ratio, viscosity ratio, and so on, on the displacement velocity and the length of two-phase interface are studied by establishing a simple and regular rough channel model. By establishing a smooth channel model as a control experiment, the overall effect of wall roughness on two-phase displacement is studied. Finally, the complex rough channel model is established to verify the correctness of the law obtained by the simple model. The results show that the existence of wall roughness will greatly increase the two-phase flow resistance, promote the occurrence of fingering problem, and reduce the displacement efficiency.

New investigation of Micro-fluidized bed: The effect of wall roughness and particle size on hydrodynamics regimes

This study presents a new design for Micro-fluidized beds (MFBs), together with a new approach to their hydrodynamic analysis. Two 4 mm MFB reactors were manufactured from glass and resin (constructed by stereo-lithographic 3D printing), to assess the influence of wall roughness on the hydrodynamic characteristics. The novel 3D printing scheme ensures air tightness and good transparency of the reactor. Four types of glass, SiC, Al2O3 and TiO2, belonging to groups B, A/C and C from Geldart's classification, were used to investigate the influence of particle size on their fluidization behavior in MFBs. Three grades of MFB wall roughness were tested. Mechanical vibration energy was applied to the MFBs to improve the quality of fluidization, and to overcome wall friction and cohesive forces. Their hydrodynamic properties were obtained by analyzing pressure fluctuations in the time and frequency domains. The results show that only an appropriate increase in wall roughness was helpful for the suppression of slugging fluidization. The use of mechanical vibrations was found to promote the attenuation or destruction of plug phenomena, thus improving the quality of fluidization of Group A/C and C particles. The feasibility of fluidizing group A/C and C particles has thus been verified in a 3D printed MFB reactors.

Effect of Wall Roughness on the Fluid Solid Interfacial Slip in Nanochannels

Effect of Wall Roughness on the Fluid Solid Interfacial Slip in Nanochannels

Effect of Wall Roughness on Secondary Flow Choking in Supersonic Air Ejector with Adjustable Motive Nozzle

Effect of Wall Roughness on Secondary Flow Choking in Supersonic Air Ejector with Adjustable Motive Nozzle

Proppant transport in scaled experiments: Effect of drainage configuration and fracture wall roughness

In recent years, there has been a number of experimental and numerical studies on the transport and settlement of proppant in scaled laboratory fractures that are of interest to understand hydraulic stimulation in hydrocarbon reservoirs. The results from these studies are generally taken with skepticism by the industry. Part of this is due to the perception that results must be very sensitive to the boundary conditions. Small changes in the experimental configuration are expected to lead to very different results. In particular, little is known on how the actual roughness of the fracture walls and the way the slurry is drained from the laboratory cell affect the results. Here, we report experimental results on the proppant transport in a scaled vertical rough cell that mimics a vertical fracture induced during stimulation. We compare results with those obtained in a smooth wall cell, which is more common in laboratory tests. We found that a rough wall allows for a significant increase in the amount of proppant deposited in the cell at high pumping rate. However, the effect of roughness seems to be marginal for low pumping rates, suggesting that most previous studies in the literature which use of a reduced pumping rate were not significantly affected by the use of smooth walls. We also designed different flow configurations at the fracture tip (where the slurry drains form the cell) to study their effect on the way in which the proppant is deposited in the fracture. We found that, in general, the deposited dune is fairly independent of the particular boundary conditions. This suggests that most results reported in the literature are also robust against changes in the selected outflow configuration.

Effects of Wall Roughness on the Separation Performance of Hydrocyclones under Different Inlet Conditions

Wall roughness is a crucial parameter in the manufacture and operation of hydrocyclones but has been underestimated. This study assesses the effects of wall roughness on the separation performance under different inlet conditions using a two-fluid model and an empirical wall roughness model that modify the logarithmic law of the wall. These models are proven to give good predictions for the in-house experiments on a ⌀150 mm hydrocyclone. The results indicate that small roughness is acceptable in use, whereas large roughness can significantly deteriorate the separation performance. The wall roughness mainly affects the pressure–kinetic energy conversion in the boundary layer by decreasing the tangential velocity and increasing the axial and radial velocities. All these changes further affect the air core diameter, the spatial distribution of particles, and finally, the separation results. The equal pressure condition can provide superior separation results with higher capacity than the equal flow rate condition but still cannot eliminate the adverse effects of wall roughness.

The effects of solid shear stress and wall roughness on the simulation of cylindrical hydrocyclone classification

Reasonable numerical conditions are critical for the precision and accuracy of the numerical consequences of hydrocyclones. In this paper, the effects of resistance coefficient, wall roughness, and friction packing limit on particle velocity and distribution as well as the separation behaviors were studied in a cylindrical hydrocyclone using the two-fluid model. The results show that the friction packing limit and wall roughness can significantly influence the numerical results, whereas the effect of the resistance coefficient is negligible. As the friction packing limit increases from 0.4 to 0.55, the cut size gradually decreases with improving separation sharpness, and it reaches the minimum when the friction parking limit is 0.5. The wall roughness monotonously increases the cut size and reduces the separation sharpness. Simulation results were also compared with experimental data, and a reasonable prediction can be obtained with resistance coefficient, friction packing limit, and wall roughness height of 0.95, 0.5, and 50 μm, respectively.