Effect Of Wall Friction Research Articles

Mass discharge rate of granular flow in eccentric silos with variable side wall friction

There are many studies on the effect of side wall friction on mass discharge rate (MDR), but the physical mechanism is lacking. In this paper, by changing side wall friction of eccentric silos, the relationship between macroscopic MDR and microscopic frictional coefficient was established, and the regulation of MDR up to a range of 40 % was realized. Furthermore, it is revealed that the variation of MDR is caused by the change in geometric structure of free-fall arch (FFA). In addition, the velocity of particles on FFA is the same under different MDR. The reason is that under high frictional coefficient, the gravitational potential energy of particles is more dissipated in the rotational motion caused by resistance. This work explains for the first time the mechanism of the frictional coefficient on MDR, and provides a data reference for improving the theoretical model of MDR.

An Experimental Study of the Frictional Behavior at the Interface Soil-Structure with Regards to Wall Friction Effect and Clogging Related to Surface Roughness Form

Abstract The interactions between soil-structure are largely governed by the shear strength at the soil-structure interface, particularly in situations involving large displacements, known as shear residual strength. This latter can be influenced by a range of factors, including normal stress, physical properties, surface roughness, wall friction. The main objective of this study is to investigate clogging related to surface roughness form, in addition to wall friction arises from consolidation. In order to investigate the impact of wall friction, experiments involving ring shear tests were carried out on kaolin. Four different consolidation durations were employed to induce varying settlements of the upper platen, while three distinct testing procedures, including single stage, multistage, and preshearing, were utilized. Additionally, different levels of surface roughness were incorporated to investigate the effect of clogging. The test results obtained from various testing procedures indicate that employing a single stage procedure with zero consolidation time is the most suitable approach for mitigating the influence of wall friction. For clogging effect, is dependent on the materials used, namely, the kaolin and the mixture, as well as, the interfaces involved.

Three-dimensional shock–flame interactions: effect of wall friction

Multi-dimensional numerical simulations were performed to study the interactions between shock wave and premixed flame. The three-dimensional (3D) fully-compressible, reactive Navier–Stokes equations were solved using a high-order numerical method on a dynamically adapting mesh. The effect of wall friction on the shock–flame interaction was examined by varying wall boundary condition on sidewall. The simulations agree with previous experiment in terms of flame perturbation and flame evolution. The results show two effects of wall friction on flame–shock interaction: (1) flame stretching and (2) damping of local flame perturbation very close to the no-slip wall. The stretch effect leads to non-uniform development of the perturbated flame and consequently a significantly higher growth rate in both global flame perturbation and averaged pressure in the no-slip case compared to the free-slip case. By contrast, the damping effect locally moderates the flame perturbation in close proximity to the no-slip wall because less vorticity is deposited on this part of flame during shock–flame interaction. Quantitative analysis of vortex dynamics suggests that vorticity generation that is produced by baroclinic torque and enhanced by the dilation term during shock–flame interaction causes a growth of flame perturbation via Richtmyer–Meshkov instability. Nevertheless, the vorticity generation is greatly weakened in close proximity to the no-slip wall by the viscous torque and viscous dissipation terms due to friction.

Research on Scavenging Flow Dynamics of Marine Two-Stroke Engines With a CFD-Derived Quasi-Dimensional Model

The complex in-cylinder gas state and flow significantly affect fuel-air mixture, combustion efficiency and emissions. However, the scavenging CFD model of large bore marine engines in digital twin system requires substantial computational resources. Therefore, a fast-run phenomenological model of the scavenging process is built in this study. The model simulates the thermal states and flow dynamics of inlet and exhaust gas based on the energy and momentum conservation principles, considering the ideal in-cylinder swirl velocity profile with effects of air mass loss, wall friction, and swirl shear. The model’s accuracy is confirmed by comparing it with CFD simulations on different engines, showing an average relative error less than 3.5%. It also analyzes the impacts of intake pressure. The model provides accurate boundary conditions for subsequent fuel spray, combustion, and emission simulations and can be combined with these models in the future, thereby applied to engine design, diagnostics, and control.

Capturing the dynamics of a two orifice silo with the [formula omitted] model and extensions

Granular material in a silo with two openings can display a ‘flow rate dip’, where a non-monotonic relationship between flow rate and orifice separation occurs. In this paper we study continuum modelling of the silo with two openings. We find that the μ(I) rheology can capture the flow rate dip if physically relevant friction parameters are used. We also extend the model by accounting for wall friction, dilatancy, and non-local effects. We find that accounting for the wall friction using a Hele-Shaw model better replicates the qualitative characteristics of the flow rate dip seen in experimental data, while dilatancy and non-local effects have very little effect on the qualitative characteristics of the mass flow rate dip. However, we find that all three of these factors have a significant impact on the mass flow rate, indicating that a continuum model which accurately predicts flow rate will need to account for these effects.

Effect of wall friction on oscillation of velocity at the head of the gravity current

Velocity oscillations at the head of the gravity current were investigated in experiments and numerical simulations of a locked-exchange flow. A comparison of the experimental and numerical simulations showed that the depth and volume of the released fluid affected the oscillations in the velocity of the gravity current. At the initial stage, the head moved forward at a constant velocity, and velocity oscillations occurred. The head maximum thickness increased at the same time as the head, which did not have a round, and accumulated buoyant fluid due to the buoyancy effect intrusion force. The period of accumulation and release of the buoyant fluid was almost the same as that observed for the head movement velocity; the head movement velocity was faster when the buoyant fluid accumulated and slower when it was released. At the viscous stage, the forward velocity decreased proportionally to the power of 1/2 of time, since the head was not disturbed from behind. As the mass concentration at the head decreased, the gravity current was slowed by the viscous stage in its effect. At the viscous stage, the mass concentration at the head was no longer present, and the velocity oscillations also decreased.

Effect of vibration parameters and wall friction on the mixing characteristics of binary particles in a vertical vibrating container subject to cohesive forces

The effect of particle cohesion, vibration parameters, and wall-particle friction conditions on the mixing process of binary spherical particles in a vertical vibrating container were studied by the discrete element method. The phenomena in the simulation can be explained by three mechanisms: void filling, convection, and self-organization. The results show that the mixing rate decreases with the increase of particle cohesion. Increasing the amplitude can increase the mixing rate, but the effect of improvement is no longer obvious when the amplitude exceeds a threshold. There is an optimal vibration frequency, and a higher or lower frequency will lead to the reduction of the mixing rate. The macro convection caused by the strong wall friction can promote the flow and disaggregation of particles, but also cause the segregation that large particles gather in the center of the convection unit and thus reduce the mixing index. The degree of segregation is approximately linearly related to the shear force difference from the sidewall to the center of the container. Cohesion changes the segregation by influencing this difference. Under proper conditions, the segregation increases first and then decreases with the increase of cohesion. The expected mixing degree can be obtained by selecting appropriate wall friction conditions and particle cohesion. • Mixing of binary particles depends on vibration parameters, wall friction and particle cohesion. • The strong wall friction effect leads to the convective segregation of particles. • The shear force difference has a linear negative correlation with the segregation. • There is an upper limit to the effect of promoting mixing by increasing amplitude. • There is an optimal value by changing frequency to promote mixing.

Influence of wall friction on flow parameters in natural gas transmission pipeline

A mathematical model and computational algorithm are derived for the prediction of natural gas pipeline flow. Non-isothermal and compressible steady-state flow is considered. Heat transfer between gas flow and surroundings is taken into account together with the heat generation due to the gas friction on the inner pipeline wall. The computational algorithm is based on the marching procedure with defined initial conditions. The predicted thermal effect of the wall friction is validated by the simulation of a case that is available in the open literature. The influence of heat generation by gas wall friction in the long transmission pipeline on gas pressure and temperature is evaluated. Differences between results obtained with and without the heat generation due to gas wall friction are analysed. The heat generation due to gas friction on the pipeline inner wall has an influence on the gas temperature change along the pipeline, while its influence on the pressure drop is negligible. These detailed results are novel since most of the previously published results on non-isothermal gas flow did not take into account the thermal effect of the gas wall friction or the influence of this effect was not evaluated. The presented results are a support to the gas pipeline design methods and operational analyses.

Measurement of boundary force and wall friction coefficient during the development of a granular discharge flow from a symmetric silo

ABSTRACT Self-developed sensing modules were implemented in a symmetric silo of flat base to study the spatial-temporal evolution of the normal and tangential boundary force from the onset of a granular discharge. The static force profiles can be reproduced by Janssen’s model but peculiar variation into non-Janssen force depth profiles was detected after the onset of discharge. Interestingly, the two force components evolved in a consistent manner at steady discharge to yield a narrow range of the effective wall friction coefficient μ w. As for the transient phase before the steady discharge, different transient evolution trends were discovered for μw (t) to a value much greater than the value at steady discharge and the type of μw (t) evolution is closely tied with the initial static force state.

Quantification of mechanisms governing the angle of repose, angle of tilting, and Hausner ratio to estimate the flowability of particulate materials

The angle of repose, angle of tilting, and Hausner ratio are frequently used to estimate the flowabilities of particulate materials. In this study, these parameters were measured for 65 samples. A visualization study showed that there are three mechanisms affecting the angle of repose. Only one is directly related to the flowability. Crucially, to eliminate the effect of wall friction, the measurement of the angle of repose should be conducted on a high-friction surface (such as one covered in glue) or within a ring. Further, although all three parameters can be described as a function of the Archimedes number, the performance of each material was different; thus, the results cannot be generalized. Only the Hausner ratio could be defined reasonably by a single function. A new flowability classification was developed for the three parameters, and the best metric to define flowability was found to be the Archimedes number.

How universal is the vibration-velocity controlled granular convection?

Recently, a number of articles have reported that granular convection induced by continuous vibration is controlled by vibration velocity, in contrast with some previous studies. We have reported such an example for the Brazil nut effect when the vibration is given discontinuously, using a one-layer granular bed in a cell with down-facing side walls. Here, we report the effect of vibration phase and wall friction using the same experimental system, to confirm rising motion of an intruder induced by granular convection is again governed by vibration velocity. We compare two different cases of vibration phase for giving intermittent vibration cycles, and found one, in which granular packing is well established before grains start to lose contacts due to vibration, provides distinctly high reproducibility. We further control the side wall friction using a microfabrication technique, and found that significantly high reproducibility is attained in a cell with vertical side walls when a millimeter texture is introduced on the side walls. Our results indicate that the granular convection is universally controlled by vibration velocity. The present study opens a way to conduct highly reproducible experiments on granular dynamics, which is indispensable for deep physical understanding of granular flow and segregation.

Research on Apparent Friction Factor in Perforated and Slotted Horizontal Filters to Dewater Opencast Mines Using Numerical Analysis

In the mining industry, the horizontal wells have been using for dewatering purpose for several years. Due to unique characteristics, those horizontal filters are recommended to balance both the mechanical strength and hydraulic performance. Pressure drop along the horizontal well is one of the essential factors that affect the performance of a wellbore, the dewatering capacity as well. The pressure drop is caused due to four separate factors: wall friction, perforation roughness, inflow acceleration and mixing effects. In this publication, numerical analysis was implemented with different types of perforated and slotted pipes to investigate the effects of two later factors: inflow acceleration and the mixing effects. The simulated data using ANSYS 14.5 was analyzed using universal velocity distribution law in combination with the roughness function and influx flow. The results revealed that the apparent friction factor is a linear function of roughness friction factor and the total inflow rate ratio.

Motion of finite-size spheres released in a turbulent boundary layer

Individual magnetic wax spheres with specific gravities of 1.006, 1.054 and 1.152 were released from rest on a smooth wall in water at friction Reynolds numbers, Reτ=680 and 1320 (sphere diameters d+=58 and 122 viscous units, respectively). Three-dimensional tracking was conducted to understand the effects of turbulence and wall friction on sphere motions. Spheres subjected to sufficient mean shear initially lifted off of the wall before descending back towards it. These lifting spheres translated with the fluid above the wall, undergoing saltation or resuspension, with minimal rotations about any axis. By contrast, spheres that did not lift off upon release mainly slid along the wall. These denser spheres lagged the fluid more significantly due to greater wall friction. As they slid downstream, they began to roll forward after which small repeated lift-off events occurred. These spheres also rotated about both the streamwise and wall-normal axes. In all cases, the sphere trajectories were limited to the buffer and logarithmic regions, and all wall collisions were completely inelastic. In the plane parallel to the wall, the spheres migrated in the spanwise direction about 12% of the streamwise distance traveled suggesting that spanwise forces are important. Variations in sphere kinematics in individual runs were likely induced by high and low momentum zones in the boundary layer, vortex shedding in the sphere wakes, and wall friction. The repeated lift-offs of the forward rolling denser spheres were attributed to a Magnus lift.

Developing A Rheological Relation for Transient Dense Granular Flows Via Discrete Element Simulation in A Rotating Drum

ABSTRACTThis work examines the μ(I) relation that describes the effective friction coefficient μ of a dense granular flow as a function of flow inertial number I, at the center of a rotating drum from its flow onset to steady state using DEM. We want to see how the internal friction coefficient of an accelerating flow may be predicted so that the associated tangential stress can be estimated with the proper knowledge of the normal stress. Under the three investigated drum speeds (3, 4.5 and 6 rpm), the bulk normal stress, σn(y), is found to be a consistent linear depth profile throughout the flow development with a slope degraded from the hydrostatic value, Ph(y), due to lateral wall friction. With the discovery of a non-constant depth-decaying effective wall friction coefficient, we derive analytically a wall-degradation factor K(h) to give σn(y)= K(h)Ph(y). The depth profile of tangential stress, however, varies in time from a concave shape upon acceleration, τa(y), to a more linear trend at the steady state, τss(y). Hence, the μa-Ia profile (with μa=τ/σn) upon flow acceleration offsets from the steady μss(Iss) relation. A pseudo-steady acceleration modification number, ΔI, is proposed to shift the inertial number in the acceleration phase to I* = Ia+ΔI so that the μa-I* data converge to μss(Iss). This finding shall allow us to predict a transient tangential stress by τa(y) = μss(I*)K(y)Ph(y) using the well-accepted knowledge of steady flow rheology, hydrostatic pressure, and the currently developed wall-degradation factor.

Discrete-element modelling of the trapdoor arching effect in sand and rubberised sand

The onset and evolution of soil arching in a trapdoor apparatus was investigated using discrete-element modelling (DEM). The effects of wall friction, particle size and material type (sand and rubber–sand) on the micro-behaviour of the granular material during arching was monitored. It was observed that when the particles are large with respect to the trapdoor width, they form a stable arch over the trapdoor element. An ‘intermediate zone’ within the granular materials is then introduced. This is the zone where inter-particle contact forces increase in the early stage of active arching and then decrease as the trapdoor displacement continues. The DEM simulation results showed that the granular particles rearranged in reaction to arching, causing uneven densities throughout the backfills. The distributions of two-dimensional stresses in the cross-section of the modelled sand and rubber–sand backfills were obtained using the kriging method and compared.

Comprehensive experimental study of proppant transport in an inclined fracture

The application of slickwater fracturing technology along with horizontal drilling has revolutionized oil and gas energy industry. Solid particles, i.e. proppant, are mixed with slickwater and injected into the formation to keep fractures opened after fracturing. Many researchers have studied the proppant transport and distribution in vertical fractures. However, in unconventional reservoirs with multiple bedding layers and preexisting natural fractures, hydraulically induced fractures with inclined angles are often generated. Thus, proppant transport in various angles of fracture geometry is necessary to study further. In this study, multiple case studies including fracture inclination angle, proppant size, and injection point location were performed to experimentally quantify proppant delivery mechanism in an inclined fracture. Laboratory parameters were scaled down from field conditions to successfully capture proppant transport mechanism without any significant boundary effect. Sand slurry was pumped into a fracture channel and proppant placement was recorded at various conditions. The effect of individual parameter was scientifically analyzed at predetermined condition to quantify the proppant transport trend within an inclined fracture.Friction due to wall roughness and different effective vertical gravitational forces completely change proppant transport behaviors in inclined plane. Frictional force acts stronger, as the fracture plane angle becomes closer to the horizontal plane. Strong wall friction effect creates a thin suspension area rather than settling and forming compact dune. Some proppants are invested for the suspension zone instead of dune area. This leads to greater proppant coverage area, yet smaller dune area. Suspended zone is the most differentiating feature compared to vertical fractures. There is almost no suspended zone observed in the vertical fractures, yet suspended zone is well observed and prevalent in inclined fractures. Fine mesh sized proppants travel farther due to low settling velocity. It therefore forms a well distributed sand pack. Fine particles are heavily influenced by the flow, therefore it follows the streamline of the incoming flow. They also experience strong wall friction. Thus suspension zone and total coverage area increase with smaller proppant size. In addition, the difference between having a single inlet point at various depth compared to having several opened inlet points provides an insight while operating in field environment. The inlet location determines the shape of the sandpack, especially near the wellbore. Lower injection point pushes the sandpack farther, yet this poses a potential wellbore closure or collapse.

Sub-channel CFD for nuclear fuel bundles

This paper presents a novel Computational Fluid Dynamics (CFD)-based sub-channel framework for nuclear power plants, which combines the advantageous features of modern CFD and traditional 1-D sub-channel codes. The new method is capable of producing CFD-level 3-D results with locally desirable refinement when coupled with embedded resolved models, but because a very coarse mesh is used over most part of the domain, the computing cost is typically significantly smaller than that of a conventional CFD simulation.In this new sub-channel CFD (SubChCFD), a dual-mesh system is used, comprising, (i) a filtering mesh which aligns with the mesh used in typical sub-channel codes, enabling the use of existing engineering correlations to account for integral wall friction and heat transfer effects, and (ii) a computing mesh, which provides a platform for the solution of the governing equations with turbulence modelled using a mixing-length-type model. The method has been implemented in an open-source finite volume CFD code Code_Saturne and validated initially using a 5 × 5 bare bundle case based on the OECD/NEA MATiS-H benchmarking experiment. It has been found that SubChCFD is able of satisfactorily predicting both the velocity and temperature fields. To further investigate the performance for complex flow conditions, SubChCFD was applied to two full 3-D cases. The first is a 5 × 5 rod bundle case with local blockage in one of the sub-channels, creating significant localised cross flows. The second is a two-parallel-assembly channel with different input mass flow rates at the inlet of each assembly, allowing strong inter-assembly mixing. For both cases, SubChCFD has produced results which agree well with experiments and simulations using resolved CFD. It has also been demonstrated that SubChCFD exhibits excellent flexibility in comparison with traditional sub-channel codes and that it has the potential to serve as a substitution to sub-channel codes.

Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition**Project supported by the National Natural Science Foundation of China (Grant Nos. 11732003 and 11521062) and the National Key Research and Development Program of China (Grant No. 2017YFC0804700).

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition (DDT) in micro-scale channel is investigated by high-resolution numerical simulations. The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front. The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT. It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction. The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer. The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat. Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction. The transition to detonation depends non-monotonically on the channel width.

A new uniaxial compression tester: development and application

Powder flow characterization with some of the conventional shear testers can be costly, time consuming and requires a trained operator, therefore the application of a cheaper, simpler, and sometimes faster uniaxial compaction tester (UCT) have often been suggested as an alternative. However, it has been known for many years that the results derived from the two methods are not necessarily the same, due to the lower state of compaction attained in the traditional design of UCT. Additionally, the traditional design of UCT tends to give a wide scatter in the results. To overcome these limitations of the UCT, this work developed an easy-to-use uniaxial tester in order to reduce the difference between flow properties reported by the two techniques. In this regard, flow functions of four powders in the cohesive and very cohesive range were measured with shear testers, conventional uniaxial compaction tester (UCT) and the new uniaxial tester at the University of Greenwich (“Greenwich Uniaxial Tester” or GUT). A method in the style of Janssen approach for correcting wall friction effect on the compaction stress of UCT was applied. Results showed the unconfined yield strength attained from GUT is in line with the results obtained from shear testers while the results from UCT are well below the shear tester results suggesting the advantage of the new GUT.

Confined uniaxial compression of granular stainless steel 316

The quasi-static compaction response of granular stainless steel 316 was studied under triaxial loading conditions using a confined crucible experiment apparatus, specifically looking at effects of the particle size (127 ± 34μm, 309 ± 88μm, 487 ± 98μm) on mechanical behavior. The material response was captured using a load washer and strain gauges to relate: the porosity effects as a function of hydro-static pressure, particle size dependency on wall friction effects, and particle size-dependent failure mechanism. Our observations revealed that the path of crushing out porosity varied based on the particle size and the frictional effects. Scanning Electron Microscope images were taken to observe the surface features of the compacted material and comment on failure mechanisms. Using these techniques it was observed that the smaller particles exhibited significant plastic deformation and flow, while the larger fragments appeared to show micro-cracking which lead to inelastic deformation and particle fracture. Altogether, these results are important because granular behavior is critical in powder flow applications such as additive manufacturing.