SIMULATION OF SUDDEN-EXPANSION AND SWIRLING GAS-PARTICLE FLOWS USING A TWO-FLUID PARTICLE-WALL COLLISION MODEL WITH CONSIDERATION OF THE WALL ROUGHNESS

A two-fluid particle-wall collision model with consideration of wall roughness is pro- posed.It takes into account the effects of the friction,restitution and in particular the wall roughness, and hence the redistribution of Reynolds stress in different directions,the absorption of turbulent en- ergy from the mean motion and the attenuation of particle motion by the wall.The proposed model is used to simulate sudden-expansion and swirling gas-particle flows and is validated by comparing with experimental results.The results show that the proposed model gives better results than those obtained by the presently used zero-gradient condition.Hence,it is suggested that the proposed model should be used as the wall boundary condition for the particle phase in place of the presently used boundary condition.

Boundary-layer receptivity under interaction of free-stream turbulence and localized wall roughness

The boundary-layer receptivity under the interaction of free-stream turbu- lence （FST） and localized wall roughness is studied by the direct numerical simulation （DNS） and the fast Fourier transform. The results show that the Tollmien-Schlichting （T-S） wave packets superposed by a group of stability, neutral, and instability T-S waves are generated in the boundary layer. The propagation speeds of the T-S wave packets are calculated. The relation among the boundary-layer receptivity response, the amplitude of the FST, the roughness height, and the roughness width is determined. The results agree well with Dietz＇s experiments. The effect of the roughness geometries on the receptivity is also studied.

EFFECTS OF WALL ROUGHNESS ON COHERENT STRUCTURE IN TURBULENT SHEAR FLOWS

There are many examples that fluid flows on rough wall, such as channel flow in nature, pipe flow, etc. In order to know the flow structure of real fluids, it is important to study the effects of wall roughness on coherent structure in turbulent shear flows. The experiments were carried out in a square glass channel, which is 600cm long, with the cross section of 30×25cm^2. The flow velocity was varied from 2 to 40 cm/s. Uniform sands whose diameters were 0.0012cm, 0.2gcm, 0.385cm, 0.594cm and 0.896cm respectively were glued to the floor of the channel. The rough Reynolds number Re_Δ= U_*Δ/ν=0.04～73, where U_*is the shear velocity, Δ is the diame- ter of uniform sand, v is the kinematic viscosity coefficient. Hydrogen bubble technique for flow visualization and HWL-II hot-film anemometer for velocity mea- surement were used in the experiments.

Wall-resolved large-eddy simulation of turbulent channel flows with rough walls

Turbulent flows over rough surfaces widely exist in nature and industry.Investigating its mechanism is of theoretical and practical significance.In this work we simulate the turbulent channel flow with rough walls using large-eddy simulation with rough elements resolved using the curvilinear immersed boundary method and compare the results obtained in this work with those in the paper by Yuan and Piomelli(J.Fluid Mech.,vol.760,pp.R1,2014),where the volume of fluid method was employed for modeling rough elements.The mean streamwise velocity profiles predicted by the two methods agree well with each other.Differences in Reynolds stresses and dispersive stresses are observed,which are attributed to the different approaches in dealing with the complex geometry of the rough surface.

Local receptivity in non-parallel boundary layer

The research on boundary-layer receptivity is the key issue for the laminarturbulent transition prediction in fluid mechanics. Many of the previous studies for local receptivity are on the basis of the parallel flow assumption which cannot accurately reflect the real physics. To overcome this disadvantage, local receptivity in the non-parallel boundary layer is studied in this paper by the direct numerical simulation （DNS）. The difference between the non-parallel and parallel boundary layers on local receptivity is investigated. In addition, the effects of the disturbance frequency, the roughness location, and the multiple roughness elements on receptivity are also determined. Besides, the relations of receptivity with the amplitude of free-stream turbulence （FST）, with the roughness height, and with the roughness length are ascertained as well. The Tollmien- Schlichting （T-S） wave packets are excited in the non-parallel boundary layer under the interaction of the FST and the localized wall roughness. A group of T-S waves are separated by the fast Fourier transform. The obtained results are in accordance with Dietz＇s measurements, Wu＇s theoretical calculations, and the linear stability theory （LST）.

Analysis of Molten Metal Distribution in the Mold of a Horizontal Centrifugal Casting

It is numerically studied the influence of the angular velocity, the molten metal viscosity, and the mold wall roughness on the molten metal distribution in the mold of a horizontal centrifugal casting process. The undesirable raining phenomenon sometimes arises in horizontal centrifugal casting. It occurs when the molten metal rains or falls from the top of the mold to the bottom while the mold is rotating. Using Computational Fluid Dynamics simulations, the conditions for the emergence of the raining phenomenon were explored in this work. For the system considered, angular velocities less than 77 rad/s cause the emergence of the raining phenomenon and accumulation of the molten metal in the lower part of the mold, whereas angular velocities greater than 77 rad/s produce a constant thickness of the molten metal and prevent raining.

Numerical simulation on proppant migration and placement within the rough and complex fractures

Hydraulic fracturing is a key technology for the development of unconventional hydrocarbon resources.The proppant placement morphology determines the fracture conductivity,thus affecting the reservoir stimulation effect.In this paper,the proppant migration and placement within complex fractures was studied by considering the fracture wall roughness through computational fluid mechanics-discrete element method(CFD-DEM)in numerical simulation,which is a key approach to study the proppant migration and placement.The results show that the proppant placement non-uniformity,proppant migration capacity,and proppant volume filled in the far-end and the secondary branched fracture are enhanced within the rough fracture compared with those within smooth fractures.The proppant migration capacity is increased within the fracture at low inclination angles(<60°)and low approach angles(<90°),and the proppant placement area is larger in the inclined fracture than that in the vertical fracture.The rise of injection rate and fracturing fluid viscosity causes more proppants migrate to far-end or secondary fractures,resulting in a non-proppant area within the near-wellbore fracture.An increase by 1.3 times in the injection rate and 3 times in the fracturing fluid viscosity leads to a decrease by 26.6%and 27%,respectively,in the proppant placement area within the near-wellbore fracture.The staged injection with small size proppants followed by large size proppants increases the proppant placement area in the primary fracture by 13%-26%,and that with large size proppants followed by small size proppants increases the proppant placement area by 19%-25%,which is due to that the latter method facilitates filling of the secondary branched fracture.The injection location mainly affects the proppant filling degree within the near-wellbore fractures.Compared with the upper injection,the middle and lower injection is not beneficial to filling of proppants within the near-wellbore fracture.

Large Eddy Simulation of Turbulent Channel Flows over Rough Walls with Stochastic Roughness Height Distributions

This paper studies the 3-D turbulent channel flows over rough walls with stochastic roughness height distributions by using the large eddy simulation and the immersed boundary method. The obtained mean and fluctuating velocity profiles for the smooth and rough channel flows agree well with the available numerical and experimental results. The stochastic surface roughness is found to have a more significant influence than the uniform surface roughness on the turbulent velocity statistics and the coherent structures, with the same average roughness height. With a greater variation in the roughness height, the mean velocity and the streamwise fluctuating velocity is decreased and the spanwise velocity and the wall-normal fluctuating velocity are increased. In addition, one observes larger and more profuse quasi-streamwise vortices, hairpin vortices and elongated structures above the crest plane of the roughness array in cases of highly stochastic rough walls. However, the low-speed streaky structures are broken up locally and the ejection and sweep events are depressed by the stochastic roughness below the average roughness height. The results of this study support Townsend’s wall-similarity hypothesis for both stochastic and uniform rough wall turbulences, demonstrating that in both cases the effects of the surface roughness on the turbulent flow are limited to the rough sub-layer.

Numerical Simulation of Proppant Dynamics in a Rough Inclined Fracture

Although the dynamics of proppant(small ceramic balls used to prevent opened fractures from closing on the release of pressure)have been the subject of several numerical studies over recent years,large-scale inclined fractures exist in unconventional reservoirs for which relevant information is still missing.In the present study,this problem is investigated numerically considering the influence of several relevant factors such as the fracture roughness,inclination,the proppant particle size,the injection rate and the fluid viscosity.The results show that a rough wall enables the proppant to travel farther and cover larger areas.The inclination angle has little effect on the dune but a significant influence on the suspension zone.The area of this zone increases with a decrease in the inclination angle,and its value for an inclination of 15°is 20 times that at 90°.Small particle size,high injection rate,and high fracturing fluid viscosity have a beneficial influence on proppant transport;vice versa they hinder settling phenomena.

Comprehensive Euler/Lagrange modelling including particle erosion for confined gas-solid flows

The present research aims to assess the capability of a comprehensive Euler/Lagrange approach for predicting gas-solid flows and the associated solid particle erosion.The open-source code OpenFOAM®4.1 was used to carry out the numerical simulations,where the standard Lagrangian libraries were substantially extended to account for all necessary models.Particles are tracked considering both translational and rotational motion as well as all relevant forces,such as gravity/buoyancy,drag and transverse lift due to shear and particle rotation.The tracking time step was dynamically adapted ac-cording to the locally relevant time scales,which drastically reduces computational times.Stochastic approaches are adopted to model particle turbulent dispersion,particle collisions with rough walls and particle-particle interactions.Five solid particle erosion models,available in the literature,were considered to estimate pipe bend erosion.Three study cases are provided to validate the adopted nu-merical approach and erosion models extensively.The first case intends to evaluate the ability of the extended CFD code to predict the behaviour of gas-solid flows in pneumatic conveying systems.This goal is achieved by comparing the numerical results with the experimental data obtained by Huber(1997)and Huber and Sommerfeld(1994,1998)in a pneumatic conveying system.Here,the importance of considering inter-particle collisions and surface roughness for predicting particle velocity,mass flux and mean diameter distributions in gas-solid flows is highlighted.The second and third case intend to evaluate the ability of the erosion models in estimating bend erosion in diluted gas-solid flows.The erosion data obtained experimentally by Mazumder et al.(2008)and Solnordal et al.(2015)in very dilut pneumatic conveying systems is used for validating the numerical results,neglecting now inter-particle collisions and two-way coupling.Besides a comprehensive analysis of the different influential properties on erosion,the innovation of the present study is as follows.For the first time also a temporal modifi-cation of the surface roughness due to the erosion was considered in the simulations obtained from previous measurements(Novelletto Ricardo&Sommerfeld,2020).As the surface roughness is increased due to erosion,eventually erosion rate becomes lower.This is the result of diminishing wall collision frequency.Simulations for several degrees of surface roughness showed that larger roughness is coupled with a drastic reduction of erosion.Hence,numerical simulations neglecting wall surface roughness are not realistic.The consideration of a particle size distribution instead of mono-sized computations showed a possible reduction of erosion rate.The detailed analysis of the different single-particle erosion models revealed that the model proposed by Oka et al.(2005)and Oka and Yoshida(2005)yields the best agreement with the measurements,however particle and wall properties are needed.

Extension of the KDO turbulence/transition model to account for roughness

Wall roughness significantly influences both laminar-turbulent transition process and fully developed turbulence.A wall roughness extension for the KDO turbulence/transition model is developed.The roughness effect is introduced via the modification of the k andνt boundary conditions.The wall is considered to be lifted to a higher position.The difference between the original position and the higher position,named as equivalent roughness height,is linked to the actual roughness height.The ratio between the two heights is determined by reasoning.With such a roughness extension,the predictions of the KDO RANS model agree well with the measurements of turbulent boundary layer with a sand grain surface,while the KDO transition model yields accurate cross-flow transition predictions of flow past a 6:1 spheroid.