A rescaling algorithm for multi-relaxation-time lattice Boltzmann method towards turbulent flows with complex configurations

Understanding and modeling flows over porous layers are of great industrial significance.To accurately solve the turbulent multi-scale flows on complex configurations,a rescaling algorithm designed for turbulent flows with the Chapman-Enskog analysis is proposed.The mesh layout and the detailed rescaling procedure are also introduced.Direct numerical simulations(DNSs)for a turbulent channel flow and a porous walled turbulent channel flow are performed with the three-dimensional nineteen-velocity(D3Q19)multiple-relaxation-time(MRT)lattice Boltzmann method(LBM)to validate the accuracy,adaptability,and computational performance of the present rescaling algorithm.The results,which are consistent with the previous DNS studies based on the finite difference method and the LBM,demonstrate that the present method can maintain the continuity of the macro values across the grid interface and is able to adapt to complex geometries.The reasonable time consumption of the rescaling procedure shows that the present method can accurately calculate various turbulent flows with multi-scale and complex configurations while maintaining high computational efficiency.

Comprehensive review of telbivudine in pregnant women with chronic hepatitis B

AIM:To achieve an evidence-based conclusion regarding the safety and efficacy of telbivudine during pregnancy.METHODS:A pooled analysis of data from a literature search reported 1739 pregnancy outcomes(1673 live births)from 1725 non-overlapping pregnant women treated with telbivudine.The prevalence of live birth defects(3.6/1000)was similar to that of the nonantiviral controls(3.0/1000)and not increased as compared with overall prevalence(14.5 to 60/1000).No target organ toxicity was identified.The prevalence of spontaneous abortion in pregnant women treated with telbivudine(4.2/1000)was not increased compared with the overall prevalence(16/1000).The mother-to-child transmission rate was significantly reduced in pregnant women treated with telbivudine(0.70%)compared to those treated with the non-antiviral controls(11.9%;P<0.0001)or compared to the historical rates of hepatitis B virus(HBV)-infected population without antiviral treatment(10%-15%).RESULTS:Cumulatively 489 pregnancy cases have been reported in the telbivudine pharmacovigilance database(with a cut-off date 31 August 2014),of those,308 had known pregnancy outcomes with 249 cases of live births(239 cases of live birth without congenital anomaly and 10 cases of live birth with congenital anomaly).In the latest antiretroviral pregnancy registry report(1 January 1989 through 31 January 2015)of27 patients exposed to telbivudine during pregnancy(18,6 and 3 during first,second and third trimester,respectively)19 live births were reported and there were no cases of birth defects reported.CONCLUSION:Telbivudine treatment during pregnancy presents a favorable safety profile without increased rates of live birth defects,spontaneous abortion or elective termination,or fetal/neonatal toxicity.Exposure to telbivudine in the first,second and third trimester of pregnancy has been shown to significantly reduce the risk of HBV transmission from mother to child on the basis of standard immune prophylaxis procedure.

Drag reduction of turbulent channel flows over an anisotropic porous wall with reduced spanwise permeability

The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.

Characteristics of turbulence transport for momentum and heat in particle-laden turbulent vertical channel flows

The dynamic and thermal performance of particle-laden turbulent flow is investigated via direction numerical simulation combined with the Lagrangian point-particle tracking under the condition of two-way coupling, with a focus on the contributions of particle feedback effect to momentum and heat transfer of turbulence. We take into account the effects of particles on flow drag and Nusselt number and explore the possibility of drag reduction in conjunction with heat transfer enhancement in particle-laden turbulent flows. The effects of particles on momentum and heat transfer are analyzed, and the possibility of drag reduction in conjunction with heat transfer enhancement for the prototypical case of particle-laden turbulent channel flows is addressed. We present results of turbulence modification and heat transfer in turbulent particle-laden channel flow, which shows the heat transfer reduction when large inertial particles with low specific heat capacity are added to the flow. However, we also found an enhancement of the heat transfer and a small reduction of the flow drag when particles with high specific heat capacity are involved. The present results show that particles, which are active agents, interact not only with the velocity field, but also the temperature field and can cause a dissimilarity in momentum and heat transport. This demonstrates that the possibility to increase heat transfer and suppress friction drag can be achieved with addition of particles with different thermal properties.

Transport of dissolved oxygen at the sediment-water interface in the spanwise oscillating flow

The distribution and concentration of dissolved oxygen(DO)play important roles in aerobic heterotroph activities and some slow chemical reactions,and can affect the water quality,biological communities,and ecosystem functions of rivers and lakes.In this work,the transport of high Schmidt number DO at the sediment-water interface of spanwise oscillating flow is investigated.The volume-averaged Navier-Stokes(VANS)equations and Monod equation are used to describe the flow in the sediment layer and the sediment oxygen demand of microorganisms.The phase-averaged velocities and concentrations of different amplitudes and periods are studied.The dependence of DO transfer on the amplitude and period is analyzed by means of phase-average statistical quantities.It is shown that the concentration in the sediment layer is positively correlated with the turbulence intensity,and the DO concentration and penetration depth in the sediment layer increases when the period and amplitude of the oscillating flow increase.Moreover,in the presence of oscillating flow,a specific scaling relationship exists between the Sherwood number/oxygen consumption of aerobic heterotrophs and the Reynolds number.

Impact of spray droplets on momentum and heat transport in a turbulent marine atmospheric boundary layer

Droplet-laden turbulent airflow(i.e. the mixture of dry air and water vapor) in the marine atmospheric boundary layer is described by an open channel flow configuration in direct numerical simulation(DNS). The dispersed phase, the spray droplets are tracked in a Lagrangian framework, and their impact on the carrier airflow is modeled with the two-way coupling between the two phases. A wide-range droplet size is typically found near the air–sea interface according to the sea spray concentration function(SSCF). The interactions of the droplets with turbulent airflow including mass, momentum, and energy exchange are investigated here. We found a balancing mechanism exists in the droplet effects on the turbulent drag coefficient, since spray droplets lead to a decreased vertical turbulent momentum transport, but also lead to an increased droplet contribution to total drag coefficient. For the heat transfer, as droplet mass loading increasing, the total Nusselt number decreases due to the depression of turbulent heat flux and enhanced negative droplet convective flux.

Investigation of turbulence and skin friction modification in particle-laden channel flow using lattice Boltzmann method

Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation(DNS) with a lattice Boltzmann(LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow,the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction.

Multiphase flow model developed for simulating gas hydrate transport in horizontal pipe

The hydrate formation or dissociation in deep subsea flow lines is a challenging problem in oil and gas transport systems. The study of multiphase flows is complex while necessary due to the phase changes （i.e., liquid, solid, and gas） that occur with increasing the temperature and decreasing the pressure. A one-dimensional multiphase flow model coupled with a transient hydrate kinetic model is developed to study the characteristics of the multiphase flows for the hydrates formed by the phase changes in the pipes. The multiphase flow model is derived from a multi-fluid model, while has been widely used in modelling multiphase flows. The heat convection between the fluid and the ambient through the pipe wall is considered in the energy balance equation. The developed multiphase flow model is used to simulate the procedure of the hydrate transport. The results show that the formation of the hydrates can cause hold-up oscillations of water and gas.

Effect of inertial particles with different specific heat capacities on heat transfer in particle-laden turbulent flow

The effect of inertial particles with different specific heat on heat transfer in particle-laden turbulent channel flows is studied using the direct numerical simulation（DNS） and the Lagrangian particle tracking method. The simulation uses a two-way coupling model to consider the momentum and thermal interactions between the particles and turbulence. The study shows that the temperature fields display differences between the particle-laden flow with different specific heat particles and the particle-free flow,indicating that the particle specific heat is an important factor that affects the heat transfer process in a particle-laden flow. It is found that the heat transfer capacity of the particle-laden flow gradually increases with the increase of the particle specific heat. This is due to the positive contribution of the particle increase to the heat transfer. In addition,the Nusselt number of a particle-laden flow is compared with that of a particle-free flow.It is found that particles with a large specific heat strengthen heat transfer of turbulent flow, while those with small specific heat weaken heat transfer of turbulent flow.

Effect of particles on turbulent thermal field of channel flow with different Prandtl numbers

The direct numerical simulation （DNS） of heat transfer in a fully developed non-isothermal particle-laden turbulent channel flow is performed. The focus of this paper is on the modulation of the particles on turbulent thermal statistics in the particle-laden flow with three Prandtl numbers （Pτ = 0.71, 1.5, and 3.0） and a shear Reynolds number （Reτ = 180）. Some typical thermal statistics, including normalized mean temperature and their fluctuations, turbulent heat fluxes, Nusselt number and so on, are analyzed. The results show that the particles have less effects on turbulent thermal fields with the increase of Prandtl number. Two reasons can explain this. First, the correlation between fluid thermal field and velocity field decreases as the Prandtl number increases, and the modulation of turbulent velocity field induced by the particles has less influence on the turbulent thermal field. Second, the heat exchange between turbulence and particles decreases for the particle-laden flow with the larger Prandtl number, and the thermal feedback of the particles to turbulence becomes weak.

Numerical investigation on fluid forces of piggyback circular cylinders in tandem arrangement at low Reynolds numbers

Numerical simulations of flows past the piggyback circular cylinders in tandem arrangement are performed by solving the variational multiscale formulation of the incompressible Navier-Stokes equations using in-house finite element method(FEM)codes.The effects of the gap-spacing-to-diameter(G/D)and the two diameter ratio(d/D)on the flow characteristics and the reductions of the root-mean-square(RMS)drag and lift coefficients are considered for Reynolds numbers(Res)are 100 and 200.The validation shows the fluid force coefficients obtained by the in-house FEM codes are in good agreement with the results in the existing literatures.The obtained results show that,with a proper placement of the smaller cylinder(d/D=0.2)behind the larger cylinder,the RMS drag and lift coefficients largely decrease compared to those of the single circular cylinder.When d/D=0.2,the largest reductions of the RMS lift coefficient of the larger cylinder and the RMS total lift coefficient appear at G/D=1.2 as Re=100 and at G/D=1.0 as Re=200.It is observed that the proper placement of the smaller cylinder causes the surrounding vorticity to take opposite sign with the vorticity in the outer region so as to suppress and postpone the vortex shedding in the wake,and that the different positions of the vortex shedding at two Res cause that the largest reductions of the RMS lift coefficient of the larger cylinder and the RMS total lift coefficient appear at different G/D as Re is different.When d/D varies,the variation of the RMS total lift coefficient behaves differently at two Res.It decreases with J/D increasing at Re=100,while it no longer monotonously varies with J/D,but reaches a minimum in the considered range of d/D at Re=200.Moreover,the larger d/D results in stronger suppression and postponement of the vortex shedding in the wake.