Two-way Coupling Effects Research Articles

The effects of cloud–aerosol interaction complexity on simulations of presummer rainfall over southern China

Abstract. Convection-permitting simulations are used to understand the effects of cloud–aerosol interactions in a case of heavy rainfall over southern China. The simulations are evaluated using radar observations from the Southern China Monsoon Rainfall Experiment (SCMREX) and remotely sensed estimates of precipitation, clouds and radiation. We focus on the effects of complexity in cloud–aerosol interactions, especially the depletion and transport of aerosol material by clouds. In particular, simulations with aerosol concentrations held constant are compared with a fully cloud–aerosol-interacting system to investigate the effects of two-way coupling between aerosols and clouds on a line of organised deep convection. It is shown that the cloud processing of aerosols can change the vertical structure of the storm by using up aerosols within the core of line, thereby maintaining a relatively clean environment which propagates with the heaviest rainfall. This induces changes in the statistics of surface rainfall, with a cleaner environment being associated with less-intense but more-frequent rainfall. These effects are shown to be related to a shortening of the timescale for converting cloud droplets to rain as the aerosol number concentration is decreased. The simulations are compared to satellite-derived estimates of surface rainfall, a condensed-water path and the outgoing flux of short-wave radiation. Simulations for fewer aerosol particles outperform the more polluted simulations for surface rainfall but give poorer representations of top-of-atmosphere (TOA) radiation.

A new collision model for ellipsoidal particles in shear flow

All natural and a growing number of manufactured solid particles are non-spherical. Interesting fluid–particle dynamics applications include the transport of granular material, piling of seeds or grains, inhalation of toxic aerosols, use of nanofluids for enhanced cooling or improved lubrication, and optimal drug-targeting of tumors. A popular approach for computer simulations of such scenarios is the multi-sphere (MS) method, where any non-spherical particle is represented by an assemblage of spheres. However, the MS approach may lead to multiple sphere-to-sphere contact points during collision, and subsequently to erroneous particle transport and deposition. In cases where non-spherical particles can be approximated as ellipsoids with arbitrary aspect ratios, a new theory for particle transport, collision and wall interaction is presented which is more accurate computationally and more efficient than the MS method. In general, with the new ellipsoidal particle interaction (EPI) model, contact points and planes of ellipsoids, rather than spheres, are obtained based on a geometric potential algorithm. Then, interaction forces and torques of the colliding particles are determined via inscribed ‘pseudo-spheres’, employing the soft-particle approach. The off-center forces and moments are then transferred to the mass center of the ellipsoids to solve the appropriate translatory and angular equations of motion. Considering ellipses to illustrate the workings and predictive power of the new collision model, turbulent fluid–particle flow with the EPI model in a 2-D channel is simulated and compared with 3-D numerical benchmark results which relied on the MS method. The 2-D concentrations of micron particles with different aspect ratios matched closely with the 3-D cases. However, interesting differences occurred when comparing the particle-velocity profiles for which the 2-D EPI model generated somewhat larger particle velocities due to out-of-plane collisions, slightly higher particle–wall interactions, and two-way coupling effects.

Electroelastic modeling of thin-laminated composite plates with surface-bonded piezo-patches using Rayleigh–Ritz method

Laminated composite panels are extensively used in various engineering applications. Piezoelectric transducers can be integrated into such composite structures for a variety of vibration control and energy harvesting applications. Analyzing the structural dynamics of such electromechanical systems requires precise modeling tools which properly consider the coupling between the piezoelectric elements and the laminates. Although previous analytical models in the literature cover vibration analysis of laminated composite plates with fully covered piezoelectric layers, they do not provide a formulation for modeling the piezoelectric patches that partially cover the plate surface. In this study, a methodology for vibration analysis of laminated composite plates with surface-bonded piezo-patches is developed. Rayleigh–Ritz method is used for solving the modal analysis and obtaining the frequency response functions. The developed model includes mass and stiffness contribution of the piezo-patches as well as the two-way electromechanical coupling effect. Moreover, an accelerated method is developed for reducing the computation time of the modal analysis solution. For validations, system-level finite element simulations are performed in ANSYS software. The results show that the developed analytical model can be utilized for accurate and efficient analysis and design of laminated composite plates with surface-bonded piezo-patches.

Settling velocity and preferential concentration of heavy particles under two-way coupling effects in homogeneous turbulence

We study settling of inertial particles in homogeneous turbulence. Under two-way coupling effects direct numerical simulations show that the local collective backward-force exerted by the particles on the fluid make the particles and fluid fall together.

An atmosphere–wave regional coupled model: improving predictions of wave heights and surface winds in the southern North Sea

Abstract. The coupling of models is a commonly used approach when addressing the complex interactions between different components of earth systems. We demonstrate that this approach can result in a reduction of errors in wave forecasting, especially in dynamically complicated coastal ocean areas, such as the southern part of the North Sea – the German Bight. Here, we study the effects of coupling of an atmospheric model (COSMO) and a wind wave model (WAM), which is enabled by implementing wave-induced drag in the atmospheric model. The numerical simulations use a regional North Sea coupled wave–atmosphere model as well as a nested-grid high-resolution German Bight wave model. Using one atmospheric and two wind wave models simultaneously allows for study of the individual and combined effects of two-way coupling and grid resolution. This approach proved to be particularly important under severe storm conditions as the German Bight is a very shallow and dynamically complex coastal area exposed to storm floods. The two-way coupling leads to a reduction of both surface wind speeds and simulated wave heights. In this study, the sensitivity of atmospheric parameters, such as wind speed and atmospheric pressure, to the wave-induced drag, in particular under storm conditions, and the impact of two-way coupling on the wave model performance, is quantified. Comparisons between data from in situ and satellite altimeter observations indicate that two-way coupling improves the simulation of wind and wave parameters of the model and justify its implementation for both operational and climate simulations.

Numerical simulation of cryogen spray cooling by a three-dimensional hybrid vortex method

Cryogen spray cooling (CSC) can protect the epidermis from unwanted thermal damage during laser dermatological procedures. Numerical simulation of CSC can provide significant guidance for clinical implementation, but relevant research is sparse. Thus, a 3D two-way coupling hybrid vortex method is developed to simulate CSC. The two-way coupling and compressibility effects are implemented by introducing a vorticity source term into the grid. The validity of this algorithm has been confirmed in a previous simulation of the evaporating acetone spray. Comparisons between the simulation results and the R134a spray cooling experimental results demonstrate the accuracy of the present method in analyzing the complex atomization and evaporation. The simulated cooling capacity achieves the maximum experimental value at a spray distance of 26mm, which is close to the optimal spray distance of 30mm in clinical practice. Particulate count reveals that the central area with a radius of 2mm in R134a spray accounts for 74% of the total cooling capacity, which is beneficial for the precise control of the therapy area in laser surgery.

Effect of two-way coupling on the calculation of forest fire spread: model development

The present work addresses the problem of how wind should be taken into account in fire spread simulations. The study was based on the software system FireStation, which incorporates a surface fire spread model and a solver for the fluid flow (Navier–Stokes) equations. The standard procedure takes the wind field computed from a single simulation in the absence of fire, but this may not be the best option, especially for large fires. The two-way coupling method, however, considers the buoyancy effects caused by the fire heat release. Fire rate of spread is computed with the semi-empirical Rothermel model, which takes as input local terrain slope, fuels properties and wind speed and direction. Wind field is obtained by solving the mass, momentum and energy equations. Effects of turbulence on the mean flow field are taken into account with the k – ϵ turbulence model. The calculation procedure consists of an interchange between the fire spread model and the wind model through a dynamic interaction. The present work describes the first part of this research, presenting the underlying models and a qualitative sensitivity analysis. It is shown that the update frequency for the dynamic interaction markedly influences the total calculation time. The best strategy for updating the wind field during the fire progression is presented. The dependence of results on mesh size is also described.

Identification of the strength of junction coupling effects in water hammer

In order to analyse the effects of junction coupling during water hammer in piping systems, a test facility has been designed and constructed. The main objective is to identify the dependence on the bend geometry and the strength of two-way coupling, and to derive a critical, geometric parameter, by means of which it is easily evaluated whether significant two-way effects occur or not. Oscillation experiments on movable U-shaped bends with various geometries were carried out. The excitation was realised by water hammer. The bend configurations differed in distance between the parallel levers, which results in different excitation forces, masses and stiffnesses. Inter alia the vertical displacement of the bend and the pressure inside the pipe were measured for various free oscillating lengths while the rest of the piping system was restrained. The results are displayed in curves of the maximum displacement and frequency spectra of the pressure and the displacement for the different bend configurations. Additional experiments with variable excitation at the coincidence frequency for two bend configurations were performed. Only for the medium and long bend configurations, two-way junction coupling effects were resolved. The strength of the coupling was found to be independent of the excitation, which just scaled the amplitudes. A quantification of the strength of the two-way coupling effects was performed by an analogy with the two-mass oscillator. This method was verified using coupled simulations. Owing to the good agreement between the results of the experiments and the simulations, additional bend configurations were calculated. Using this data, it is possible to evaluate the strength of the coupling on the basis of the geometry and mass of the bend.

Direct Numerical Simulation of biomass pyrolysis and combustion with gas phase reactions

We present Direct Numerical Simulation of biomass pyrolysis and combustion in a turbulent channel flow. The model includes simplified models for biomass pyrolysis and char combustion along with a model for particle tracking. The gas phase is modelled as a mixture of reacting gas species. The gas-particle interactions for mass, momentum, and energy exchange are included by two-way coupling terms. The effect of two-way coupling on the conversion time of biomass particles is found noticeable for particle volume fractions > 10-5. We also observe that at constant volume fraction the effect of two-way coupling increases as the particle size is reduced, due to the higher total heat exchange area in case of smaller particles. The inclusion of gas phase homogeneous reactions in the DNS model decreases the biomass pyrolysis time due to higher gas temperatures. In contrast, including gas phase reactions increases the combustion time of biomass due to the lower concentration of oxygen at the particle surface.

Numerical simulation of the accumulation of heavy particles in a circular bounded vortex flow

In present work, an Eulerian–Lagrangian CFD model based on the discrete element method (DEM) and immersed boundary method (IBM) has been developed, validated and used to investigate the accumulation of heavy particles in a circular bounded viscous vortex flow. The inter-particle and particle-wall collisions are resolved by a hard-sphere model. Effects of one-way and two-way coupling, Reynolds number, and particle diameter are systematically explored. Results show that, in case of one-way coupling, the majority of particles will spiral into an accumulation point located near the stagnation point of the flow field. The accumulation point represents a stable equilibrium point as the drag created by the flow field balances the destabilizing centrifugal force on the particle. However, in case of two-way coupling, there does not exist a stable accumulation point due to the strong interaction between the particles and fluid dynamics. Instead most particles are expelled from the circular domain and accumulate on the confining wall. The percentage of accumulated particles on the wall increases with increasing Reynolds number and particle diameter. Moreover, influence of three well-known drag models is also studied and they give consistent results on the particle accumulation behavior, although small quantitative differences can still be discerned.

Inter-phase heat transfer and energy coupling in turbulent dispersed multiphase flows

The present paper addresses important fundamental issues of inter-phase heat transfer and energy coupling in turbulent dispersed multiphase flows through scaling analysis. In typical point-particle or two-fluid approaches, the fluid motion and convective heat transfer at the particle scale are not resolved and the momentum and energy coupling between fluid and particles are provided by proper closure models. By examining the kinetic energy transfer due to the coupling forces from the macroscale to microscale fluid motion, closure models are obtained for the contributions of the coupling forces to the energy coupling. Due to the inviscid origin of the added-mass force, its contribution to the microscale kinetic energy does not contribute to dissipative transfer to fluid internal energy as was done by the quasi-steady force. Time scale analysis shows that when the particle is larger than a critical diameter, the diffusive-unsteady kernel decays at a time scale that is smaller than the Kolmogorov time scale. As a result, the computationally costly Basset-like integral form of diffusive-unsteady heat transfer can be simplified to a non-integral form. Conventionally, the fluid-to-particle volumetric heat capacity ratio is used to evaluate the relative importance of the unsteady heat transfer to the energy balance of the particles. Therefore, for gas-particle flows, where the fluid-to-particle volumetric heat capacity ratio is small, unsteady heat transfer is usually ignored. However, the present scaling analysis shows that for small fluid-to-particle volumetric heat capacity ratio, the importance of the unsteady heat transfer actually depends on the ratio between the particle size and the Kolmogorov scale. Furthermore, the particle mass loading multiplied by the heat capacity ratio is usually used to estimate the importance of the thermal two-way coupling effect. Through scaling argument, improved estimates are established for the energy coupling parameters of each energy exchange mechanism between the phases.

Exact regularized point particle method for multiphase flows in the two-way coupling regime

Particulate flows have mainly been studied under the simplifying assumption of a one-way coupling regime where the disperse phase does not modify the carrier fluid. A more complete view of multiphase flows can be gained calling into play two-way coupling effects, i.e. by accounting for the inter-phase momentum exchange, which is certainly relevant at increasing mass loading. In this paper we present a new methodology rigorously designed to capture the inter-phase momentum exchange for particles smaller than the smallest hydrodynamical scale, e.g. the Kolmogorov scale in a turbulent flow. The momentum coupling mechanism exploits the unsteady Stokes flow around a small rigid sphere, where the transient disturbance produced by each particle is evaluated in a closed form. The particles are described as lumped point masses, which would lead to the appearance of singularities. A rigorous regularization procedure is conceived to extract the physically relevant interactions between the particles and the fluid which avoids any ‘ad hoc’ assumption. The approach is suited for high-efficiency implementation on massively parallel machines since the transient disturbance produced by the particles is strongly localized in space. We will show that hundreds of thousands of particles can be handled at an affordable computational cost, as demonstrated by a preliminary application to a particle-laden turbulent shear flow.

Delay of biomass pyrolysis by gas–particle interaction

We apply a biomass pyrolysis model, based on the model developed by Haseli et al. [4], which can be used in combination with Direct Numerical Simulation. The pyrolysis model is combined with a model for particle tracking to simulate 3D turbulent particle-laden channel flow with biomass particles undergoing pyrolysis in nitrogen. Transfer of momentum, heat and mass between gas and particles are fully taken into account. The effects of this transfer are analyzed and quantified in terms of the delay in the conversion or pyrolysis time. The delay is shown to depend on the initial volume fraction (number of particles) and on the size of the particles. The two-way coupling effects are relevant at volume fractions >10−5. For a fixed volume fraction, gas–particle interaction induces a delay in the devolatilization, decreasing with increasing particle size. Using this model, we also performed simulations of realistic biomass particle size distributions in order to compare two-way and one-way coupling.

LBE–DEM simulation of inter-phase heat transfer in gas–solid cross jets

Lattice-Boltzmann equation (LBE)–Discrete element method (DEM) coupled simulation of a two-dimensional gas–solid cross jet is performed, focusing on the gas-particle two-way coupling effect on heat transfer characteristics. The Reynolds number is 1000, and particle Stokes numbers are 10, 25, and 50 under the same number flow rate of particles. The gas phase temperature field and particle distribution as well as the inter-phase heat transfer characteristics are studied and analyzed. The dominating effects, i.e. the mean temperature difference and mean heat transfer coefficient between the gas–solid phases, for the pre- and post- collision stages of the cross jets are illustrated respectively. The change of dominating roles is related to the dynamical response characteristics of particles.

Unsteady Euler/Lagrange simulation of a confined bluff-body gas–solid turbulent flow

An unsteady Euler–Lagrangian approach is adopted to predict the gaseous carrier and disperse phases flow dynamics. The turbulence is captured using two different methods, i.e. the unsteady Reynolds averaging based numerical simulation (URANS) and the large eddy simulation (LES). In the latter one, the dynamic Smagorinsky approach is used to model the sub-grid scale stresses. The time-dependent solid particle and gas phase flow properties of a confined bluff-body turbulent flow including two-way coupling effects are evaluated through comparisons with experimental data. The configuration under study features an important recirculation zone and has a mass loading of 22%. So, collision effects are not considered while tracking the disperse phase that consists of glass beads. A thermodynamically consistent turbulence modulation approach is applied for the determination of the source terms that account for the effect of particles on the turbulence level of the carrier phase. Within the URANS technique the dispersion of particles is captured by the Markov sequence approach; this model is modified by integrating a drift factor term while modeling the pressure gradient. A particular emphasis is put on the disperse phase feedback on the carrier phase and coupling procedure within each Eulerian time step along with an unsteady coupling of both codes, the (Eulerian) FASTEST3D and the (Lagrangian) LAG3D codes. Quantitative results of the disperse phase properties as well as those of the carrier phase are presented at different positions around the recirculation zone. The numerical results using both, the LES and/or the URANS delivered comparable results that agree reasonably with experimental data. However, a slight advantage of LES over URANS could be observed.

Numerical simulation of the interaction between vortex ring and bubble plume

In present paper a three-dimensional Vortex-In-Cell method with two-way coupling effect was developed to study the bubble plume entrainment by a vortex ring. In this method the continuous flow was calculated by the three-dimensional Vortex-In-Cell method and the bubbles are tracked through bubble motion equation. Two-way coupling effect between continuous flow and dispersed bubbles is considered by introducing a vorticity source term, which is induced by the change of void fraction gradient in each computational cell. After validated by the comparison between experimental measurements and simulation results for the motion of vortex rings and the rising velocity of bubble plume, present method is implemented to simulate the interaction between an evolving vortex ring and a rising bubble plume. It was found that there is little effect of the bubble entrainment to the total circulation of vortex ring while the effect of bubble entrainment to the vortex ring structure is quite obvious. The bubble entrainment by the vortex ring not only changed the vorticity distribution in the vortex structure, but also displaced the positions of the vortex cores. The vorticity in the lower vortex core of the vortex ring decreases more than that in the upper vortex core of the vortex ring while the vortex core in the upper part of the vortex ring is displaced to the center of vortex ring by the entrained bubbles. Smaller bubbles are easier to be entrained by the large scale vortex structure and the transportation distance is in inverse proportion to bubble diameter.

A DNS study of the effect of particle feedback in a gas–solid three dimensional plane jet

A direct numerical simulation of a three dimensional plane jet is carried out to study the turbulence modulation and particle dispersion in gas–solid flows. The particle motion is simulated in a deterministic way under the Lagrangian frame. The point-force approximation is used for the two-way coupling method. The effects of particle feedback on the modification of turbulence characteristics, such as the vortex structure, velocity profiles, turbulence intensity and kinetic energy, Reynolds stress tensor, and the particle dispersion characteristics are explored in detail, under the condition of different mass loadings. With the particle feedback effects, fluid vortex structures are changed greatly. The mean velocity profiles of fluid are changed in different manners in either the stream-wise or the lateral-wise directions. For the same Stokes number, the degree of modulation of turbulence depends on the mass loadings. The turbulence kinetic energy and the Reynolds stress tensor are suppressed by addition of small particles. On the other hand, the effect of two-way coupling changes the particle dispersion characteristics greatly. The concentration is enhanced due to the two-way coupling effect, and the degree of enhancement is proportional to the mass loadings.

Modelling of turbulent gas-particle flows with focus on two-way coupling effects on turbophoresis

An Eulerian model was developed for turbulent gas-particle flow that takes into account the influence of particles on the gas-phase turbulence. For the description of the particle-phase stress the kinetic theory of granular flow and the simpler Hinze model were adopted. A K-ω model was used as the gas phase turbulence model. The difference between one- and two-way coupling was investigated for different particle volume fractions and particle diameters. It was found that particles with a much higher density than the fluid substantially affect the gas-phase in turbulent channel flow for particle volume fractions as low as 10−4. The models with the particle-phase stress described by the kinetic theory of granular flow and the simpler Hinze model produce similar results for particles with small response times but deviate for larger response times. The study shows that two-way coupling and the turbophoretic effect must be taken into account in models even at relatively low particle volume fractions.

DNS experiments on the settling of heavy particles in homogeneous turbulence: two-way coupling and Reynolds number effects

The present study addresses the effects of two-way fluid-particle interaction ("two-way coupling") on the settling of heavy particles in homogeneous turbulence. The modification of turbulence by the particles and the resulting effects on the settling velocity are analyzed by taking into account Reynolds number effects. The turbulent carrier fluid phase is resolved by direct numerical simulation (DNS) and the particle phase by the Lagrangian point-particle approximation. Results are presented for two Taylor microscale Reynolds numbers Rλ = 40 and 130. While for Rλ = 40 the Kolmogorov time scale, tη, is found almost unaltered by the two-way coupling, it is found significantly decreased for Rλ = 130. The consequent modification of the particle Stokes number St = τp/tη, τp being the particle response time, shows that comparisons between one-way coupling numerical data and two-way coupling numerical or experimental data may be inconsistent. This is particularly relevant for the particle settling in turbulence which has been previously shown to be strongly influenced by the dynamical interaction of the small flow scales with the particles.

Particle-laden jets: particle distribution and back-reaction on the flow

DNS data of particle-laden jets are discussed both in the one- and two-way coupling regimes. Dynamics of inertial particles in turbulent jets is characterized by an anomalous transport that leads to the formation of particle concentration peaks along the jet axis. Larger is the particle inertia farther the peak location occurs. The controlling parameter is found to be the local large-scale Stokes number which decreases quadratically with the axial distance and is order one in coincidence of the peaks. The centerline mean particle velocity is characterized by two scaling laws. The former occurs upstream the location where the Stokes number is order one, and is linear in the axial distance with negative coefficient. The latter, occurring downstream where the local Stokes number is small, coincides with that of the centerline mean fluid velocity. This behavior affects the development of the particle-laden jet when the mass load of the particulate phase increases and two-way coupling effects become relevant. Two distinct behaviors for the jet development are found behind and beyond the location of unity local Stokes number leading to different scaling laws for the mean centerline fluid velocity.