Direct Numerical Simulation Study Research Articles

Understanding Low-Speed Streaks and Their Function and Control through Movable Shark Scales Acting as a Passive Separation Control Mechanism

The passive bristling mechanism of the scales on the shortfin mako shark (Isurus oxyrinchus) is hypothesized to play a crucial role in controlling flow separation. In the hypothesized mechanism, the scales are triggered in response to patches of reversed flow at the onset of separation occurring in the low-speed streaks that form in a turbulent boundary layer. The two goals of this investigation were as follows: (1) to measure the reversing flow occurring within the low-speed streaks in a separating turbulent boundary layer; (2) to understand the passive flow control mechanism of movable shark skin scales that inhibit reversing flow within the low-speed streaks. Experiments were conducted using digital particle image velocimetry (DPIV). DPIV was used to analyze the flow in a turbulent boundary layer subjected to an adverse pressure gradient formation over both a smooth flat plate and a flat plate on which shark skin specimens were affixed. The experimental analysis of the flow over the smooth flat plate corroborated the findings of previous direct numerical simulation studies, which indicated that the average spanwise spacing of the low-speed streaks increases in the presence of adverse pressure gradients upstream of the point of separation. However, the characteristics of the flow over the shark skin specimen more closely resemble that of a zero-pressure gradient turbulent boundary layer. A comparative analysis of the width and velocity of the reversed streaks between flat plate and shark skin cases reveals that the mean spanwise spacing decreases, and thus, the number of streaks increases over the shark skin. Additionally, the reversed streaks observed over shark scales are thinner and the highest negative velocity within the streaks falls within the range required to bristle the scales.

Modulation of flow, heat, and mass transfer in turbulent double-diffusive convection

The present study addresses numerical simulations of the double-diffusive convection in a turbulent regime. The characteristic concentration and temperature Prandtl numbers (PrC = 700 and PrT = 7) correspond to the typical seawater properties. Here, we applied the Large-Eddy Simulation (LES) approach, with relatively simple subgrid turbulence closures of momentum, concentration, and temperature for the unresolved scales. The wall-resolved LES approach proved to work well on significantly coarser numerical mesh than used in recent Direct Numerical Simulation (DNS) studies of Yang et al. (2016) in the intermediate range of working parameters, 107 ≤ RaC ≤ 109, 0 ≤ RaT ≤ 106, which covers the quasi-Rayleigh-Bénard, fingering, and damping flow regimes. A good agreement between concentration and temperature Nusselt numbers is obtained. The instantaneous Nusselt numbers distribution revealed a significant impact of the imposed strong thermal stratification (RaT = 106) in comparison to the neutral case (RaT = 0).

Characteristics of slurry transport regimes: Insights from experiments and interface-resolved Direct Numerical Simulations

A common way to transport solids in large quantities is by using a carrier fluid to transport the solids as a concentrated solid/liquid mixture or slurry through a pipeline. Typical examples are found in dredging, mining and drilling applications. Dependent on the slurry properties and flow conditions, horizontal slurry pipe flow is either in the fixed-bed, sliding-bed or fully-suspended regime. In terms of non-dimensional numbers, the flow is fully characterized by the bulk liquid Reynolds number (Re), the Galileo number (Ga, a measure for the tendency of particles to settle under gravity), the solid bulk concentration (ϕb), the particle/fluid density ratio (ρp/ρf), the particle/pipe diameter ratio (Dp/Dpipe), and parameters related to direct particle interactions such as the Coulomb coefficient of sliding friction (μc). To further our fundamental understanding of the flow dynamics, we performed experiments and interface-resolved Direct Numerical Simulations (DNS) of slurry flow in a horizontal pipe. The experiments were performed in a transparent flow loop with Dpipe=4 cm. We measured the pressure drop along the pipeline, the spatial solid concentration distribution in the cross-flow plane through Electrical Resistance Tomography (ERT), and used a high-speed camera for flow visualization. The slurry consisted of polystyrene beads in water with Dp=2mm, ρp/ρf=1.02, Ga between 40–45 and ϕb between 0.26–0.33. The different flow regimes were studied by varying the flow rate, with Re varying from 3272 till 13830. The simulations were performed for the same flow parameters as in the experiments. Taking the experimental uncertainty into account, the results from the DNS and the experiments are in reasonably good agreement. The results for the pressure drop agree also fairly well with popular empirical models from literature. In addition, we performed a parametric DNS study in which we solely varied Re and Ga. In all flow regimes, a secondary flow of Prandtl’s second kind is present, ascribed to the presence of internal flow corners and a ridge of densely packed particles at the pipe bottom during transition towards the fully-suspended regime. In the bulk of the turbulent flow above the bed, secondary flow transport of streamwise momentum dominates over turbulent diffusion in regions where the secondary flow is strong and vice versa where it is weak. The transition between flow regimes appears to be governed by the competition between the net gravity force on the particles and shear-induced particle migration from particle–particle interactions. This competition can be expressed by the Shields number, θ. For θ≲0.75, gravity is dominant and the flow is in the fixed-bed regime. For θ≳0.75, shear-induced migration becomes progressively more important for increasing θ. Low-concentration zones flanking the sliding bed start to form at the top corners of the bed, and gradually expand downwards along the pipe wall till the pipe bottom is reached. For θ≳1.5, shear-induced migration is responsible for lifting the particle bed away from the wall, associated with the onset of the suspended regime. For θ≫1, gravity is of minor importance and the mean flow eventually reaches axi-symmetry with a high-concentration particle core at the pipe center and negligible secondary flow.

Direct Numerical Simulation of Involute Channel Turbulence

Abstract A direct numerical simulation (DNS) study was performed on turbulent flow in the high flux isotope reactor involute channel geometry to develop a numerical database and determine the differences compared with a flat parallel channel. The varying channel curvature along the walls was studied for differences in mean profiles. Parameters of interest include streamwise velocity, turbulent kinetic energy (TKE), and turbulence dissipation rate, as well as Reynolds stresses and turbulence transport terms. Profile sampling was carried out at 10 locations along the span of the involute. Additional DNS studies were performed on smaller domains of comparable curvature to the involute domain: a high curvature channel (high circular), a low curvature channel (low circular), and a flat channel (flat). Each of these four cases was compared against each other and to other DNS studies performed on parallel flows. The results indicate that the bulk involute channel flow does not differ significantly from a flat parallel channel flow and that the curvature of the walls does not significantly alter the mean flow parameters. However, the regions of the involute channel near the side walls exhibit relatively low magnitude twin recirculation structures driven toward the side walls from the centerline of the channel, which warrants further study.

Interparticle friction in sheared dense suspensions: Comparison of the viscous and frictional rheology descriptions

In the literature, two different frameworks exist for describing the rheology of solid/liquid suspensions: (1) the “viscous” framework in terms of the relative suspension viscosity, ηr, as a function of the reduced solid volume fraction, ϕ/ϕm, with ϕm the maximum flowable packing fraction, and (2) the “frictional” framework in terms of a macroscopic friction coefficient, μ, as a function of the viscous number, Iv, defined as the ratio of the viscous shear to the wall-normal particle stress. Our goal is to compare the two different frameworks, focusing on the effect of friction between particles. We have conducted a particle-resolved direct numerical simulation study of a dense non-Brownian suspension of neutrally buoyant spheres in slow plane Couette flow. We varied the bulk solid volume fraction from ϕb=0.1 to 0.6 and considered three different Coulomb friction coefficients: μc=0, 0.2, and 0.39. We find that ηr scales well with ϕ/ϕm, with ϕm obtained from fitting the Maron–Pierce correlation. We also find that μ scales well with Iv. Furthermore, we find a monotonic relation between ϕ/ϕm and Iv, which depends only weakly on μc. Since ηr=μ/Iv, we thus find that the two frameworks are largely equivalent and that both account implicitly for Coulomb friction. However, we find that the normal particle stress differences, N1 and N2, when normalized with the total shear stress and plotted against either ϕ/ϕm or Iv, remain explicitly dependent on μc in a manner that is not yet fully understood.

A flow control strategy for a near-wall square cylinder using porous media: A direct numerical simulation study

The topic of flows around a near-wall square cylinder has garnered increasing attention in recent decades. However, there are a few publications that have focused on mitigating the occurrence of a substantial negative lift in near-wall flows. In light of this, the present study has developed a novel flow control strategy that covers porous media at inward corners of a near-wall square cylinder to address this problem. We achieve such a control strategy with the aid of a high-fidelity computational framework at Re = 1000. Direct numerical simulations are employed to account for accurate flow behaviors, and the Cartesian cut-cell method as well as an adaptive mesh refinement algorithm are advocated to simplify grid generation and reduce computational costs. Additionally, a quasi-microscopic flow model is introduced to model the porous medium pore structure, providing an intuitive and accurate description of internal flows within the porous medium. Six porous medium layouts are first designed, and their influences and mechanisms on flow control are assessed using the presented computational framework to identify an optimal strategy. The optimal strategy yields a notable reduction of 52.472% in the lift coefficient. The identified strategy is then applied to a case involving a near-wall square cylinder with a substantial negative lift, where a gap ratio of 0.6 is determined via parameterization. The capacity of the presented strategy in flow control of the near-wall square cylinder is fully explored and demonstrated via the consideration of different porosities. The results indicate that the most effective flow control is achieved when the porosity exceeds 90%, leading to a near-zero lift coefficient. Furthermore, the underlying mechanism contributing to the variations in flow control effectiveness due to different porosities is analyzed.

A comprehensive study of large eddy simulation models in capturing the dynamics of gas-particle turbulent suspensions

The particle phase attenuates the fluid fluctuations with an increase in volume fraction, and a sudden collapse in the turbulence is observed at a particular particle volume fraction, called critical particle volume loading (CPVL) [P. Muramulla et al. J. Fluid Mech. 889, A28 (2020)]. The present study reports the capability of two different classes of large eddy simulation (LES), viz. anisotropic and eddy viscosity-based, models to capture the turbulence modulation and the sudden disruption of the fluid fluctuations in the particle-laden vertical channel flows. The simulations are performed at two bulk Reynolds numbers of 3300 and 5600 based on the channel width and the bulk averaged fluid velocity. Our study on different LES models shows that approximate deconvolution (ADM) and scale similarity (SS) models accurately predict the critical loading for the Reynolds number of 3300. However, these models predict the critical loading qualitatively only for the Reynolds number of 5600 in the sense that they fail to predict the discontinuity as shown by the direct numerical simulation (DNS) study. The coherent structure model (CSM) predicts the critical loading with an 80% accuracy at both Reynolds numbers. The energy spectral density, production, and particle-induced dissipation spectra are plotted to analyze the distribution across wavenumbers. For all the LES models, a decrease in more than one order of magnitude is observed in the energy spectrum density at the critical loading compared to the unladen flow. The energy density decreases more in the channel center than in the near-wall region for the same particle volume loading. The mean component of particle-induced dissipation is almost two orders of magnitude larger than the particle dissipation spectra of fluctuating energy. The magnitude of streamwise and spanwise dissipation spectra of fluctuating components is higher in the near-wall region than the channel center. However, the magnitude of wall-normal dissipation spectra is higher in the channel center than near the wall region.

Direct numerical simulation of bubble collision, bounce and coalescence in bubble-induced turbulence

A direct numerical simulation study has been carried out to better understand the dynamics of deformable bubbles in bubble-induced turbulence. A mathematical model for bubble collision, bounce and coalescence is developed based on the Volume-of-Fluid (VoF) method. A surface-tension-holding film thickness δc is introduced to determine whether the bubbles will coalesce or bounce after collision. Bubbly flows with the gas volume fractions αg=5.6% (low), 11.2% (medium) and 22.1% (high) are considered in the study. The DNS results show that the dissipation of turbulent kinetic energy in the cases under consideration takes place when the wavenumber κ is smaller than twice the wavenumber for the bubble diameter 2κd. The bubble-induced turbulence is anisotropic at the length scales close to the bubble diameter due to the elongated turbulent structures in the bubble-rising direction. The bubbles collide intensively with each other when they are in a cluster surrounded by the liquid with low pressure. Parallel clustering of bubbles is found when the gas volume fraction has a low or medium value. Three-dimensional clustering is found when the bubbles are densely populated. Two different mechanisms of bubble collision have been identified from the DNS results, termed parallel collision and vertical collision. Parallel collision is often observed when bubbles are sparsely populated. In a parallel collision, the relative velocity of the bubbles slows down as two bubbles approach each other due to the jet flow between them, while the relative velocity increases sharply after the bounce. In a vertical collision, by contrast, the relative velocity of bubbles accelerates as two bubbles approach each other, while it slows down during the collision. Vertical collisions occur when the bubbles are more densely populated. The numerical results also show the significant effects of δc on bubble coalescence.

Overcoming computational challenges to realize meter- to submeter-scale resolution in cloud simulations using the super-droplet method

Abstract. A particle-based cloud model was developed for meter- to submeter-scale-resolution simulations of warm clouds. Simplified cloud microphysics schemes have already made meter-scale-resolution simulations feasible; however, such schemes are based on empirical assumptions, and hence they contain huge uncertainties. The super-droplet method (SDM) is a promising candidate for cloud microphysical process modeling and is a particle-based approach, making fewer assumptions for the droplet size distributions. However, meter-scale-resolution simulations using the SDM are not feasible even on existing high-end supercomputers because of high computational cost. In the present study, we overcame challenges to realize such simulations. The contributions of our work are as follows: (1) the uniform sampling method is not suitable when dealing with a large number of super-droplets (SDs). Hence, we developed a new initialization method for sampling SDs from a real droplet population. These SDs can be used for simulating spatial resolutions between meter and submeter scales. (2) We optimized the SDM algorithm to achieve high performance by reducing data movement and simplifying loop bodies using the concept of effective resolution. The optimized algorithms can be applied to a Fujitsu A64FX processor, and most of them are also effective on other many-core CPUs and possibly graphics processing units (GPUs). Warm-bubble experiments revealed that the throughput of particle calculations per second for the improved algorithms is 61.3 times faster than those for the original SDM. In the case of shallow cumulous, the simulation time when using the new SDM with 32–64 SDs per cell is shorter than that of a bin method with 32 bins and comparable to that of a two-moment bulk method. (3) Using the supercomputer Fugaku, we demonstrated that a numerical experiment with 2 m resolution and 128 SDs per cell covering 13 8242×3072 m3 domain is possible. The number of grid points and SDs are 104 and 442 times, respectively, those of the highest-resolution simulation performed so far. Our numerical model exhibited 98 % weak scaling for 36 864 nodes, accounting for 23 % of the total system. The simulation achieves 7.97 PFLOPS, 7.04 % of the peak ratio for overall performance, and a simulation time for SDM of 2.86×1013 particle ⋅ steps per second. Several challenges, such as incorporating mixed-phase processes, inclusion of terrain, and long-time integrations, remain, and our study will also contribute to solving them. The developed model enables us to study turbulence and microphysics processes over a wide range of scales using combinations of direct numerical simulation (DNS), laboratory experiments, and field studies. We believe that our approach advances the scientific understanding of clouds and contributes to reducing the uncertainties of weather simulation and climate projection.

Effect of flow shear on the onset of dynamos

Understanding the origin and structure of mean magnetic fields in astrophysical conditions is a major challenge. Shear flows often coexist in such astrophysical conditions, and the role of flow shear on the dynamo mechanism is of great interest. Here, we present a direct numerical simulation study of the effect of flow shear on dynamo instability for EPI2D flows [Yoshida et al., Phys. Rev. Lett. 119, 244501 (2017)] with controllable mirror symmetry (i.e., fluid helicity). Our numerical observations suggest that for helical base flows, the effect of shear is to reduce the small-scale dynamo (SSD) growth rate moderately. For non-helical base flows, flow shear has the opposite effect of amplifying the SSD action. The magnetic energy growth rate (γ) for non-helical base flows has been found to follow an algebraic nature of the form, γ=−aS+bS23, where a,b>0 are real constants, S is the shear flow strength, and γ is found to be independent of the scale of flow shear. Studies with different shear profiles and shear scale lengths for non-helical base flows have been performed to test the universality of our finding.

Modulation of Turbulence Flux Budgets by Varying Fluid Properties in Heated High Prandtl Number Flow

The accurate computational modelling of momentum and heat transfer in turbulent wall-bounded flow is still a challenging task, especially for fluids with high molecular Prandtl numbers. Due to the steep temperature gradients, the strong variation of fluid properties becomes an additional important issue. The present Direct Numerical Simulation study specially addresses this problem considering a representative working fluid in terms of a real heat transfer oil with temperature-dependent material properties. Near the heated wall, the increase in molecular viscosity significantly attenuates the turbulent convective fluxes of both momentum and heat, despite the markedly increased enthalpy fluctuations. As the thermal conductivity decreases markedly less with temperature than the viscosity, the dampened turbulent mixing significantly reduces the Nusselt number, whereas the skin friction coefficient only marginally differs from the constant fluid property case. The attenuation of the radial velocity fluctuation consistently reduces the production terms in the budgets of the turbulent momentum and heat fluxes, while the production of the enthalpy fluctuations is only shifted away from the wall without significant change in peak level. The also attenuated redistribution in the turbulent axial stress budget reduces the pressure-strain based steering of turbulence towards isotropy. Facing a consequently more anisotropic turbulent motion poses a further difficulty to standard turbulence modelling mostly developed for isotropic turbulence.

Supersonic Base Flow With and Without Propulsive Jets: A Brief Review

Supersonic base flow is encountered in many aerospace applications related to satellite launch vehicle, missiles and projectiles. The origin and evolution of large scale structures which govern the dynamics of the base flow is not properly understood. The presence of central propulsive jet further complicates the base flow characteristics. Different semi-empirical, experimental and numerical studies carried out in supersonic base flow is briefly reviewed. Various issues and challenges faced in CFD simulation of base flow problems are described. Apart from resolution of numerics, proper choice of turbulence model remains the key aspect in the Reynolds Averaged Navier Stokes (RANS) simulations. Prediction of axial distribution of centerline velocities and recirculation bubble size remain problematic in RANS simulation. Detached Eddy Simulation (DES) results show dependence on model constants. Large Eddy Simulation (LES) studies with proper filter and Sub Grid Scale (SGS) model is promising, but its applications are limited to canonical problems due to very high computational requirements. Direct Numerical Simulation (DNS) studies reported in the literature consider much lower Reynolds number and hence could not be compared with the experimental data. It is suggested to use RANS and DES methods with anchored turbulence model from well-designed experiments for design data generation. Efforts should be made to use higher order numerical methods (LES and DNS) to explore the origin and evolution of large scale structures observed in compressible base flow.

Asymmetric secondary flows above geometrically symmetric surface roughness

The mean flow behaviour of a turbulent boundary layer over rough walls is expected to exhibit symmetries that govern the flow dynamics. In particular, when roughness elements are arranged in a spanwise symmetric manner, the mean flow above them should also exhibit spanwise symmetry. This symmetrical consideration has garnered substantial empirical support. We conduct direct numerical simulations (DNS) of flow over aligned cube arrays to test such symmetry considerations further. We vary the surface coverage density from 0.25 % to 6.25 %, and employ an averaging time of about 100 large-eddy turnover times, which is longer than the typical averaging time in prior DNS studies of rough-wall boundary layers. The results suggest the presence of spanwise asymmetry in the mean flow. Specifically, we observe the development of a prominent secondary vortex on one side of the cubical roughness, accompanied by a relatively smaller secondary vortex on the other side. This asymmetry becomes most pronounced when the surface coverage density is approximately 0.59 %, and diminishes as the coverage density approaches either a low or a high value. We also establish that this mean flow asymmetry is robust across variations in the domain size, the initial condition, and the placement of the cubes in the spanwise direction.

Structured input–output analysis of oblique laminar–turbulent patterns in plane Couette–Poiseuille flow

In this work, we employ structured input–output analysis to study the flow patterns in transitional plane Couette–Poiseuille flow (CPF). First, we focus on the well-studied intermediate laminar profile, which balances the shear and pressure effects. We show that the highest structured gain corresponds to perturbations with wavelengths associated with the oblique turbulent bands observed in experiments. In addition, the inclination angles of these structures show a Reynolds number dependence consistent with experimentally observed trends. We then examine the Reynolds number scaling of the maximal structured frequency response as the velocity profile is varied from plane Couette to Poiseuille flow. Our results demonstrate that, as expected, the scaling exponent increases over this range, but this increase is not monotonic. We attribute the variation in the shape of the scaling exponent curve as a function of the velocity profile shape to changes in flow patterns that dominate each associated flow regime. We then focus on the particular case of plane Couette flow and compute the structured response modes. Their behavior and structural features are consistent with results obtained through direct numerical simulation (DNS) studies. Finally, we employ the structured analysis framework to examine the temporal evolution of the dominant structures. For the well-studied cases of plane Couette and plane Poiseuille flows, the computed advection speeds of the oblique structures are consistent with those predicted through DNS.

A Direct Numerical Simulation Study for Flame Structure and Propagation Characteristics of Multi-Jet Flames

ABSTRACT Lifted multi-jet flames of hydrogen and steam-diluted oxygen are studied using direct numerical simulations (DNS). The DNSs have been carried out for laminar and turbulent flame configurations. The simulation data are analyzed to investigate the flame and flame-base structures and the flame-base propagation in a configuration akin to a gas turbine with multiple fuel jets. The laminar flames have a tribrachial structure, although lean premixed and diffusi on flame branches collide downstream owing to preferential diffusion. The turbulent multi-jet flames have a single connected base region, whereas the laminar flames have separate bases, each of which is associated with the corresponding fuel jet. Although turbulent multi-jet flames have substantially different flame-base characteristics from laminar multi-jet flames (and conventional single-jet lifted flames), the propagation speed is similar for both conditions. The results suggest that the well-known flame-base stabilization theory can be straightforwardly applied to the multi-jet configuration, despite the turbulent flame structure’s unconventional features.

Reynolds number induced growth of the large-scale rolls in plane Couette flow using resolvent analysis

In direct numerical simulations (DNS) of turbulent Couette flow, the observation has been made that the long streamwise rolls increase in length with the Reynolds number (Lee & Moser, J. Fluid Mech., vol. 842, 2018, pp. 128–145). To understand this, we employ both linear stability theory and its extension to resolvent analysis. For this, we emphasise the high Reynolds number ( $Re \rightarrow \infty$ ) and small streamwise wavenumbers ( $\alpha \rightarrow 0$ ) limit, imposing the distinguished limit $Re_{\alpha }=Re \, \alpha = O(1)$ . We find that in case of linear stability theory, $Re_{\alpha }$ acts as a global invariant in the resulting eigenvalue problem, while in case of resolvent analysis, $Re_{\alpha }$ acts as a local invariant in the behaviour of the energy of the system characterised through the first singular value $\sigma _1$ of the resolvent operator within the investigated asymptotic limit. In order to obtain constant streamwise structures for increasing Reynolds numbers, the respective streamwise wavenumber has to decrease, which verifies the observations from DNS studies of an increasing length of the streamwise structures with the Reynolds number. In linear stability theory, a parameter reduction is achieved for the above asymptotic limit, resulting in the modified Orr–Sommerfeld and Squire equations being dependent only on $Re_{\alpha }$ . The behaviour of both the coherent structures obtained from linear stability theory and resolvent analysis are compared with each other and show similar behaviours over $Re_{\alpha }$ .

Controlling second-mode oblique breakdown in high-speed boundary layers using streak: A direct numerical simulation study

The suppression effect of streaks on the second mode has been studied by the stability analysis in the past decade. In the present study, we conduct direct numerical simulations (DNS) to investigate the direct nonlinear control in the complete laminar-to-turbulence transition scenario within a high Mach number supersonic boundary layer. Our study aims to provide a comprehensive understanding of the mechanisms underlying the influence of streaks on the transition of high-speed boundary layers. Our work demonstrates the effective control effect of the streaks generated by blowing and suction strips on laminar-to-turbulence transition via the second-mode oblique breakdown at a Mach number 4.5 high-speed boundary layer using DNS. Modal analysis, nonlinear disturbance formulation, and stability analysis are used to provide insight into the stabilization effect of control streaks. Crucially, the role of three-dimensional control modes and mean-flow distortion generated by the control streak are investigated. Our findings indicate that both mean-flow distortion and three-dimensional control modes effectively stabilize the fundamental oblique second mode, particularly when the amplitude of control streaks is strong.

A direct numerical simulation study to elucidate the enhancement of heat transfer for nucleate boiling on surfaces with micro-pillars

Recent experimental studies have demonstrated great potential of surface engineering in enhancing nucleate boiling heat transfer performance. However, the underlying mechanism remains unclear, especially the role of microlayer evaporation underneath bubbles. In this work, we investigate the heat transfer from microlayer evaporation underneath a growing bubble on micro-pillar arrayed surfaces using Direct Numerical Simulations (DNS). The evolution of the microlayer is reproduced in the DNS by considering a bubble growth driven by the local temperature gradient. The effects of micro-pillar structures on the microlayer profile and the heat transfer performance are systematically studied and analyzed. Our simulation results reveal three distinct microlayer morphologies related to micro-pillar structures: the undisturbed microlayer, the disturbed microlayer, and the disrupted microlayer. It can be further generalized as the greater the spacing and height of the micro-pillars, the more disrupted the microlayer becomes. Due to the reduction of microlayer thickness, more disruption means higher microlayer heat transfer coefficient. However, this accelerates microlayer depletion and thus reduces the overall heat transfer potential from microlayer evaporation during its life cycle in nucleate boiling. Based on these findings, a strategy is revealed for the design of micro-pillar arrayed surfaces to achieve optimal heat transfer performance of the microlayer.

Effects of bubble-induced turbulence on interfacial species transport: A direct numerical simulation study

A direct numerical simulation (DNS) study is carried out to understand the effects of bubble-induced turbulence on the interfacial species transport. A volume-of-fluid (VOF) method is used in the DNS to simulate the multiphase flow in a box containing one or two deformable bubbles. The bubbly flows with Schmidt numbers 1≤Sc≤64, Reynolds numbers 100≤ReB≤750, Eötvös number Eo=1.21 and gas volume fractions 2.8%≤αg≤22.1% have been investigated. The DNS results indicate that the one-dimensional energy spectra evolve as the power −3 of the wavenumber at large scales. At a high Schmidt number, an inertial subrange characterized by the -5/3 slope can be found in the power spectra of species concentration. The power spectra of species concentration for different Schmidt numbers become close to each other when the wavenumber is scaled with Sc0.5. The bubble-induced turbulence enhances the oscillation of the transient Sherwood number, however, it has marginal effects on the time-averaged value. By contrast, the time-averaged Sherwood number for a fixed Peclet number increases with an increase of αg, indicating that convection plays a more important role in species transport when the bubbles are more densely populated. A possible reason is that, the neighboring bubbles enhance the spatial fluctuations of the velocity, which favor the interfacial species transfer. The energy spectrum confirms that an increase of αg leads to stronger spatial fluctuations of the vertical velocity component at low wavenumbers.

A DNS study of pulverized coal combustion in a hot turbulent environment: Effects of particle size, mass loading and preferential concentration

In the present study, pulverized coal combustion in a hot turbulent environment is investigated using direct numerical simulation (DNS). The coal particles are tracked in the Lagrangian framework while the flow is solved in an Eulerian way. A model involving two competing reactions is used for describing the process of coal devolatilization. The volatile matter is treated as a postulated substance (CaHbOc) and a two-step global reaction mechanism for volatile combustion is adopted. The char combustion is modeled using a one-step global reaction model. The effects of various factors, such as particle size, mass loading and preferential concentration, on the combustion process of coal particles were explored. It was found that, the location of reaction zones appears more upstream and the volatile matter concentration is higher for the cases with smaller particles. The combustion modes were examined. It was shown that the contribution of premixed combustion decreases with increasing particle size while non-premixed combustion prevails in all cases. The increase of particle mass loading results in an increase of volatile matter mass fraction, which leads to higher heat release rate and combustion temperature. Through Voronoï analysis, the preferential concentration of particles was examined. It was found that the preferential concentration behavior of particles is more prominent when the particle Stokes number (St) is close to unity. The volatile matter concentration is higher when particles are preferentially concentrated, facilitating coal particles to ignite.