L.M. Portela

Numerical study of the near-wall behaviour of particles in turbulent pipe flows

Abstract The near-wall behaviour of particles is important in terms of predicting both the particle-deposition and the near-wall particle-concentration. In this paper, we study the near-wall behaviour of elastic-bouncing small heavy spheric particles in fully developed turbulent pipe flows without gravity, using direct numerical simulations (DNS) with a one-way point-particle approach. The particle-concentration is assumed to be small enough such that the influence of the particles on the fluid and interparticle interactions can be neglected. The focus of the paper is on: (i) the understanding of the differences between elastic-bouncing and absorbing walls, and (ii) the evaluation of simple “local-equilibrium” models. Our results show that the near-wall behaviour of elastic-bouncing walls is very different from absorbing walls. The absence of a mean radial particle-velocity leads to a much higher particle-concentration near the wall than in the case of absorbing walls. This can be explained by the absence of a “mean drag force”: the “turbophoretic effect”, due to the gradient in the particle-velocity fluctuation in the radial direction, is balanced only by the “drift-velocity”, due to a gradient in the particle-concentration. Our results indicate that, from a pragmatic perspective, simple “local-equilibrium” models for the “turbophoretic effect”, assuming a proportionality between the particle and fluid “Reynolds-stresses”, are adequate, except very close to the wall, where the reduction in the radial fluid-velocity fluctuation is not accompanied by an equivalent reduction in the radial particle-velocity fluctuation.

Direct numerical simulation analysis of local flow topology in a particle-laden turbulent channel flow

The results of point-particle Eulerian–Lagrangian direct numerical simulation (DNS) calculations of dilute particle-laden turbulent channel flow are used to study the effect of the particles on the local flow topology. It is found that in the viscous sublayer, the flow becomes increasingly more two-dimensional as the two-way coupling effect (due to interaction between particles and fluid flow) increases with increasing particle load. Beyond the viscous sublayer the modifications in flow topology are not strongly related to the preferential concentration of particles in the flow field, which is in contrast to previous channel flow simulations. The effect of particles on the turbulent flow beyond the viscous sublayer is mostly a result of the overall changing near-wall dynamics of the fluid flow.

Numerical Data for Reliability of LES for Non-isothermal Multiphase Turbulent Channel Flow

Turbulent non-isothermal fully-developed channel flow laden with small particles is investigated through numerical simulation combined with the tracking of the individual particles using DNS and LES. The simulations are performed at Reτ=180 and 395, with Pr=1.0, using the point-particle approach and neglecting the influence of the particles on the fluid and inter-particle interactions. The focus is on the interactions between particles and turbulence and their effect on the particles using DNS and LES. Presented data obtained through direct numerical simulation show new effects related to clustering and heat exchange for particles. The LES results shows that particles behaviour is very complex and to have proper results additional subgrid modelling for dispersed phase is required.

Modeling of low-capillary number segmented flows in microchannels using OpenFOAM

Modeling of low-Capillary number segmented flows in microchannels is important for the design of microfluidic devices. We present numerical validations of microfluidic flow simulations using the volume-of-fluid (VOF) method as implemented in OpenFOAM. Two benchmark cases were investigated to ensure the reliability of OpenFOAM in modeling complex physical phenomena in microfluidics, viz. 1) the steady motion of bubbles in capillaries, and 2) the formation of bubbles in T-junctions. We found that it is crucial to reduce spurious currents and to apply local grid refinement to capture the relevant flow physics. With these, we obtain good agreement between our numerical simulations and previously published theoretical and experimental data.

Benchmark test on particle-laden channel flow with point-particle LES

Dispersion of particles in a turbulent wall-bounded flow is crucial in many practical applications. For numerical simulation of particle-laden turbulent flow various approaches are available. Among these, LES is perhaps the most promising because its computational cost is lower than that of DNS and its predictive capability is much higher than Reynolds-Averaged Navier-Stokes methods especially in case of particles-turbulence interaction in boundary layers. Various subgrid models are available which have proved their validity for several types of flow. However, the treatment of particles in LES is still a relatively new topic with open questions regarding e.g. Sub-Grid Scales (SGS) effects on particle behavior and the modeling of particle-particle, particle-fluid or particle-wall interactions. To address these issues, an international collaborative benchmark test has been proposed as part of the activity of the COST Action P20 LESAID. The objective is to gather a large database of results obtained with different numerical methods, SGS models and physical models in order to resolve questions about the validity of these models. In this paper the first statistics of the benchmark for a base Eulerian-Lagrangian simulation of particle-laden channel flow, are presented. The specific simulation parameters have been chosen also to allow estimate of the quality of the LES results upon comparison with available DNS results (Marchioli et al., 2008) for the same test case. The groups participating in the benchmark are: UUD-UPI (Marchioli, Soldati, Salvetti); TUE (Kuerten); IMFT-ASU (Konan, Fede, Simonin, Squires); TUM (Gobert, Manhart); TUK-TUD (Jaszczur, Portela). Results provided by each group refer to a statistically stationary situation in which the particle concentration has reached a steady state. The time taken to reach steady concentration is very long (up to 2⋅104 in wall units (Marchioli et al., 2008)) thus making the required computational effort quite high even for a LES-based calculation.

Subgrid Particle-Fluid Coupling Evaluation in Large-Eddy Simulations of Particle-Laden Flows

Point-particle Eulerian-Lagrangian DNS/LES simulations allow us to deal with a large number of small particles, using relatively modest computer resources. When doing LES, one can consider the subgrid particle-fluid coupling, using a subgrid model, or simply ignore it. We present a criterion to evaluate the importance of the subgrid particle-fluid coupling on: (i) the particle motion, and (ii) the resolved fluid-motion. The criterion assumes that the particles can be treated as point-particles, from the perspective of both the resolved and subgrid motions, and it is based on simple “local equilibrium” models for the interaction between the particles and the subgrid fluid-motion. The criterion was applied to a high-resolution channel flow LES, with a moderate particle-loading. The results indicate that: (i) for heavy particles, the common practice of ignoring the subgrid particle-fluid coupling is adequate, (ii) for very-light particles a model for the subgrid-driven particle-velocity fluctuations might be important.Copyright

EFFECT OF THE PARTICLE-INDUCED TURBULENCE-MODIFICATION ON TWO-EQUATION MODELS FOR PARTICLE-LADEN WALL-BOUNDED TURBULENT FLOWS

In particle-laden flows, the particles can promote large changes in the turbulence characteristics of the fluid. Current two-equation models — in particular, the k – ɛ model — take into account only the “direct interaction” between the particles and the surrounding fluid, and do not consider the “indirect effects”, due to the disruption the particles promote in the turbulence dynamics itself. This disruption can promote large changes in the “standard values” of the “constants” of the k – ɛ model, which are not currently taken into account. In this paper, we study the influence of the particle-turbulence interaction on the k – ɛ model, using point-particle Eulerian-Lagrangian DNS/LES simulations, in a fully-developed turbulent channel flow laden with small heavy spheric particles. We focus on the effect the particles have on the constant C µ , associated with the kinematic eddy-viscosity. The particles promote large non-uniform changes in the value of C µ , which can increase or decrease, depending on the distance to the wall and the Reynolds number. The changes in the value of C µ can be understood in terms of the particle-effects on: (i) the balance between the Reynolds shear-stress and the turbulence kinetic energy, and (ii) the balance between production and dissipation of turbulence kinetic energy. The particles promote a large (very roughly) uniform decrease in the value of the kinematic eddy-viscosity; this suggests that in simple wall-bounded flows it might be possible to take into account the effect of the particles on the kinematic eddy-viscosity using simpler mixing-length models, without the use of a two-equation framework.

The Effect of Surfactants on the Flow Patterns in Vertical Air-Water Pipe-Flow

In this work, we consider the influence of surfactants on the flow pattern transitions of air-water flow in vertical pipes. Surfactants cause the formation of foam, which suppresses the irregularities in the flow. Thereby, the foam significantly decreases the gas flow rate associated with the transition between annular and churn flow. Furthermore, this transition is no longer independent of the liquid flow rate, as the foam can more easily suppress the churning at low liquid flow rates. At sufficiently large surfactant concentrations, the foam suppresses all churning, leading to a direct transition between annular and slug flow. Using results from flow visualisation, the effect of the surfactants on the morphology of the different flow patterns is analysed. The results provide important subsidies for a mechanistic model of air-water-foam flow.Copyright