Djamel Lakehal

Direct numerical simulation of turbulence in a sheared air-water flow with a deformable interface

Direct numerical simulation has been performed to explore the turbulence near a freely deformable interface in a countercurrent air–water flow, at a shear Reynolds number Re� = 171. The deformations of the interface fall in the range of capillary waves of waveslope ak =0 .01, and very small phase speed-to-friction velocity ratio, c/u� .T heresults for the gas side are compared to open-channel flow data at the same shear Reynolds number, placing emphasis upon the influence of the waves in the interfacial viscosity-affected region, and away from it in the outer core flow. Comparison shows a similarity in the distribution of the turbulence intensities near the interface, confirming that for the range of flow conditions considered, the lighter phase perceives the interface like a flexible solid surface, at least in the limit of non-breaking waves. Overall, in a time-averaged sense, the interfacial motion affects the turbulence in the near-interface region; the most pertinent effect is a general dampening of the turbulent fluctuating field which, in turn, leads to a reduction in the interfacial dissipation. Furthermore, the turbulence is found to be less anisotropic at the interface than at the wall. This is confirmed by the analysis of the pressure– rate-of-strain tensor, where the effect of interfacial motion is shown to decrease the pressure strain correlation in the direction normal to the interface and in the spanwise direction. The analysis of the turbulent kinetic energy and Reynolds stress budgets reveals that the interface deformations mainly affect the so-called boundary term involving the redistribution of energy, i.e. by the action of pressure, turbulent fluctuations and molecular viscosity, and the dissipation terms, leaving the production terms almost unchanged. The non-zero value of the turbulent kinetic energy at the interface, together with the reduced dissipation, implies that the turbulent activity persists near the interface and contributes to accelerating the turbulent transfer mechanisms. Away from the interface, the decomposition of the fluctuating velocity gradient tensor demonstrates that the fluctuating rate-of-strain and rate-of-rotation at the interface influence the flow throughout the boundary layer more vigorously. The study also reveals the streaky structure over the deformable interface to be less organized than over a rigid wall. However, the elongation of the streaks does not seem to be much affected by the interfacial motion. A simple qualitative analysis of the quasi–streamwise vortices using different eduction techniques shows that the interfacial turbulent structures do not change with a change of boundary conditions.

Interface tracking towards the direct simulation of heat and mass transfer in multiphase flows

Abstract The paper presents recent trends in the development of prediction methods for the direct numerical simulation of multiphase flows based on the one-fluid formalism coupled with various interface tracking algorithms. The methods are based on solving a single set of transport equations for the whole computational domain and treating the different phases as a single fluid with variable material properties. Changes in these properties are accounted for by advecting a phase indicator function. Interfacial exchange terms are incorporated by adding the appropriate sources as delta functions or smoothed gradients of the composition field at or across the interface. The strategies are first discussed within the isothermal phase context and then for situations featuring inter-phase heat and mass transfer. Various aspects such as the treatment of capillary forces are discussed, supported by selected examples demonstrating recent progress drawn from the current work of the authors.

Mechanisms of particle deposition in a fully developed turbulent open channel flow

Particle dispersion and deposition in the region near the wall of a turbulent open channel is studied using direct numerical simulation of the flow, combined with Lagrangian particle tracking under conditions of one-way coupling. Particles with response times of 5 and 15, normalized using the wall friction velocity and the fluid kinematic viscosity, are considered. The simulations were performed until the particle phase reached a statistically stationary state before calculating relevant statistics. For both response times, particles are seen to accumulate strongly very close to the wall in the form of streamwise oriented streaks. Deposited particles were divided into two distinct populations; those with large wall-normal deposition velocities and small near-wall residence times referred to as the free-flight population, and particles depositing with negligible wall-normal velocities and large near-wall residence times (more than 1000 wall time units), referred to as the diffusional deposition population....

Large-eddy simulation of bubbly turbulent shear flows

This paper reports on recent advances in the application of the large-eddy simulation (LES) approach to turbulent, vertical mixing layers containing bubbles at low void fraction. The method is based on the filtered multi-fluid equations derived from the application of a single component-weighted volume-averaging process. The subgrid-scale (SGS) modelling is based on the Smagorinsky kernel in both its original form and the dynamic procedure of Germano. Parameter studies have been undertaken to determine the effects of the ratio of the cut-off filter to the typical length scale characterizing the dispersed phase, the influence of the lift coefficient, the performance of the SGS models and the importance of inlet turbulence levels. A new model is proposed for possible bubble-induced turbulence modulation, in which the mixing length of the dispersed phase at the SGS is inferred dynamically from the resolved flow field. By averaging over times longer than the dynamic time scales of the turbulent fluctuations, ...

Multi-physics treatment in the vicinity of arbitrarily deformable gas-liquid interfaces

A novel three-dimensional problem formulation is introduced for the simulation of turbulent interfacial multi-fluid flows. The strategy is built around the large eddy simulation (LES) concept, and can be employed for interfacial heat and mass transfer problems in which use can be made of either scalar transfer correlations, or exact mass/energy jump conditions. This multi-physics treatment capability at arbitrarily deformable interfaces translates into two main features: (i) a reconstructed distance function (RDF) is introduced to define a level-set interface-normal length scale, and (ii) an interfacial shear velocity is defined on the distance function support for further use in near-interface transport models. The solution algorithm uses VOF with piecewise planar interface reconstructions on a twice-as-fine mesh, and infers the convective mass fluxes from the interface solution to promote momentum conservation. The interfacial shear velocity defined on the distance function support is introduced to accommodate the asymptotic behaviour of turbulence approaching the interface in a proximity-dependent manner. Provided with highly accurate distance function data, the scheme generates near-interface damping functions that are second-order accurate and independent of interface orientation. The damping was found to be affected by errors introduced into shear velocity estimates by the high-frequency errors in the RDF scheme near the interface. The methodology has been applied for the simulation of a wave breaking scenario featuring multiple modes and interfacial length scales.

Direct Numerical Simulation of Turbulent Heat Transfer Across a Mobile, Sheared Gas-Liquid Interface

The impact of interfacial dynamics or turbulent heat transfer at a deformable, sheared gas-liquid interface is studied using Direct Numerical Simulation (DNS). The flow system comprises a gas and a liquid phase flowing in opposite directions. The governing equations for the two fluids are alternated solved in separate domains and then coupled at the interface by imposing continuity of velocity and stress. The deformations of the interface fall in the range of capillary waves of waveslope ak=0.01 (wave amplitude a times wavenumber k), and very small phase speed-to-friction velocity ratio, c/u*. The influence of low-to-moderate molecular Prandtl numbers (Pr) on the transport in the immediate vicinity of the interface is examined for the gas phase, and results are compared to existing will-bounded flow data. The shear-based Reynolds number Re* is 171 and Prandtl numbers of 1, 5, and 10 were studied. The effects induced by changes in Pr in both wall-bounded flow and over a gas-liquid interface were analyzed by comparing the relevant statistical flow properties, including the budgets for the temperature variance and the turbulent heat fluxes

On the modelling of multiphase turbulent flows for environmental and hydrodynamic applications

Abstract The paper examines a selection of well-established prediction methods employed for the modelling of multiphase turbulent flows presented in typical environmental and hydrodynamic applications. The main objective is to provide a basic understanding of the subject with a deliberate intention to simplifying the presentation. Turbulence is approached on the basis of the conventional one-point closure context. The experience gathered by the author and by others with various predictive strategies all based on the Eulerian–Eulerian (field description) and the Eulerian–Lagrangian methods are discussed and summarized; the goals, limitations, and required developments are described. Typical applications of each calculation method are presented, in which the interaction between the transported dispersed-phase and the field turbulence is treated on the basis of both one-way and two-way coupling. The case studies in question include aerosol production and transport over the oceans, pollutant dispersion in the atmospheric surface layer, hydrometeor impact on urban canopies, sedimentation of active sludge in secondary water clarifiers, and mixing and circulation within confined bubble plumes. Analysis of the various models reveals that for most of the reported applications the Reynolds averaged Navier–Stokes approach is inherently ill-posed and should be transcended by the promising large-eddy simulation concept.

Large-eddy simulation of sheared interfacial flow

Large-eddy simulations (LES) of a turbulent interfacial gas-liquid flows are described in this paper. The variational multiscale approach (VMS) introduced by Hughes for single-phase flows is systematically assessed against direct numerical simulation (DNS) data obtained at a shear Reynolds number Re⋆=171, and compared to LES results obtained with the Smagorinsky model, modified by a near-interface turbulence decay treatment. The models are incorporated in the same pseudospectral DNS solver built within the boundary fitting method used by Fulgosi et al. for air-water flow. The LES are performed for physical conditions allowing low interface deformations that fall in the range of capillary waves of wave slope ak=0.01. The LES results show that both the modified Smagorinsky model and the VMS are capable to predict the boundary layer structure in the gas side, including the decay process, and to cope with the anisotropy of turbulence in the liquid blockage layer underneath the interface. Higher-order turbulen...

Near-Wall Modeling of Turbulent Convective Heat Transport in Film Cooling of Turbine Blades With the Aid of Direct Numerical Simulation Data

The paper presents novel developments in the DNS-based, turbulence modeling strategy of Lakehal et al. developed for calculating jets in crossflow. The particular features of the model include: 1) dynamic coupling of the high-Re k« with a one-equation model resolving the near-wall viscosity-affected layer; 2) inclusion of the anisotropy of turbulent transport coefficients for all transport equations; 3) near-wall variation of the turbulent Prandtl number as a function of the local Reynolds number. Most of the important aspects of the proposed model are based on known DNS statistics of channel and boundary layer flows. The model is validated against experiments for the case of film cooling of a flat plate, where coolant air is injected from a row of streamwise inclined jets. Excellent results were obtained for this configuration as compared to earlier numerical investigations reported in the open literature. The model is then extended to calculate film cooling of a symmetrical turbine blade by a row of laterally injected jets for various blowing rates. Comparison of the calculated and measured wall-temperature distributions show that only with this anisotropy eddy-viscosity/diffusivity model can the spanwise spreading of the temperature field be well predicted and the strength of the secondary vortices reduced. Furthermore, results of additional calculations show that combining the anisotropy eddy viscosity model with the DNS-based relation for turbulent Prandtl number promotes the eddy diffusivity of heat vis-a `-vis that of momentum further, leading to an enhanced spanwise spreading of the jet. The performance of this new approach improves with increasing blowing rate. @DOI: 10.1115/1.1482408#

Two-Phase Convective Heat Transfer in Miniature Pipes Under Normal and Microgravity Conditions

Detailed numerical simulations have been performed to study the effect of flow orientation with respect to gravity on two-phase flow heat transfer (without phase change) in small diameter pipes. The Nusselt number distribution shows that the bubbly, slug, and slug-train regimes transport as much as three to four times more heat from the tube wall to the bulk flow than pure water flow. The flow blockage effect of the inclusions results in a circulating liquid flow superimposed on the mean flow. For upflow, the breakup into bubbles/slugs occurs earlier and at a higher frequency. The average Nusselt numbers are not significantly affected by the flow orientation with respect to gravity. A mechanistic heat transfer model based on frequency and length scale of inclusions is also presented.