Turbulence structure and interaction with steep breaking waves

Large-eddy and interface simulation using an interface tracking-based multi-fluid flow solver is conducted to investigate the breaking of steep water waves on a beach of constant bed slope. The present investigation focuses mainly on the ‘weak plunger' breaking wave type and provides a detailed analysis of the two-way interaction between the mean fluid flow and the sub-modal motions, encompassing wave dynamics and turbulence. The flow is analysed from two points of views: mean to sub-modal exchange, and wave to turbulence interaction within the sub-modal range. Wave growth and propagation are due to energy transfer from the mean flow to the waves, and transport of mean momentum by these waves. The vigorous downwelling-upwelling patterns developing at the head and tail of each breaker are shown to generate both negative- and positive-signed energy exchange contributions in the thin sublayer underneath the water surface. The details of these exchange mechanisms are thoroughly discussed in this paper, together with the interplay between three-dimensional small-scale breaking associated with turbulence and the dominant two-dimensional wave motion. A conditional zonal analysis is proposed for the first time to understand the transient mechanisms of turbulent kinetic energy production, decay, diffusion and transport and their dependence and/or impact on surface wrinkling over the entire breaking process. The simulations provide a thorough picture of air-liquid coherent structures that develop over the breaking process, and link them to the transient mechanisms responsible for their local incidenc

DNS and LES of turbulent flow in a closed channel featuring a pattern of hemispherical roughness elements

Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) were performed for fully-developed turbulent flow in channels with smooth walls and walls featuring hemispherical roughness elements at shear Reynolds numbers Reτ = 180 and 400, with the goal of studying the effect of these roughness elements on the wall-layer structure and on the friction factor. The LES and DNS approaches were verified first by comparison with existing DNS databases for smooth walls. Then, a parametric study for the hemispherical roughness elements was conducted, including the effects of shear Reynolds number, normalized roughness height (k⁺ = 10–20) and relative roughness spacing (s⁺/k⁺ = 2–6). The sensitivity study also included the effect of distribution pattern (regular square lattice vs. random pattern) of the roughness elements on the walls. The hemispherical roughness elements generate turbulence, thus increasing the friction factor with respect to the smooth-wall case, and causing a downward shift in the mean velocity profiles. The simulations revealed that the friction factor decreases with increasing Reynolds number and roughness spacing, and increases strongly with increasing roughness height. The effect of random element distribution on friction factor and mean velocities is however weak. In all cases, there is a clear cut between the inner layer near the wall, which is affected by the presence of the roughness elements, and the outer layer, which remains relatively unaffected. The study reveals that the presence of roughness elements of this shape promotes locally the instantaneous flow motion in the lateral direction in the wall layer, causing a transfer of energy from the streamwise Reynolds stress to the lateral component. The study indicates also that the coherent structures developing in the wall layer are rather similar to the smooth case but are lifted up by almost a constant wall-unit shift y⁺(∼10–15), which, interestingly, corresponds to the relative roughness k⁺ = 10

Modélisation des écoulements avec interface gaz-liquide par simulation des grands tourbouillons (LES) des échelles de sous maille

The focus of the paper is on extending the Large-Eddy Simulation (LES) approach for turbulent, interfacial gas-liquid flows. While the direct numerical simulation (DNS) is generally not applied for practical multi-fluid flows, it provides relevant information about the flow physics, which may be of great help in deriving closure laws for the LES. The illustrative example provided in this paper consists of the DNS of counter-current gas-liquid flow studied by [?], which has revealed that the turbulence at sheared interfaces presents similarities with wall flows. Our first attempt to simulate the same flow using LES within the eddy-viscosity sub-grid scale (SGS) modelling context, has been revealed that a near-interface treatment is necessary to accommodate the asymptotic behavior of turbulence, in very much the same way as in wall flows. The LES approach is shown to be potentially promising for tackling interfacial flows, and is clearly best suited for a range of similar problems in which immiscible phases are in direct contact, rather than using more conventional kinetic energy dissipation-based models

Les représailles non armées: droit et réalité dans les relations interétatiques contemporaines

Pas de résuméDoctorat en sciences politiquesinfo:eu-repo/semantics/nonPublishe

Large Eddy & Interface Simulation (LEIS) of disturbance waves and heat transfer in annular flows

A numerical method for forced convective boiling in an annulus needs to be developed in order to elucidate the reason for nucleation enhancement by disturbance waves. We first developed a numerical strategy to model the development of disturbance waves in annular flows where the highly turbulent gas core flow drives the laminar liquid flow upwards using advanced CFD tool TransAT. In which, the interface tracking method (e.g. Level-set) combined with a scale-resolving turbulence simulation technique (Large Eddy Simulation) was employed to capture dominant turbulence and interfacial scales. Then, the disturbance wave phenomenon in a vertical steam-water annulus system was investigated and analyzed. The finding reported in the present work provides insight into the evolution of disturbance wave and its influence on the heat transfer in annular flow. The modeling results revealed that locally hot ‘spots’ occurred upstream of disturbance wave. These locally overheated zones could play key roles in activating the nucleation boiling sites. In addition, the inception criteria of disturbance wave were explored by adjusting the mass flux of saturated water. And it was found no disturbance waves occurred at liquid film Reynolds number lower than the critical value, 225

Turbulent transport mechanisms in oscillating bubble plumes

International audienceThe detailed investigation of an unstable meandering bubble plume created in a 2-m-diameter vessel with a water depth of 1.5 m is reported for void fractions up to 4% and bubble size of the order of 2.5 mm. Simultaneous particle image velocity (PIV) measurements of bubble and liquid velocities and video recordings of the projection of the plume on two vertical perpendicular planes were produced in order to characterize the state of the plume by the location of its centreline and its equivalent diameter. The data were conditionally ensemble averaged using only PIV sets corresponding to plume states in a range as narrow as possible, separating the small-scale fluctuations of the flow from the large-scale motions, namely plume meandering and instantaneous cross-sectional area fluctuations. Meandering produces an apparent spreading of the average plume velocity and void fraction profiles that were shown to remain self-similar in the instantaneous plume cross-section. Differences between the true local time-average relative velocities and the difference of the averaged phase velocities were measured; the complex variation of the relative velocity was explained by the effects of passing vortices and by the fact that the bubbles do not reach an equilibrium velocity as they migrate radially, producing momentum exchanges between high- and low-velocity regions. Local entrainment effects decrease with larger plume diameters, contradicting the classical dependence of entrainment on the time-averaged plume diameter. Small plume diameters tend to trigger ‘entrainment eddies' that promote the inward-flow motion. The global turbulent kinetic energy was found to be dominated by the vertical stresses. Conditional averages according to the plume diameter showed that the large-scale motions did not affect the instantaneous turbulent kinetic energy distribution in the plume, suggesting that large scales and small scales are not correlated. With conditional averaging, meandering was a minor effect on the global kinetic energy and the Reynolds stresses. In contrast, plume diameter fluctuations produce a substantial effect on these quantities