Numerical study of the propagation of lean hydrogen-air flames in Hele-Shaw cells

International audienc

Legitimacy of the narrow-channel approximation for the study of flames propagating between two closely-spaced parallel plates

International audienc

Evaluation of fast atmospheric dispersion models in a regular street network

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications

Comparison between LES and RANS computations for the study of contaminant dispersion in the MUST field experiment

The Seventh Conference on Coastal Atmospheric and Oceanic Prediction and Processes joint with the Seventh Symposium on the Urban Environment (10-13 September 2007) (San Diego, CA)Depto. de Análisis Matemático y Matemática AplicadaFac. de Ciencias MatemáticasTRUEpu

Preferential concentration and settling of heavy particles in homogeneous turbulence

International audienceVoronoï diagrams are used to analyze one-way coupling direct numerical simulation data of heavy particles settling in homogeneous turbulence. Preferential concentration and clustering of the inertial particles are analyzed for an extended range of particle Stokes and Rouse numbers. Influence of preferential concentration on the settling velocity enhancement is addressed from statistics of particle and flow field quantities conditioned on the local concentration. While gravity is found to have almost no influence on the global characteristics of preferential concentration, the conditional statistics bring out a refined preferential sampling of the flow field resulting from the gravitational effects. This preferential sampling shows that beside the descending fluid velocity contribution, the settling velocity is further increased by the descending fluid acceleration. This effect cannot be detected from global estimations of the particle concentration field. A 2D analysis of the Voronoï cells is also presented to investigate their shape and orientation. It is found that clusters can be represented as 2D elongated manifolds. Their shape is shown to be similar in zero and non-zero gravity fields while Voronoï cells tend to be more elongated for Stokes numbers around unity. Under the gravity effects, they tend to be preferentially oriented perpendicularly to the gravitational axis

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

International audienceThe settling velocity of inertial particles falling in homogeneous turbulence is investigated by making use of Direct Numerical Simulation (DNS) at moderate Reynolds number that include momentum exchange between both phases (two-way coupling approach). Effects of particle volume fraction, particle inertia and gravity are presented for flow and particle parameters similar to the experiments of Aliseda et al. [1]. A good agreement is obtained between the DNS and the experiments for the settling velocity statistics, when overall averaged but as well when conditioned on the local particle concentration. Both DNS and experiments show that the settling velocity further increases with increasing volume fraction and local concentration. At the considered particle loading the effects of two-way coupling is negligible on the mean statistics of turbulence. Nevertheless , the DNS results show that fluid quantities are locally altered by the particles. In particular, the conditional average on the local particle concentration of the slip-velocity shows that the main contribution to the settling enhancement results from the increase of the fluid velocity surrounding the particles along the gravitational direction induced by the collective particle back-reaction force. Particles and the surrounding fluid are observed to fall together, which in turn results in an amplification of the sampling of particles in the downward fluid motion. Effects of two-way coupling on preferential concentration are also reported. Increase of both volume fraction and gravity is shown to lower preferential concentration of small inertia particles while a reverse tendency is observed for large inertia particles. This behavior is found to be related to an attenuation of the centrifuge effects and to an increase of particle accumulation along gravity direction, as particle loading and gravity become large

LES and RANS simulations of the MUST experiment. Study of incident wind direction effects on the flow and plume dispersion

In this study, we propose to assess and compare the performance of LES and RANS methodologies for the simulation of pollutant dispersion in an urban environment by making use of field and wind tunnel measurements of the MUST experiment configuration. First, the proposed analysis addresses the relevance of taking into account the small geometrical irregularities of the obstacle array in the computations. For this, local and spatial averaged time mean flow properties are compared for two geometries, one with a perfect alignment of the containers and another one including the irregularities present in the experiment. In both geometries the incident flow is orthogonal to the front array of obstacles. The second part of this study presents simulations with different approaching wind directions to analyse the effect of small changes in the incident wind direction on the flow and on the plume dispersion. In this second part, the mean concentration field is compared with the experimental data and an analysis that relates the channelling effects with the plume deflection is provided

A TROPICAL PACIFIC PREDICTION SYSTEM OF INTERMEDIATE COMPLEXITY: ROLE OF THE VERTICAL STRUCTURE VARIABILITY

An intermediate ocean-atmosphere coupled model of the tropical Pacific is used to investigate the sensitivity of the seasonal forecasts to the configuration of the oceanic vertical structure. The models consist in a three baroclinic mode tropical Pacific Ocean model and a Gill (1980)'s tropical atmosphere. The predictive skill of the model using a simple nudging method for the initialization is first presented from 1970 and compared to the results of other prediction system of similar complexity, which emphasizes the modulation of the skill on decadal timescales. It is then demonstrated that the skill is critically dependant on the energy distribution on the baroclinic mode, higher-order mode contributions being favored at some period and not at others. Linear Green's function are used to assimilate satellite observations (SST and wind) and derive the optimized set of parameters that determines the vertical structure in the model for a particular period of time. The scheme is first tested for the period prior to the development of the 1997 El Niño. It is shown that substantial improvement in forecasting the event is realized for an increase of the relative contribution of the higher-order modes through the model parameters setting. Assimilation experiments of satellite data for the September 2003-February 2004 period are carried for producing initial conditions for the coupled model. Results of forecasts runs for 2004 are presented and discusse

Piston effect in a supercritical fluid sample cell : A phenomenological approach of the mechanisms

We report on the analysis of the response of a near-critical fluid sample cell submitted to heating at different distances from the critical temperature Tc, in the Mir station microgravity environment. We recall the hydrodynamic and thermodynamic bases of the mechanisms of fast adiabatic heating, also called "thermalization by Piston Effect" (PE). We give a phenomenological approach of these mechanisms in a highly compressible critical fluid submitted to heating until a steady state is reached, and present the main results of numerical experiments in a Van der Waals fluid. We then use this phenomenological understanding to analyze an experimental case of heating in reduced gravity onboard Mir, using the Alice 2 facility between 1996 and 1999 (Cassiopea to Perseus missions). We have measured the characteristic time of the PE as a function of the critical temperature distance, and investigated the crossover behavior from the pure adiabatic regime for a surrounding wall of infinite conductivity, to the bottleneck conductivity regime induced by a real cell. Close to Tc, we have evidenced the key influence of the geometry of the cell and of the thermal properties of the wall materials