M. Ayrault

Passive scalar dispersion in a turbulent boundary layer from a line source at the wall and downstream of an obstacle

Simultaneous measurements of the velocity and scalar concentration fields have been made in the plume emitting from a two-dimensional line source at the wall. The source is one obstacle height, h , downstream of a two-dimensional square obstacle located on the wall of a turbulent boundary layer. These measurements were made in two fluid media: water and air. In both media particle image velocimetry (PIV) was used for the velocity field measurements. For the scalar concentration measurements laser-induced uorescence (LIF) was used for the water flow and Mie scattering diffusion (MSD) for the air flow. Profiles of the mean and root-mean-square streamwise and wall-normal velocity components, Reynolds shear stress and mean and root-mean-square scalar concentration were determined at x = 4 h and 6 h downstream of the obstacle in the recirculation region and above it in the mixing region. At these streamwise stations the scalar fluxes, uc and vc , were also determined from the simultaneous velocity and scalar concentration field data. Both of these fluxes change sign from negative to positive with increasing distance from the wall in the recirculating region at 4 h . A conditional analysis of the data was carried out by sorting them into the eight categories (octants) given by the sign combinations of the three variables: ± u , ± v and ± c . The octants with combinations of these three variables that correspond to types of scalar concentration flux motions that can be approximated by mean gradient scalar transport models are the octants that make the dominant contributions to uc and vc . However, in the recirculating zone, counter-gradient transport type motions also make significant contributions. Based on this conditional analysis, second-order mean gradient models of the scalar and the momentum uxes were constructed; they compare well to the measured values at 4 h and 6 h , particularly for the streamwise scalar flux, uc . Additional measurements of the velocity and concentration fields were made further downstream of the reattachment location in the wake region of the air flow. The mean velocity deficit profile determined from these measurements at x = 20 h compares quite well to a similarity solution profile obtained by Counihan, Hunt & Jackson (1974). Their analysis was extended in the present investigation to the concentration field. The similarity solution obtained for the mean concentration compares well to profiles measured at x = 12 h , 15 h , and 20 h , up to about three obstacle heights above the wall.

Concentration flux measurements of a scalar quantity in turbulent flows

A method for determination of velocity-concentration fluxes is presented that combines two conventional imaging techniques, particle image velocimetry (P.I.V.) and planar laser-induced fluorescence (P.L.I.F.). The passive concentration jet was a perfect mixture of fluorescein dye and solid particles submerged in an isotropic homogeneous turbulent channel. The light intensity fluoresced by the dye and the light intensity scattered by the particles were recorded separately on two synchronized cameras by using appropriate high and low-pass filters. Two different sets of images were thus obtained simultaneously. Once digitized and numerically processed, they provide the space and time evolution of velocity and concentration instantaneous fields. Thus, the velocity-concentration correlations can easily be determined. The statistical results for velocity and concentration are compared with classical results in order to validate the technique. We finally report some results giving velocity-concentration fluxes.

Negative buoyancy effects on the dispersion of continuous gas plumes downwind solid obstacles

An investigation of continuous dense gas plumes released from a surface area source and dispersing downwind solid obstacles is presented. Comparison with passive plumes released in the same experimental conditions enables us to emphasize the buoyancy effects. Some quantitative statistical results such as mean concentration, root-mean-square fluctuations, skewness and kurtosis are given. The experimental technique used, visualizations and digital image processing, allows us to characterize the geometrical evolution of the plumes.

An experimental study on the evolution and dispersion of a cloud of gas heavier than air

Abstract This paper presents an experimental study of the evolution and dispersion of a cloud of gas heavier than air in a simulated neutral atmospheric boundary layer. First, a comparative study of instantaneous releases for two density cases of 1 and 1.87 are presented. The results were obtained by laser tomography technique. Then, we report some statistics on the fluctuating concentration field in a two-dimensional plane. The measurements were obtained by a technique based on visualization and digital image processing. The method is well suited for this type of study, provided that the number of observations is sufficient.

Effects of a two-dimensional low hill in a thermally neutral and stably stratified turbulent boundary layer

Abstract This paper presents an experimental study of the effects of a two-dimensional low hill on the mean and turbulent structure of a thermally neutral and a stably stratified boundary layer. The experiments are carried out in an atmospheric diffusion wind tunnel. The thermal stable stratification is obtained by an intense cooling of the floor by means of liquid and gaseous nitrogen. The hill, having a height of about 1 8 the boundary layer thickness and an aspect ratio equal to 1 3 , is placed perpendicular to the air flow. The measurements are made for the two stratification cases in different sections downwind of the hill and compared with the results obtained in flow without the hill. A mean velocity and temperature deficit is noted. Observations indicate that the thermal wake thickens more strongly than the neutral. The hill effects induce large perturbations on the Reynolds stresses, the vertical R.M.S. values and turbulent heat flux. The values in mean and fluctuant perturbations in the wake are found to decay as [X- X c L ] −1 .

Effects of thermal stable stratification on turbulent boundary layer characteristics

Abstract An experimental study of the influence of the buoyancy forces on the turbulent characteristics of a stable thermally stratified boundary layer, is carried out with the help of a device requiring only a comparatively small heat output. The stratification is obtained by an intense cooling of the floor of the wind tunnel used, by means of liquid nitrogen; the degree of stability reached is observable over a layer of about 30 cm thickness. The first results submitted here are in good accordance with those obtained by other authors by a bulk heating of the outflow, and allow us to contemplate studies on complex topography site in stable situations.

Experimental study on passive scalar diffusion in a turbulent shearless mixing-layer by Mie scattering diffusion and PIV

The experiments are run in an inhomogeneous turbulent shearless layer generated in a wind tunnel with two different meshes. A PIV system synchronized simultaneously with a Mie scattering diffusion concentration system is used in order to obtain the velocity and concentration fields at the same instant. We present here some statistical results for both velocity and concentration and also the mean flux. Filtering and subgrid scale modeling are applied to experimental velocity and concentration field in order to check the effect of the length of filter and the accuracy of subgrid model of LES.

Modèle stochastique de diffusion continu en temps: approximation différentielle de l'équation d'évolution de la densité de probabilité de la concentration et sa solution asymptotique dans le cas d'une turbulence homogène

This Note presents the results obtained from the theoretical analysis of the continuous stochastic mixing model (CSM model) and its discretised counterpart. The CSM model contains an unspecified coefficient ξ. When ξ is a random variable uniformly distributed in the range [0, 1], the CSM model is reduced to the Hsu and Chen (1991) model. Differential approximation of the concentration pdf equation corresponding to the discretised CSM model for homogeneous turbulence is derived. The analysis shows that the pdf shape tends to a Gaussian shape only in the case when ξ is a deterministic variable. The model is then in agreement with experimental data (Jayesh and Warhaft, 1992).

Effect of Complex Terrain on Turbulent Diffusion

The diffusion from sources placed near a step is studied through a quantitative visualization system. The geometry of the flow consists in a step of 0.15 m placed in a boundary layer of 0.6 m thick. Two angles of the step with the free stream velocity are studied (90°–60°).