Serge Simoëns

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.

Modeling aeolian erosion in presence of vegetation

Semiarid landscapes are characterized by vegetated surfaces. Understanding the impact of vegetation on aeolian soil erosion is important for reducing soil erosion or limiting crop damage through abrasion or burial. In the present study, a saltation model fully coupled with a large-eddy simulation airflow model is extended to vegetated landscapes. From this model, the sensitivity of sand erosion to different arrangements and type of plants (shrub versus tree) representative of semiarid landscapes is investigated and the wind erosion reduction induced by plants is quantified. We show that saltation processes over vegetated surfaces have a limited impact on the mean wind statistics, the momentum extracted from the flow by saltating particles being negligible compared to that extracted by plants. Simulated sand erosion patterns resulting from plant distribution, i.e., accumulation and erosion areas, appear qualitatively consistent with previous observations. It is shown that sand erosion reduction depends not only on vegetation cover but also on plant morphology and plant distribution relative to the mean wind direction. A simple shear stress partitioning approach applied in shrub cases gives similar trends of sand erosion reduction as the present model following wind direction and vegetation cover. However, the magnitude of the reduction appears significantly different from one approach to another. Although shrubs trap saltating particles, trees appear more efficient than shrubs to reduce sand erosion. This is explained by the large-scale sheltering effect of trees compared to the local shrub one.

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.

Large-eddy simulation and Lagrangian stochastic modeling of passive scalar dispersion in a turbulent boundary layer

A large-eddy simulation (LES) with the dynamic Smagorinsky–Germano subgrid scale (SGS) model is used to study the passive scalar dispersion in a turbulent boundary layer. Instead of resolving the passive scalar transport equation, fluid particles containing scalar are tracked in a Lagrangian way. The Lagrangian velocity of each fluid particle is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The movement of fluid elements containing scalar at a subgrid level is given by a three-dimensional Langevin model. The stochastic model is written in terms of SGS statistics at a mesh level. The results of the LES are compared with the wind-tunnel experiments of Fackrell and Robins (1982, Journal of Fluid Mechanics, 117, 1–26) and with the LES results of Sykes and Henn (1992, Atmospheric Environment A, 26, 3127–3144), who used a completely Eulerian approach with a non-dynamic SGS model. Our simulations predict the quantitative features of the experiments of Fackrell and Robins (1982, Journal Fluid Mechanics, 117, 1–26). Moreover, by using the Lagrangian approach, scalar fluxes are computed with no additional modeling assumptions and show good agreement with the experimental data. A classic mean-gradient model of the scalar flux is calculated from the computed results. The agreement between the directly computed fluxes and the classic mean-gradient model calculation is remarkable.

Three-dimensional solid particle positions in a flow via multiangle off-axis digital holography

We present a new development of digital off-axis (OA) holography for determining the instantaneous solid particle positions in a flow. This holographic imaging method uses a CCD camera for the simultaneous digital recording of two views of digital Fresnel OA holograms on the same support. The reconstruction is obtained numerically. The method provides two orthogonal views of the same flow area of interest at the same instant. It helps to overcome the depth of focus problem existing for the particle image reconstructions and that is inherent to the method. This method has the advantage of being simpler than the methods presently available, and it does not suffer from the flaws of in-line holographic configuration. Furthermore it is completely digital and thus avoids the cumbersome analysis following hologram recording. Digital holograms and digital reconstructions are obtained for solid particles of 200 mum moving into a stirred flow cell of 5 cm(3).

Scalar dispersion by a large-eddy simulation and a Lagrangian stochastic subgrid model

A hybrid Eulerian-Lagrangian large-eddy simulation (LES) is used to compute scalar dispersion in a turbulent flow. Instead of resolving the passive scalar transport equation, fluid particles are tracked in a Lagrangian way. In order to obtain the subgrid scale velocity component of fluid particles, a Lagrangian stochastic subgrid model is coupled with the Eulerian LES. The Lagrangian stochastic subgrid model is written in terms of subgrid scale statistics. The coupling is applied to the study of turbulent scalar dispersion. The results of our simulation are compared with a direct numerical simulation and with experimental results.

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.

Two beams two orthogonal views particle detection

This paper presents a new technique for recording the two views off-axis digital holography using only two beams, each one acting as an object beam for its proper view and as a reference for the other view. This technique allows one to obtain two orthogonal views of the same volume simultaneously using only two beams. This leads one to avoid the large focusing distance inherent to off-axis digital holography and gives the real position of any object in the working volume by crossing the two view data. Furthermore, the lateral resolution should be the same as the vertical one. The proposed technique was improved experimentally using a metallic wire in an L shape and four moving particles.

Two-Frame Optical Flow Formulation in an Unwarping Multiresolution Scheme

In this paper, we propose a new formulation of the Differential Optical Flow Equation (DOFE) between two consecutive images considering spatial and temporal information from both. The displacement field is computed in a Markov Random Field (MRF) framework. The solution is done by minimization of the Gibbs energy using a Direct Descent Energy (DDE) algorithm. A hybrid multiresolution approach, combining pyramidal decomposition and two-step multigrid techniques, is used to estimate small and large displacements. A new pyramidal decomposition method without warping process between pyramid levels is introduced. The experiments carried out on benchmark dataset sequences show the effectiveness of the new optical flow formulation using the proposed unwarped pyramid decomposition schema.

Optical flow robust estimation in a hybrid multi-resolution MRF framework

We propose in this paper a robust multi-resolution technique to estimate dense velocity field from image sequences. It couples a Gaussian pyramidal down-sampling decomposition together with a multi-grid approach. At each pyramid level, bilinear interpolation and efficient warping techniques are performed to generate a residual images. The displacement field is computed in a Markov random field (MRF) framework. We compare two different methods to minimize the Gibbs energy: a modified iterative conditional mode (ICM) and a graph-cut algorithm extended to multi-grid scheme. We validate and demonstrate the robustness of our approach on synthetic and real images for fluid experiment applications.