Nejla Mahjoub Said

Three-Dimensional Numerical Calculations of a Jet in an External Cross Flow: Application to Pollutant Dispersion

We present a three-dimensional numerical simulation of a circular turbulent jet issuing transversely into a uniform air stream. In the first part an air-helium jet is considered and the three-dimensional structure of the flow field is discussed. Then, a comparison between the numerical results of four turbulence closure models (three first-order models and a second-order one) are presented and compared with the experimental data given by Crabb et al. , and Andreopoulos et al.

Flow Field Measurement in a Crossflowing Elevated Jet

Structural features resulting from the interaction of a turbulent round jet issuing transversely into a uniform stream are described with the help of flow visualization and the PIV technique. The jet exits from a rigidly mounted pipe projecting at a distance from the floor of a tunnel. The aim of the present work is to investigate the flow structure in the near-field jet-pipe exit. Jet-to-crossflow velocity ratios from 0.375 to 3 were revealed at Reynolds numbers from 1660 to 6330. Flows in the vertical symmetry plane and horizontal plane across the jet-wake, jet-exit, and pipe-wake regions are investigated. The measured velocity fields present quantitative characteristics of the streamlines, vortices, and topological features of the flow structures. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies.

Dispersion of Twin Inclined Fume Jets of a Variable Height within a CrossFlow

Twin elliptic inclined tandem jets are emitted within an oncoming cooler crossflow. The jets contain a non reactive fume whose dispersion is tracked all over the surrounding domain. Such a configuration may be found in chimney stacks, ships’ chimneys, etc. We propose to evaluate in the present paper the impact of the jets’ height on the resulting dispersion process. To reach this goal, a numerical simulation of a double jet model of variable height is carried out by means of the finite volume method together with a non uniform grid system. The model, validated by previous experimental data, allowed the tracking of the emitted fume by studying the evolution of a single particle contained within this fume, the Carbone dioxide (CO2) mass fraction. This is possible thanks to the assumption of handling a non reactive fume, which is adopted only to simplify the calculations. The CO2 mass fraction was mainly tracked between the emitting nozzles, in a try to find out the changes brought by the extension of the emitting jet nozzles on the flow trapped between them.

Assessment of a Chimney Jet Flowing Around an Obstacle

A numerical modeling study was undertaken in order to examine the structure of the flow issued from a bent chimney around a parallelepiped obstacle. The main purpose of this work is to track the overall evolution of the flow and determine the thermal and mass transfer features that characterize the resulting flow field. We first validated the numerical model with experimental data. The experimental data are depicted by means of a particle image velocimetry technique. The numerical model is simulated by solving the mass, momentum, energy, and species equations. The finite-volume method is used, together with the second-order Reynolds stress model. A good level of agreement was achieved between the experimental data and numerical calculations. The comparison concerned both the mean and fluctuating dynamic features. Once validated, our model allowed the evaluation of the effect of different parameters on the heat and mass transfer features. These parameters consist of the velocity ratio (0.5, 1, and 1.6), the distance separating the chimney and the obstacle (100 mm and 200 mm), the height of the obstacle (50 mm, 90 mm, and 150 mm) and the orientation of the bent chimney (15°, 30°, and 45°). The evaluation of the effects of these different parameters has the ultimate purpose of determining the optimum conditions for the weakest concentration of pollutants.

Dispersion of Twin Inclined Fume Jets of Variable Temperature within a Crossflow

This paper deals with the interaction of twin elliptic jets with a cooler oncoming crossflow. The jet nozzles are placed tandem with the main flows, three diameters one from another and initially inclined with a 60° angle. The exploration of the resulting flowfield was carried out numerically by means of the finite volume method together with the RSM (Reynolds Stress Model) second order turbulent closure model and non uniform grid system that was particularly refined near the nozzles. After validation with reference to PIV (particle image velocimetry) experimental data, the model was upgraded by discharging a non reactive fume from the jet nozzles and introducing a variable temperature gradient between the interacting flows. We focused mainly in the present work on the determinant role of the temperature gradient on the dispersion of the discharged pollutants within the domain. Such a study is likely to optimize (control, reduce, eliminate, etc.) one of the most alarming nowadays’ environmental concern: the atmospheric pollution.

Thermal Field of Twin Variably Elevated Tandem Jets in Crossflow

Consideration is given to twin inline elliptical fume jets issuing within an oncoming cooler environmental crossflow. Jets are emitted from similar nozzles, characterized by a variable injection height. Such a configuration is found at large scale, in the industrial urban zones, and more particularly in multiple chimney power plants. It is found at small scale as well like in cooling in electronic devices. The present study is carried out numerically by means of the finite volume method together with the Reynolds Stress Model (RSM) second order turbulent closure model and non uniform grid system particularly refined around the emitting nozzles. Emphasis is put on the temperature distribution around the emitting nozzles in order to highlight the joint effect of the jets elevation and temperature. It was mainly found that both parameters are complementary and help straitening the discharged jets, leading their thermal mixing away from the injection ground. Nomenclature

Flow Structure Issued from a Bent Chimney around a Cylindrical Obstacle: Effect of the Aspect Ratio

We present in this work a numerical study of pollutants’ dispersion resulting from a bent chimney around a cylindrical obstacle. A three-dimensional numerical model, based upon the RSM (Reynolds Stress Model) turbulence closure model was used. The adopted grid is not uniform, particularly refined near the chimney and around the cylindrical building. The domain is large enough to be able to visualize the recirculation zones and the vortices created by the building. The results obtained in three dimensional configurations make possible the description of the dynamics and mass features under different aspect rations.

Effect of the Separating Distance of Twin Buildings on the Generated Flow Structure

The present work consists in a numerical examination of the dispersion of pollutants discharged from a bent chimney and crossing twin similar cubic obstacles placed in the lee side of the source. The resulting flow is assumed to be steady, three-dimensional and turbulent. Its modelling is based upon the resolution of the Navier Stokes equations by means of the finite volume method together with the RSM (Reynolds Stress Model) turbulent model. This examination aims essentially at detailing the wind flow perturbations, the recirculation and turbulence generated by the presence of the twin cubic obstacles placed tandem at different spacing distances (gaps): W = 4 h, W = 2 h and W = 1 h where W is the distance separating both buildings.

Experimental and Numeric Study of Flow Around a Parallelepiped Obstacle Issued from a Bent Chimney

Our work consisted in carrying out an experimental investigation in order to study the structure of the flow field issuing from a bent chimney and deviated when meeting a parallelepiped obstacle. For the matter we used: the Particle Image Velocimetry (PIV). A parallel numeric simulation of the problem was also elaborated and compared with the above mentioned experimental results. A three-dimensional numerical model based on the RMS turbulence closure model was used. The adopted grid is not uniform, particularly refined near the chimney and around the obstacle. A good level of agreement was achieved between the experimental data and numerical calculations. Once the model validated, we studied the effects of the distance separating the chimney and the obstacle. The tested values are Dch-obs= 10 cm and Dch-obs= 20 cm.