Nejla Mahjoub Saïd

Computational study of mixing behaviour of a turbulent jet issuing in a uniform counterflow at low velocity ratios

This paper proposes a computational study for the analysis of the velocity and the scalar concentration field of a round turbulent jet flowing into a uniform stream in opposite direction. The investigation is carried out for a range of low jet-to-counterflow velocity ratios; R = 1.3, 1.6, 2.2, 3.1 and 3.4. The Reynolds stress model is applied in numerical simulation to compare obtained results with experimental data from the literature. It is found that predicted results are in good agreement with the experimental data and that the jet fluid decays faster in the presence of a counterflow. The linearity between the penetration distance and the velocity ratio is verified and the axial fluctuating velocities along jet centreline appear to have two distinct peaks, except for the stronger counterflow. The enhanced mixing efficiency of the counterflowing jet is verified through the radial distribution of velocity and scalar concentration at different streamwise stations.

Experimental and numerical study of an offset jet with different velocity and offset ratios

The present study examines the configuration of an offset jet issuing into either a quiescent medium or a moving stream (co-flowing). The mean velocity and turbulence characteristics of the turbulent offset jet have been investigated using a particle image velocimetry technique at three velocity ratios and for two offset ratios. A numerical simulation of a three-dimensional offset jet has also been carried out by comparing the corresponding results with previous experimental data and our measurements. The numerical investigation was performed by means of the finite volume method together with a second-order turbulent closure model – the Reynolds stress model (RSM) – to examine the behavior of the flow for different velocity ratio and offset ratios. Results give a satisfactory agreement between the experimental data and the calculations. Data from the early flow region clearly show a significant influence of the velocity ratio and the offset ratio on the mean flow and turbulence characteristics.

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

ABSTRACT In the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–ϵ turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.

Numerical Simulation of Wave-Structure Interaction around an Obstacle

In this work, we study a turbulent two-phase free surface flow around an obstacle in unsteady regime. A dynamic study relating to the formation of coherent vortex structures enables us to determine the shape of the flow and to clarify its main characteristics (shear layer, recirculation and reattachment). We determine first the dynamic structure of the flow through a numerical approach using the computer code ANSYS Fluent (closure model is k-e). In the second part we study the impact of these vortices on such configurations. A series of numerical simulations have been conducted to further verify the applicability of this model for wave simulations interaction with vortex structures of various shapes.

Numerical Study of a Turbulent Offset Jet Flow

A dynamic study of the mean flow behavior of a three-dimensional turbulent offset jet issuing into a quiescent ambient is presented. The flow is characterized by a longitudinal variation of curvature, skewed impingement onto a flat surface, a recirculating region, and the development of a wall jet region. A numerical simulation is used, by means of the finite volume method with the second order turbulent closure model: the Reynolds stress Model (RSM), to investigate the influence of certain parameters such as jet discharge height and the geometric nozzle. Flow structure is described in the preimpingement, recirculation and impingement regions. Interdependence is shown among the offset height (h) and the geometric nozzle (plane jet and circular jet). The obtained results are presented in terms of the jet dimensionless velocity distribution, maximum velocity decay and vectors velocity of the flow. The jet decay is presented. The recirculation region is fed by a relatively strong backflow for the reported high offset height and it is shown that the reattachment point depends strongly with the jet form and the offset height.

Numerical Study of a Gas Jet Impinging on a Liquid Surface

The present study introduces a numerical model for one of the most important fluid–fluid interaction problems in industrial engineering applications, mainly a gas jet impinging perpendicularly onto a liquid interface. A better understanding of the process of the interaction of this type of flow was performed using the Reynolds Stress Model (RSM). The Volume Of Fluid (VOF) method is employed to follow the deformation of the liquid surface. The results from the numerical tests are comparable with those presented by Munoz (Appl Math Model 36:2687-2700, 2012) and it is found that computational results agreed well with experimental data. The obtained numerical results provide useful insight and a better understanding of the highly complex flow encountered in such processes. Moreover, we propose to examine the effect of the nature of the liquid on the development of the global flow. Dynamic characters of liquid surface such as the presence of the cavity and the formation of the wave were displayed.

CFD Modeling of Wastewater Discharges in a Sewer System

The malfunctioning of combined sewer systems can lead to an uncontrolled discharge of wastewater into receiving environments causing very serious pollution. The protection of these environments requires a control of the flows and the pollutants concentration. This approach takes into account the hydraulic operation of the sewer systems and the mechanism of the pollutant transfer. In this work, we are interested in a portion of a sewer system of Monastir city in order to reproduce the hydraulic phenomena that occur there. The numerical study was treated using “ANSYS Fluent” software. The standard k-e turbulence model and the multiphase VOF model are used in this work. The exploitation of the results is mainly carried out on the flow rates, the water velocities, and the free surface. Then, we described the evolution of the pollutant concentration, the free surface, and the sediment deposition by examining the various mechanisms of the turbulent flow.

Numerical Study of Wall Horizontal Turbulent Jet of Freshwater in a Homogeneous Co-flow Stream of Saltwater

The disposal of effluents in nature and more particularly in the sea is a very frequent practice. The discharge of wastewater into a receiving environment such as seawater is generally in the form of a turbulent jet. The efficiency of the dispersion of the jet depends on its characteristics of mixing with the ambient environment and when the latter is in motion, it is significantly modified. This paper investigates a numerical study about buoyant wall turbulent jet in co-flow stream. A light fluid of freshwater is injected horizontally and tangentially to a plane wall into homogenous moving environment of saltwater. Since the domain temperature is assumed to be constant, the density of the mixture is a function of the salt concentration only. The mathematical model is based on the finite volume method and reports on an application of standard turbulence model k-e for steady flow with densimetric Froude number of 11 and Reynolds number of 3800. The aim of this work is to predict the influence of the co-flow stream on the dispersion of the jet and the mixing processes between freshwater jet and ambient saltwater. The concentration contours, the cling length, and the central trajectory of the jet are determined. It is found that the jet behavior depends on the co-flow-to-jet velocity ratio.

Turbulent-Heated Plane Compressible Jet Emerging in a Directed Co-Flowing Stream

In this present work, we have studied the directed co-flow effects on mean and turbulent flow properties of a turbulent heated plane jet with variable density discharging into a directed co-flowing stream. The first order k-eturbulence model is applied and has been compared with the existing experimental data. The Finite Volume Method (FVM) is used to discretize governing equations. First of all, it is found that predicted results are in satisfactory agreement with the experimental findings. Moreover, the numerical results of the mean and turbulent quantities has been presented and discussed in this work. The major interest of presenting this model is that to show the importance of the directed co-flow with a positive angle, which is enhancing the mixing. Furthermore, a qualitative analysis of the air entrainment would suggest that the higher inlet hot air jet temperature affect more the lateral in flow of air into the jet and jet lateral expansion is augmented when the inlet hot air jet temperature increases. Therefore, the increase of the inlet hot air jet temperature decreases faster the axial mean velocity and thus more entrainment air is required.

Comparative Investigation of Turbulence Modeling in Counterflowing Jet Predictions

In this chapter, a comparison between a number of turbulent models is done in order to investigate turbulence modeling in a turbulent round jet flowing into a counterflow stream. For this purpose, three turbulence closure models are tested: Standard k-e model, k-e RNG model, and RSM model. The studied cases of jet-to-counterflow velocity ratios are ranging between R = 3 and R = 15. Velocity and concentration fields are predicted through the centerline velocity and dilution decay along the downstream jet axis. Predicted results are compared with available experimental data from the literature. It is found that the three tested models behave in a similar way in the first region, however, in the further region, a difference between curves from different models appears. This discrepancy seems to depend on the jet-to-current velocity ratio. The k-e models are found to underestimate the experimental data, while the second-order closure model RSM is found to be the best model to predict the jet diffusion.