Mahmoud El Hajem - Academia.edu (original) (raw)

Papers by Mahmoud El Hajem

WIT transactions on modelling and simulation, 1970

In order to provide a set of validation data for a computational technique, an experimental inves... more In order to provide a set of validation data for a computational technique, an experimental investigation of the flow field at the exit of a centrifugal pump impeller was carried out. Measurements have been conducted at design and offdesign flow rates. These data are compared to the results of a quasi-three dimensional computational technique. At design conditions, the two sets of data coincide well. While, at low flow rate the numerical model fails to give a prediction of the flow field.

HAL (Le Centre pour la Communication Scientifique Directe), Sep 17, 2018

Journal of Biomedical Science and Engineering, 2023

In dialysis treatment, the radio-cephalic arteriovenous fistula (RCAVF) is a commonly used fistul... more In dialysis treatment, the radio-cephalic arteriovenous fistula (RCAVF) is a commonly used fistula, yet its low maturation rate remains a challenge. To enhance surgical outcomes, the relationship between stenosis-prone locations and RCAVF anastomosis angle is studied during maturation by developing two sets of RCAVF models for early (non-mature) and mature RCAVFs at five anastomosis angles. The impact of hemodynamics and wall shear stress (WSS) is examined to determine optimal anastomotic angles. Results indicate that acute angles produce more physiological WSS distributions and fewer disturbed regions, with early stenosis-prone regions located near the anastomosis that shift to the bending venous segment during remodeling. A pilot study comparing clinical and numerical results is conducted for validation.

Physical review fluids, Mar 31, 2020

Understanding the behaviour of liquid phase turbulence near gas-liquid interfaces is of great int... more Understanding the behaviour of liquid phase turbulence near gas-liquid interfaces is of great interest in many fundamental, environmental, or industrial applications. For example, near-surface liquid side turbulence is known to enhance the mass transfers between the two phases. Descriptions of this behaviour for air-water systems exist in the literature, but the case of turbulence in a shearthinning liquid phase below a flat gas-liquid interface has never been considered so far. This paper consists in an experimental characterization of low Reynolds number, oscillating grid generated, near-surface turbulence in shear-thinning dilute polymer solutions, in the surface-influenced and in the viscous sub-layers. The energy transfer mechanism, known in the water case, is evidenced in dilute polymer solutions (DPS). An horizontal damping mechanism, similar to the one introduced by surfactants, is evidenced. The evolution of the viscous sub-layer depth can be explained by both viscous and shear-thinning effects, and it appears that a critical polymer concentration may exist within the dilute regime.

Experiments in Fluids, Mar 27, 2014

Fig. 2 Specification of the influence of surfactants in the medium comparing experimentally measu... more Fig. 2 Specification of the influence of surfactants in the medium comparing experimentally measured rising bubble velocities with data from the literature (Clift et al. 1978) for contaminated and pure water The online version of the original article can be found under

HAL (Le Centre pour la Communication Scientifique Directe), Jun 28, 2021

European Journal of Mechanics - B/Fluids, 2017

In this paper, a numerical study of air bubble formation and detachment in water (S-1) and in gly... more In this paper, a numerical study of air bubble formation and detachment in water (S-1) and in glycerin solutions (S-2 and S-3) was investigated using the volume of fluid (VOF) method. The full formation process was studied under the effect of operating conditions such as wettability of the orifice plate (static contact angle  S), orifice diameter and orifice air velocity. In addition, the influence of the viscosity of the liquid phase was examined. The numerical simulations were carried out for different orifice velocities satisfying the quasi-static bubble growth condition at low Capillary and Bond numbers. Under such conditions, the surface tension effect is dominant over viscous drag and buoyancy effects. The bubble growth at different instants predicted by the VOF simulation was experimentally validated in water. During the expansion/elongation stage significant bubble shape oscillations have been observed by the simulation. In water, bubble shape is mainly dominated by inertial and surface tension forces, and the influence caused by the viscous drag force could be neglected. However, when the viscosity of the solution increases, the bubble shape oscillations can be reduced due to the effect of viscous drag force.

Physics of Fluids, 2019

Oscillating grid apparatuses are well known and convenient tools for the fundamental study of tur... more Oscillating grid apparatuses are well known and convenient tools for the fundamental study of turbulence and its interaction with other phenomena since they allow to generate turbulence supposedly homogeneous, isotropic, and free of mean shear. They could, in particular, be used to study turbulence and mass transfer near the interface between non-Newtonian liquids and a gas, as already done in air-water situations. Although frequently used in water and Newtonian fluids, oscillating grid turbulence (OGT) generation has yet been rarely applied and never characterized in non-Newtonian media. The present work consists of a first experimental characterization of the flow properties of shear-thinning polymer (Xanthan Gum, XG) solutions stirred by an oscillating grid. Various polymer concentrations are tested for a single grid stirring condition. The dilute and semidilute entanglement concentration regimes are considered. Liquid phase velocities are measured by Particle Image Velocimetry. ...

HAL (Le Centre pour la Communication Scientifique Directe), Aug 1, 2019

La maîtrise du procédé de grenaillage par jet d'eau cavitant nécessite une meilleure compréhensio... more La maîtrise du procédé de grenaillage par jet d'eau cavitant nécessite une meilleure compréhension des mécanismes physiques en jeu. Ceci implique le recours à une instrumentation fine afin d'exploiter les paramètres pertinents de fonctionnement. Dans cette perspective, un banc expérimental basique a été mis en place. Il permet de montrer la faisabilité d'une mesure quantitative de pression vue par une surface au moment de l'implosion dans son voisinage d'une bulle de cavitation. Cette mesure est réalisée grâce à un capteur en poly(fluorure de vinylidène) PVDF. Ces mesures sont corrélées à un modèle analytique s'appuyant sur des enregistrements vidéo à haute cadence.

HAL (Le Centre pour la Communication Scientifique Directe), May 1, 2021

Shot peening is commonly used as an effective method to extend the service life of parts and redu... more Shot peening is commonly used as an effective method to extend the service life of parts and reduce the residual tensile stresses. The main principle of this surface treatment method is to introduce compressive residual stresses into parts, thereby preventing the crack-propagation. Water cavitation peening (WCP) is a recent technology that allows generating high-intensity pressure waves and micro-jet at the surface of parts via a cavitating jet. However, it is a challenge to predict the pressure intensity at the treatment surface and therefore being able to make predictive modelization of the process. To anticipate the pressure distribution around the cavitating jet, a multiple steps approach has been developed. First, through the study of a single cavitation bubble, understand the pressure pulse generated by a single bubble during its collapse process. The second step is to establish a numerical simulation of the cavitating jet near the nozzle with ANSYS Fluent. Accordingly, the cavitation area and the vapor volume fraction at different section of cavitating jet will be predicted. Then, the mechanical loading at the surface of parts will be defined by these sources. The second part of the approach is here studied. The simulation results will be compared with experimental high-speed camera imaging of the cavitating jet.

Tribology Letters, 2017

This paper deals with optical experimental methods for improving current knowledge on particle en... more This paper deals with optical experimental methods for improving current knowledge on particle entrapment in elastohydrodynamic contacts. Particular attention was paid to the possibilities offered by fluorescence-based techniques for mapping lubricant flow and tracking contaminant trajectories. A ball-on-disk tribometer, a fluorescent medium, and particle image velocimetry equipment were used together for this purpose. This original experimental setup enabled us to obtain lubricant streamlines and velocity vector maps in the contact area. Moreover, it allowed us, for the first time, to capture the details of trapping as the rejection of contaminating particles, in the context of lubrication, as it happened. Dynamic in situ observations clearly showed that the entrapment of debris-like particles was competing against the backflow occurring upstream, in particular in elliptical contacts. The results reported in this work confirm and expand previous results obtained with different methods, i.e., using a twindisk machine and CFD computations. Finally, we briefly describe as future work the potential for applying these fluorescence-based methods to questions that are still under debate in the lubrication community.

International Journal of Heat and Mass Transfer

Turbulence is known to enhance gas-liquid mass transfer and mixing of high Schmidt number dissolv... more Turbulence is known to enhance gas-liquid mass transfer and mixing of high Schmidt number dissolved gases in water by deforming the concentration boundary layer that develops at the interface. Fundamental mechanisms of surface renewal and injection have been progressively evidenced throughout the last decades, via fundamental experiments of low mean shear turbulence interacting with flat interfaces in water. However, and despite the obvious influence of non-Newtonian behaviours on gas liquid mass transfer in industrial and environmental applications, not such study exists (to the best of the author's knowledge) on whether and how these mechanisms apply in shear-thinning dilute polymer solutions (DPS). Following a previous work on near surface hydrodynamics, turbulent mass transfer and mixing is studied in a weakly shear-thinning fluid, and compared to the Newtonian, water case. Stereoscopic Particle Image Velocimetry (SPIV) and Inhibited Planar Laser Induced fluorescence are used simultaneously to measure the local liquid phase velocity and dissolved gas concentration fields respectively. Coupled measurements are used to estimate the turbulent mass fluxes, which are interpreted using a conditional quadrant analysis. Results show that in DPS as well as in water, surface renewal is the most frequent mechanism, but injection events are the most efficient in terms of mass transfer. Even

Computers & Fluids, 2018

The dynamics of a single air bubble, the wake structure, the instantaneous liquid velocity field ... more The dynamics of a single air bubble, the wake structure, the instantaneous liquid velocity field around it and the coaxial coalescence of two successive bubbles have been widely studied in this work by using the VOF method on the software platform of Fluent. It is observed that the bubble rising trajectory changes from one dimension to three dimensions by decreasing the viscosity of the liquid phase. The different behaviors of air bubbles introduce various instantaneous bubbles wake structures which strongly depend on their shape and on the physical properties of the liquid phase. Indeed, as the solution viscosity decreases, the bubbles' shape changes from non-deformed (ellipsoidal) to the deformed shape. In the case of bubbles chain, the wake of the leading bubble significantly affects the shape, trajectory and velocity of the trailing bubble, as well as the velocity field of the liquid phase surrounding it. For high orifice air velocities and due to the wake of the leading bubble, the trailing bubble accelerates and approaches to the leading bubble and finally coalescence phenomenon occurs. During this process, the shape of the leading bubble becomes oblate while the shape of the trailing bubble is stretched in a vertical direction. Thus, the coalescence time and position of two successive bubbles generally increase with increasing the surface tension of the liquid and reducing its viscosity.

Cardiovascular Engineering and Technology

Purpose-Pulse Wave Velocity (PWV) is an indicator of arterial stiffness used in the prediction of... more Purpose-Pulse Wave Velocity (PWV) is an indicator of arterial stiffness used in the prediction of cardiovascular disease such as atherosclerosis. Non-invasive methods performed with ultrasound probes allow one to compute PWV and aortic stiffness through the measurement of the aortic diameter (D) and blood flow velocity (U) with the lnD-U method. This technique based on in vivo acquisitions lacks validation since the aortic elasticity modulus cannot be verified with mechanical strength tests. Method-In the present study, an alternative validation is carried out on an aorta phantom hosted in an aortic flow simulator which mimics pulsatile inflow conditions. This in vitro setup included a Particle Image Velocimetry device to visualize flow in a 2D longitudinal section of the phantom, compute velocity fields (U), and track wall displacements in the aorta phantom to measure the apparent diameter (AD) variations throughout cycles. Results-The lnD-U method was then applied to evaluate PWV (5.92 ± 0.32 m/s) and calculate the Young's's modulus of the aorta phantom (0.66 ± 0.08 MPa). This last value was compared to the elasticity modulus (0.53 ± 0.07 MPa) evaluated with tensile strength tests on samples cut from the silicone phantom. Conclusion-Considering the uncertainties from the two methods, the measured elasticities are consistent and close to a 50-60 years old male aortic behavior. A comparison with in vivo data shows that the choice of silicone for the phantom material is a relevant and promising option to mimic the human aorta on in vitro systems.

The International Journal of Rotating Machinery, 2004

International Journal of Rotating Machinery, 2006

The paper presents a comparison between two sets of experimental results in a centrifugal flow pu... more The paper presents a comparison between two sets of experimental results in a centrifugal flow pump. The tested impeller is the so-called SHF impeller for which many experimental data have been continuously produced to built databases for CFD code validations with various levels of approximation. Measurements have been performed using optical techniques: 2D particle image velocimetry (PIV) technique on an air test model and 2D laser doppler velocimetry (LDV) technique on a water model, both for different flow rates. For the present study, results obtained by these optical techniques are compared together in terms of phase averaged velocity and velocity fluctuations inside the impeller flow passage for design flow rate.

La Houille Blanche, 2001

... Toutefois, le mélange de l'écoulement sortant de la roue semble dépendre de la position ... more ... Toutefois, le mélange de l'écoulement sortant de la roue semble dépendre de la position azimutale par rap-port aux aubes du diffuseur. [II] JOHNSON M. w.. MOORE J. - Secondary flow mixing losses in a centri-fugai impeller. Journal of Engineering for Power. 1983. Vol. 105. ...