Mohamed Farhat

Biographie

M. Farhat was born in Casablanca in 1962 (Moroccan citizen). He graduated at Ecole Nationale Supérieure d'Hydraulique et de Mécanique de Grenoble (France. He joined The LMH laboratory in 1986 as research assistant. He completed in 1994 a Ph.D. thesis on Cavitation. He joined the R&D department of Hydro-Quebec in Montréal (Canada) in 1995 where he was in charge of several research projects in the areas of production and transportation of hydropower and mainly the monitoring of large hydro turbines. Since 2001, he is senior scientist at the LMH laboratory, head of the cavitation group. He is also lecturer in Master and Doctoral programs. He is member of the Doctoral Committee in Mechanics.

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https://people.epfl.ch/mohamed.farhat?lang=fr

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In recent years, considerable interest has been devoted to the interactions between cavitation bubbles and tissue-mimicking materials due to their promising applications in medicine and biomedical sciences. The strong fluid-structure interaction between a cavitation bubble and these elastic surfaces triggers unique collapse dynamics, characterized by bubble splitting and subsequent microjetting phenomena that can damage adjacent boundaries. In this work, we investigate how the elasticity of the boundary and the distance between the bubble and the elastic surface affect the bubble dynamics and the velocity of its microjet. To this end, we generate single laser-induced cavitation bubbles in the vicinity of agarose hydrogels with different degrees of elasticity and follow the bubble dynamics using high-speed imaging techniques, with a special focus on the formation and evolution of the microjets. We provide a time-resolved evidence of the atomization of the liquid microjet within the bubble, which precedes the establishment of a fully liquid microjet. The atomized portion of the microjet can reach supersonic velocities of up to 2000 ms( -1), while the ensuing fully developed liquid microjet travels at averaged speeds of up to 1000 ms (-1). To gain further insight into the bubble dynamics leading to the formation of these very fast microjets, we also propose a numerical model based on the boundary integral method and observe a remarkable agreement between the numerical simulations and the experimental observations.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0136577

AIP Publishing2023

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Near-wall hemodynamic parameters quantification in in vitro intracranial aneurysms with 7 T PC-MRI

Mohamed Farhat, Philippe Reymond

ObjectiveWall shear stress (WSS) and its derived spatiotemporal parameters have proven to play a major role on intracranial aneurysms (IAs) growth and rupture. This study aims to demonstrate how ultra-high field (UHF) 7 T phase contrast magnetic resonance imaging (PC-MRI) coupled with advanced image acceleration techniques allows a highly resolved visualization of near-wall hemodynamic parameters patterns in in vitro IAs, paving the way for more robust risk assessment of their growth and rupture.Materials and methodsWe performed pulsatile flow measurements inside three in vitro models of patient-specific IAs using 7 T PC-MRI. To this end, we built an MRI-compatible test bench, which faithfully reproduced a typical physiological intracranial flow rate in the models.ResultsThe ultra-high field 7 T images revealed WSS patterns with high spatiotemporal resolution. Interestingly, the high oscillatory shear index values were found in the core of low WSS vortical structures and in flow stream intersecting regions. In contrast, maxima of WSS occurred around the impinging jet sites.ConclusionsWe showed that the elevated signal-to-noise ratio arising from 7 T PC-MRI enabled to resolve high and low WSS patterns with a high degree of detail.

SPRINGER2023

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Numerical study on the scale effect of tip vortex cavitation induced by incoming velocities and scale ratios

Huaiyu Cheng, Mohamed Farhat

Large eddy simulations of tip vortex cavitation (TVC) around an elliptical hydrofoil is performed to study its scale effect. A satisfying agreement is obtained between the numerical and experimental data. It indicates that the scale effect of TVC is remarkable. With the increase of scale ratio or incoming velocity, the intensity of TVC significantly increases. Based on our results, two mechanisms for the scale effect of TVC are proposed, i.e., the roll-up of boundary layer and the interaction between tip vortex (TV), secondary vortex and trailing vortex. As incoming velocity increases, the fusion of TV and secondary vortex enhances and the boundary layer thickness decreases, which lead to larger circulation and smaller vortex core radius, tending to intensify TV. As scale ratio increases, the fusion of TV and secondary vortex enhances and the boundary layer thickness increases slightly, which lead to much higher circulation and slight increase of vortex core radius, promoting the occurrence of TVC in general. In addition, with equal cavitation number and Reynolds number, TVC in the large scale is stronger than that in the small scale, indicating that it seems not so reasonable to equate the velocity-induced scale effect with the scale-ratio-induced one.

PERGAMON-ELSEVIER SCIENCE LTD2023

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