Publications de Mohamed Farhat

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It is well known that metals and alloys erode at lower impact threshold velocities than expected. This was studied by Thomas and Brunton (1970) who reported “discrepancy factors” in the range 4–10. These authors suggested that liquid impact was a more sear ...

Sonosensitive perfluorocarbon F(8)TAC(18)-PFOB emulsion is under development to enhance heating, increase thermal contrast, and reduce treatment times during focused ultrasound tumor ablation of highly perfused tissue. The emulsion previously showed enhanc ...

An experimental study of flow in a Francis hydraulic turbine, especially designed for the GAMM Workshop, has been carried out. In order to provide a data base for the validation and comparison of computational fluid dynamics codes, a Francis turbine mode! and adequate instrumentation have been specially developed. Measurements of both global and local quantities have been performed for the best operating point of the turbine. A five-hole probe mounted on a remote traversing system provides static pressure a

When in operation, pump-turbine runners have to withstand large pressure pulsations generated by the rotor-stator interaction. The analysis of the dynamic behavior of these structures is necessary to avoid damage. For this analysis a realistic model of the ...

The collapse of a cavitation bubble is always associated with the radiation of intense shock waves, which are highly relevant in a variety of applications. To radiate a strong shock wave, it is necessary to converge energy at the collapse, and understandin ...

Experimental flow analysis of hydraulic machines is of prime importance for improving the design of such a unit. Moreover, computational fluid dynamics codes need an experimental data reference in order to check their ability to simulate the main flow features. To obtain such an experimental data base a Francis turbine model and an associated instrumentation is specially developed. Measurements of both global and local quantities are performed for five different operating points of the turbine. A five-hole

This study proposes a novel method for measuring the pressure inside a laser-induced cavitation bubble during its expansion. Based on Paschen's theory, which relates electrical breakdown to gas pressure, applied voltage, and electrode distance, our approac ...

Three different methods of cavitation erosion measurement are compared and correlated with weight loss rates in two cavitation set-ups: a high-pressure cavitation jet producing a high repetition rate of a wide range of impact intensities and a single vortex machine producing a low repetition rate of large collapses. The electrochemical cavitation activated current measurements on pure titanium sensors are calibrated on the jet and used to evaluate erosion rates on the vortex. The cavitation pit counting tec

Most of the severe erosion of hydraulic machines are found to be associated with the collapse of transient cavitation vortices downstream of a leading edge cavity. The dynamics of such a type of cavitation is investigated in a Cavitation Vortex Generator (CVG). By producing the cyclic growth and collapse of single cavitation vortex, this device provides a way to investigate the mechanisms involved in the final stage of the cavity collapse. Very high-speed visualization, at a rate of 500 '000 frames per seco

In the present study, the identification of hydrodynamic damping is investigated in the case of 2D blade placed in the test section of the EPFL High Speed Cavitation Tunnel. A non intrusive technique, based on a spark generated bubble, is used to generate ...

We present high-precision experiments conducted with the aim to better characterise weak deformations of single cavitation bubbles. Using two needle hydrophones and a high-speed photodetector, we record the timings of shock waves and luminescence emitted a ...

Transient cavities generated from unsteady leading-edge cavitation may undergo aggressive collapses which are responsible for cavitation erosion. In this paper, we studied the hydrodynamic mechanisms of these events in the leading edge cavitation formed ov ...

An experimental investigation on Francis turbines suffering from inlet cavitation erosion on the runner blades has permitted to assess several aspects of the actual cavitation erosion detection and prediction methods based on vibrations. Detection tests in three prototypes have consisted in the measurement of vibrations in turbine guide bearing, guide vanes and draft tube during operation. Discussion about RMS levels, auto power spectra and amplitude demodulation results is provided. A dynamic calibration h

In 2009, l’Hydroptère broke the symbolic barrier of 50 knots and became the world fastest sailing boat over both 500 meters and 1 nautical mile. This major achievement relied on the high skills of the sailing team but also on technical advances of the boat, resulting from the scientific collaboration between the Hydroptère Design Team and the Ecole Polytechnique Fédérale de Lausanne (EPFL). In the present article, we highlight the multidisciplinary research activity performed within EPFL in the course of th

The appearance of gas-filled vortices at the tip of hydraulic machines blades is usually associated with increase of losses, vibrations, noise and erosion risk. The understanding of the roll-up region, where the boundary layer on the pressure side crosses ...

An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-strea

The paper below is a follow up of the authors contribution to the IAHR 2002, in which the simulations of the flow through Pelton buckets were presented. In 2002, a good consistency between the predicted and measured efficiency was presented. This paper will first present a discussion of several possible strategies for bucket flow simulation. It will be described, why the authors favour the 3 bucket approach instead of a periodic simulation. The second part describes the detailed validation of the simulation

We studied spark-generated bubble inside stable water drops produced in microgravity. The closed and isolated liquid geometry results in a unique confinement of subsequent shockwaves, since the latter bounce of the free surface. This setting results in an ...

The effect of inflow waveform on the hemodynamics of a real-size idealized sidewall intracranial aneurysm (IA) model was investigated using particle imaging velocimetry (PIV). For this purpose, we implemented an error analysis based on several PIV measurements with different time lags to ensure high precision of velocity fields measured in both the IA and the parent artery. The relative error measured in the main part of the circulating volume was

In the present paper, Large Eddy Simulations combined with the Zwart-Gerber-Belamri cavitation model are conducted to study the flow characteristics in a tip-leakage flow. A reasonable agreement is obtained between the numerical and experimental data. With ...

Mohamed Farhat

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Cavitation in impacted drops and jets and the effect on erosion damage thresholds

Perfluorocarbon emulsion enhances MR-ARFI displacement and temperature in vitro: Evaluating the response with MRI, NMR, and hydrophone

Experimental flow study of the GAMM turbine model

Analysis of the dynamic response of pump-turbine runners- Part I: Experiment

Observation of the Formation of Multiple Shock Waves at the Collapse of Cavitation Bubbles for Improvement of Energy Convergence

Flow survey and blade pressure measurements in a Francis turbine model

An innovative method of pressure measurement inside a laser-induced cavitation bubble

Cavitation erosion, impact intensity and pit size distribution of jet and vortex cavitation

Shock pressure generated by cavitation vortex collapse

Hydrodynamic Damping Identification from an Impulse Response of a Vibrating Blade

Detailed experiments on weakly deformed cavitation bubbles

Hydrodynamic mechanisms of aggressive collapse events in leading edge cavitation

Field Assessment of Cavitation Detection Methods in Hydropower Plants

L'HYDROPTERE: HOW MULTIDISCIPLINARY SCIENTIFIC RESEARCH MAY HELP BREAK THE SAILING SPEED RECORD

Roll-up region of tip vortex: numerical investigation

Obstacle-induced spiral vortex breakdown

Validation of Bucket Flow Simulation using Dynamic Pressure Measurements

Cavitation Bubbles inside Spherical Water Drops in Microgravity

Multi-time-lag PIV analysis of steady and pulsatile flows in a sidewall aneurysm

LES investigation of the influence of cavitation on flow patterns in a confined tip-leakage flow