arXiv subjects

Stephane Dorbolo

Publications and source records attributed to Stephane Dorbolo.

6 recordsLinked to original sources

arXiv2025

Wake dynamics of a square cylinder while moving upward in quiescent water

We experimentally investigate the wake dynamics of a square cylinder rising through quiescent water over a range of Froude numbers ($\mathrm{Fr}$). Time-resolved Particle Image Velocimetry provides velocity and vorticity fields that enable pressure reconstruction and vortex characterization. Diagnostics based on swirl strength ($\lambda_{ci}$), the Okubo-Weiss parameter ($W$), and a shear-vortex interaction measure ($\Lambda$) reveal that the wake is governed by a persistent pair of counter-rotating vortices rather than by periodic shedding. Circulation exhibits a two-regime dependence on $\mathrm{Fr}$, with a sharp increase below $\mathrm{Fr}\approx 1$ and saturation above this threshold, mirroring entrainment force scaling reported previously. While vortex area remains nearly constant, swirl strength and negative-$W$ regions expand with $\mathrm{Fr}$, indicating that entrainment enhancement arises from intensified rotation rather than an enlarged vortex footprint. These findings provide new physical insight into vortex-free-surface interactions and enrich the understanding of entrainment mechanisms in unsteady wakes, with implications for multiphase flows and the hydrodynamic design of naval and offshore structures.

physics.flu-dyn↗

arXiv2025

Experimental investigation of the water exit of the accelerating circular cylinder

We investigate the vortical dynamics generated during the forced exit of a circular cylinder through a quiescent free surface. While previous work on exit flows has largely emphasized cavity collapse and force generation, the formation and evolution of the starting vortex beneath the body have received much less attention. Using high-speed particle image velocimetry at up to 2000 Hz, we isolate and track the primary vortex shed during exit. The body motion is driven by constant vertical acceleration from rest, such that the cylinder kinematics satisfy $v_{cyl}^2$ = $2$$\alpha$$\Delta$$y_{cyl}$, with $\Delta$ $y_{cyl}$ the displacement since release. This controlled protocol allows systematic variation of the acceleration alpha while keeping the maximum crossing velocity fixed. We characterize vortex trajectories, equivalent radius, peak vorticity, and circulation, and compare across accelerations. The results show that circulation growth collapses under the impulse-based scaling $\Gamma / \sqrt(\alpha(\Delta y_{cyl})^3)$, independent of the imposed acceleration. A consistent transition is observed as the vortex centroid approaches the free surface, where its evolution is modified by surface interaction. The trajectories remain predominantly vertical with only weak lateral drift, robust across all accelerations. Taken together, these findings establish a systematic framework for water-exit vortex dynamics under constant acceleration and highlight scaling features that parallel the classical starting-vortex problem in entry flows.

physics.flu-dyn↗

arXiv2024

Experimental investigation of exit dynamics of a circular cylinder out of water and silicone oil

Experimental investigations of the exit dynamics of a horizontal cylindrical object were performed in water and silicone oil (50 cSt). The fully immersed cylinder was initially at rest in a still fluid tank before being pushed (or pulled according to the measurement procedure) upwards at a constant velocity. Firstly, we demonstrate that these conditions are better satisfied for a large aspect ratio cylinder equipped with vertical side plates. Secondly, the influence of the initial depth on the liquid entrained and the wake generated by the cylinder is discussed. The deformation of the bath is found to be independent of the starting depth when the starting depth is larger than 6 times the cylinder diameter. In the present case, this criterion reflects also the finite acceleration of the cylinder to reach the determined constant exit velocity. Measurements in a range of exit speeds between 0.1 and 1 m/s indicate that the thickness of the liquid above the cylinder, when the cylinder starts crossing the interface, increases with the speed according to a logarithmic law of the Froude number. During the subsequent drainage, the evolution of the coated liquid thickness is found to first decrease exponentially with time just after the crossing of the interface. At later times, a change of regime occurs and the drainage follows the inverse of the square root of time irrespective of the crossing speed. Finally, the force necessary to maintain a constant exit speed during the motion of the cylinder inside and outside the bath is analyzed. This global measurement of the entrained liquid confirms the square root scaling of the thinning with time during the drainage process.

physics.flu-dyn↗

arXiv2023

Effect of the Surface Dimples on the Exit Dynamics of a Sphere at a Constant Velocity

This article experimentally investigates the exit dynamics of two different spheres i.e. a smooth sphere and a sphere with dimples at a constant speed. Employing high-speed experimental observations, the study investigates the key characteristic lengths and hydrodynamic forces between the sphere's surface and water. The experiments were performed at different traveling speeds (or Froude numbers). The results shed light on the drag force coefficient, entrainment force coefficient, and cross-over force coefficient. In this paper, we have shown that the surface dimples not only affect the drag force but also the entrainment force. However, the cross-over force coefficient remains the same.

physics.flu-dyn↗

arXiv2023

Exit Dynamics of a Square Cylinder

In this paper, we experimentally investigate the exit dynamics of a square cylinder, that is initially fully immersed in a water tank and that crosses the interface perpendicularly to its symmetry axis. The cylinder moves upwards at a constant velocity in the vertical direction till the cylinder exits out of the water into the air. The experiments were performed at different traveling speeds. The images of the cylinder crossing the interface were taken using a high-speed camera. The images were used to track the interface deformation when the cylinder approaches and crosses the interface. On top of these measurements, the force required to move the cylinder was simultaneously measured in order to estimate the drag force during the travel in the tank, the force of entrainment, and the force of crossing over the interface. Particle image velocimetry was performed to visualize the flow. Correlations between the different measurements are inspected.

physics.flu-dyn↗

arXiv2013

Electrically charged droplet: Influence of the generator configuration and the nature of the liquid

We studied how to charge droplets by induction and how to maximize this charge. In order to aim this objective, we developed an innovative device that avoids non-linear effects and that is able to charge liquids of different nature. The device developed resembles a planar capacitor. The influence of the nature of the liquid (i.e. presence of ions in solution, polarity, surface tension and conductivity) on the charge induced was measured. We deduced that there exists a threshold of electrical conductivity for the fluid below which it does not charge according to the "perfect conductor" model.

physics.flu-dyn↗