arXiv subjects

Marine Aulnette

Publications and source records attributed to Marine Aulnette.

4 recordsLinked to original sources

arXiv2026

Orientation Dynamics of Rigid Fibers in a Microfluidic Burgers-like Vortex

Fiber suspensions are common in biological and environmental flows and are widely used in industrial applications. Fiber transport and orientation dynamics are affected by interactions with the surrounding fluid and strongly depend on the nature of the flow. The complexity of realistic flows, which are often heterogeneous or time-dependent, hinders a full understanding of fiber dynamics. In this study, we combine microfluidic experiments, theory and numerical simulations to investigate the orientation dynamics of rigid neutrally buoyant fibers in a well-controlled model system, a streamwise stationary vortex at moderate Reynolds number. Despite the three-dimensional nature of the flow, the orientation dynamics are remarkably simple: the fiber orientation is accurately described by Jeffery equations coupled with the Burgers-vortex model. We show that fibers undergo uniform precession about the vortex axis driven by fluid vorticity while simultaneously aligning with the latter due to strain in the vortex core. These two motions are decoupled, with the alignment timescale determined by the local strain rate and the fiber aspect ratio. Finite particle size and inertia induce weak deviations from the base flow streamlines while leaving the orientational dynamics largely unaffected. These results establish a simple framework for understanding the behavior of elongated particles in stretched vortex flows, which constitute key building blocks of turbulence

physics.flu-dyn↗

arXiv2025

Vertical velocities in quasigeostrophic laboratory vortices

In the present study, we test the predictions of the {\omega}-Equation against laboratory experiments with direct measurements of the vertical velocity w. Our results are further completed through the use of theoretical models of oceanic vortices, with the aim of helping oceanographers in better quantifying regions of upwelling and downwelling in the ocean. Using a rotating table and density stratification, we investigate non-axisymmetric surface vortices. The predicted vertical velocities calculated from the {\omega}-Equation are relatively small (|w| ~ 20 {\mu}m/s) and primarily appear at the vortex edges, where the vorticity sign changes, acting to restore flow stratification. However, our estimates of w, obtained from the divergence of the horizontal velocity field measured by PIV, are five times larger. This discrepancy is further confirmed by direct particle tracking measurements, which indicate a magnitude of approximately 100 {\mu}m/s for w. To address this inconsistency, we incorporate dissipative terms into the {\omega}-Equation to assess the role of viscous diffusion in enhancing internal recirculation in the vortex and thus vertical velocity magnitude. This hypothesis is favorably tested on a Gaussian vortex model.

physics.flu-dyn↗

arXiv2025

Transport of spherical microparticles in a 3D vortex flow

Particles are common in biological and environmental flows and are widely used in industrial and pharmaceutical applications. Their motion and flow dynamics are strongly affected by interactions with the surrounding flow structure. While particle-flow interactions have been extensively studied in low Reynolds number (Re) flows as well as in fully developed turbulence, the transport mechanisms of these particles in intermediate flow regimes remain less explored. Here, we investigate the response of neutrally buoyant spherical particles to a single vortex flow field. Using a microfluidic cross-slot geometry, we generate a well-characterized, stationary, three-dimensional streamwise vortex at moderate $\text{Re}$ ($\sim 50$). Our experimental results, supported by numerical simulations, show that with increasing particle diameter, they are progressively excluded from the vortex core. Initially, small particles follow a Burgers vortex-like self-similar motion, but for larger particle diameters, deviations from this trend emerge due to fluid inertia and finite-size effects. These findings enhance our understanding of particle dynamics in vortical flows and have implications for microfluidic applications involving particle sorting and separation.

physics.flu-dyn↗

arXiv2022

Kelvin-Helmholtz instability and formation of viscous solitons on highly viscous liquids

Viscous solitons are strongly non-linear surface deformations generated by blowing wind over a liquid beyond a critical viscosity. Their shape and dynamics result from a balance between wind drag, surface tension and viscous dissipation in the liquid. We investigate here the influence of the liquid viscosity in their generation and propagation. Experiments are carried out using silicon oils, covering a wide range of kinematic viscosities $ν_\ell$ between 20 and 5000~mm$^2$~s$^{-1}$. \modif{We show that, for $ν_\ell> 200$~mm$^2$~s$^{-1}$, viscous solitons are sub-critically generated from an unstable initial wave train at small fetch, where the wind shear stress is larger. The properties of this initial wave train are those expected from Miles's theory of the Kelvin-Helmholtz instability of a highly viscous fluid sheared by a turbulent wind: the critical friction velocity and critical wavelength are independent of $ν_\ell$, and the phase velocity decreases as $ν_\ell^{-1}$.} We demonstrate the subcritical nature of the transition to viscous solitons by triggering them using a wavemaker for a wind velocity below the natural threshold. Finally, we analyze the flow field induced by a viscous soliton, and show that it is well described by a two-dimensional Stokeslet singularity in the far field. The resulting viscous drag implies a propagation velocity with a logarithmic correction in liquid depth, in good agreement with our measurements.

physics.flu-dyn↗