arXiv subjects

David Fabre

Publications and source records attributed to David Fabre.

4 recordsLinked to original sources

arXiv2026

Stability prediction of vortex induced vibrations of multiple freely oscillating bodies

The vortex-induced vibration of multiple spring-mounted bodies free to move in the orthogonal direction of the flow is investigated. In a first step, we derive a Linear Arbitrary Lagrangian Eulerian (L-ALE) method to solve the fluid-structure linear problem as well as a forced problem where a harmonic motion of the bodies is imposed. We then propose a low computational-cost impedance-based criterion to predict the instability thresholds. A global stability analysis of the fluid-structure system is then performed for a tandem of cylinders and the instability thresholds obtained are found to be in perfect agreement with the predictions of the impedance-based criterion. An extensive parametric study is then performed for a tandem of cylinders and the effects of mass, damping and spacing between the bodies are investigated. Finally we also apply the impedance-based method to a three-body system to show its validity to a higher number of bodies.

physics.flu-dyn↗

arXiv2025

Deformation and stability of a gas bubble in a biaxial straining flow

Taking advantage of the recently developed L-ALE framework [Sierra-Ausin \textit{et al.}, Phys. Rev. Fluids {\bf{7}}, 113603 (2022)], we characterize the linear dynamics of an incompressible gas bubble immersed in a biaxial straining flow. We show that the system undergoes a saddle-node bifurcation with strongly different equilibrium shapes when varying the Ohnesorge number, $\Oh$, which compares viscous and capillary effects. Equilibrium shapes are found to be oblate for sufficiently large $\Oh$ while, counter-intuitively, they are prolate for low-enough $\Oh$. The bifurcation diagram is found to contain also two sets of disconnected branches that cannot be obtained by continuation starting from a spherical shape. One set corresponds to bubble shapes expected to be unstable, while the second set comprises a wide region exhibiting stable shapes that might be observed in practice. We then characterize the linear stability of the various branches. In addition to the unstable axisymmetric mode arising at the saddle-node bifurcation, two non-oscillating drift modes are also identified, together with a new unstable non-oscillating mode with azimuthal wave number $m=2$. This mode might be responsible for some type of bubble breakup observed in experiments.

physics.flu-dyn↗

arXiv2025

Is the transition to unsteadiness in the wake of slender bodies an artefact of boundary conditions ?

This work considers the transition to unsteadiness in the wake of 2D slender bodies, and questions the relevance of the generally accepted scenario involving a region of absolute instability within the near wake. The case of a thin plate at zero incidence is first considered. Despite the absence of absolute instability region, global stability analysis reveals the existence of numerous unstable eigenvalues organized along a characteristic "arc-branch" whose properties significantly depends upon the size of the numerical domain. These arc-branch modes are explained as resulting from a non-local pressure perturbation spuriously generated at the outlet of the domain due to the no-stress boundary condition, which then triggers the shedding of vortical structures at the trailing edge of the plate. The case of NACA0012 wing profiles at small incidences is then considered. Global stability analysis reveals that both the non-local spurious feedback mechanism and the classical local feedback mechanism are active. Trying to suppress the spurious feedback by enlarging the size of the numerical domain is shown to be inefficient. On the other hand, filtering methods suppressing the exponential spatial growth of perturbations, with either a sponge or a complex mapping, are found to be efficient. Thanks to these ideas, the critical Reynolds number and Strouhal number at onset can eventually be computed and are mapped for incidences in the range $ \alpha \in [0^o - 5 ^o]$. It is postulated that the non-local feedback mechanism evidenced here could be at play in other strongly convective flows.

physics.flu-dyn↗

arXiv2023

Mode selection in concentric jets. The steady-steady 1:2 resonant mode interaction with O(2) symmetry

In this article, a thorough characterization of the configuration composed by two concentric jets at a low Reynolds number is presented. The analysis comprises a layout with a wide range for the velocity ratio between the inner and outer jets, defined within the interval [0, 2], and also details the influence of the distance between jets, where the wall thicknesses separating the two jets is [0.5, 4]. Global linear stability analysis identifies the most significant modes driving the changes in the flow dynamics. The neutral lines revealing the critical Reynolds number connected to the presence of the main (steady and unsteady) flow bifurcations, which are presented by global azimuthal modes, show the high complexity of the problem under study, where hysteresis and other types of complex cycles are pointed out. Finally, the mode interaction is analysed, highlighting the presence of travelling waves emerging from the interaction of steady states, and the existence of robust heteroclinic cycles that are asymptotically stable. The high level of detail in the results presented, makes this work as a reference for future research development in the field of concentric jets.

physics.flu-dyn↗