An experimental investigation on elliptical instability of a strongly asymmetric vortex pair in a stable density stratification

We investigate the elliptical instability of a strongly asymmetric vortex pair in a stratified fluid, generated by the acceleration and deceleration of the rotation of a single flap. The dominant parameter is the Froude number, Fr=U/(NR), based on the maximum azimuthal velocity, U, and corresponding radius, R, of the strongest vortex, i.e.&nbsp;the principal vortex, and buoyancy frequency N. For Fr>1, both vortices are elliptically unstable while the instability is suppressed for Fr<1. In an asymmetric vortex pair, the principal vortex is less &ndash; and the secondary vortex more &ndash; elliptical than the vortices in an equivalent symmetric dipolar vortex. The far more unstable secondary vortex interacts with the principal vortex and increases the strain on the latter, thus increasing its ellipticity and its instability growth rate. The nonlinear interactions render the elliptical instability more relevant. An asymmetric dipole can be more unstable than an equivalent symmetric dipole. Further, the wavelength of the instability is shown to be a function of the Froude number for strong stratifications corresponding to small Froude numbers, whereas it remains constant in the limit of a homogenous fluid

Experimental study on subcooled nucleate boiling from a single artificial cavity

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Bubbles condensing in subcooled water were experimentally studied in the aim of characterizing the nucleation behavior dependence on the water pool subcooled temperature. A single artificial cavity was created on a brass plate and heated by a laser beam in a water pool and high speed camera was used to capture bubble growth dynamics. Postprocessing allows the measurement of important parameters such as bubble volume, rising velocity, departure diameter, detachment frequency, volume variation versus time and temperature and dry spot diameter. Three growth regimes were identified according to the bulk temperature and the heat flux supplied: an equilibrium regime, a standard nucleate boiling regime and a boiling crisis regime in which we observed the formation of a large dry spot on the solid surface. The absence of rewetting of the cavity is a key feature of this regime. The obtained results are compared with the few experimental correlations available in the literature.dc201

Theory and Validation of Magnetic Resonance Fluid Motion Estimation Using Intensity Flow Data

15 p.Background Motion tracking based on spatial-temporal radio-frequency signals from the pixel representation of magnetic resonance (MR) imaging of a non-stationary fluid is able to provide two dimensional vector field maps. This supports the underlying fundamentals of magnetic resonance fluid motion estimation and generates a new methodology for flow measurement that is based on registration of nuclear signals from moving hydrogen nuclei in fluid. However, there is a need to validate the computational aspect of the approach by using velocity flow field data that we will assume as the true reference information or ground truth. Methodology/Principal Findings In this study, we create flow vectors based on an ideal analytical vortex, and generate artificial signal-motion image data to verify our computational approach. The analytical and computed flow fields are compared to provide an error estimate of our methodology. The comparison shows that the fluid motion estimation approach using simulated MR data is accurate and robust enough for flow field mapping. To verify our methodology, we have tested the computational configuration on magnetic resonance images of cardiac blood and proved that the theory of magnetic resonance fluid motion estimation can be applicable practically. Conclusions/Significance The results of this work will allow us to progress further in the investigation of fluid motion prediction based on imaging modalities that do not require velocity encoding. This article describes a novel theory of motion estimation based on magnetic resonating blood, which may be directly applied to cardiac flow imaging.Kelvin Kian Loong Wong, Richard Malcolm Kelso, Stephen Grant Worthley, Prashanthan Sanders, Jagannath Mazumdar, Derek Abbot

An experimental investigation on elliptical instability of a strongly asymmetric vortex pair in a stable density stratification

We investigate the elliptical instability of a strongly asymmetric vortex pair in a stratified fluid, generated by the acceleration and deceleration of the rotation of a single flap. The dominant parameter is the Froude number, Fr=U/(NR), based on the maximum azimuthal velocity, U, and corresponding radius, R, of the strongest vortex, i.e. the principal vortex, and buoyancy frequency N. For Fr>1, both vortices are elliptically unstable while the instability is suppressed for Fr<1. In an asymmetric vortex pair, the principal vortex is less – and the secondary vortex more – elliptical than the vortices in an equivalent symmetric dipolar vortex. The far more unstable secondary vortex interacts with the principal vortex and increases the strain on the latter, thus increasing its ellipticity and its instability growth rate. The nonlinear interactions render the elliptical instability more relevant. An asymmetric dipole can be more unstable than an equivalent symmetric dipole. Further, the wavelength of the instability is shown to be a function of the Froude number for strong stratifications corresponding to small Froude numbers, whereas it remains constant in the limit of a homogenous fluid

Etude de la stabilité d'un vortex en fluide faiblement stratifié

International audienc

Instability of a columnar vortex in a stratified fluid

We experimentally investigate the stability of a monopolar columnar vortex in a linearly stratified fluid. The vortex was generated by a single flap of which the rotation about the vertical axis was accelerated and subsequently decelerated, resulting in an approximately Gaussian vorticity profile. The Reynolds number Re=UR/n was of order O(1000), with U the maximum azimuthal velocity and R the corresponding radius, while the Froude number Fr=U/(RN), with N the buoyancy frequency, ranged between 0.5 and 5. Though these vortices are centrifugally and elliptically stable for Fr<2, they are unstable due to a resonant interaction between inertial and gravitational oscillations that occur near the radius where the angular velocity approaches N. This instability is apparent as a helical wave, of which the wavelength varies monotonically with F. To our knowledge this instability has not been observed before, and differs from the so-called zigzag instability observed by Billant and Chomaz (2000)