Libration-driven flows in ellipsoidal shells

Planets and satellites can undergo physical librations, which consist of forced periodic variations in their rotation rate induced by gravitational interactions with nearby bodies. This mechanical forcing may drive turbulence in interior fluid layers such as subsurface oceans and metallic liquid cores through a libration‐driven elliptical instability (LDEI) that refers to the resonance of two inertial modes with the libration‐induced base flow. LDEI has been studied in the case of a full ellipsoid. Here we address for the first time the question of the persistence of LDEI in the more geophysically relevant ellipsoidal shell geometries. In the experimental setup, an ellipsoidal container with spherical inner cores of different sizes is filled with water. Direct side view flow visualizations are made in the librating frame using Kalliroscope particles. A Fourier analysis of the light intensity fluctuations extracted from recorded movies shows that the presence of an inner core leads to spatial heterogeneities but does not prevent LDEI. Particle image velocimetry and direct numerical simulations are performed on selected cases to confirm our results. Additionally, our survey at a fixed forcing frequency and variable rotation period (i.e., variable Ekman number, E) shows that the libration amplitude at the instability threshold varies as ∼E⁰·⁶⁵. This scaling is explained by a competition between surface and bulk dissipation. When extrapolating to planetary interior conditions, this leads to the E1/2 scaling commonly considered. We argue that Enceladus' subsurface ocean and the core of the exoplanet 55 CnC e should both be unstable to LDEI

Analysis and experiments on flow-induced hemolysis.

Hemolysis (red cells lysis) caused by fluid stresses in flows within hypodermic needles, blood pumps, artificial hearts and other cardiovascular devices, is one of the major concerns during the design and use of cardiovascular or blood-processing extracorporeal devices. A non-invasive experimental method which does not interfere directly with red blood cells was designed to investigate the red cells\u27 deformations in response to a range of flow conditions. The designed flow chamber and pump system provided a controlled two-dimensional Poiseuille flow with average velocity of up to 4 m/s and a range of fluid stresses up to 5000 dyn/cm 2 . The dimension of deformed cells and positions was measured to obtain the aspect ratio of red cells under stress from images captured by the microscope-laser-camera system. A strain-based blood damage model from Rand\u27s viscoelastic model was built to predict cell strain. The empirical coefficients in the blood damage model were calibrated by the measurements from the experiments. Flow-induced hemolysis in the blood flow through hypodermic needles was investigated. The flow-induced hemolysis of the needles may be reduced by a modified design of the entrance geometry of the needle. Three groups of 16 gauge needles were compared: one standard group with sharp entrance, another with beveled entrance and a third with rounded entrance. The CFD analysis combined with the strain-based blood damage model, Heuser et al. model and Giersiepen et al. model respectively was used to analyze the flow-induced hemolysis of the three needles. The predicted results were compared to the experimental results, which showed the rounded entrance reduced hemolysis by 34%, but the beveled entrance increased hemolysis by 38%. The strain-based blood damage model predicted the reduced hemolysis by 7.4% in rounded needle and the increased hemolysis by 13% in beveled needle. Both Heuser et al. model and Giersiepen et al. model predicted increased hemolysis in rounded needle

PS Poster Session - All

This document includes all poster sessions at the IBPC 2018

Nonlinear interactions of internal gravity waves in a continuously stratified fluid

Internal gravity waves are buoyancy-driven oscillations which can arise in a density-stratified fluid. They exist throughout the oceans and atmosphere, the oceanic internal wavefield being sufficiently energetic for nonlinear effects to play an important role in the internal wave dynamics. Numerical studies of oceanic internal waves (such as Broutman & Young, 1986) have suggested that under the right conditions a weak internal gravity wave can be strongly refracted and frequency-shifted by the time-varying shear of a large-amplitude internal wave. My project aimed to experimentally observe this type of strongly nonlinear interaction by generating the required internal gravity waves in a continuously stratified aqueous solution. The waves were observed using a colour schlieren system, and power spectra of the internal wavefield were obtained using conductivity probes and polarimetry. Several nonlinear phenomena were observed, including anharmonic waves and forced sum and difference frequencies, as well as second-harmonic generation from the wave sources. However a combination of wavelength limitations imposed by viscosity, inescapable restrictions on the strong wave amplitude and severe observational difficulties all conspired to prevent detection of the particular nonlinear interaction of interest. A proposed apparatus could overcome these difficulties, but its construction would be quite beyond the scope of an Honours project

Full Proceedings, 2018

Full conference proceedings for the 2018 International Building Physics Association Conference hosted at Syracuse University