A multiscale modeling approach for bubble-vortex interactions in hydro-propulsion systems

Graduation date: 2010Multiphase flow dynamics are important in many naval and turbomachinery applications, where bubble interaction with low pressure, vortical structures can lead to undesirable cavitation effects. To better understand the underlying phenomena, a multiscale numerical framework is developed to simulate bubble laden flow in dynamic hydro propulsion systems. Several subproblems in the pre-cavitation regime are identified and simulated, with a particular emphasis on bubble-vortex interactions. Two simulation techniques are used throughout. First, a Hybrid Lagrangian Eulerian (HLE) method, based on fictitious domain techniques is applied to both forced and free motion of arbitrarily shaped rigid bodies (i.e. propellers, hydrofoils & bubbles). This technique uses cartesian, body non-conformal grids to capture rigid body motion interactions with the fluid phase without grid regeneration. Next, a Discrete Element Model (DEM) is applied to flows where the bubbly phase is not resolved by the grid (sub-grid scale). In this lower order model, DNS and Lagrangian bubble tracking are used. Various closures account for bubble forces, momentum transfer (2-way coupling), and bubble size effects (volumetric coupling). Several validation cases are presented to show the accuracy and flexibility of the methods. Two studies of bubble vortex interaction are presented in detail to show the applicability of both methods to bubble laden, propulsion generated turbulence. First, bubble interactions with a traveling vortex tube are studied using the DEM approach. Use of the volumetric coupling model results in distorted vortexes that are qualitatively similar to the distorted vortex rings observed by Sridhar & Katz [JFM, 1999], and good agreement is obtained with the experimental data for bubble settling location in the vortex core. Second, both models are compared in simulations of bubble entrainment in and subsequent distortion of a Gaussian vortex. The HLE approach, which fully resolves the bubble surface, is able to capture the transient distortion process, and gives insight into the mechanisms leading to this phenomena