Observations of Cavitating Flow

This paper will present a review of some of the recent advances in our understanding of the dynamics and acoustics of cavitating flows. We focus first on the individual events which evolve from a single travelling nucleus and describe observations of the intricate micro-fluid-mechanics which affect both the bubble shape and the subsequent emission of noise. These phenomena have important consequences in terms of their implications for the scaling of cavitation damage and noise. We also present calculations of the interaction between the individual traveling bubbles and the irrotational flow outside of the boundary layer of the headform. Comparisons of predicted and experimentally observed bubble shapes show qualitative agreement but further work is necessary to understand the details of the interactions between the viscous boundary layer and the bubble. To model the processes of cavitation inception, noise and damage it is necessary to generate a model of the cavitation event rate which can then be coupled with the consequences of the individual events. In the second part of this paper we describe recent efforts to connect the observed event rates to the measured distributions of cavitation nuclei in the oncoming stream. Such studies necessarily raise questions regarding the nuclei distributions in water tunnels and in the ocean and it would seem that we still know little of the nuclei population dynamics in either context. This is illustrated by a few observations of the population dynamics in a particular facility. The third subject addressed in this paper is the question of the noise produced by an individual travelling cavitation event. It is shown that the distortions in the shape of cavitation bubbles leads to acoustic impulses which are about an order of magnitude smaller than those predicted by the spherical bubble dynamics of the Rayleigh-Plesset equation. However, at the higher cavitation numbers, the upper bound on the experimental impulses scales with speed and size much as one would expect from the spherical bubble analysis. Initially, as the cavitation number is decreased, the impulse increases as expected. But, beyond a certain critical cavitation number, the noise again decreases in contrast to the expected increase. This phenomenon is probably caused by two effects, namely the interaction between events at the higher event densities and the reduction in the impulse due to a change in the dominant type of cavitation event. From the single event we then move to the larger scale structures and the interactions which occur when the density of the events becomes large and individual bubbles begin to interact. One of the more important interaction phenomena which occur results from the behaviour of a cloud of cavitation bubbles. Most previous theoretical studies of the dynamics of cavitating clouds have been linear or weakly non-linear analyses which have identified the natural frequencies and modes of cloud oscillation but have not, as yet, shown how a cloud would behave during the massively non-linear response in a cavitating flow. We present non-linear calculations which show the development of an inwardly propagating shock wave during the collapse phase of the motion. These observations confirm the earlier speculation of Mørch and his co-workers

Bond-Order Time Series Analysis for Detecting Reaction Events in Ab Initio Molecular Dynamics Simulations.

Ab initio molecular dynamics is able to predict novel reaction mechanisms by directly observing the individual reaction events that occur in simulation trajectories. In this article, we describe an approach for detecting reaction events from simulation trajectories using a physically motivated model based on time series analysis of ab initio bond orders. We found that applying a threshold to the bond order was insufficient for accurate detection, whereas peak finding on the first time derivative resulted in significantly improved accuracy. The model is trained on a reference set of reaction events representing the ideal result given unlimited computing resources. Our study includes two model systems: a heptanylium carbocation that undergoes hydride shifts and an unsaturated iron carbonyl cluster that features CO ligand migration and bridging behavior. The results indicate a high level of promise for this analysis approach to be used in mechanistic analysis of reactive AIMD simulations more generally

Assessing load-sharing within optimistic simulation platforms

The advent of multi-core machines has lead to the need for revising the architecture of modern simulation platforms. One recent proposal we made attempted to explore the viability of load-sharing for optimistic simulators run on top of these types of machines. In this article, we provide an extensive experimental study for an assessment of the effects on run-time dynamics by a load-sharing architecture that has been implemented within the ROOT-Sim package, namely an open source simulation platform adhering to the optimistic synchronization paradigm. This experimental study is essentially aimed at evaluating possible sources of overheads when supporting load-sharing. It has been based on differentiated workloads allowing us to generate different execution profiles in terms of, e.g., granularity/locality of the simulation events. © 2012 IEEE

Determination of Scale Effects for a Scaled Physical Model of a Labyrinth Weir Using CFD

In the present study the three dimensional (3D) Computational Fluid Dynamics (CFD) Volume of Fluid (VOF) model is employed to reproduce the complex hydraulic flows over a labyrinth weir and a spillway for two flow rates, 40 m3/s and the PMF event of the scheme, 159.5 m3/s. The VOF model is implemented in two solvers: the open source platform OpenFOAM and the commercial CFD package ANSYS Fluent. Validation is undertaken by modelling the scaled physical model of the scheme. Prototype scale simulations of the two flow rates are undertaken, with comparisons between predictions at the two scales being used to establish discrepancies between the two scales. Overall the two solvers predict the prototype flows to be shallower and with higher velocities than those at model scale, but with these scale effects becoming less prominent for increasing flow rates. In the 40 m3/s case the wave structures in the prototype present elongation compared to those at model scale. In the PMF case, in addition to elongation, the wave structures also change in position. Work is currently underway with the modelling of further flow rates in order to investigate the discrepancies between scale and prototype simulations with increased detail and determine limits to minimise impact of scaling

Dynamical system analysis and forecasting of deformation produced by an earthquake fault

We present a method of constructing low-dimensional nonlinear models describing the main dynamical features of a discrete 2D cellular fault zone, with many degrees of freedom, embedded in a 3D elastic solid. A given fault system is characterized by a set of parameters that describe the dynamics, rheology, property disorder, and fault geometry. Depending on the location in the system parameter space we show that the coarse dynamics of the fault can be confined to an attractor whose dimension is significantly smaller than the space in which the dynamics takes place. Our strategy of system reduction is to search for a few coherent structures that dominate the dynamics and to capture the interaction between these coherent structures. The identification of the basic interacting structures is obtained by applying the Proper Orthogonal Decomposition (POD) to the surface deformations fields that accompany strike-slip faulting accumulated over equal time intervals. We use a feed-forward artificial neural network (ANN) architecture for the identification of the system dynamics projected onto the subspace (model space) spanned by the most energetic coherent structures. The ANN is trained using a standard back-propagation algorithm to predict (map) the values of the observed model state at a future time given the observed model state at the present time. This ANN provides an approximate, large scale, dynamical model for the fault.Comment: 30 pages, 12 figure