Session B01: Free-Surface Flows: Interaction with Physical Structures II

Statistics of extreme body motions in nonlinear wave fields.

We consider the statistics of extreme body motion in a nonlinear irregular wave field. In addition to the excitation by extreme waves, the extreme motions can be physically resulted from wave-body resonance or parametric excitation. Therefore, their statistics cannot be directly derived from extreme wave statistics which has been studied extensively. The computation of the statistics, however, requires the Monte-Carlo method, which can become computationally intractable (when coupled with high-fidelity simulations) due to the rareness of the extreme events. In this work, we develop a general framework, which enables an efficient resolution of the statistics of extreme body motions in a nonlinear wave field. This leverages a range of physics and learning based approaches, including nonlinear wave simulations, body response simulations, dimension-reduction techniques, sequential sampling and Gaussian process regression (Kriging). The developed method is benchmarked for its effectiveness in accurately resolving the statistics, and applied to study the extreme statistics of ship roll motions in an evolving narrow-banded nonlinear wave field.   

Experimental study of improving power conversion capability of a floating point absorber with reactive control technique.

One of the biggest challenges for wave energy conversion (WEC) system to be competitive in the renewable market is to maintain a high and stable efficiency from stochastic ocean waves with wide spectrum. To solve this issue, there is an emerging need to develop a reactive control system (RCS) for WEC according to incoming wave condition via feedback control. An advanced boundary element method with viscous correction was developed to analyze the system dynamics. Scaled model tests were conducted in the newly developed multiple function towing tank at SJTU to study the power generation of a floating point absorber with RC. Incoming wave information is well integrated into control loop with a group of gauges. The results show that power output of RCS can be about 2-3 times higher than that of passive control system (PCS). High efficiency spectrum of RC-WEC ranges 1.5 times wider than PC-WEC's. Viscous effect increases with power generation coefficient, indicating the significance of geometry optimization with control system optimization together. Power output efficiency decreases as wave height increases, indicating that increasing nonlinear hydrodynamics can degrade the performance of linear control.   

Wave attenuation by flexible vegetation

The study of fluid flows interacting with vegetative structures in coastal regions presents a significant challenge on account of its multi-scale nature. In this talk, we show how compact asymptotic reductions allow us to study surface waves over submerged vegetative regions. The vegetation is modeled as a collection of elastic cantilever beams in cross-flow and is coupled to fluid momentum equations. Our simplified framework provides some advantages to existing models. For example, for rigid plants over a horizontal bed, previous work on linear waves predicts a quadratic decay in wave amplitude. However, by accounting for plant flexibility, we predict a sub-quadratic decay that agrees more closely with experimental observations. Consideration of a varying depth further demonstrates competition between shoaling and vegetative dissipation.   

The scaling laws of an exploding liquid cylinder irradiated by an ultrashort X-ray pulse

A general formulation of the partial destruction of a liquid object in vacuum after the sudden deposition of a very large amount of energy is proposed. That energy instantaneously raises the pressure of a portion of the liquid to extreme values and changes its state, which causes its explosive expansion into vacuum and against the rest of the liquid object. When the deformable object is a liquid capillary cylinder or column, the model reduces to a universal equation for the evolution of the expanding gap between the two sides of the exploding liquid column. The theoretical analysis contemplates two asymptotic stages for small and large times from the initiation of the blast, whose asymptotic solutions are fitted to available experimental data. An universal approximate analytical solution is obtained. A complete dimensional analysis of the problem and an optimal collapse of experimental data reveal that the proposed solution is in remarkable agreement with experiments of a jet exploding after being irradiated by an ultrashort and intense X-ray pulse from a X-ray free electron laser (XFEL).   

The Dynamics of Liquid Sorbents in Open Capillary Channels

Direct contact liquid-gas sorbent beds offer unique benefits for spacecraft air quality control. In a recent ISS technology demonstration experiment (CSELS---Capillary Sorbent), two 16-parallel open capillary channel contactors plumbed in series demonstrated passive `thin film' control, modeling both absorption and desorption functions for a potential low-gravity gas scrubbing system for spacecraft. The open wedge-shaped channels mimic terrestrial falling film reactors by exploiting capillary pressure gradients instead of gravity. In this presentation we highlight the fluid mechanics of the process~with and without the effects of~CO$_{\mathrm{2}}$~absorption across the surface. We identify the limits of operation, stability, and transients for systems as functions of wedge geometry and working fluid thermo-physical properties. Rare analytical solutions are found that may be applied to enormous systems of~$n$-parallel channels. The analytical approach serves as the building block for massively parallel systems requiring large surface areas to achieve the desired performance