题目: Two-fluids modeling of wave breaking flows: numerical methods and physical aspects
报告人： Alessandro Iafrati 高级研究员 （意大利CNR-INSEAN）
时间：2018年9月14号；09:00-10:30
地点：海动A区会议室
报告摘要：
The results of two-fluids numerical simulations of the wave breaking induced by modulations instability are presented and comparisons between wind and no-wind conditions are discussed. Simulations are performed by using two-fluids Navier-Stokes solvers, which are briefly introduced. Most of the resultspresented are obtained by using a two-dimensional assumption, but some of the inherent limits are discussed.
The breaking is induced via the Benjamin-Feir instability mechanism, triggered by side-band perturbations added to a fundamental component. Simulations cover the initial development of the instability till the end of the whole breaking process. In order to investigate the role of wind on the modulational instability and on the resulting breaking, a uniform wind, twice the phase speed, is assigned in the air phase.
The physical analysis is focused on the growth of the modulational instability and on the energy dissipation induced by the breaking. In order to better investigate the exchanges taking place at the air water interface, the air and water domains are mapped onto rectangular domains. Thanks to such transformation, it is possible to compute the pressures and tangential stresses acting ah the interface and thus to highlight the differences between the wind and no-wind case. It is shown that due to the flow separation there is a favorable pressure gradient about the wave crests whereas the tangential stresses are generally in favor of the wave propagation on the back of the wave but are opposed to the propagation along the forward face and in the wave trough. In agreement with what experimentally found, in presence of wind the growth rate of the side-bands is reduced compared to the corresponding no-wind solution. No substantial differences have been found in terms of the total energy dissipated by the whole breaking process although the dissipation rate for the wind case is noticeably higher.
题目: Domain decomposition approaches combining potential and two-fluids Navier-Stokes models
报告人： Alessandro Iafrati 高级研究员 （意大利CNR-INSEAN）
时间：2018年9月15号；13:30-15:00
地点：海动A区会议室
报告摘要：
Two-fluids numerical methods based on Navier-Stokes equations are generally extremely expensive from the computational viewpoint. For applications in the context of free surface waves, two-fluid modeling is only needed in limited portions of the computational domain whereas the rest of the fluiddomain can be efficiently modeled by potential flow approaches. Based on the above consideration, a domain decomposition approach combining together fully-nonlinear potential flow model and Navier-Stokes solvers for two-fluid modeling is presented and discussed in details. The two numerical methods are presented first. Hence, two possible coupling strategies, named direct and indirect, and their respective pros and cons are discussed. Applications to a bottom bump or to a submerged body are shown.
Beside the decomposition of the spatial domain, a decomposition based on the time axis is presented as well. The time evolution of modulated wave groups is followed by a fully-nonlinear potential flow model showing the development of the modulational instability. If the initial steepness is too large, the waveamplification leads the steepness to exceed a limiting value and the breaking starts, so that the next stage which cannot be described by potential flow models. Hence, a combined method is presented which uses the potential flow solution to initialize the two-fluids Navier-Stokes simulation. A detailed discussion on the coupling procedure with both fully-nonlinear panel methods and with HOS simulation is presented.
报告人简介：
Alessandro Iafrati is a senior research scientist in INstitute of Marine engineering – National Research Council Rome, Italy (CNR-INSEAN), He received his Master Degree of Mechanical Engineering in 1991 from University of Rome, and Ph.D degree from University of East Anglia (UK) in 2009. His research interest include numerical simulation of water waves and its interaction with marine structures, experiments on fluid mechanics. He has received many research grants from Italian government, European Union and industry companies. He is the Chair of the Specialist Committee on Modelling of Environment Conditions of ITTC.