PhD Defense - Young-Myung CHOI - ED SPI

Laboratory: LHEEA

The Ph.D. defense will be held at 2.30 pm on 21st November in Lecture Theatre S.

ABSTRACT:

The present study proposes the two-way coupling methodology between potential and viscous flows by the nonlinear model for incident potential and viscous flows, and linearized complementary flow model. The literature survey on the previous coupling methodology and the preliminary study on the coupling are firstly conducted to see what is necessary for two-way coupling in a marine application.
The nonlinear potential theory on the incident waves is summarized for regular and irregular waves. The reconstruction methodology of nonlinear HOS waves is proposed and it is based on cubic interpolation. The strategy is validated with the numerical simulation by Higher order spectral(HOS) method and experiments for the generation/absorption of irregular waves in the viscous flow model.
The Poincaré velocity representation, which is the modification of the boundary integral equation, is newly proposed for an unsteady time domain free surface flow. New velocity representation is validated with an arbitrary matching surface for the field point underwater, but shows a singular behavior when the field point approaches the mean free surface. A circular cylindrical matching surface with Fourier-Laguerre flow approximation is introduced for weak formulation. It necessitates the evaluation of elementary functions that the surface and water line integral of a Green function multiplied by shape functions. Two numerical algorithms are introduced for the evaluation. The wave elevation and the velocity above mean free surface are reconstructed by linear kinematic free surface condition and Wheeler stretching by Laguerre function. The representation with the cylindrical matching surface is validated for diffraction and radiation problem.

The viscous flow model based on the spectral wave explicit Navier-Stokes equations(SWENSE) is proposed for multi-phase flows with Level set interface modeling. The Euler terms in the momentum equation are canceled by employing the extension of kinematic pressure up to air which is proposed by Li(2018), and the Level-set equation is split into the complementary and incident terms that are similar to Vukcevic(2016). The proposed method is validated for propagating waves in the numerical wave tank and the cylinder diffraction problem.

Finally, two-way coupling algorithm between potential and viscous flows are applied. Both functional decomposition(FD) and domain decomposition(DD) are applied for coupling. Total fields are decomposed into incident and complementary parts by functional decomposition. It is assumed that nonlinear incident waves are available for whole computational domain. The computational domain for complementary fields are decomposed. The coupling between potential and viscous flows are done for complementary fields with overlapped coupling region. The complementary velocity and wave elevation, obtained from viscous flow solver, are given to linear potential solve for the generation of complementary waves in the relaxation zone of viscous flows. In the relaxation zones, the complementary fluid velocity and Level-set are updated by linear scattering potential theory.