Chih-Min Hsieh

Criticality of flow transition behind two side-by-side elliptic cylinders

In this study, near-critical bifurcations of low Reynolds number (Re) flows past a pair of elliptic cylinders in the side-by-side arrangement are numerically investigated, and onsets of several distinct transition scenarios are addressed. A nested Cartesian-grid formulation, in combination with an effective immersed boundary method and a two-step fractional-step procedure, has been adopted to simulate the flows. The transition scenarios associated with various periodic, quasi-periodic, and biased flows, their bifurcation characteristics, corresponding critical Reynolds numbers, and phase-portraits are exploited to better understand the governing physics. From the global point of view, there appear variety of flow patterns within the investigated parameter space, 40 ⩽ Re ⩽ 300, 0.2 ⩽ G ⩽ 3.0 (G being the gap-ratio of the cylinders), and 1.5 ⩽ A ⩽ 3 (A is the cylinder aspect-ratio), which include, symmetric vortex shedding mode, semi-single/twin vortex street formations, asymmetric/deflected flows, stationa...

NUMERICAL MODELING OF WAVE-INDUCED ROTATIONS OF A BOTTOM-HINGED FLAPPER WITH A SPH MODEL

In this study, a Smoothed Particle Hydrodynamics model for simulating wave-induced rotations of a bottom-hinged flapper was established in a 2-D numerical wave flume. The simulated rotating angles illustrated that the flapper could swing back and forth following harmonic wave loadings. The simulations were also seen to be in good agreement with experimental data, confirming the applicability of the present numerical model. The simulated hydrodynamic behaviors at different phases showed that the flapper moved downstream under the wave crest and upstream under the wave trough following the elliptical form of water particle trajectory. The energy conversions of a flapper during an average wave cycle showed that larger rotating angle ranges could result in higher energy conversions. However, smaller rotating angle ranges could result in higher captured efficiency.

Nonlinear interaction and wave breaking with a submerged porous structure

Numerical simulations are performed to investigate interactive velocity, streamline, turbulent kinetic energy, and vorticity perturbations in the near-field of a submerged offshore porous triangular structure, as Stokes waves of different heights pass through. The wave-structure interaction and free-surface breaking for the investigated flow situations are established based on solutions of 2D Reynolds Averaged Navier-Stokes equations in a Cartesian grid in combination with K-e turbulent closure and the volume of fluid methodology. The accuracy and stability of the adopted model are ascertained by extensive comparisons of computed data with the existing experimental and theoretical findings and through efficient predictions of the internal physical kinetics. Simulations unfold “clockwise” and “anticlockwise” rotation of fluid below the trough and the crest of the viscous waves, and the penetrated wave energy creates systematic flow perturbation in the porous body. The interfacial growths of the turbulent k...

Effects of initial amplitude and pycnocline thickness on the evolution of mode-2 internal solitary waves

Numerical simulations are performed to investigate the effects of the initial amplitude and pycnocline thickness on the evolutions of convex mode-2 internal solitary waves propagating on the flat bottom. A finite volume method based on a Cartesian grid system is adopted to solve the Navier-Stokes equations using the improved delayed detached eddy simulation turbulent closure model. Mode-2 internal solitary waves (ISWs) are found to become stable at t = 15 s after lifting a vertical sluice gate by a gravity collapse mechanism. Numerical results from three cases of pycnocline thickness reveal the following: (1) the occurrence of a smooth mode-2 ISW when the wave amplitude is small; (2) the PacMan phenomenon for large amplitude waves; and (3) pseudo vortex shedding in the case of very large amplitudes. In general, basic wave properties (wave amplitude, wave speed, vorticity, and wave energy) increase as the wave amplitude increases for a specific value of the pycnocline thickness. Moreover, the pycnocline th...

Effect of Plateau Length on the Transformation of Internal Solitary Waves

Abstract—The propagation and dissipation of internal waves over continental shelf bathymetry are complex phenomenon. The waveform would be re-generated while transmitting a submerged deep-shallow-deep topography. To study the effect of the marine topography on the evolution of an internal wave, numerical simulation is utilized to perform the flow evolution and waveform inversion of a large depression internal wave over a trapezoidal obstacle with different plateau. A finite volume based Cartesian grid method is adopted to solve the Reynolds averaged Navier-Stokes equations using a k-ε model for the turbulence closure. Numerical results reveal that the re-generated waveform does not occur due to baroclinic wave. The shorter plateau length would induce strong vortex in back of the obstacle. Moreover, the wave amplitude, vorticity and turbulent kinetic energy are dissipated significantly. However, the level of the phenomenon decreases as the plateau length is larger than the wavelength.