Jiahn-Horng Chen

Measurement of Pressure Distribution from PIV Experiments

In this study, a non-staggered grid SIMPLER pressure solution algorithm, which is able to produce correct pressure distribution directly if correct velocities are given, is proposed to solve the pressure distribution for PIV experiments. The cell face pseudo velocity required in the pressure equation is approximated by a simple linear average of the adjacent nodal pseudo velocities so that the velocity and pressure are collocated without causing the checkerboard pressure distribution problem. In addition, the proposed pressure solution algorithm has the features that upwind effects of the convective terms are considered, boundary conditions are not required, and the pressure distribution obtained can be used to correct the velocity field so that the continuity equation is satisfied. These features make the present algorithm a superior method to calculate the pressure distribution for PIV experiments. The pressure field solved is realistic and accurate. The proposed pressure equation solver is first calibrated with a two-dimensional cavity flow. It is found that the results are almost identical to the exact solution of the test flow. The algorithm is then applied to analyze a uniform flow past two side-by-side circular cylinders in a soap film channel. With the velocity and pressure distributions successfully measured, the structures of the complex shedding flow patterns are clearly manifested.

Theoretical Analysis and Sph Simulation for the Wave Energy Captured by a Bottom-Hinged OWSC

This paper examines one type of wave energy converter, the Oscillating Wave Surge Converter (OWSC) with its bottom hinged on the sea bed. The simplest form of OWSC is a flapper connected with a power take-off (PTO). Theoretical analysis based on a 2D linear potential theory elucidates the mechanisms of the wave energy capture by a bottom-hinged OWSC in terms of impedances associated with the wave field, the flapper body, and the PTO. Criteria of impedance canceling and matching to maximize the energy capture factor can be deduced. Smoothed Particle Hydrodynamics (SPH) is established for simulating the same problem, and the trends of SPH results match that of the theoretical prediction well. Therefore, SPH can be regarded as a reliable numerical tool for designing and optimizing OWSCs.

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.

Optimization of particle image distortion for PIV measurements

In this study, a jump matching correlation scheme is proposed. For the multi-grid, iterative particle image distortion analysis, the coarse grid solutions are not obtained from coarse pixel image constructed by binning small pixels to large pixel, but by jumping the interrogation window the number of pixels specified during the particle image matching process. With the jump matching scheme applied and particle image interpolated, the central difference particle image pattern matching and image distortion analysis are easily linked together to resolve flow fields from coarsest grid to super-resolution grid. Optimization of particle image distortion is achieved by averaging a forward and a backward image distortion correlation for every iteration process. The proposed method is first verified by standard particle images of impinging jet flow that has exact solution available, and then applied to analyze uniform flow past two side-by-side circular cylinders. Satisfactory results are obtained from the jump matching scheme proposed.

Wall effects on separation of flow past a circular cylinder

Abstract The nature of the flow past a circular cylinder has been a classical problem, raising many questions concerning the various wake phenomena that have been observed. This study focused on the critical Reynolds number for the onset of the recirculation region for a flow restricted in a channel. The influence of the bounded walls is examined. The trend is that larger critical Reynolds numbers are accomplished with larger values of blockage ratio (defined as the ratio of cylinder diameter to the channel width). Furthermore, as the blockage ratio tends to zero, the trend seems to imply that the critical Reynolds number approaches the experimental value for flows in an unbounded domain.

Hydrodynamic consideration in ocean current turbine design

Ocean currents are one of important resources of ocean energy. Although it is not widely harnessed at present, ocean current power has a vital potential for future electricity generation. In fact, several turbine systems have been proposed in the world. In the present, we consider what factors should be considered in designing the system from the perspective of hydrodynamics. As an example, a floating Kuroshio turbine system which is under development in Taiwan is employed to serve as the case study. The system consists of five major parts; i.e. a foil float which can be employed to adjust the system submergence depth, a twin contrarotating turbine system for taking off the current energy, two nacelles housing power generators, a cross beam to connect two nacelle-and-turbine systems, and two vertical support to connect the foil float and the rest of the system.

A DOMAIN-EXTENSION RADIAL BASIS FUNCTION COLLOCATION METHOD FOR HEAT TRANSFER IN IRREGULAR DOMAINS

Abstract A fictitious domain extension approach is introduced to solve the heat transfer problem in arbitrary domains by the radial basis function collocation method. The concept is derived from the superposition principle of potential problems. In this approach, arbitrary physical geometries which are usually not convex and/or topo‐logical rectangular are extended to domains which are topologically rectangular. The solution domain is also extended to the fictitious area and assumed to satisfy the same governing equation in it and on its boundaries. The boundary conditions are still specified on the boundaries of the original physical domain. The problem in the extended domain becomes ill‐posed. However, it can be easily circumvented by the collocation method. Applying this approach to several test problems with complicated geometries, we demonstrate that the solution can be directly obtained without domain decompositions and iterations. The new approach is simple, efficient and accurate.

Horseshoe Vortex Suppression with a Strake

We conducted computationally a parametric study to investigate the horseshoe vortex suppression due to a boundary-layer flow past a wing of finite span by a leading-edge strake. Both boundary-layer flows over a flat and curved wall were explored. In total, 48 cases were studied for various strake geometries. The Spalart-Allmaras model (1-equation model) is employed for the turbulence effect. The computational results show that different ratios lead to different flow development. Some of them can effectively suppress the horseshoe vortex. The detailed flow fields near the leading edge of the wing and the wake development are also investigated for different cases.

Computations of 2‐d cavitating flows using a dipole‐only BEM

Abstract A new boundary element procedure which employs dipole‐only singularities is developed to predict the flow past a two‐dimensional cavitating hydrofoil. In the present approach, the unknowns in the discretized simultaneous equation system consist of the dipole strengths at the control points on the wet surface of hydrofoil, the adjusted cavity increments at the nodal points on the cavity surface and the constant tangential speed on the cavity surface. As usual, the nonlinear iteration procedure is adopted; that is, the dynamic and kinematic boundary conditions of the cavity surface are specified on the correct cavity boundary which is a part of the flow solution and unknown before the problem is solved. Some numerical experiments have been carefully conducted. The results show that the present simpler approach successfully predicts the flow, compared to the methods in the literature.

Wall effects on emergence of vortex shedding in flows past a circular cylinder

Abstract This paper investigates a two‐dimensional flow past a circular cylinder placed within a channel. The study is focused on the emergence of a periodic flow pattern subjected to steady boundary conditions. Effects of domain truncation and the cylinder size on flow developments are also examined. The results indicate that the unsteady flow appears to be periodic and there is only one period. The period approaches the one in an infinite flow domain as the cylinder size is decreased.