Ching Jer Huang

Structural permeability effects on the interaction of a solitary wave and a submerged breakwater

Abstract The unsteady two-dimensional Navier–Stokes equations and Navier–Stokes type model equations for porous flows were solved numerically to simulate the interaction between a solitary wave and a submerged porous breakwater. The free surface boundary conditions and the interfacial boundary conditions between the water and the porous media are in complete form. A piston-type wavemaker, set-up in the computational domain, generated the incident solitary wave. The accuracy of the numerical model was verified by comparing the numerical results with the experimental data. Having verified the accuracy of the numerical model, the effects of several parameters on the interaction of a solitary wave and a submerged breakwater were systematically investigated. These parameters include the incident wave height, the aspect ratio of the breakwater, and the porosity including the impermeable case. The flow fields near the breakwater are discussed in terms of the velocity vectors, the vortex shedding and the trajectories of the fluid particles. The pressure drag acting on the breakwater was also calculated. The numerical results reveal that if the breakwater width is small compared with the effective wave length, the structure permeability has no apparent effect on wave transformation. For wide porous breakwaters, if the structure porosity is small, the increase in the porosity results in the reduction of the transmission coefficient; otherwise the transmission coefficient increases with porosity.

On the interaction of a solitary wave and a submerged dike

Abstract The unsteady, two-dimensional Navier–Stokes equations and the exact free surface boundary conditions were solved to study the interaction of a solitary wave and a submerged dike. A piston-type wavemaker was set up in the computational domain to produce the incident solitary waves. The incident wave and the associated boundary layer flow in a wave tank with a flat bed were compared with the analytical solutions to verify the accuracy of this numerical scheme. Effects of the incident wave height and the size of the dike on the wave transformation, the flow fields, and the drag forces on the dike were discussed. Our numerical results showed that even though the induced local shear stress on the top surface of the dike is large at some particular locations, the resultant pressure drag is much larger than the friction drag. The primary vortex generated at the lee side of the dike and the secondary vortex at the right toe of the dike may scour the bottom and cause a severe problem for the dike.

Wave deformation and vortex generation in water waves propagating over a submerged dike

The Navier–Stokes equations and the exact free surface boundary conditions are solved to simulate wave deformation and vortex generation in water waves propagating over a submerged dike. Incident waves are generated by a piston-type wavemaker set up in the computational domain. Numerical results are compared with experimental data in order to confirm the validity of the numerical model. The fast Fourier transform and a wave resolution technique are applied to decompose the transformed waves and the higher harmonics. Effects of different parameters on wave transformation and vortex generation are studied systematically. These parameters include the Ursell number, the Keulegan–Carpenter number, the water depth ratio, the Reynolds number, the length aspect ratio of the dike, and the type of dike.

Model of sheared granular material and application to surface-driven granular flows under gravity

This work presents a novel model of sheared granular materials that consist of two-dimensional, slightly inelastic, circular disks. To capture the static and kinetic features of the granular flow involving different regimes, both the shear stress and pressure are superimposed by a rate-independent component (representing the static contribution) and a rate-dependent component (representing the kinetic contribution), as determined using granular kinetic theory. The dilatancy law is adopted to close the set of equations, and the constraint that static pressure is non-negative is utilized to determine the transition between the dense regime and the inertial regime. The balance equation of granular temperature incorporates the works done by both the static and kinetic components of shear stress. This enabled the proposed model to predict the hysteretic flow thresholds and the shear bands. Additionally, a thick, surface-driven granular flow under gravity is investigated using the proposed model. The predicted ...

Kinetic-theory-based model of dense granular flows down inclined planes

This work extends a continuum model of sheared granular material comprising two-dimensional disks [C. H. Lee and C. J. Huang, Phys. Fluids 22, 043307 (2010)10.1063/1.3400203] to elucidate the dynamics of three-dimensional spheres. The proposed model is applied to investigate dense granular flows down an inclined plane. In the model, stress has a static component and a kinetic component. The constitutive model for shear stress reduces to the Bagnold model when the diffusion of granular temperature is small. The predicted rheological characteristics are identical to those observed in the preceding experiments and numerical simulations, validating the present model. The predicted rheological characteristics reveal that dense granular flows down an inclined plane are characterized by three special angles that determine the phase diagram. The predicted thick granular flow on an inclined plane exhibits the Bagnold velocity profile and a uniform volume fraction throughout its depth. The governing equation of gra...

The present development of debris flow monitoring technology in Taiwan - A case study presentation

In order to document the on-site debris flow events, the Soil and Water Conservation Bureau (SWCB), Council of Agriculture, Taiwan, has de- voted to develop the debris flow monitoring system since 2002. This paper introduces the technology of 17 on-site and 3 mobile debris flow monitoring sta- tions established by SWCB in Taiwan. In each on-site monitoring station, several observation instruments including rain gauges, CCD cameras, wire sensors, geophones, and water level meters were installed to collect the dynamic debris flow information that can be used as the references for countermeasures of de- bris flow disaster mitigation. Besides, several mete - orological sensors are also adopted recently in order to record the long-term climate change effects on the slopeland of Taiwan. The framework of the debris flow monitoring system consists of monitoring sen- sors, instrumental cabin (vehicle platform for mobile station), transmission system and web-based display system. During the typhoon Mindulle period in 2004, a debris flow event in Aiyuzih creek was observed by the Shenmu debris flow monitoring station on July 2, Nantou County, central Taiwan. On-site observation data including the rainfall patterns, video images, wire sensor ruptures and ground vibrations caused by de- birs flows are analyzed in detail.

Calibration and Deployment of a Fiber-Optic Sensing System for Monitoring Debris Flows

This work presents a novel fiber-optic sensing system, capable of monitoring debris flows or other natural hazards that produce ground vibrations. The proposed sensing system comprises a demodulator (BraggSCOPE, FS5500), which includes a broadband light source and a data logger, a four-port coupler and four Fiber Bragg Grating (FBG) accelerometers. Based on field tests, the performance of the proposed fiber-optic sensing system is compared with that of a conventional sensing system that includes a geophone or a microphone. Following confirmation of the reliability of the proposed sensing system, the fiber-optic sensing systems are deployed along the Ai-Yu-Zi and Chu-Shui Creeks in Nautou County of central Taiwan for monitoring debris flows. Sensitivity test of the deployed fiber-optic sensing system along the creek banks is also performed. Analysis results of the seismic data recorded by the systems reveal in detail the frequency characteristics of the artificially generated ground vibrations. Results of this study demonstrate that the proposed fiber-optic sensing system is highly promising for use in monitoring natural disasters that generate ground vibrations.

Statistical analysis on the long-Term observations of typhoon waves in the Taiwan sea

Seventeen data buoys were deployed in the Taiwan Sea since 1997. These buoys have made measurement for more than 100 typhoons. The purpose of this paper is to study the statistical characteristics of the observed typhoon waves. High resolution directional wave spectra are obtained by analyzing the buoy data. The Significant Typhoon Wave Height (SWTH) and the Duration of Large Waves (DLW) are proposed to indicate the sea severity of typhoons. The joint effect of the SWTH and the DLW is used to assess possible impact by the typhoon waves to the coast, using the damage curve calibrated from historical events. In addition analysis on the slope of the equilibrium range on the high-frequency side of the typhoon wave frequency spectra shows that its slope is less than 3.5 in absolute value for coastal water and larger than 3.8 for the deep sea. The JONSWAP spectrum model is then used to fit the mean wave spectra of the largest sea state in typhoons. It is found that the peak-enhancement factors  obtained were 2.48 and 2.19 for the deep sea and the coastal ocean, respectively, showing that typhoon waves in Taiwan sea are in the developing stage.

An Adaptation Due to Climate Change in Southwest Coast of Taiwan

The adaptive capacity of coastal disasters caused by climate change in order to strengthen southwestern Taiwan against natural calamities in the future is investigated. In Taiwan, the coastal zone suffers from approximately four typhoons each year, and the exceptionally high sea levels caused by storm surges frequently results in coastal disasters and hinders the development of the coastal area. The problems of sea-level rises and frequent typhoons induced by climate change have threatened the Taiwanese coastal environments. These influences as well as serious land subsidence upon a scenario year were carried out in the southwest coastal areas. The present study focuses on the construction of the disaster characteristics, model establishment for situation analysis of water environmental factors, impact estimation and indefinite analysis on disasters, and vulnerability and risk estimation of coastal disasters on the study area. These results could provide useful information to establish strategies to implement as well as how to analyze the benefits of such a program.

Fiber optic probe for local void fraction measurements in bubbly flows

A technique based on Fresnel reflection at the tip of a fiber optic probe is used to investigate local void fractions in bubbly flows. The fiber optic system with multiplexing scheme is designed for demonstrating the feasibility of measuring local void fractions in bubbly flows. Processing the signal acquired from the fiber optic system, local void fractions can be obtained. Linear regression with least square method is applied to analyze data. The correlation coefficient of 0.94 indicates that the technique seems to be suited for measuring local void fractions in the range from 2% to 20%.