Ho-Cheng Lien

Flow Structure around Bridge Piers of Varying Geometrical Complexity

Piers with back-to-back stems or columns and piers for which part of the foundation becomes exposed as a result of the development of scour over large periods of time or because of severe flood events are fairly common at bridge waterways. The present paper uses eddy-resolving numerical simulations to study flow and turbulence structure at piers of complex shape and/or with multiple components. In particular, the study considers cases with one and two back-to-back pier columns for which the section of the main column is neither circular nor rectangular. In addition to a design case for which the foundation of each pier column is submerged, the study analyzes a case when scour exposes part of the foundation of the main column. The results show that the shape and size of the pier column have a significant effect on the spatial and temporal distributions of the bed friction velocity induced by the horseshoe vortex system. The large-scale shedding behind the main column greatly influences flow structure and increases bed friction velocity around the downstream column for piers with two back-to-back columns that are aligned with the incoming flow direction. The present study shows that the presence of large-scale unsteady coherent structures in the vicinity of the bed around piers of complex shapes results in very complex distributions of the bed friction velocity and in large-scale temporal oscillations of the bed friction velocity. The results of eddy-resolving simulations strongly suggest the need to account for the effect of these large-scale oscillations around the mean value when bed friction velocity distributions are used to estimate the flux of entrained sediment in movable bed simulations that do not resolve the large-scale turbulent flow structures.

Development of a real-time monitoring system as a decision-support system for flood hazard mitigation

This study proposes a real-time monitoring system to integrate the supportive flood-related information for hazard mitigation purposes. Through integrating photography, communication, geometric information display and network technology, this system can provide the real-time monitoring images at the critical gauge stations, inundation-prone areas and important hydraulic facilities during floods. With the development of mobile monitoring modules, the maneuverability of users and facilities could be significantly improved. Users can easily employ mobile phones to receive or report real-time flood-related information at any place and time. Furthermore, the water-level image recognition method is developed and applied to the selected monitoring stations for acquiring early warning of flood events. In final, an application scenario is given to illustrate how this system can greatly improve the effectiveness and efficiency of emergency responses to flood hazard mitigation for the decision-makers.

The integration and application of the computational grid structure in flood forecast system

The grid technology is the third network current after the traditional Internet and WWW, and could be specified as the third generation of network application. The main topic of the grid technology is to integrate all the resources in the network that include the computing capacity, the storage system, the communication system, software, information, and knowledge. In another word, the grid structure is a huge sharing platform that integrates all kinds of resources from Internet. National Center for High-performance Computing, Taiwan, is the main computing center taking charge of the integration and application of high-performance computing and networking in Taiwan. The grid technology is applied in this article to integrate techniques of network, database, hydrology, fluid mechanics, and related applications. Experts are called together to join the project. The main goal in this study is to build a networked, Web-based, and distributed flood forecast system with the integration of computation grid. Users could access this system simultaneously through Web browser. A plurality of flood forecast projects could be as well managed at the same time. Results of the flood forecast projects would be provided to the Central Disaster Emergency Center and local government units to help process the flood forecast and the exigent disaster-reducing assignments through Internet.

An efficienct way for sharing real-time data and the application of remote network video system

This study is to integrate the current monitoring systems and to develop the multicasting and peer-to-peer network environment for end users. Because the bandwidth of network is limited where the camera is located, the transmission of images will delay or crash when over a few numbers of end user login simultaneously. In this paper, the middleware for sharing realtime data will be developed to solve the problem of limitation for multi-connection from fields. The monitoring systems for flood events, that are located in the rivers, creeks, reservoirs, and pump stations of Taiwan, will be integrated as a demo case to show the capability of the system integration. And in this case will also show how the performance is that sharing real-time data for unlimited number of users with limited network bandwidth.

Simulation of free surface by surface capturing method

In the present study, a vertical, two-dimensional, two-phase flow model is developed, which is capable of simulating the fluid and air flow fields simultaneously to capture the positions of the free surface. To deal with the discontinuity properties in density field near the free surface, the TVD-MUSCL scheme is employed to overcome the numerical oscillation problem. The numerical algorithm is developed using a conservative, explicit, finite volume discretization of the motion equation. The dam-breaking problem is employed to validate the proposed model. The good agreements between the computed and measured results demonstrate the applicability of the model.