Response prediction of long flexible risers subject to forced harmonic vibration

Several research efforts have been directed toward the development of models for response prediction of flexible risers. The main difficulties arise from the fact that the dynamic response of flexible risers involves highly nonlinear behavior and a self-regulated process. This article presents a quasi-steady approach for response prediction of oscillating flexible risers. Amplitude-dependent lift coefficients are considered, as is an increased mean drag coefficient model during synchronization events. Experimental validation of the proposed model was carried out using a 20-m riser model excited by forced harmonic vibration at its top end. Large variations in the hydrodynamic force coefficients, a low mass ratio value, and synchronization events are the main features of the model presented in this article. Experimental validation was provided for the asymmetric, transverse, diagonal, and third vortex regimes.     

On the mechanism of the bursting phenomena of propeller tip vortex cavitation

 The bursting phenomenon of tip vortex cavitation of a propeller sometimes causes severe high-frequency vibration, but its mechanism has not yet been elucidated. In this study, we carried out model experiments by changing the propellers, wake distributions, thrust coefficients, and cavitation numbers parametrically, examined the bursting phenomenon with a high-speed video camera, and measured the pressure fluctuations caused by the phenomenon. We also measured flow distribution around the tip vortex. As a result, we found that in the bursting phenomenon, large pressure fluctuations occurred twice, and that they strongly depended on the wake distribution. Two means were suggested to suppress the bursting phenomenon, other than changing the wake distribution: stabilizing tip vortex cavitation or reducing the cavity volume. Numerical fluid simulations around a propeller in noncavitating, unsteady conditions were also conducted, and the strength of the tip vortex along the circumference and its derivative were examined. As a result, the phenomena were parameterized by the time derivative of the strength of the tip vortex, and if it was higher than a threshold value, the tip vortex cavitation burst. Therefore, it is possible to predict the occurrence of the bursting phenomenon by numerical analysis. Received: November 6, 2001 / Accepted: January 24, 2002     

Education of truck drivers using the assist driver support system ‐a pilot survey‐

We have developed a driver support system, ASSIST, to decrease automobile driving accidents. Most traffic accidents involve collisions of two objects. A collision occurs when a vehicle's headway is shorter than the stopping distance. Therefore, we plan to warn the driver when the vehicle's headway is shorter than the estimated stopping distance. This driver support system performs exactly that task. Results of experiments verify that this system increases that distance gap by warning the driver to increase the headway.