On the fluid kinematics of common types of greenwater events: An experimental study

Abstract

Three common types of greenwater events – plunging dam breaker (PDB), hammer fist (HF) and plunging wave (PW) – are experimentally modeled in a laboratory wave flume on a rectangular structure, with a focus on investigating their fluid kinematics. To ensure high repeatability for type PW and type HF, a specific wave focusing method was employed, while PDB-type events were generated using a regular wave train. Utilizing a combination of PIV (particle image velocimetry) and BIV (bubble image velocimetry) techniques, ensemble-averaged flow fields were obtained from 20 repeated tests for each event type. The flow patterns at high speed, along with corresponding velocity fields, facilitated a comprehensive examination of flow behaviors, particularly for HF-type events which have received limited study. The maximum dominant speed for type PW was measured at up to 2.76$C$ during the run-up phase, where $C$ denotes the celerity of the incoming wave. For type HF and type PDB, the maximum dominant speeds occurred during the greenwater phase, with magnitudes of 1.37$C$ and 0.79$C$, respectively. The velocity deviation during the greenwater phase is <0.62C for all type events. The greenwater front velocity was measured at 1.36$C$ for type PW and approximately 0.8$C$ for both type HF and type PDB. Moreover, an attempt to evaluating the potential greenwater loads of high spatial resolution is demonstrated by the measured velocity fields for all event types. In this study, the simplest dam break solution is found to effectively capture the horizontal greenwater velocity distribution for all event types. Additionally, other mathematical expressions for the horizontal greenwater velocity have been derived based on flow self-similarity.