Ship-Wave Impact Generated Sea Spray. Part 2: Formulating Spray Frequency

Water breakup affects the variety of droplet sizes and velocities in a cloud of spray resulting from a sea wave striking a vessel bow. The Weber and Reynolds numbers of droplets are the main parameters for water breakup phenomena. “Stripping breakup” is a faster phenomenon than “bag breakup” and occurs at higher velocities and with larger diameters of droplets. A water breakup model employs droplet trajectories to develop a predictive model for the extent of spray cloud. The governing equations of breakup and trajectories of droplets are solved numerically. Stripping breakup is found as the major phenomenon in the process of the formation of wave-impact sea spray. Bag breakup acts as a complementary phenomenon to the stripping breakup. The extent of the spray as well as wet heights, for a Mediumsized Fishing Vessel (MFV), are obtained by numerical solutions. The results show that bag breakup occurs at higher heights. In addition, there is no breakup when droplets move over the deck.

Download Full-text

Ship-Wave Impact Generated Sea Spray: Part 2 — Formulating Spray Frequency

Volume 6A: Ocean Engineering ◽

10.1115/omae2020-18224 ◽

2020 ◽

Author(s):

Shafiul A. Mintu ◽

David Molyneux ◽

Bruce Colbourne

Keyword(s):

Field Measurements ◽

Operating Conditions ◽

Sea Spray ◽

Ship Motions ◽

Empirical Formulas ◽

Wave Impact ◽

Quick Estimate ◽

Semi Empirical ◽

Wave Induced ◽

Analytical Expressions

Abstract In certain, but not all, circumstances a cloud of spray forms after a wave impacts a ship. The frequency of spray events affects the icing process. Previous spray frequency formulas are derived empirically from field observations considering only the ship’s forward speed and oceanographic conditions. The significance of various degrees of ship motions on the spray frequency is ignored. However in reality, the interrelationships of heave and pitch motions under wave actions together with surge motion determine the number of spray events that a ship may experience in a given period of time. This paper introduces a theoretical model for estimating the frequency of sea spray considering ship motions. Ship motions can be easily estimated by strip/panel methods. However, in this work, the aim was to develop a simple framework for a quick estimate of spray frequency. The model inputs are, therefore, restricted to ship’s principal particulars, its operating conditions, and the environmental conditions. The wave-induced motions are estimated by semi empirical analytical expressions. A novel spray threshold is developed to keep the deck wetness frequency separated from the spray frequency. The proposed spray frequency formula is validated against available full-scale field measurements from a Russian fishing vessel, MFV Narva, and reasonable agreement is found. Limitations of previous empirical formulas are also discussed.

Download Full-text

Multi-Phase Simulation of Droplet Trajectories of Wave-Impact Sea Spray Over a Vessel

Volume 2: CFD and FSI ◽

10.1115/omae2019-95799 ◽

2019 ◽

Cited By ~ 1

Author(s):

Shafiul A. Mintu ◽

David Molyneux ◽

Bruce Colbourne

Keyword(s):

Theoretical Model ◽

Gpu Computing ◽

Sea Spray ◽

Maximum Height ◽

Observation Data ◽

Cfd Model ◽

Wave Impact ◽

Trajectory Model ◽

Droplet Trajectory ◽

Multi Phase

Abstract Sea spray, generated by ship-wave collisions, is the main source of marine icing. In certain, but not all, circumstances a cloud of spray forms after a wave impacts a ship. The spray cloud comprises numerous water droplets of various sizes. These droplets are dispersed and transported over the vessel deck by the surrounding wind and fall onto the deck or into the ocean under the effect of gravity. The motion of these droplets is important since they determine the extent of the spray cloud and its duration over the deck, which consequently affects the distribution of icing accumulation on a ship in freezing weather. In this paper, a multi-phase air-water simulation of droplet trajectory is used to predict the cloud motion of various size droplets. A smooth particle hydrodynamics (SPH) computational fluid dynamics (CFD) model is implemented and the simulation is accelerated using GPU computing. The field observation data is used to simulate the trajectory. The results of the simulations are compared with an available theoretical model and reasonable agreement is found. The inverse dependence of size and velocity for droplets after the breakup process is examined. The simulation results are consistent with the theoretical model in that neither the largest nor the smallest droplets reach the maximum height of the spray cloud, but the mid-size droplets do. The spray cloud spreads faster and crosses the front of the vessel quicker than predicted by the theoretical model. It is also found that the trajectory of a single droplet is significantly affected by surrounding droplets in a multi-droplet trajectory model. A mono-droplet theoretical trajectory model, therefore, is not as accurate as the multi-droplet CFD model.

Download Full-text