Steady streaming of viscous surface layer in waves

An investigation of the drift velocity induced by water waves of a contminated surface layer is carried out. The theoretical analysis is based on a thin boundary layer on the free surface. The results of the analysis reveal that the drift velocity of a viscous layer on the water surface is 7/4 times the Stokes prediction ofc(ak) ² wherec is the wave celerity andak the wave slope. The present experimental investigation confirms the validity of the theoretical prediction for the drift velocity for a lightly contaminated surface layer; however, for a heavily contaminated surface layer, the experimental results exceed the theoretical prediction. An investigation for a heavily contaminated layer is carried out assuming an inextensible surface layer. Thus, in the experiment, vinyl sheets are used to substitute the contaminated layers. By balancing the wave-induced mean thrust force with the mean drag force, the drift velocity is obtained and compared with the experimental results. Based on the theoretical and experimental analyses, formulae for predicting the drift velocities for laminar and turbulent flow conditions are proposed.     

Assessment of residual ultimate strength of an asymmetrically damaged tanker considering rotational and translational shifts of neutral axis plane

This paper provides two convergence criteria to find translational and rotational locations of the neutral axis plane (NAP) for intact and damaged vessels. Definition of three types of asymmetries of a ship section is proposed: material-, load-, and geometry-induced asymmetries. Concept of moment plane (MP) is introduced to define the heeling angle of ship section. It is suggested that force equilibrium and force vector equilibrium criteria are simultaneously necessary to determine new position of NAP due to both translational and rotational shifts. In order to verify the applicability of the convergence criteria, midship section of a VLCC is selected with two types of asymmetries: one is due to heeling of a section and the other due to hull damages. 0° and 30° heeling conditions and collision-induced and grounding-induced damage extents based on ABS Guides and DNV Ship Rules are taken into account. The various section properties are compared according to the area reduction ratios for each heeling and damage cases. It is shown that ultimate hull girder capacities are closely related to the area reduction due to the damages. Using new convergence criterion, mobility of NAPs and force centroids in elastic and inelastic regimes are visually provided.     

Comparative measurements on the flow structure of a marine propeller wake between an open free surface and closed surface flows

In order to investigate the effects of a free surface on the wake behind a rotating propeller, experiments were carried out in a circulating water channel for two cases: one with an open free surface and one with a closed free surface covered by a rigid plate. Four hundred instantaneous velocity fields were measured using a two-frame particle image velocimetry (PIV) technique at four different blade phases. These were ensemble-averaged to investigate the time-averaged flow structure in the near-wake region. For a surface ship, the flow behind the propeller is influenced by the hull wake and the free surface. The phase-averaged mean velocity fields show the potential wake and the viscous wake formed by the boundary layers developed on the blade surfaces. The interaction between the bilge vortices and the incoming flow along the ship’s hull deforms the wake structure. Tip vortices are generated periodically, and the slipstream contraction occurs in the near-wake region. The free surface was found to affect the axial velocity component and vortex structure behind the propeller. As the flow goes downstream, the tip and trailing vortices dissipate due to turbulent diffusion and active mixing with adjacent vortices.     

Investigation of the effectiveness of tandem oil fences under currents

The behavior of the flow passing a tandem oil fence, and the performance of the fence, were investigated by experimental and numerical methods. The flow characteristics between tandem fences were measured by the particle image velocimetry (PIV) method for the rigid and open free surface between the two fences in order to gather reference data for numerical investigations. A method of assessing a tandem fence by tracing the movement of an oil droplet around the fence is introduced. The effect of the current speed, the separation distance between the two fences, the relative draft of the two fences, and the water depth on the oil containment between the fences was investigated.