Jeonghwa Seo

Experimental Study on the Six Degree-of-Freedom Motions of a Damaged Ship Floating in Regular Waves

One of the most critical issues for ship owners, shipbuilders, and insurance companies is the operational safety. In particular, keeping damaged ships stable in waves is of great interest, because more nonconventional hull forms are being introduced for military and passenger vessels while international rules and regulations are becoming stricter. However, ship stability for damaged ships is quite different from that for intact ships as the assessment is very complicated and difficult due to the highly nonlinear behavior. Computational fluid dynamics (CFD) methods that solve the Navier-Stokes equations are acknowledged as the only viable approach to simulate and analyze these complex physical phenomena. Although there have been a number of research activities reported on damaged ship stability recently, most of them are not designed to validate CFD studies. For a data set to be valuable for CFD validation and development, model tests should eliminate unclear factors as much as possible. The main objective of this study is to establish an experimental database for CFD validation by collecting data from towing tank tests of a ship hulls six degree-of-freedom (6DOF) motion responses in regular waves for both intact and damaged conditions. A mooring system was designed to prevent drift motions of the ship model. Parametric roll was not observed when the ship was damaged, although it was observed for the intact ship in the same wave conditions. The mooring force acting on the ship model due to spring tension was also calculated.

Numerical Analysis of a Tip Vortex Flow for Propeller Tip Shapes

In order to control the tip vortex cavitation occurring around the tip of a rotating propeller blade, researches on the propeller cavitation and blade tip vortex flows have been increased. In this paper, the propeller tip vortex flow for a blunt and sharp tips was studied using an unsteady Reynolds-averaged Navier-Stokes equations solver based on a cell-centered finite volume method. In numerical open water test, torques, thrusts, pressure distributions and vortex flows were compared for various rotating speeds. To consider a hull wake, the nominal wake was specified in inlet boundary condition. Pressure distributions and vortex flows with the hull wake were investigated for various propeller rotating angles. From the results, it was confirmed that the blunt tip propeller delayed the tip vortex flow Ke ywords :P ropeller blade tip(추진기 날개 끝), Tip vortex flow(날개 끝 보오텍스 유동), Computational fluid dynamics(CFD, 전산유체역학)

Uncertainty Assessment of a Towed Underwater Stereoscopic PIV System

Test uncertainty of a towed underwater Stereoscopic Particle Image Velocimetry (SPIV) system was assessed in a towing tank. To estimate the systematic error and random error of mean velocity and turbulence properties measurement, velocity field of uniform flow was measured. Total uncertainty of the axial component of mean velocity was 1.45% of the uniform flow speed and total uncertainty of turbulence properties was 3.03%. Besides, variation of particle displacement was applied to identify the change of error distribution. In results for variation of particle displacement, the error rapidly increases with particle movement under one pixel. In addition, a nominal wake of a model ship was measured and compared with existing experimental data by five-hole Pitot tubes, Pitot-static tube, and hot wire anemometer. For mean velocity, small local vortex was identified with high spatial resolution of SPIV, but has serious disagreement in local maxima of turbulence properties due to limited sampling rate.

Propeller Wake Measurement of a Model Ship in Self Propulsion Condition using Towed Underwater PIV

A two-dimensional particle image velocimetry (2D PIV) system in a towing tank is employed to measure a wake field of a very large crude oil carrier model with rotating propeller in self propulsion condition, to identify characteristics of wake of a propeller working behind a ship. Phase-averaged and time-averaged flow fields are measured for a horizontal plane. Scale ratio of the model ship is 1/100 and Froude number is 0.142. By phase-averaging technique, trajectories of tip vortex and hub vortex are identified and characteristic secondary vortex distribution is observed in the hub vortex region. Propeller wake on the starboard side is more accelerated than that on the port side, due to the difference of inflow of propeller blades. The hub vortex trajectory tends to face the port side. With the fluctuation part of the phase-averaged velocity field, turbulent kinetic energy (TKE) is also derived. In the center of tip vortex and hub vortex region, high TKE concentration is observed. In addition, a time-averaged vector field is also measured and compared with phase-averaged vector field.