S.H. Kwon

Wave profile measurement by wavelet transform

A new technique for measuring wave profiles by wavelet transform using the Mexican Hat wavelet as the mother wavelet is introduced. This technique has the potential to provide low cost, high resolution field measurements of wave profiles in the laboratory. The experiments to capture the video image of the wave profile were carried out in a wave flume. Then, the Mexican Hat wavelet was adopted to trace out the exact profiles of the waves from the captured video images. The series of tests on numerical data and video images show promise as means of detecting two-dimensional profiles of waves.

The effects of drag reduction by ribbons attached to cylindrical pipes

This study proposes a drag reduction device that uses three ribbons attached 120 degree apart to vertical pipes. Experiments were conducted in a circulating water channel to investigate the effects of the ribbon length and the direction of the flow on various current velocities. Drag on a vertical cylinder was measured by a resistance dynamometer. Flow visualizations were conducted using laser sheet beams. Laser Doppler Velocimetry (LDV) was used to measure the velocity field in the wake. This experiment demonstrates that attached ribbons can be used to reduce the drag force on vertical pipes for various directions of incoming flows. The ribbon-type device is very simple and easy to fabricate for field applications. The results are promising for the application to offshore structures.

Carbon nanotube reinforced shape memory polyurethane foam

Multiwalled carbon nanotube (MWCNT)/polyurethane (PU) foams have been synthesized from polypropylene glycol and 2,4/2,6-toluene diisocyanate following the one-shot foaming method in the presence of water as the chemical blowing agent. The effects of CNT content on the performances of the foams have been analyzed in terms of reactivity, mechanical and dynamic mechanical properties, and shape memory properties of the foams. It was found that the cream time, rise time, tensile, and compressive strengths at room temperature, glassy and rubbery state moduli, glass-transitional temperature (Tg), and shape fixity and shape recovery increased with the addition and increasing amount of MWCNT.

Experimental and numerical studies on the development of a new wave absorber

This paper proposes a new wave absorber made of flexible net structures. To test the efficiency of the proposed water absorber, experiments were done on wave absorbers of various lengths of and the thicknesses of the wave absorber. To perform a numerical modeling of the proposed wave absorber, damping terms were introduced in linearized free-surface boundary conditions. The length and the thickness of the wave absorber were modeled by the length and the coefficient of the damping zones. The boundary element method was adopted to solve the system. Series of experiments were performed to obtain the data for the coefficients of the damping term needed in numerical calculations. The predicted wave heights agreed very closely with those of experiments when the lengths of the incoming waves were within the order of the length of the wave absorber.

A Study on Gap Influence on Slamming Experiment

This study presents an investigation of gap effect on slamming experiment. Two gaps were considered; a gap between transverse side of a model and tank wall and a gap between longitudinal end and end of a model. The deadrise angle was fixed 0°. The pressure was measured at the central location and compared. Three different drop heights were chosen to see the difference. A High speed camera was used to record the flow field. Therefore pressure at the center, and flow field around corner of the specimen were analyzed to investigate the gap effect in slamming experiment. The results showed that there was clear influence of the gap in slamming experiment.Copyright

A Study on Ullage Effect in 2-D Sloshing Experiment

This study presents investigation on ullage effect in sloshing experiment. The experiment was done with two dimensional tank. Sloshing experiments were carried out in the tank with 6 different ullage pressures. The tested filling ratio was 30% of the tank height. The flow field was recorded with high speed camera. The sloshing impact pressure were measured at 18 locations. It was shown that the variation of ullage pressures influences the magnitude of pressure and flow field. This study demonstrated the importance of ullage pressure in sloshing test.

Comparison of Potential and Viscous Codes for Water Entry Problem

This paper presents a comparison of potential and viscous computational codes for the water entry problem. A po-tential code was developed which adopted the boundary element method to solve the problem. A nonlinear free surface boundary condition was integrated to find new locations of free surface. The dynamic boundary condition was simplified by taking constant potential values for every time steps. The simplified dynamic boundary condition was applied in the new position of the free surface not at the mean level, which is the usual practice for linearized theory. The commercial code FLUENT was used to solve the water entry problem from the viscosity point of view. The movement of the air-liquid interface is traced by distribution of the volume fraction of water in a computational cell. The pressure coefficients were compared with each other, while experimental results published by other researchers were also examined. The characteristics of each method were discussed to clarify merits and limitations when they were applied to the water entry problems.

The deformation of a free surface due to the impact of a water droplet

An attempt was made to compute the free surface deformation due to the impact of a water droplet. The Cauchy Poisson, i.e. the initial value problem, was solved with the kinematic and dynamic free surface boundary conditions linearized. The zero order Hankel transformation and Laplace transform were applied to the related equations. The initial condition for the free surface profile was derived from a captured video image. The effect of the surface tension was not significant with the water mass used in this investigation. The computed and observed free surface deformations were compared.

Application of wavelet transforms to the decoupling of free-roll decay data

This study presents the results of the decoupling of a free-roll decay test by wavelet transforms. A free-roll decay test was performed to determine the coefficients of damping terms in equations of motion. No motion of the model was restricted during the experiment. A slight yaw motion was found while the model was in the free-roll decay motion. It was necessary to extract a pure roll motion from the experimental data. Wavelet transforms were applied to the signals to extract the pure roll motion. The results were compared to those found through the Fourier transform. Discrete wavelet transforms were able to efficiently decouple the test signals, while the continuous wavelet transform and the Fourier transform could not.

ANALYSIS OF RINGING BY CONTINUOUS WAVELET TRANSFORM

This paper investigates the ringing phenomenon by using continuous wavelet transform (CWT). Experiments were conducted in a wave tank. Breaking waves were generated to understand the ringing phenomenon. The model tested was a surface piercing circular cylinder and the time series of the wave height and force on the cylinder was measured. An analysis was conducted using continuous wavelet transform. The scalograms of the time series of the wave height and those of the forces showed that high frequency components were generated at the onset of the breaking wave impact in the time domain. It was concluded that, ringing is a natural frequency response due to the breaking wave impact.