A time-domain simulation of an oscillating water column with irregular waves

Koo, Weoncheol,Kim, Moo-Hyun Techno-Press 2012 Ocean systems engineering Vol.2 No.2

A time-domain simulation of a land-based Oscillating Water Column (OWC) with various irregular waves as a form of PM spectrum is performed by using a two-dimensional fully nonlinear numerical wave tank (NWT) based on the potential theory, mixed Eulerian-Lagrangian (MEL) approach, and boundary element method. The nonlinear free-surface condition inside the OWC chamber was specially devised to describe both the pneumatic effect of the time-varying pressure and the viscous energy loss due to water column motions. The quadratic models for pneumatic pressure and viscous loss are applied to the air and free surface inside the chamber, and their numerical results are compared with those with equivalent linear ones. Various wave spectra are applied to the OWC system to predict the efficiency of wave-energy take-off for various wave conditions. The cases of regular and irregular waves are also compared.

Free surface simulation of a two-layer fluid by boundary element method

Weoncheol Koo 대한조선학회 2010 International Journal of Naval Architecture and Oc Vol.2 No.3

A two-layer fluid with free surface is simulated in the time domain by a two-dimensional potential-based Numerical Wave Tank (NWT). The developed NWT is based on the boundary element method and a leap-frog time integration scheme. A whole domain scheme including interaction terms between two layers is applied to solve the boundary integral equation. The time histories of surface elevations on both fluid layers in the respective wave modes are verified with analytic results. The amplitude ratios of upper to lower elevation for various density ratios and water depths are also compared.

Simplified formulas of heave added mass coefficients at high frequency for various two-dimensional bodies in a finite water depth

Koo, Weoncheol,Kim, Jun-Dong The Society of Naval Architects of Korea 2015 International Journal of Naval Architecture and Oc Vol.7 No.1

The aim of this study is to develop a simplified formula for added mass coefficients of a two-dimensional floating body moving vertically in a finite water depth. Floating bodies with various sectional areas may represent simplified structure sections transformed by Lewis form, and can be used for floating body motion analysis using strip theory or another relevant method. Since the added mass of a floating body varies with wave frequency and water depth, a correction factor is developed to take these effects into account. Using a developed two-dimensional numerical wave tank technique, the reference added masses are calculated for various water depths at high frequency, and used them as basis values to formulate the correction factors. To verify the effectiveness of the developed formulas, the predicted heave added mass coefficients for various wetted body sections and wave frequencies are compared with numerical results from the Numerical Wave Tank (NWT) technique.

Simplified formulas of heave added mass coefficients at high frequency for various two-dimensional bodies in a finite water depth

Weoncheol Koo,Jun-Dong Kim 대한조선학회 2015 International Journal of Naval Architecture and Oc Vol.7 No.1

The aim of this study is to develop a simplified formula for added mass coefficients of a two-dimensional floating body moving vertically in a finite water depth. Floating bodies with various sectional areas may represent simplified structure sections transformed by Lewis form, and can be used for floating body motion analysis using strip theory or another relevant method. Since the added mass of a floating body varies with wave frequency and water depth, a correction factor is developed to take these effects into account. Using a developed two-dimensional numerical wave tank technique, the reference added masses are calculated for various water depths at high frequency, and used them as basis values to formulate the correction factors. To verify the effectiveness of the developed formulas, the predicted heave added mass coefficients for various wetted body sections and wave frequencies are compared with numerical results from the Numerical Wave Tank (NWT) technique.

유한 수심에서 반원형 부유체의 부가질량계수 약산식 개발

구원철(Weoncheol Koo),김준동(Jun-Dong Kim) 한국해양공학회 2013 韓國海洋工學會誌 Vol.27 No.1

This study is to develop the simplified formulae for added mass coefficient of a 20D floating body with a semi-circle section in a finite water depth. The semi-circle floating body may represent a simplified midship section transformed by Lewis form, which can be used for the ship motion analysis by strip theory. Since the added mass coefficient varies with motion frequencies and sea bottom effect, the correction factor representing the effect of water depth and frequencies is developed for accurate prediction of added mass. Using a two-dimensional numerical wave tank (NWT) technique based on the boundary element method(BEM) including sea bottom boundary the reference values of added mass are calculated to develop the correction factor. For verification and effectiveness of the formulae, the predicted added mass coefficients for various frequencies and water depth ratios are compared with the calculated values from NWT technique.

DYNAMIC RESPONSE ANALYSIS OF WAVE ENERGY CONVERTER PLATFORM WITH OSCILLATING MULTI-BUOY

Sanghwan HEO,WeonCheol KOO,Min-Su PARK 한국신재생에너지학회 2017 AFORE Vol.2017 No.11

Dynamic Response Analysis of a Bottom-Fixed 5-MW Jacket Foundation Offshore Wind Turbine

Sanghwan Heo,Weoncheol Koo 한국신재생에너지학회 2023 AFORE Vol.2023 No.11

Dynamic Response and Reliability of Six-Leg Jack-Up Type Wind Turbine Installation Vessel

Heo, Sanghwan,Koo, Weoncheol,Park, Min-Su World Scientific Publishing Company 2017 International journal of structural stability and Vol.17 No.3

<P>A fast, reliable and optimized numerical procedure of the hydrodynamic response analysis of a slender-body structure is presented. With this method, the dynamic response and reliability of a six-leg jack-up-type wind turbine installation vessel under various environmental conditions is analyzed. The modified Morison equation is used to calculate the wave and wind-driven current excitation forces on the slender-body members. The Det Norske Veritas (DNV) rule-based formula is used to calculate the wind loads acting on the superstructure of the jack-up leg. From the modal analysis, the natural period and standardized displacement of the structure are determined. The Newmark-beta time-integration method is used to solve the equation of motion generating the time-varying dynamic responses of the structure. A parametric study is carried out for various current velocities and wind speeds. In addition, a reliability analysis is conducted to predict the effects of uncertainty of the wave period and wave height on the safety of structural design, using the reliability index to indicate the reliability of the dynamic response on the critical structural members.</P>

Analysis of Dynamic Response Characteristics for 5 MW Jacket-type Fixed Offshore Wind Turbine

Jaewook Kim,Sanghwan Heo,WeonCheol Koo 한국해양공학회 2021 韓國海洋工學會誌 Vol.35 No.5

This study aims to evaluate the dynamic responses of the jacket-type offshore wind turbine using FAST software (Fatigue, Aerodynamics, Structures, and Turbulence). A systematic series of simulation cases of a 5 MW jacket-type offshore wind turbine, including wind-only, wave-only, wind & wave load cases are conducted. The dynamic responses of the wind turbine structure are obtained, including the structure displacement, rotor speed, thrust force, nacelle acceleration, bending moment at the tower bottom, and shear force on the jacket leg. The calculated time-domain results are transformed to frequency domain results using FFT and the environmental load with more impact on each dynamic response is identified. It is confirmed that the dynamic displacements of the wind turbine are dominant in the wave frequency under the incident wave alone condition, and the rotor thrust, nacelle acceleration, and bending moment at the bottom of the tower exhibit high responses in the natural frequency band of the wind turbine. In the wind only condition, all responses except the vertical displacement of the wind turbine are dominant at three times the rotor rotation frequency (considering the number of blades) generated by the wind. In a combined external force with wind and waves, it was observed that the horizontal displacement is dominant by the wind load. Additionally, the bending moment on the tower base is highly affected by the wind. The shear force of the jacket leg is basically influenced by the wave loads, but it can be affected by both the wind and wave loads especially under the turbulent wind and irregular wave conditions.

Nonlinear time-domain NWT simulations for two types of a backward bent duct buoy (BBDB) compared with 2D wave-tank experiments

Kim, Sung-Jae,Koo, Weoncheol,Kim, Moo-Hyun Elsevier 2015 Ocean engineering Vol.108 No.-

<P><B>Abstract</B></P> <P>A fully nonlinear time-domain NWT-simulation tool has been developed with nonlinear viscous-damping and pneumatic pressure terms. The tool was applied to the performance evaluation of fixed/floating sharp-corner and round-corner BBDB WECs. Shape-induced and motion-induced viscous damping for the BBDB system was considered in the potential-flow-based simulation with quadratic pneumatic pressure and fully nonlinear free-surface conditions. Systematic experiments were also conducted in a two-dimensional wave tank to verify the NWT-simulation results. The numerical results were further verified by checking the total power conservation of the entire system. The nonlinear simulations were also compared with linear ones. Since the round-corner BBDB has less energy loss caused by body shape and motion, the available power of the round-corner BBDB is generally bigger than that of the sharp-corner BBDB. The phenomenon of negative drift motion for only the sharp-corner BBDB for a certain wave-length range was also reproduced by the fully nonlinear BBDB simulations. This numerical simulation tool can be used for the reliable design of prototype systems for a given site and environmental conditions.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A fully nonlinear time-domain NWT-simulation tool for BBDB WEC was developed. </LI> <LI> Systematic experiments were conducted to verify the NWT-simulation results. </LI> <LI> The shape-induced and the motion-induced viscous damping were evaluated. </LI> <LI> Comparing to the sharp-corner BBDB, the round-corner BBDB has more wave power. </LI> </UL> </P>