Roll Damping Coefficient Research Articles

Experimental and numerical investigation on the influence of bilge keel shape on roll damping

Excessive roll amplitudes due to roll motion are undesirable in marine ships. Consequently, it is imperative to conduct a detailed analysis of roll motion and the associated roll damping characteristics. This study experimentally and numerically investigates the roll damping characteristics of bilge keels with various geometric shapes on a ship model under different roll amplitudes. By comparing the non-dimensional roll damping coefficients obtained from experiments and numerical analyses, it is observed that bilge keels with geometries differing from the conventional plate shape exhibit distinct roll damping coefficients. Specifically, bilge keels with sharper tip ends demonstrate higher roll damping coefficients. Based on these findings, it is recommended that the corners and tip end of bilge keels be sharpened to enhance the roll damping coefficient.

Numerical Study on the Effect of Weight Variables on the Roll Damping Coefficients for a Fishing Vessel

Abstract The roll damping coefficient is essential when considering the viscous effect in the potential-based hydrodynamic analysis of fishing vessels; it is an important factor in the roll motion response. The present study performs free roll decay simulations, altering weight variables using Computational Fluid Dynamics (CFD) to investigate the correlation between the roll damping coefficient and the weight variation of a fishing vessel. The time series of roll amplitude and roll damping coefficient are compared, for varying vertical and longitudinal centres of gravity and radii of gyration in roll motion. As the vertical centre of gravity increases, both the roll decay period and the roll damping coefficient also increase. The roll decay period tends to increase with the increase in the radius of gyration during roll motion, while the roll damping coefficient exhibits a decrease. A longitudinal centre of gravity has a limited effect on free roll decay characteristics. The roll damping coefficients between the maximum and minimum combinations of weight variables show significant differences. The findings of the present study could enhance the understanding of the safety of fishing vessels based on their loading conditions. Consequently, future research could further improve the results obtained in the present study by considering various hull shapes and speeds.

Eksperimentalno istraživanje o učinku položaja ljuljne kobilice na desantnim pomoćnim brodovima

The issue of energy efficiency in using fossil fuels and reducing the effects of greenhouse gas emissions is an urgent problem. So fuel-saving measures, including in the marine transportation sector, are needed, even if only by a small percentage. Landing Craft Utility (LCU) is a type of sea transport for defense matters that requires the addition of a rolling motion stabilizer for the safety and comfort of the ship, cargo, and passengers. The use of roll-damping devices can affect the increase in the resistance value of the ship and cause an increase in fuel consumption. The bilge keel is a roll-damping device that is suitable for LCU vessels. The experimental study roll decay tests and resistance tests were carried out to determine the effect of the bilge keel placement on roll damping and additional ship resistance. The flow around the vessel’s surface was observed with a paint smear test to determine the placement position. The bilge keel installed in the transitional position between the bottom and side hull has the highest total roll damping coefficient, up to 28.57%, compared to the bare hull condition. However, this placement has increased the average resistance up to 8.84%. Alternately, placement close to the draft line has a fairly good roll-damping coefficient of up to 21.27%, increasing the resistance to only 3.66%.

무인반잠수정의 진자식 횡동요 저감 장치 설계 및 감쇠계수 기반 검증

무인반잠수정의 진자식 횡동요 저감 장치 설계 및 감쇠계수 기반 검증

Experimental and numerical estimation on roll damping and pressure on a 2-D rectangular structure in free roll decay test

Experimental and numerical studies were performed to estimate roll damping and hydrodynamic pressure on a 2-D rectangular structure. Free roll decay test was simulated by experiment in 2-D wave tank with Particle Image Velocimetry (PIV) measurement, and by numerical simulations using CFD based on an unsteady incompressible Reynolds-Averaged Navier-Stokes (RANS) approach. The dynamic characteristics of the roll motion and the roll damping coefficients from the experiment and CFD were analyzed and compared. Velocity and vorticity fields obtained from the PIV measurement were examined to understand the fluid fields in the vicinity of the structure and compared with them from CFD to validate the numerical methods applied in the study. The PIV-based pressure estimation method was applied to predict pressures on the structure, and results were compared with them from CFD to assess the feasibility of the PIV-based estimation approach. The overall agreement between experimental and CFD results was good for roll damping coefficient, velocity and vorticity fields, and pressure considering the complexity of fluid physics in the roll motion. It was found that the PIV-based pressure estimation method would be alternative technique to estimate the pressure and loads on the structure.

DETERMINATION OF LINEAR AND NONLINEAR ROLL DAMPING COEFFICIENTS OF A SHIP SECTION USING CFD

The most prevalently used method to obtain the nonlinear roll damping coefficient is the free roll decay test. However, this method can only be conducted at the resonance frequency and thus cannot consider the effect of the frequency. This is a certain limitation as the resonance frequency can be changed at any time by the ship’s loading conditions. Therefore, it is worth investigating the frequency dependency of the nonlinear roll damping coefficients. In this study, a numerical method was proposed to derive the linear and nonlinear roll damping coefficients of ships at different frequencies. Fully nonlinear CFD simulations of forced harmonic roll motion were conducted and the roll damping coefficients were calculated. Then, the damping coefficients were decomposed into the linear and nonlinear components using the linear regression analysis. The linear roll damping coefficients were compared with potential coefficients and showed a good agreement, while the nonlinear roll damping coefficients were compared with the coefficients calculated using a semi-empirical method. The nonlinear roll damping coefficients calculated from the proposed method showed a strong frequency dependency. Finally, possible rationales for the frequency dependence of the nonlinear roll damping coefficient were investigated.

Ship Roll Damping Coefficient Prediction Using CFD

For a ship undergoing roll motion at a resonance frequency, the roll restoring moment and virtual moment of inertia counteract each other and only the damping moment resists the roll exciting moments. Hence, accurate computation of the roll damping moment improves the roll motion prediction. There are several methods to calculate the damping moment; however, most of them fail to consider the viscous effects, such as the potential theory-based method. The experimental measurements take into account the viscous effect; however, they are expensive. The Computational Fluid Dynamics (CFD) method is an alternative to accurately consider the viscous effect. In this study, a CFD approach based on harmonic exciting roll motion technique is adopted to compute the roll motion characteristics and damping coefficients at different conditions. The impact of appendages, Froude number, and degrees of freedom on roll motion characteristics and damping coefficients are investigated for the model scale and full scale to study the scale effects. The results at the model scale are compared against experimental measurements and good correlation was found. 1. Introduction Controlling the roll motion of a ship in a seaway is essential for the safety and habitability. The roll motion of a ship is nonlinear, and the nonlinearity increases at the resonance as the ship experiences larger roll motion. Accurately computing the roll damping at the resonance frequency is important because the moment of inertia and restoring moment counteract each other and the damping moment opposes the roll motion. The total roll damping consists of friction damping, body eddy shedding damping, surface wave damping, lift force damping, and appendages damping. The wave and lift force damping can be calculated by potential flow theory based methods, whereas others are influenced by viscosity. In general, most roll damping calculation methods are based on empirical models, and Ikeda's method (Ikeda et al. 1978) is most common one. Although this method works quite well for conventional ships, the predicted results are sometimes conservative or underestimated for unconventional ships (Gu et al. 2013). Large roll damping is strongly nonlinear and is influenced by fluid viscosity and flow characteristics such as flow separation and vortex shedding. Empirical or semi-empirical methods cannot account the flow characteristics of such a complex flow. Roll damping calculation for conventional ships by Ikeda's simplified method can fit experimental data quite well at small roll angles. However, when the roll angle is large, it is out of the acceptable range of Ikeda's method, and the accuracy of the computed damping coefficient is low (Kawahara et al. 2012).

The effects of roll motion of the floating platform on hydrodynamics performance of horizontal-axis tidal current turbine

Under the condition of actual sea state, hydrodynamic characteristics of floating horizontal-axis turbine are related to wave characteristics and floating carrier motion responses. We recently published the hydrodynamic performance and axial damping coefficient of the horizontal-axis tidal current turbine influenced by surge motion in constant inflow using CFD simulation (Zhang et al. Renew Energy 74:796–802, 2015). Encouraged by this result, this paper uses sliding mesh to analyze the hydrodynamic characteristics in uniform stream when the turbine is forced to roll and studies influences of different roll frequency, roll amplitude, and tip speed ratio on turbine’s performance. Roll-damping coefficient and added mass coefficient can be derived by torque almanacs curve of rolling turbine by the least square method. Results show that the turbine axial load, roll moment, and energy utilization ratio will fluctuate in roll motion; the more roll frequency and roll amplitude, and the more load and moment wave amplitude of momentary value. The crest value occurs in the balance position of rolling, while the amplitude of oscillation depends on the angular speed of rolling and rotating speed of the turbine. The frequency and amplitude of the roll have little impact on damping coefficient, but rotational speed of the turbine has positive impact on this coefficient. Results of this study can provide data to study motion response of floating carrier for floating tidal current turbine system and check the structural design and control of the electric output.

URANS prediction of roll damping for a ship hull section at shallow draft

It is analytically difficult to calculate roll damping of ships due to the effects of viscosity. Therefore, computational fluid dynamics (CFD) has become a powerful tool in predicting roll damping recently. The unsteady flow around a forced rolling hull section with bilge keels can be calculated using a commercial URANS code which includes the viscous effects. In this study, two-dimensional (2D) roll damping calculations for a S60 midsection with bilge keels including free surface effects are performed for shallow draft case. The first objective of the study is to show whether the URANS code can be used to predict roll damping coefficient correctly. The second one is to show why Ikeda’s estimation method is insufficient at shallow draft case. Sinusoidal forced roll motion calculation method of roll damping moment with the help of a sliding interface and a fixed roll axis is successfully applied to predict roll damping coefficient. The calculations are carried out for different roll motion periods and amplitudes to validate the accuracy of the URANS code for different cases. Numerical results are compared with experiments, which were carried out at the towing tank facility of Osaka Prefecture University (OPU), and Ikeda’s estimation method. The results show that the URANS code is capable of predicting roll damping coefficients in a good agreement with experimental results and can be used further to develop a better model for prediction of roll damping.

Measurement of Hydrodynamic Coefficients on a Planing Hull Using Forced Roll Oscillations

For planing hulls, dynamic lift reduces the submergence of the hull, allowing small motions to result in large changes in hydrodynamic forces and moments. This article explores various modeling assumptions associated with transverse plane dynamics in roll and the hydrodynamic forces in the planing regime. The significance of nonlinearities in the roll hydrodynamic forces for planing hulls was investigated with regard to roll amplitude, model speed, model displacement, and roll oscillation frequency. A wooden 20° deadrise prismatic planing hull was tested at steady roll angles and in dynamic roll at various roll amplitudes and frequencies. Two displacements, three model speeds, three roll amplitudes, and four roll oscillation frequencies were tested. The measured wetted lengths, roll-restoring coefficient, roll-added inertia coefficient, and roll-damping coefficient are presented. The roll-restoring moment is found to be nonlinear with roll amplitude, the added inertia coefficient shows some dependence on model speed, and the roll-damping coefficient shows some amplitude dependence.

On the Roll Damping Coefficient obtained from the Irregular and Large Amplitude Forced Rolling Experiments

The authors developed a compact new apparatus for making the irregular and large amplitude forced roll, and after the experiments, compared the obtained equivalent linearlized roll damping coefficient with that from regular forced roll experiment made by another investigator.From this result, it is said that the equivalent linearlized damping, similar to that from regular or transient forced roll experiment, is also obtained from large amplitude irregular forced roll.