Roll Damping Research Articles

Bilge-Keel Influence on Free Decay of Roll Motion of a Realistic Hull1

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random-Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous-flow solver developed to run on a standard desktop computer. It features a quasi-three-dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier–Stokes solver Free-Surface Random-Vortex Method (FSRVM) can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll-decay motion resulting from simulations and from experimental measurements. Furthermore, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental Digital Particle Image Velocimetry (DPIV) images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll-decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity isosurface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.

A hybrid empirical–analytical model for predicting the roll motion of prismatic planing hulls

During the last 5 years, there have been some major advances in roll motion modeling of planing boats. In this article, previous studies have been reviewed, and an attempt has been made to propose a relatively straightforward method for computing roll motion coefficients and thus predicting the roll response of a planing vessel in calm water on a straight path with constant speed. The proposed method has been developed by modifying, combining, and using previous empirical and analytical methods. Roll restoring moment is determined by implementing an asymmetric parameter in previous relations of lift of planing hulls. Also, an empirical method is proposed for computing righting arm of this moment. Roll damping is computed using previous relations related to damping of lift force and considering the asymmetrical effects. Roll added mass is obtained by utilizing an analytical approach for the roll of a flat plate. However, an effective half-wetted beam computed by asymmetrical effects has been proposed for this purpose. Finally, it has been suggested that current hypothesis be used for time-domain simulation. Validity of the proposed method is evaluated by comparing the obtained righting moment, response amplitude operator, and roll hydrodynamic coefficients against previously published experimental data. This comparison confirms that the proposed method has relatively good accuracy against experimental results. Using the current method, the influence of Froude beam number on roll hydrodynamic coefficients and roll responses has been investigated. It is observed that increase in Froude beam number results in a decrease in roll added mass and an increase in the roll damping. It has also been concluded that for larger load coefficients, the roll hydrodynamic coefficients are larger. Results associated with roll response indicate that an increase in Froude beam number would yield an increase in the amplitude of the response. However, an increase in load coefficient results in a decrease in the response.

The influence of suspension components friction on race car vertical dynamics

ABSTRACTThis work analyses the effect of friction in suspension components on a race car vertical dynamics. It is a matter of fact that race cars aim at maximising their performance, focusing the attention mostly on aerodynamics and suspension tuning: suspension vertical and rolling stiffness and damping are parameters to be taken into account for an optimal setup. Furthermore, friction in suspension components must not be ignored. After a test session carried out with a F4 on a Four Poster rig, friction was detected on the front suspension. The real data gathered allow the validation of an analytical model with friction, confirming that its influence is relevant for low frequency values closed to the car pitch natural frequency. Finally, some setup proposals are presented to describe what should be done on actual race cars in order to correct vehicle behaviour when friction occurs.

Bilge keel–free surface interaction and vortex shedding effect on roll damping

Prediction of the roll damping of a ship with bilge keels using traditional techniques, such as Ikeda’s method, is no longer sufficient when the bilge keels interact with the free surface. The discrepancies occurring between this method and actual roll damping are due to free surface interaction and vortex shedding, which are not considered in this method. Reynolds-averaged Navier–Stokes (RANS) solvers can be used to capture these effects, and the widely used Ikeda’s method can be modified. In this study, two-dimensional (2D) roll damping calculations for a hull section with bilge keels, including the free surface effects, are calculated numerically and experimentally for different draft cases. The normal forces acting on the bilge keels are calculated numerically to show the free surface effect on bilge keel roll damping. The generated vortices and vortex shedding from the bilge keels are analyzed to investigate the effect of the bilge keels–free surface interaction on the roll damping coefficients. The results show that the RANS solver is capable of predicting the roll damping coefficients in good agreement with experimental results and can be further used to modify Ikeda’s method. It is concluded that the bilge keel forces decrease when the bilge keels interact with the free surface, whereas Ikeda’s method gives a constant value for each draft condition. The interaction of the bilge keels and free surface affects the generation of the vorticity and vortex shedding from the bilge keels.

Experimental and numerical study of a containership under parametric rolling conditions in waves

A set of scaled model experiments have been conducted at HSVA towing tank to study the occurrence of parametric rolling on a containership in regular and irregular waves. Herein the performed model tests are described and the experimental results are presented. Forced rolling tests were also performed with the ship model in order to investigate its roll damping characteristics. By means of these tests the roll damping coefficients were determined for different maximum roll amplitudes and advance speeds. The experimental data of this containership is compared with results from a non-linear time domain model, where the prediction of maximum roll amplitude under parametric rolling conditions associated with different viscous roll damping models is thoroughly discussed.

A novel fusion algorithm for estimation of the side-slip angle and the roll angle of a vehicle with optimized key parameters

As is known, estimation of the dynamic states of a vehicle plays an important role in safety control, but it is difficult to obtain the vehicle states accurately without expensive measurement instruments because of the non-linear and huge hysteretic characteristics. Nowadays, many methods have been adopted to solve this problem, the results of which are not ideal because it is assumed that the key parameters are constant, in particular in severe manoeuvres. This paper develops a novel estimation method for the vehicle states using the extended Kalman filter with a fusion algorithm. First, the optimal key parameters (the equivalent roll stiffness and the equivalent roll damping) are identified by genetic algorithms using the data from the relationship between the key parameters and the vehicle real-time states. Then, a novel non-linear observer for the side-slip angle and the roll angle is established on the basis of a four-degree-of-freedom vehicle model by utilizing the identified key parameters and the sensors mounted on normal vehicles. The performance of the observer is investigated using both simulations and real-vehicle experiments. The results demonstrate the reasonable accuracy of the estimation method proposed in this paper.

A study of hybrid prediction method for ship parametric rolling

The parametric rolling (PR) in the head or following waves has been considered as one of the main stability failure modes in the development of the 2nd generation Intact Stability criterion by the International Maritime Organization (IMO). According to previous studies, the estimation methods of the roll damping affect the prediction of the PR significantly, and most of them are based on experiment data or Ikeda's empirical formula. The accuracy of the estimation method for the roll damping could be a key aspect for the validity of its prediction for the full scale ship. In this research, a hybrid prediction method is developed for the numerical prediction of the parametric rolling when experiment data are not available for the roll damping. Comparison study is also carried out between the hybrid method and a nonlinear dynamics method, where the roll damping is estimated by the simplified Ikeda's method and the direct CFD prediction method in a direct non-linear simulation based on the 3-D CFD approach in the model scale. It is shown that the results of the hybrid method are in satisfactory agreements with the model experiment results, and the method can be used for analysis especially at the early design stage where experiment data are often not available.

High-Alpha Prediction of Roll Damping and Magnus Stability Coefficients for Finned Projectiles

The roll damping and Magnus dynamic stability coefficients, as well as total force and moment coefficients, were numerically computed at high angles of attack up to 90 deg for two generic fin-stabilized flight munitions at a supersonic Mach number of 2.49. The aerodynamic coefficients were computed via time-accurate Reynolds-averaged Navier–Stokes numerical methods, and they were compared with archival wind-tunnel data. Fair to excellent comparisons with the experiment were obtained for the coefficients for the full angle-of-attack range, except for the axial force, which was overpredicted at higher angles of attack. Numerical modeling parameter studies were conducted to include dependence on spin rate, grid resolution, temporal resolution, and other numerical convergence parameters.

Melnikov Chaos in a Modified Rayleigh–Duffing Oscillator with ϕ6 Potential

The chaotic behavior of the modified Rayleigh–Duffing oscillator with [Formula: see text] potential and external excitation is investigated both analytically and numerically. The so-called oscillator models, for example, ship rolling motions. The single well and triple well potential cases are considered. Melnikov method is applied and the conditions for the existence of homoclinic and heteroclinic chaos are obtained. The effects of nonlinear damping on roll motion of ships are analyzed in detail. As it is known, nonlinear roll damping is a very important parameter in estimating ship responses. It is noted that the pure and unpure quadratic damping parameters affect the Melnikov criterion in the heteroclinic and homoclinic cases respectively while the pure cubic parameter affects the amplitude in both cases. The predictions have been tested with numerical simulations based on the basin of attraction. It is pointed out that certain quadratic damping effects are contrary to cubic damping effect.

Estimation of ship roll damping—A comparison of the decay and the harmonic excited roll motion technique for a post panamax container ship

The decay motion as well as the harmonic excited roll motion are established techniques to estimate roll damping for ships. This paper compares the advantages and disadvantages of both techniques and focuses on their applicability. Different analysis methods for both techniques to determine the nonlinear roll damping moment are investigated with the aim of developing an accurate estimation approach without additional filtering, curve fitting and offset manipulation of the recorded time series. Damping coefficients of both techniques are compared for available experiments of the benchmarking post panamax container ship model Duisburg Test Case (DTC). Reasons for deviations are investigated, and the importance of an accurate estimation of the current nonlinear hydrostatic roll moment will be shown. A method for the determination of the nonlinear hydrostatic moment during a harmonic excited roll motion test is introduced. Different approximations of roll damping based on series expansion are investigated. Limitations of the widely used linear-quadratic and linear-cubic function approaches are discussed based on the results.

Direct lateral maneuvers in hawkmoths.

ABSTRACTWe used videography to investigate direct lateral maneuvers, i.e. ‘sideslips’, of the hawkmoth Manduca sexta. M. sexta sideslip by rolling their entire body and wings to reorient their net force vector. During sideslip they increase net aerodynamic force by flapping with greater amplitude, (in both wing elevation and sweep), allowing them to continue to support body weight while rolled. To execute the roll maneuver we observed in sideslips, they use an asymmetric wing stroke; increasing the pitch of the roll-contralateral wing pair, while decreasing that of the roll-ipsilateral pair. They also increase the wing sweep amplitude of, and decrease the elevation amplitude of, the contralateral wing pair relative to the ipsilateral pair. The roll maneuver unfolds in a stairstep manner, with orientation changing more during downstroke than upstroke. This is due to smaller upstroke wing pitch angle asymmetries as well as increased upstroke flapping counter-torque from left-right differences in global reference frame wing velocity about the moth's roll axis. Rolls are also opposed by stabilizing aerodynamic moments from lateral motion, such that rightward roll velocity will be opposed by rightward motion. Computational modeling using blade-element approaches confirm the plausibility of a causal linkage between the previously mentioned wing kinematics and roll/sideslip. Model results also predict high degrees of axial and lateral damping. On the time scale of whole and half wing strokes, left-right wing pair asymmetries directly relate to the first, but not second, derivative of roll. Collectively, these results strongly support a roll-based sideslip with a high degree of roll damping in M. sexta.

Nonlinear roll damping of a barge with and without liquid cargo in spherical tanks

Damping plays a significant role on the maximum amplitude of a vessel's roll motion, in particular near the resonant frequency. It is a common practice to predict roll damping using a linear radiation–diffraction code and add that to a linearized viscous damping component, which can be obtained through empirical, semi-empirical equations or free decay tests in calm water. However, it is evident that the viscous roll damping is nonlinear with roll velocity and amplitude. Nonlinear liquid cargo motions inside cargo tanks also contribute to roll damping, which when ignored impedes the accurate prediction of maximum roll motions. In this study, a series of free decay model tests is conducted on a barge-like vessel with two spherical tanks, which allows a better understanding of the nonlinear roll damping components considering the effects of the liquid cargo motion. To examine the effects of the cargo motion on the damping levels, a nonlinear model is adopted to calculate the damping coefficients. The liquid cargo motion is observed to affect both the linear and the quadratic components of the roll damping. The flow memory effect on the roll damping is also studied. The nonlinear damping coefficients of the vessel with liquid cargo motions in spherical tanks are obtained, which are expected to contribute in configurations involving spherical tanks.

Use of Tunnel- and Azimuthing Thrusters for Roll Damping of Ships

Abstract: This paper presents a novel technique of active roll damping in marine vessels by sole use of conventional thrusters. Many marine operations, such as crane operation and helicopter landing, should be carried out in small and steady roll motions. However, active roll damping devices such as fins and rudders lose their efficiency in low-speed conditions. This paper, by use of already installed thrusters, provides a new methodology for roll reduction by adjusting the shaft speed and pitch of the propellers. The numerical simulations, presented in the paper, shows promising results with significant roll damping.

Optimal controllers for rudder roll damping with an autopilot in the loop

Abstract: In this paper we propose a new approach to the design of a rudder roll stabilization system, integrated with a predesigned heading controller (autopilot). Whereas a number of methods for the roll oscillation damping have been proposed in the literature, this problem still remains challenging. In this paper we consider a linear-quadratic optimization problem, where the cost functional penalizes the roll angle, the heading deviation and the control effort (rudder angle). Approximating the wave disturbance by a polyharmonic signal with a known spectrum, we design a controller, which optimizes this quadratic performance index, providing thus a tradeoff between the maintenance of the desired heading, roll damping and the steering system utilization. The practical applicability of our algorithms is illustrated by numerical simulations.

Roll Damping Using Voith Schneider Propeller: a Repetitive Control Approach

Abstract: Voith Schneider propellers are potential propulsion systems for dynamic positioning in offshore applications because of their fast and accurate thrust generation. An overlaid roll damping control by the same propulsion system is possible to reduce the influence of external disturbances. The paper at hand presents simulation results of roll damping control using a repetitive control approach. Repetitive control is a highly effective control technique for the compensation of periodic disturbances, or for exact tracking of periodic reference signals. Simulation results of repetitive control in roll damping will be compared with those from a classical approach. The simulation models as well as the disturbance signals are based on theoretical considerations as well as on verifying experiments in a towing tank. The robustness of the method has been investigated according to deviations from a purely periodic excitation as well as to the influence of propulsion saturation.

감쇠계수 산출을 위한 자유 횡동요 감쇠실험 연구

In general ships and FPSOs, roll damping is very small and consequently roll motion is very large at the roll resonance frequency. Proper evaluation of the roll damping coefficient at the resonance frequency is an important task in the study of roll motion and usually it is done by the analysis of free roll decay tests. The relative decrement method based on energy relation has been used mainly for the evaluation of roll damping coefficient from the roll decay test so far. As another method, the logarithmic decrement method based on equivalent linear decay assumption can be used for the same purpose and it is relatively simple. In this paper, both of the relative decrement method and the logarithmic decrement method are used for the evaluation of roll damping coefficient including quadratic damping from the free roll decay tests, and their results are cross-checked for verifying the obtained damping coefficients. Through applications to a box-type floating body equiped with bilge keels, it is shown that the two methods give almost the same damping coefficients in a practical view point and the cross-check of their results is to be a good tool to prevent a possible error. And also the quantitative effects of the bilge keels on the roll damping of box-type floating body are shown and discussed.

CFD approach to roll damping of ship with bilge keel with experimental validation

The most common method of reducing roll motion of ship-shaped floating systems is the use of bilge keel which act as damping elements. The estimation of the damping introduced by bilge keel is still largely based on empirical methods. The present work adopts the CFD approach to the estimation of roll damping, both without and with bilge keel and validates the results with experiments conducted in a wave flume. Specifically, free oscillation tests are conducted at model scale to obtain roll damping, both by experiments and CFD simulation and reasonably good comparisons are obtained. The experiments also include PIV study of the flow field and attempt has been made to correlate the measured flow field with that obtained by CFD. The CFD methodology has the potential to determine rationally the size and orientation of bilge keels in design with reasonably accurate estimate of the additional roll damping that it provides to ship's roll motion.

Identification of the nonlinear roll damping and restoring moment of a FPSO using Hilbert transform

Roll motion is a major concern of ship and offshore operators because it has a direct impact on the boarding comfort of crews, downtime of the process plant and structural integrity of the hull and appurtenance system. Nevertheless, the nonlinear effects involved in the roll motion of ship and offshore structures are not only incompletely understood, but also inaccurately predicted during the design stage. Therefore, efforts have been made to identify the dynamic characteristics of the roll motion of a FPSO using a novel dynamic system identification scheme based upon the Hilbert transform. First, the system identification scheme using the Hilbert transform was applied to the idealized single DOF nonlinear oscillator problem with quadratic damping and quintic stiffness terms. The solution of the single DOF problem was solved numerically using the 4th order Runge–Kutta method and the target damping and stiffness coefficients were determined by applying the Hilbert transform to the numerically obtained free decay signal. A comparison was also made with traditional logarithmic decrement technique for the damping coefficient. Second, the system identification method was applied to the model test results of a FPSO with a bilge keel, where the nonlinear effect of roll damping and restoring is expected to be dominant. This methodology can be applied to full scale measurement data to achieve a clearer understanding on the nonlinear effect of roll motion.

Computational fluid dynamics uncertainty analysis for simulations of roll motions for a 3D ship

The roll motions are influenced by significant viscous effects such as the flow separation. The 3D simulations of free decay roll motions for the ship model DTMB 5512 are carried out by Reynold averaged Navier-Stokes (RANS) method based on the dynamic mesh technique. A new moving mesh technique is adopted and discussed in details for the present simulations. The purpose of the research is to obtain accurate numerical prediction for roll motions with their respective numerical/modeling errors and uncertainties. Errors and uncertainties are estimated by performing the modern verification and validation (V&V) procedures. Simulation results for the free-floating surface combatant are used to calculate the linear, nonlinear damping coefficients and resonant frequencies including a wide range of forward speed. The present work can provide a useful reference to calculate roll damping by computational fluid dynamics (CFD) method and simulate a general ship motions in waves.

Bilge keel loads and hull pressures created by bilge keels fitted to a rotating cylinder

This paper presents bilge keel loads and hull pressure measurements carried out on a rotating cylinder in a free surface water basin. A flat plate bilge keel and one more complex shaped bilge keel were studied to investigate the geometry effect. The draft of the cylinder was varied to study the effect of the vicinity of the free surface on the bilge keel loads and hull pressures. The rotation axis of the cylinder was fixed to define a pure roll experiment (one degree of freedom).The cylinder was subject to forced oscillations of varying amplitude leading to a KC range of 0.3–16. Using Fourier analysis the first three harmonic coefficients representing the normal bilge keel load were derived. The first harmonic drag and inertia coefficients are in good agreement to existing experimental data obtained for wall bounded flat plates fitted in a U-shaped water tunnel as reported by Sarpkaya and O’Keefe (1996). New insight is gained by the fact that the addition of higher harmonic contributions is essential to capture the time varying bilge keel normal force.The pressure measurements next to the bilge keel are compared to measurements reported by Ikeda et al. (1979). Similar findings are obtained, showing that the pressure on the hull in front of the moving bilge keel is KC independent while the vortex system in the wake of the bilge keel leads to KC dependent hull pressure distributions. The hull pressure jump over the bilge keel correlates well to the force coefficient on the bilge keel. The complex nature of the vortex induced hull pressures is manifested. The empirically derived hull pressure distribution by Ikeda et al. (1979) for the time instant of maximum velocity is shown to correlate reasonably well to the measured data with some conservatism in the absolute value.Although a cylinder is very different from a ship-shaped section, the experiments provide essential insight into the physics associated with roll damping and into the factors that should be included in a roll damping prediction method.