Rolling Amplitude Research Articles

Experimental and theoretical study on single-phase natural circulation flow and heat transfer under rolling motion condition

Experimental and theoretical studies of single-phase natural circulation flow and heat transfer under a rolling motion condition are performed. Experiments with and without rolling motions are conducted so the effects of rolling motion on natural circulation flow and heat transfer are obtained. The experimental results show the additional inertia caused by rolling motion easily causes the natural circulation flow to fluctuate. The average mass flow rate of natural circulation decreases with increases in rolling amplitude and frequency. Rolling motion enhances the heat transfer, and the heat transfer coefficient of natural circulation flow increases with the rolling amplitude and frequency. An empirical equation for the heat transfer coefficient under rolling motion is achieved, and a mathematical model is also developed to calculate the natural circulation flow under a rolling motion condition. The calculated results agree well with experimental data. Effects of the rolling motion on natural circulation flow are analyzed using the model. The increase in the flow resistance coefficient is the main reason why the natural circulation capacity decreases under a rolling motion condition.

Theoretical research for natural circulation operational characteristic of ship nuclear machinery under ocean conditions

Based on the two-phase drift flux model and the multi-pressure nodes matrix solving method, natural circulation thermal hydraulic analysis models for the Nuclear Machinery (NM) under ocean conditions are developed. The neutron physical activities and the responses of the reactivity control systems are described by the two-group, 3-dimensional space and time dependent neutron kinetics model. Reactivity feedback is calculated by coupling the neutron physics and thermal hydraulic codes, and is tested by comparison with experiments. Using the models developed, the natural circulation operating characteristics of NM in rolling and pitching motions and the transitions between forced circulation (FC) to natural circulation (NC) are analyzed. The results show that the influence of the rolling motion increases as the rolling amplitude is increased, and as the rolling period becomes shorter. The results also show that for this NM, with the same rolling period and rolling angle, the influence of pitching motion on natural circulation is greater than that of rolling motion. Furthermore, the oscillation period for pitching motion is the same as the pitching period, while the oscillation period for rolling is one half of the rolling period. In the ocean environment, excessive flow oscillation of the natural circulation may cause the control rods to respond so frequently that the NM would not be able to realize the transition from the FC to NC steadily. However, the influence of ocean environment on the transition from NC to FC is limited.

Experimental study on two-phase flow instability of natural circulation under rolling motion condition

Two-phase flow instability of natural circulation under a rolling motion condition is experimentally studied. The experimental results show the rolling motion induces a fluid flow fluctuation. At the trough point of the flow fluctuation, rolling motion can cause the early occurrence of natural circulation two-phase flow instability, and this case is defined as trough-type flow oscillation. The system stability decreases with increasing rolling amplitude and effect of rolling frequency is nonlinear. The complex overlap effect of trough-type flow oscillation and density wave oscillation can enhance the system coolant fluctuation; this case is defined as complex flow oscillation. Complex flow oscillation may be divided into two types: regular and irregular complex flow oscillations. Irregular complex flow oscillation is a transition type from trough-type flow oscillation to regular complex flow oscillation. Under the same thermal hydraulic conditions, the marginal stability boundary (MSB) of regular complex flow oscillation is similar to that of density wave oscillation without rolling motion, and the influences of rolling parameters on the MSB are slight.

On the parametric rolling of ships using a numerical simulation method

This paper has shown a numerical motion simulation method which can be employed to study on parametric rolling of ships in a seaway. The method takes account of the main nonlinear terms in the rolling equation which stabilize parametric rolling, including the nonlinear shape of the righting arm curve, nonlinear damping and cross coupling among all 6 degrees of freedom. For the heave, pitch, sway and yaw motions, the method uses response amplitude operators determined by means of the strip method, whereas the roll and surge motions of the ship are simulated, using nonlinear motion equations coupled with the other 4 degrees of freedom. For computing righting arms in seaways, Grim's effective wave concept is used. Using these transfer functions of effective wave together with the heave and pitch transfer functions, the mean ship immersion, its trim and the effective regular wave height are computed for every time step during the simulation. The righting arm is interpolated from tables, computed before starting the simulation, depending on these three quantities and the heel angle. The nonlinear damping moment and the effect of bilge keels are also taken into account. The numerical simulation tool has shown to be able to model the basic mechanism of parametric rolling motions. Some main characteristics of parametric rolling of ships in a seaway can be good reproduced by means of the method. Comprehensive parametric analyses on parametric rolling amplitude in regular waves have been carried out, with that the complicated parametric rolling phenomena can be understood better.

An analytical and experimental study on dynamic stability of a vessel

Many researchers have studied the roll stability of ships since it is the most critical case of capsizing for conventional vessels. In this study external exerted forces from waves or winds on a vessel are modeled as a step exciting moment. Calculation of dynamic angle of stability Φ dy , which is the first overshoot angle of vessel response to exciting moment, is the main objective of this paper. For this purpose two methods are used. The first one, so-called traditional method, determines Φ dy by equalizing the external heeling moment energy and righting moment energy; while the other solves the rolling equation assuming step exciting moment and linear damping. Additionally, a model experiment is carried out with a step exciting moment then model rolling responses are registered. The results of both theoretical methods are compared with experimental results. The comparison shows that for large rolling amplitude such as 35 degrees, inclusion of linear damping does not have any considerable effect dynamic angle of stability. It means that the traditional method is reliable.

Model Experiment on the Mechanism of Capsizing of a Small Ship in Beam Seas

In recent research works on capsizing in beam sea conditions it is reported by some authors that the model ships were capsized by breaking waves which gave them a large overturning moment due to the direct attack of high speed wave fronts. But the model shapes were different from the ones of small ships those are mainly reported in the record of maritime accidents to have capsized.This paper deals with the capsizing of a small fishing boat and shows the different patterns of capsizing phenomena from other papers. Many capsizing could be seen in concentric transient waves with two successive crests, which excited a large rolling amplitude beyond the stability vanishing angle of the vessel. The model ship was of about 1/6 scale of an actual fishing boat and set free-floating abeam in the model basin. The GM values were varied in a wide range so that critical GM values could be determined. The effect of the initial heeling angle and the initial position of the model was also investigated. The maximum wave height was about the same order as the model breadth.As for the mechanism of capsizing it is concluded that the model ship was capsized not by the impulsive moment of breaking waves directly but the loss of stability due to a large rolling motion developed by the transient large waves. The breakers played a significant roll to change the conditions of encountering the next critical waves.

The radial damping of agricultural tractor tyres

The damping coefficients of rolling agricultural tractor tyres in the radial (or vertical) direction have been measured. Six different tyres ranging in age from 1 to 16 years have been measured. Factors which most affect the apparent radial damping of the tyre are the inflation pressure, the tyre age, and the surface over which it rolls. The effects of rolling speed, load, torque, amplitude, frequency, ply rating and lug length are also discussed.

Study on motion suppression system for marine structures

In recent years, ocean exploration and development activities have moved into deeper and more exposed seas, with progress of offshore technology. In these offshore construction works, it is very important to minimize the downtime of work vessels which have special functions, such as crane, pipe laying, diving support, etc.. Therefore, the reduction of motions of such vessels and floating structures has much significance on progress of offshore projects. In order to reduce the motions of floating structures, the effectiveness of new motion suppression system is dealt with based on modified Bell's type tanks which are composed of air chamber, water columns opened to sea and air duct between tanks.In this paper, as fundamental study, hydrodynamic forces acting on rectangular barge and its water columns in tanks are calculated by two-dimensional singular distribution method. Then, after modelization of air flow in chamber and duct, motion equations are derived. Results of calculation are compared with the experimental data. The results of motion calculation agree well with experimental ones.The results of this study are following.(1) The main factor of effects of this motion suppression system is shift of resonant period of structure to longer due to reduced GM.(2) The magnitude of rolling amplitude of structure with motion suppression system at its resonant period becomes abt.1/5 compared with amplitude without suppression system.(3) By handling of valve in air duct, very high performance of motion characteristic of structure on overall range of frequency is obtained.

A stochastic model for the statistics of rolling motion and rolling speed due to wind gusts of a modulated amplitude

This paper gives a stochastic model for the statistics of the non-stationary response process representing rolling amplitude and, as a related result, the statistics of the response derivative process representing roll speed for a ship in a regular b

Bilge Keels with Discontinuous End Plate

Bilge keels are effective devices to reduce rolling of ships. Normally, they can reduce rolling amplitude by about 20 to 50%. The initial cost of the bilge keel is minor, and the construction is simple. Many theoretical as well as experimental results regarding configuration, performance, and construction methods of bilge keels have been published. This Note is intended to present some experimental results of flanged bilge keels with and without slots in the end plate. It was found that flanged bilge keels without slots in the end plate yield less reduction of roll than those which are unflanged. However, the performance of flanged bilge keels can be improved by cutting slots in the end plate.

浅い水域での横搖れについて(第1報) : 非連成横搖れ

When a large tanker is approaching her berth in open road under seas and swell, the ship's under keel clearance is decreased by rolling and some troubles on ship handling are introduced. Therefore, it is necessary to estimate the rolling amplitude exactly for keeping good under keel clearance in restricted water. This paper relates to model experimental investigation about the shallow water effect on ship's rolling in the range of under 10 degrees of amplitude in beam seas. Model tests were carried out for 2 models of a tanker and a cargo ship, and the following results are obtained. 1) the increasing of virtual mass moment of inertia for rolling is remarkable in the depth of water under twice of ship's draft, 2) on the case of ship stationary in beam seas, the rolling amplitude calculated by uncoupled equation of motion is in good agreement with the model test results except in cases of nearly synchronous motion.

On the Characteristics of Unsymmetrical Rolling of Ships (No. 1)

Experimental results on unsymmetrical rolling of two ship models, a fishing boat and a cargo ship, are presented.The model was supported freely, except yawing, in an experimental tank under the action of beam seas, beam wind or both.The heel angle, the amplitude of rolling and the velocity of drifting were obtained.In the fisher-boat experiment, observation of unsymmetrical rolling in regular waves due to shipping water was the main subject, while, in the cargo ship experiment, the wave characteristics were kept constant and the wind velocity was changed from zero to 8 metres per second so as to obtain the informations on the combined action of wind and waves.Unsymmetrical rolling due to shipping water with weatherside heel took place under wave action alone when freeboard was too small or/and effective area of freeing port was insufficient.It is noticeable that the principle of superposition seems to fail sometimes as to motions of ships under combined action of wind and waves. In other words, these experiments revealed that the rolling amplitude varied with increasing wind speed, the heel angle due to the same wind velocity increased more or less under the additive action of waves and the velocity of drifting was independent of the existence of waves except in case of very low wind speed.

Regularity of Rolling Velocity and Acceleration

Randomness of ship's oscillations among waves is almost as the same as that of wave heights. Precisely speaking, even in one component of oscillation, namely rolling, randomnesses of amplitude, velocity, and acceleration are a little different to each other. This is because of response operator, as higher time differentials exaggerate high frequency part of wave spectrum, so that regularity increases from amplitude towards to velocity, acceleration, …If we prefer the irregularity of amplitude n=2, which means Rayleigh's distribution, for velocity becomes n=3, and for accelerantion n=4 for moderately rough sea surface. As waves grow up fully from seas to swells, these regularities gradually increase.The index number “n” is almost constant for every magnitude of maximum of oscillation, but if There are other strong resonances and there are several maxima in frequency distribution, we must consider regularity for these each component separately.The occurrence of “n=2” is to he expected most frequently in the case of pitching and rolling amplitude.

Effect of Magnitude of Ships' Natural Period of Rolling on the Statistically Composed Amplitude' on Rough Sea

Rolling amplitude of a ship decreases as the natural period increase. This fact is true not only on smooth swells, but also on confused seas.If we suppose that there are two ships simultaneously whose each period is T1 and T2, then the ratio of expected maximum value of rolling amplitude is approximatelyθ1/θ2=1/(T1/T2)2so that if T1<T2, θ1 is considerablly larger than θ2.The author examined these results on the long records of weather ships at X-ray (39 N, 153 E) and Tango (29 N, 135 E), whose types are different and the period is about 7.60 sec. and 12.32 sec respectively.For a ship whose period is long, rolling amplitudes are comparatively stationally and regular as the ship tends to prefer and select swells, but on the contrary to this, for a ship which has short period resonance waves are usally confused seas, so that statistical amplitudes are not narrow banded but widely spread and these distributions are the same as that of Drs. Longuet-Higgins.