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When a very large floating structure (VLFS) is subjected to successive regular waves, it may sometimes suffer from “beating vibration”. This phenomenon occurs when the wave period approaches the natural periods of the structure. The beating action occurs with the period far longer than the incident wave period and sometimes has a conspicuous ups and downs. The vibration amplitude due to regular wave will be alternatively increased and decreased as the synergistic effect caused by the beating. The mechanism of the beating vibration has been investigated through FEM simulation based on the time-domain analysis then the example cases of beating phenomena are demonstrated focusing on the VLFS which was preliminarily designed as a supply base for a deep ocean. It has turned out that the fundamental characteristics of the beating action are mainly contributed by the magnitude of period gap between the incident wave and the natural period of the structure, and also by the magnitude of the damping force. [ABSTRACT FROM AUTHOR]

An investigation into the dynamic behavior of the piston pump is an important method to reduce its vibration and noise. In this paper, an approach to predict the dynamics of a seawater hydraulic piston pump (SWHPP) from output-only transmissibility measurements is introduced. In order to eliminate the harmonic interference generated by spindle rotation, the excitation in this method is the pressure which caused by the SWHPP operating at a constant speed and under different loading conditions. The first four modes of the SWHPP were estimated utilizing the method proposed in this paper, and the influence of the pressure on the dynamics of the SWHPP structure was further studied. The experimental results indicated that peak occurred around the frequency of 20 Hz, and the reason is that the estimated second-order natural frequency (19.8 Hz) is near to the frequency of 20 Hz which is the excitation frequency of the swash plate and the flat valve of the SWHPP. In addition, this method can identify modal parameters of the piston pump structure in the working state, so it can effectively predict the dynamic behaviors of the piston pump. [ABSTRACT FROM AUTHOR]

The response of thin, elastic filament attached to a circular cylinder is numerically investigated under uniform flow at different wall proximities, G D (0.5, 0.6, 0.8, 1, 1.5, 2 and 2.5). The Reynolds number based on the diameter of the cylinder is 200. Numerical simulations were conducted for a range of reduced velocities U r (3,4,5 and 6), where U r  =  U ∞ f n D which appropriately captures the synchronization range of vortex-induced vibration(VIV) of the structure. U r is varied by changing the density of the material. The force coefficients, the frequency of shedding/flapping are observed to be the function of G D and U r . The positive vortices diffuses faster as G D decreases. The skewness in the amplitude attained by the flapping filament on either side of the cylinder causes unsteady shedding of vortices. Filament flaps in frequency that is closer to its natural frequency at higher G D and at lower G D , the frequency of flaps for all U r is similar. For lower gap ratios, 0.5 and 0.6, the modulations in flapping amplitude is observed. The modulation is predominant as the U r increases. During the growth and damping phase of the amplitudes in each modulation cycle, the vortex shedding is observed to be oblique towards the wall. The filament flaps in single mode shape. The frequency of the flapping depends on the U r and G D . The frequency of the flapping is in resonance with shedding frequency. [ABSTRACT FROM AUTHOR]

Wavy cylinders have been proven effective in controlling the flow and reducing the fluid induced forces. However, the hydro-elastic behavior of a flexibly mounted wavy cylinder remains unclear. The current paper endeavours to present a systematic study of the flow around a spring mounted wavy cylinder mainly at a moderate Reynolds number of 5000, the results of which are compared with a normal cylinder with the same fluid and structural attributions. It is discovered that the wavy cylinder, although almost eliminates the Kármán vortices in the fixed configuration, shows only limited efficacy on the mitigation of the flow induced vibration in the case of zero structural damping, and the typical initial-upper-lower type response curve is manifested. The hydrodynamic forces of the wavy cylinder also magnify significantly in the flexibly mounted cases during synchronization. Moreover, the phase lag between the lift coefficient and the displacement displays a clear change from 0° at the initial branch to 180° at the lower branch. The association of the 2 S and 2 P vortex shedding modes to the different branches is also similar to that of the normal cylinder, in which the 2 S mode corresponds to the initial branch and the 2 P to the lower branch. In general, despite the absence of the primary shedding frequency in the fixed configuration, the flexibly mounted wavy cylinder exhibits many features that is also found in the normal cylinder. This implies that the vortex induced vibration may not be initiated by the Kármán vortex shedding, and thus may defy the conventional view on the mechanism of the vortex induced vibrations. Additional simulations are performed with non-zero structural damping. It is disclosed that with sufficiently high structural damping, the vibration of the wavy cylinder could be reduced more efficiently than the normal cylinder. [ABSTRACT FROM AUTHOR]

Abstracts Underwater surface of man-made structures is quickly covered by unwanted aquatic organisms. They change the flow regime around the structure. Despite the important contributions of previous studies on the vortex induced vibration (VIV) of bluff bodies, the flow induced vibration of non-stationary cylinders covered by marine fouling appears not to have received due attentions in the literature. The current paper reports on an attempt for better understanding the marine fouling effects on the VIV of circular cylinders. A structured review of the literature related to the fluid structure interactions in marine fouled cylinders is first presented. Results of an experimental study on the VIV of low mass-damping circular cylinders with artificial marine fouling are then reported. The fouling was simulated by uniformly distributed pyramids on the surface of the test cylinders. The Reynolds number ranged from 3.5 × 10 3 to 3.5 × 10 4 . The experimental results showed that, the peak VIV amplitude, the synchronisation range, the lift force coefficient, the mean drag coefficient and the RMS of the fluctuating drag coefficient were decreased by the fouling. Flow visualisation showed that the separation was delayed and the vortices size and recirculation length became shorter in the fouled cylinder. In general, it appeared that the fouling was acting, to some degrees, as a VIV suppression tool. [ABSTRACT FROM AUTHOR]

This study investigates a method for suppressing the vortex-induced vibration (VIV) of a riser pipe using a mathematical model of beam vibration and numerical computation. The proposed method uses damping devices that partially cover the riser pipe such that the integral measuring the overlap of the drag distribution with the eigenfunction squared is maximized. By considering the isolines of reduced velocity and vertical profiles of magnitude factor of lift force, we specified a mode that would grow most prominently if no damping device is used. We regard this mode as the one that ought to be intensively damped, thereby configuring an added damping distribution function according to the approximate maximization of the overlap integral. We applied this method to a riser pipe in linearly sheared flows and found that the appropriate specification of the damped mode can suppress the VIV to the same degree as the entire coverage, although this effect varies with the strength of the flow velocity shear. It follows that the proposed method is useful for suppressing the VIV with a small number of damping devices attached. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) of a flexible cylinder in oscillatory flow was experimentally investigated in an ocean basin. An intermittent VIV was confirmed to have occurred during the tests. The fatigue damage caused by VIV was calculated based on rainflow counting and a standard S–N curve. There are 3 main observations for fatigue damage from VIV in oscillatory flow: 1) the damage varied significantly with the KC number, which is a unique feature for VIV in oscillatory flow. 2) Fatigue damage at small KC number cases was found to be larger compared to those at large KC numbers owing to the fact that number of vortex shedding cycles per half of the motion cycle is low, and damping within half of the motion cycle will hence become low. The fact that vortices from the previous cycle still are active during the next may also contribute to the large response at small KC numbers. 3) ‘Amplitude modulation’ and ‘mode transition’, two specific features for VIV in oscillatory flow, were found to have a strong influence on fatigue. Fatigue damage has also been calculated by an empirical VIV prediction model assuming that all cases have steady flow at an equivalent velocity. Finally, a simplified method for calculating fatigue damage from VIV in oscillatory flow based on steady flow conditions is proposed. A modification factor diagram is presented, but the scope of the present study is too limited to provide a good basis for a general model for this factor. A general model for how to apply results from constant current analysis to predict fatigue in oscillatory flow will therefore need further research. [ABSTRACT FROM AUTHOR]

Abstract: This paper concerns the response and efficiency of a two-body wave power conversion device. The physical problem of the device is simplified as a forced vibration system with viscous damping in two degrees of freedom. Based on the linear wave theory, wave excitation forces, added masses and damping are derived by use of an eigenfunction expansion matching method. The expressions of the response and efficiency are deduced from the motion equations of the device, and the optimal principle is presented as well. Numerical results indicate that damping optimal curve has a wave peak that is independent of the spring, and the value of the peak and the corresponding frequency are only related to the calculation conditions. Optimal curve presents two peaks when the spring exists, and the corresponding frequency width decreases with the increase of elastic coefficient. The damping is relative small at low frequency peak, while the response is relative high. The damping is relative high at high frequency optimization peak, while the response is relative small and almost smaller than the amplitude of the incident wave. With the decrease of the external damping coefficient, the relative motion amplitude and the efficiency increase, while the width of crest decreases. [Copyright &y& Elsevier]

Experimental results are presented in the paper of two elastically supported rigid circular cylinders subjected to steady flows in a flume. The two cylinders were initially placed at various tandem and staggered positions with one in the wake of the other when subject to the steady flows. The in-line centre-to-centre distance varied from 2 to 5 diameters whilst the cross-flow distance from 0 to 2 diameters. The nominal Reynolds numbers were in the sub-critical regime and ranged from 1.12 × 104 to 5.52 × 104, and the nominal reduced velocities from 1.78 to 8.77. The damping ratio of the test set-up is low at 0.003 which gives a combined mass-damping parameter of 0.0046. Both the cylinders were free to respond in both the in-line and the cross-flow directions. The cylinder motion was measured simultaneously with the hydrodynamic loading in the two directions. It was found that the motion trajectories of the downstream cylinder show qualitative difference depending upon whether it is in tandem with the upstream cylinder or in the wake with a transverse offset. The VIV response of the downstream cylinder is dependent upon the reduced velocity of the upstream cylinder and its own reduced velocity based upon the actual mean wake velocity. The drag amplification of the downstream cylinder in the wake appears to be fundamentally different from that of a single VIV cylinder in isolation. Furthermore, unlike the two fixed cylinders in cross flow, the downstream cylinder undergoing VIV no longer experiences a marked non-zero mean lift. The upstream cylinder is largely unaffected by the downstream cylinder when the initial spacing is greater than 3 diameters. On the other hand, the motion response of and the fluid loading on the downstream cylinder are strongly influenced by the upstream cylinder in the spacing range tested. [ABSTRACT FROM AUTHOR]

Seeking to expand the range of operation and durability of wood articles, in many cases they must be impregnated with relevant materials. The process consumes a lot of time. Different technologies are applied for modification of depth and speed of impregnation. A method of vibrational wood impregnation is presented in this paper. It was determined that the transverse vibrations of scantlings in water are analogous to those in air, only the frequencies are different by 1.5 times. This study reveals that the amplitudes of vibrations in water and in air are close to each other, while the coefficient of damping in water is larger by 50 %. It is demonstrated that in many cases the scantlings undergoing resonant vibrations in the same time absorb up to 30 % more water than in the case of non-vibrant scantlings. The results of these investigations may be applied in wood impregnation industry. [ABSTRACT FROM AUTHOR]

The potential of chaotifying vibration isolation systems to reduce line spectra of underwater vehicle and improve its capability of concealment have recently reported. Notice that for an isolation system, the amplitude of chaotifying vibration is, in general, larger than that of its specified periodic motion subject to the inherent characteristic of chaotic behavior. Therefore, there exists a conflict between the line spectra reduction based on chaotifying vibration responses and the capability of vibration isolation. In this paper, a method based on the modified projective synchronization (MPS) is developed to improve vibration isolation while the chaotifying vibration signals with broad band frequency are utilized to achieve line spectra reduction. The key to this solution is the scaling factors of MPS which enable us to proportionally diminish the vibration amplitudes of the isolated equipment. The feasibility of the methodology in practical engineering is illustrated with the application to acoustic stealth of submarines. [Copyright &y& Elsevier]