Piston Mode Research Articles

Coupled vessel and moonpool responses in regular and irregular waves

Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experiments are carried out in the Ocean Basin at Sintef Ocean. Three different moonpool sizes are studied, where the moonpool length is 1/20, 1/10 and 1/2 of the ship length. The two former are square moonpools, while the latter is a rectangular moonpool with a width 1/2 of the ship’s beam. Numerical simulations are carried out using WAMIT. The coupling between the ship and moonpool is seen to be highly dependent on the volume of the moonpool relative to the submerged ship volume. WAMIT greatly over-predicts the moonpool responses in the proximity of resonance, which indicates that wave radiation damping is small, and that damping due to flow separation at sharp edges at the moonpool entrance is dominant. Two main nonlinear effects are observed; Swirling-type sloshing and secondary resonance. These nonlinear effects are excited in a range of wave periods where it is expected that a realistic sea environment will contain significant wave energy. This is most prominent in the largest moonpool. The piston mode shape in freely floating conditions is signficantly different relative to the one in forced heave.

Experimental Research on Water Column Oscillations in Moonpool of a Drillship in Downtime Period

A drillship is a kind of merchant vessel with a self-propulsion unit and drilling equipment used for oil exploration. The major difference with the merchant vessel is the moonpool. A moonpool is a vertical opening from the continuous deck to the keel plate of the vessel for drilling operations and other applications like the launching of measuring instruments. This moonpool opening allowing the entry of water into the vessel. The water motion within the moonpool is mostly related to the encountering wave frequency, the geometry of the moonpool and the draft condition of the vessel. The major amplitude of the water particle motion within the moonpool, either may be in the sloshing mode or in piston mode. This water motion leads to the entry of green water on the deck during the rough weather condition. This is known as the downtime period of a drillship, during this time the operation of the drillship is in off-mode. This paper presents the study about the downtime period of drillship experimentally with rectangular moonpool

On natural frequencies of three-dimensional moonpool of vessels in the fixed and free-floating conditions

The hydrodynamic interactions between the vessel motion and resonant wave response inside the three-dimensional moonpool are studied in the present paper. In particular, the shift of natural frequencies of a three-dimensional moonpool in the fixed and free-floating conditions is computed and examined systematically. Barges with different moonpool dimensions are analyzed to examine how the moonpool configurations affect the shift of resonant frequencies. Detailed analyses are performed for cases with and without recess. Numerical computations are performed for regular waves with wave headings θ=0∘, 45∘ and 90∘. Free-surface elevations inside the moonpool with respect to the incident wave frequencies are computed for both fixed and free-floating conditions. It is observed that natural frequencies of the moonpool in the free-floating condition are higher than those in the fixed condition. For cases without recess, the shift of resonant frequency is more prominent in piston mode resonance, in particular for shallow or long moonpools. In contrast, for cases with a recess, the shift of resonant frequency in the first sloshing mode is relatively larger than that in piston mode. In addition, the effects of motion RAOs on the free-surface elevation inside the moonpool in the free-floating condition are also examined. It is found that the moonpool resonance can induce a local hump in heave and pitch responses. Moreover, through deriving a modified frozen mode approximation model, we reveal that, comparing to the fixed condition, the vessel motion in the free-floating condition yields a reduction in the added mass due to the fluid underneath the moonpool so that the piston mode resonant frequency increases.

Investigation of the dynamics of expansion of the near-surface light erosion plasma formed during the evaporation of a material by broadband high-brightness radiation

The dynamics of the vapor-plasma flows formed at impact of erosion type magnetoplasma compressor (MPC) discharge radiation on Ti, Al and PTFE targets with use of methods of a holographic interferometry and schlieren photos (Toepler’s scheme in the light field mode) is investigated. In the schlieren and interferogram photos, zones characteristic of the studied process of radiation interaction with matter are well visible: the gas-dynamic evaporation mode is realized (plasma piston mode), there is a shock wave in the gas, the contact boundary between the shock-compressed gas and the vapor plasma. The interferograms analysis indicates that the regime of developed evaporation is preceded by a regime of diffusion evaporation. On the interferograms above the targets at different distances from the radiation source (from the MPC), we observe 3 types of gas-dynamic perturbations: an acoustic wave, a simple wave (Riemann wave), and a shock wave. Qualitative and quantitative description of processes in these zones is discussed in the report. The effect of the radiation spectral composition (we use the method of gas spectral selection), the targets materials, and the degree of surface treatment on the evaporation is shown experimentally.

A dynamic solution for predicting resonant frequency of piston mode fluid oscillation in moonpools/narrow gaps

The objective of this study is to develop a simple tool for predicting the resonant frequency of the piston mode wave oscillation in a moonpool or a narrow gap between twin floating barges. Based on the dynamic analysis and the domain-decomposition, a linear harmonic oscillator model for the fluid oscillation in a confined space is formulated, which immediately leads to an explicit dynamic solution for the resonant frequency. In the model, it is assumed that the water depth is equal to or greater than a critical value dependent on the ratio of the draft to the breadth of the floating body. The predicted resonant frequencies are compared with the numerical results of the linear potential model and the viscous fluid model, the semi-analytical potential solutions and the experimental measurements. Good agreements are observed for a wide range of geometric parameters of the moonpool and the narrow gap of either sharp or round edge shapes. The present dynamic method is also applied to predict the resonant frequency of the piston mode oscillation in the gap between a floating body and a vertical wall with a reasonable success.

Experimental studies of a damaged ship section in forced heave motion

This work documents a detailed series of experiments performed in a wave flume on a thin walled prismatic hull form. The model consists of a rectangular opening located on the side. The length of the model is slightly smaller than the flume breadth to achieve two-dimensional (2D) behavior in the experiments. Forced oscillatory heave tests in calm water have been carried out by varying the model-motion parameters and examining both intact and damaged conditions. Video recordings, measurements of the wave elevation inside the damaged compartment and of the force on the model were performed in all the experiments. The effect of damage opening in the model on hydrodynamic loads is examined by comparing with an intact section. A theoretical analysis is used to explain the behavior of added mass and damping coefficients in heave for a 2D damaged section. The presented results demonstrate occurrence of sloshing and piston mode resonances in the tests and their influence on the hydrodynamics loads of a damaged ship. Detailed physical investigations are presented at these resonance frequencies for the damaged section. Effect of filling level in the damage compartment, damage-opening length and air compressibility in the airtight compartment is examined. Nonlinear effects are documented and appear dominant, especially, for lowest filling level where we have shallow-water depth conditions in the damaged compartment. Resonance phenomena that can lead to significant local loads are identified for the shallow water condition. Air compressibility in the airtight compartment and floodwater act as a coupled system and influence inflow/outflow of floodwater in the compartment. It has a significant effect on local floodwater behavior in the damaged compartment.

Holographic Interferograms of a Vibrating Thin Piezoceramic Disk Resonator under Asymmetric Point-Mass Loading

Deformations of the base of a free piezoceramic resonator that represents a thin disk are visualized using holographic interferometry under high-intensity electric excitation at antiresonance frequencies of radial modes. Motion of a loading point mass along the radial coordinate is used to specify identification of partial modes and the total number and energy interaction of such modes. Four topologically compatible modes form a single mode on which the pattern of Lamb modes is superimposed under certain experimental conditions, and an implicit piston mode is obtained.

Autonomous piston engine guided by the transduction of radiation pressure inoptomechanical Brownian motors

We consider the autonomous piston engine in a coupled cavity scheme in an optomechanical setup. Treating the radiation field as working fluid, we investigate the transduction of virtual photons due to ground state excitations. Rabi-like energy splittings and Casimir effect are shown by avoided crossing in the eigenfunction spectrum. Second order coherence functions are employed to analyze the bunching of the cavity and mechanical mode in optomechanical implementation of Brownian motors. The imbalance in occupation numbers of the working modes is investigated for the power for work extraction. We explore the guidance of the radiation field in work extraction. We study the synchronicity between virtual photon conversion and power in the mechanical mode. Generation and amplification of virtual photon transduction are modulated by mutual interactions between the qubit, piston, and mechanical mode in the absence of external driving fields.

On wave diffraction of two-dimensional moonpools in a two-layer fluid with finite depth

This paper studies the wave diffraction problem of two-dimensional moonpools in a two-layer fluid by using domain decomposition scheme and the method of eigenfunction expansion. Wave exciting forces, free surface and internal wave elevations are computed and analyzed for both surface wave and internal wave modes. The present model is validated by comparing a limiting case with a single-layer fluid case. Both piston mode and sloshing mode resonances have been identified and analyzed. It is observed that, compared with the solutions in surface wave mode, the wave exciting forces in internal wave mode are much smaller, and show more peaks and valleys in low-frequency region. As the wave frequency increases, the bandwidth of sloshing mode resonances decreases. Extensive parametric studies have been performed to examine the effects of moonpool geometry and density stratification on the resonant wave motion and exciting forces. It is found that, for twin bodies with deep draft in surface wave mode, the decreasing density ratio has little effects on the sloshing mode resonance frequencies but can somehow suppress the horizontal wave exciting forces and surface wave elevations around piston mode resonance region. In addition, the presence of lower-layer fluid can lead to the reduction of piston mode resonance frequency.

Influence of moonpool on the total resistance of a drillship by the effect of water motions inside the moonpool

Moonpools are openings right through the hull from continuous deck to bottom of the ship, allowing equipment or mini-submarines to be put into the water at a location on the vessel with minimum ship motion. Open moonpools in a drillship are causing additional resistance when the ship is in forward speed. It was shown that the water inside the moonpool started to oscillate at forward speed. The water mass in the moonpool is subjected to sloshing and piston modes. The vertical motion is piston mode and the longitudinal one is called as sloshing mode. This water particle motion inside the moonpool is mainly depended on the geometry, moonpool depth, and encountered wave frequency. Out of this, moonpool geometry is one of the key factors for the performance of the moonpool. The varying cross-section geometry is one of the practically possible and economically feasible solutions to reduce the oscillation to a considerable level is attempted in this paper. Also the resistance caused by the moonpool and the free surface generated around the hull is investigated with the use of computer simulation.

Transverse Spurious Mode Compensation for AlN Lamb Wave Resonators

Lamb wave modes with type I dispersion characteristics exhibit strong affinity toward multi transverse modes behavior above resonance frequency (f r ) in the AlN Lamb wave resonators (LWRs), especially the high-transduction-efficiency modes: S 0 and S 1 mode. For conventional interdigital transducer (IDT) design, the IDT aperture and IDT gap are the two main factors impacting the transverse mode placements and strengths, according to the wave vector analysis and finite element method (FEM) simulation. Moreover, the convex slowness curve of the Lamb wave modes propagating in AlN platelets allows the waveguiding and weak lateral leakage into busbars by the high-velocity IDT gap region. Apodization, the standard technique to suppress the transverse modes for IDT-excited resonators, suffers from drawbacks, such as additional loss and reduction of the effective coupling coefficient (k eff 2 ). Type I Lamb wave modes in AlN show positive slope in the dispersion branch so that a border region of lower Eigen-resonance frequency is required to form piston mode structure for transverse spurious mode suppression and lateral leakage reduction. Based on dispersion calculations and 2.5D FEM simulations, we demonstrate that by designing the low-velocity border region, such as simply changing the IDT layout, the guiding can be improved and a piston mode can be obtained for the type I Lamb wave modes.

Голографические интерферограммы вибрирующего тонкого дискового резонатора из пьезокерамики при асимметричном нагружении точечной массой

AbstractDeformations of the base of a free piezoceramic resonator that represents a thin disk are visualized using holographic interferometry under high-intensity electric excitation at antiresonance frequencies of radial modes. Motion of a loading point mass along the radial coordinate is used to specify identification of partial modes and the total number and energy interaction of such modes. Four topologically compatible modes form a single mode on which the pattern of Lamb modes is superimposed under certain experimental conditions, and an implicit piston mode is obtained.

Impact of Coupling Between Multiple SAW Modes on Piston Mode Operation of SAW Resonators.

This paper investigates piston mode operation (PMO) when two kinds of surface acoustic waves (SAWs) exist and are coupled to each other. The extended thin plate model is used for the analysis. First, a PMO condition is derived in a closed form. It is verified that PMO is possible without adding phase shifters by the use of two SAW coupling. It is also shown that this operation offers suppression of the transverse modes for a wider frequency range than the conventional PMO. Then, parameters defined in the model are extracted for various SiO2-overlay/Al-grating/128° YX-LiNbO3 substrate structures, and the structure satisfying the PMO condition is designed. Finally, the structure is also analyzed using the 3-D finite element method. The result agrees well with that calculated by the extended thin plate model.

Use of double-raised-border structure for quality factor enhancement of type II piston mode FBAR

This paper investigates applicability of double raised borders to the piston mode film bulk acoustic resonator (FBAR). It is shown that it offers significant improvement of the quality factor at the anti-resonance without sacrificing the spurious level when the structure is designed appropriately. This enhancement is attributed to the reduction of the mode-conversion and the increase of the reflection coefficient at the border region.

Model parameter extraction of lateral propagating surface acoustic waves with coupling on SiO2/grating/LiNbO3 structure

This paper investigates how lateral propagation of Rayleigh and shear horizontal (SH) surface acoustic waves (SAWs) changes with rotation angle θ and SiO2 and electrode thicknesses, hSiO2 and hCu, respectively. The extended thin plate model is used for purpose. First, the extraction method is presented for determining parameters appearing in the extended thin plate model. Then, the model parameters are expressed in polynomials in terms of hSiO2, hCu, and θ. Finally, a piston mode structure without phase shifters is designed using the extracted parameters. The possible piston mode structures can be searched automatically by use of the polynomial expression. The resonance characteristics are analyzed by both the extended thin plate model and three-dimensional (3D) finite element method (FEM). Agreement between the results of both methods confirms validity and effectiveness of the parameter extraction process and the design technique.

Hydrodynamic Study on Water Column Oscillation of Varying Cross-Sectional Moonpool and Its Effect on Resistance of a Drill Ship

A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes results in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin's (2001, “On the Piston and Sloshing Modes in Moonpools,” J. Fluid Mech., 430, pp. 27–50.) theoretical and Fukuda's (1977, “Behavior of Water in Vertical Well With Bottom Opening of Ship and Its Effects on Ship-Motion,” J. Soc. Nav. Archit. Jpn., 1977(141), pp. 107–122.) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross section. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and nonzero Froude number especially in the presence of waves.

Catalysis of heat-to-work conversion in quantum machines

We propose a hitherto-unexplored concept in quantum thermodynamics: catalysis of heat-to-work conversion by quantum nonlinear pumping of the piston mode which extracts work from the machine. This concept is analogous to chemical reaction catalysis: Small energy investment by the catalyst (pump) may yield a large increase in heat-to-work conversion. Since it is powered by thermal baths, the catalyzed machine adheres to the Carnot bound, but may strongly enhance its efficiency and power compared with its noncatalyzed counterparts. This enhancement stems from the increased ability of the squeezed piston to store work. Remarkably, the fraction of piston energy that is convertible into work may then approach unity. The present machine and its counterparts powered by squeezed baths share a common feature: Neither is a genuine heat engine. However, a squeezed pump that catalyzes heat-to-work conversion by small investment of work is much more advantageous than a squeezed bath that simply transduces part of the work invested in its squeezing into work performed by the machine.

Amplitude Induced Nonlinearity in Piston Mode Resonant Flow: A Fully Viscous Numerical Analysis

Near field flow characteristics around catamarans close to resonant conditions involve violent viscous flow such as energetic vortex shedding and steep wave making. This paper presents a systematic and comprehensive numerical investigation of these phenomena at various oscillating frequencies and separation distances of twin sections. The numerical model is based on the solution of Navier–Stokes equations assuming laminar-flow conditions with a volume of fluid (VOF) approach which has proven to be particularly effective in predicting strongly nonlinear radiated waves which directly affect the magnitude of the hydrodynamic forces around resonant frequencies. Considered nonlinear effects include wave breaking, vortex shedding and wave-body wave-wave interactions. The method is first validated using available experiments on twin circular sections: the agreement in a very wide frequency range is improved over traditional linear potential flow based solutions. Particular attention is given to the prediction of added mass and damping coefficients at resonant conditions where linear potential flow methods fail, if empirical viscous corrections are not included. The results of the systematic investigation show for the first time how the so-called piston-mode motion characteristics are nonlinearly dependent on the gap width and motion amplitude. At low oscillation amplitudes, flow velocity reduces and so does the energy lost for viscous effects. On the other hand for higher oscillation amplitude, the internal free surface breaks dissipating energy hence reducing the piston mode amplitude. These effects cannot be numerically demonstrated without a computational technique able to capture free surface nonlinearity and viscous effects.

Theoretical and numerical investigations of wave resonance between two floating bodies in close proximity

A simple theoretical dynamic model with a linearized damping coefficient is proposed for the gap resonance problem, as often referred to as the piston mode wave motion in a narrow gap formed by floating bodies. The relationship among the resonant response amplitude and frequency, the reflection and transmission coefficients, the gap width, and the damping coefficient is obtained. A quantitative link between the damping coefficient of the theoretical dynamic model (ɛ) and that devised for the modified potential flow model (up) is established, namely, up = 3πɛω (where ωn is the natural frequency). This link clarifies the physical meaning of the damping term introduced into the modified potential flow model. A new explicit approach to determine the damping coefficient for the modified potential model is proposed, without resorting to numerically tuning the damping coefficient by trial and error tests. The effects of the body breadth ratio on the characteristics of the gap resonance are numerically investigated by using both the modified potential flow model and the viscous RNG turbulent model. It is found that the body breadth ratio has a significant nonlinear influence on the resonant wave amplitude and the resonant frequency. With the modified potential flow model with the explicit damping coefficient, reasonable predictions are made in good agreement with the numerical solutions of the viscous fluid model.

On natural modes in moonpools with recesses

The theoretical model of Molin [6] is extended to the case of rectangular moonpools with one or two recesses, as can be found in some drillships. Obtained natural frequencies and modal shapes of the piston and first sloshing modes are compared with experimental results available in literature, with good agreement. An approximation easy to implement is proposed for the natural frequency of the piston mode. Further illustrative results are presented when some geometrical parameters of the moonpool are being varied.