Hydroelastic Response Research Articles

Dynamic response and acoustic characteristics of composite hydrofoil under cavitation-induced vibration

Nonconstant characteristics of cavitating flow can cause adverse effects, including vibration, noise, and cavitation. With the application of composite materials, the vibration and deformation of hydrofoils are becoming increasingly obvious, and the fluid–solid coupling problem is becoming very important. Herein, the cavitating flow field of National Advisory Committee for Aeronautics 0012 hydrofoil composed of bronze alloy and composite material is numerically investigated. The hydro-elastic response of a hydrofoil is obtained via a fluid–structure coupling method under a tight coupling strategy. The Schnerr–Sauer model is used to describe the cavitation process, and the large eddy simulation method is used to solve the turbulence problem. Additionally, the finite element method is used to determine the structural deformation. A comparison of the numerical calculation results of the fluid–solid acoustic coupling under two operating conditions shows that the composite materials can suppress the sheet cavitation attached to the hydrofoil suction surface and the tip leakage vortex (TLV) cavitation and can decrease the flow field pressure pulsation and increase the lift-to-drag ratio of the hydrofoil. Additionally, the composite material significantly improves the wake field turbulence, reducing the turbulence intensity and integration scale, and nearly eliminates the large-scale vortex. Moreover, the composite material changes the vortex structure evolution at the gap flow field, results in smoother TLV development, and enhances the flow field velocity gradient effect. Finally, the monitoring of the flow noise radiation characteristics shows that the composite material effectively reduces the sound pressure level of the self-noise and far-field radiation noise.

Eliminating the irregular frequencies in wave-body interaction: An investigation for accuracy assessment with higher-order elements

The presence and elimination of irregular frequencies in wave-body interaction are reviewed by referring to the fundamental properties of the integral equations and a predictive measure that can be beneficial for wave-induced response assessment is proposed through numerical applications. By adopting higher-order elements and considering the hydroelastic response of the floating body, this study explores three interrelated questions: (i) how widely adopted irregular frequency suppression techniques perform, (ii) how, if possible, can these techniques be effectively applied, and (iii) how can the estimations be accepted as accurate for a given discretization, that is, finding a quantitative measure to relate the employed discretization to the frequency range for which the specified error threshold is not exceeded. The investigation involves three hull forms, a barge model along with the DTMB 5415 and KCS ships, and two techniques that extend the solution domain for suppression, namely the extended boundary integral equation method (EBIEM) and combined boundary integral equation method (CBIEM). The barge model is used for preliminary analyses to identify the existing patterns. DTMB 5415 and KCS allow the transfer of findings to practical applications and assess their validity. Three assessment measures in terms of frequency are employed by considering the hydrodynamic coefficients. In the present setting, it is observed that EBIEM is far more effective, and CBIEM cannot provide adequate suppression. The frequency envelope obtained by taking two quadratic elements per wave length is found to be a proper measure for estimating the frequency range that can be confidently studied.

Hydroelastic response of concrete shells during impact on calm water

Many ocean structures located offshore are supported by large vertical concrete columns. High and steep storm waves – in the process of breaking – may induce large local slamming loads on these columns. The present work is related to the fundamental physics of the local hydroelastic shell response due to slamming.The concrete columns supporting typical offshore structures are large. The size means that full scale tests of a segment of the column is impractical and expensive. Model-scale testing in a wave tank is also challenging. Firstly, the scaling of structural properties need to adhere to the scaling laws of hydrodynamics. Secondly, the manufacturing of realistic Froude scaled elastic shell models is hard since curved shells carries loads by a combination of bending and membrane action. The challenge is to scale both the bending and membrane action properly. One part of this study shows how realistic Froude scaled elastic shells representing concrete shells can be designed.The second part of this study presents results from experimental and numerical analysis of drop tests. Numerical hydroelastic analyses of both the elastic model shells and the real concrete shells are presented. The results show that even large and thick concrete shells experience significant hydroelastic effects during slamming.The hydroelastic response of the concrete shells is dominated by only a few structural eigenmodes. This means that the calculated dynamic amplification factors, DAF, resemble those of one-degree-of-freedom mass–spring systems exposed to loads of finite duration.The structural responses are seen to significantly modify the hydrodynamic loads. This hydrodynamic load modification consists of the well-known added mass term but also a time dependent slam damping term which reduce the structural response when properly accounted for. Both terms are necessary to calculate the concrete shell response accurately.

A Fully Coupled CFD-DMB Approach on the Ship Hydroelasticity of a Containership in Extreme Wave Conditions

In this paper, we present a fully coupled computational fluid dynamic (CFD) and discrete module beam (DMB) method for the numerical prediction of nonlinear hydroelastic responses of a ship advancing in regular and focused wave conditions. A two-way data communication scheme is applied between two solvers, whereby the external fluid pressure exported from the CFD simulation is used to derive the structural responses in the DMB solver, and the structural deformations are fed back into the CFD solver to deform the mesh. We first conduct a series of verification and validation studies by using the present CFD–DMB method to investigate the global ship motion, vertical bending moments (VBMs), and green water phenomenon of the ship in different regular wave conditions. The numerical results agreed favourably with the CFD–FEA model and experimental measurements. Then, the extreme ship motions are studied in focused wave conditions to represent extreme sea conditions that a ship may experience in a real sea state. According to the conclusion drawn from the numerical simulations, it is founded that the focused wave case will lead to the increase of the longitudinal responses of the hull compared to regular wave condition, i.e., the heave, pitch, and total VBMs rise about 25%, 20% and 9%, respectively. In focused wave conditions, intensive ship responses and severe waves cause stronger slamming phenomena. It is found that the instantaneous impact pressure from the focused wave is higher and sharper compared to the regular waves and comes along with the obvious green-water-on-deck phenomena.

Research on Structural Response Characteristics of Trapezoidal Floating Body in Waves

Floating structures plays an important role in extending and developing ocean resources, and their response evaluation is a hot topic of global important research due to the large dimensions. With characteristics including small depth and large horizontal plane, it is easy to induce the hydro-elastic resonant responses due to total stiffness. In this paper, first, the model design is performed to satisfy hydro-elastic similarity. Then, the model test is carried out in a wave tank to measure the structural response of a trapezoidal floating body in a series of waves. Secondly, the 3D hydro-elastic computational platform HOMER is applied to calculate the stress response of a trapezoidal floating body in numerical waves. The model test results and numerical simulation results are analyzed and compared and the conclusions are drawn, which indicate that a numerical method is effective to predict the structural response characteristics of a trapezoidal floating body. Above all, it is found that the significant response of a floating model is generated in some cases. The methods and conclusions of this study are used to provide reference and guidance for structural design of a trapezoidal floating body.

Hydroelastic response and electromagnetic energy harvesting of square oscillators: Effects of free and fixed square wakes

The unbounded linearly increasing responses in galloping instability make sharp-edge oscillators a beneficial solution for bladeless energy harvesting. While the performance of single square oscillators has been the subject of several studies in the past, there is still little knowledge about their hydrokinetic energy performance in tandem configurations. An optimal oscillator should display high performance in a group when all bodies are free to harness energy. In this study, the energy harvesting performance of square oscillators is examined in multiple tandem configurations under free and fixed square wakes. The harmonic oscillator model is coupled with the electromagnetic transducer to evaluate the performance of the system at the optimum coupling coefficient. For a single galloping square, flow interference by a similar upstream square affects the fluid elastic response of the oscillator and degrades its overall energy harvesting performance. The upstream fixed wake is productive in terms of efficiency (123% increase) but counterproductive for the electrical current and electromagnetic power. The free square wake generally diminishes both the power and efficiency. For a double tandem oscillator device, the synergy of tandem configuration at close spacing improves the maximum efficiency of the energy harvester by 97% as compared to two separate isolated oscillators.

Numerical and experimental fluid–structure interaction analysis of a flexible propeller

ABSTRACT The increasing interest in using flexible materials to design marine propellers, considering deformations due to flow loads. A numerical procedure for analysing two-way fluid-structure interactions, based on the commercial STAR-CCM+ multiphysics software, is described and applied to predict the hydroelastic response of the flexible marine propeller P1790 to hydrodynamic forces in open water conditions. The influence of the deformation on the performance of the flexible propeller was analysed by comparison with its rigid counterpart. The procedure has been validated by means of experiments performed in the cavitation tunnel K27 of the Technical University Berlin with both, the flexible and the rigid propeller. The predicted performance coefficients and the axial deformation of the blades agree well with measured values. This suggests the feasibility of using the passive bending and twisting behaviour of a flexible propeller to adapt the pressure distribution on the blade to improve the propeller performance over a range of advance ratios.

A numerical method to compute flexible vertical responses of containerships in regular waves

In this paper, a series of simulations of hydroelastic problems of containerships are conducted by a nonlinear 3D panel method which is based on the generalized Taylor Expansion Boundary Element Method (TEBEM). In the structure part, the Transfer Matrix Method is selected to perform a modal analysis which provides key parameters of hydroelastic calculation. In the hydrodynamic part, the nonlinearities arising from the variations of instantaneous wetted surface area are considered in the solutions of hydrostatic restoring forces as well as 1st order incident wave forces. Meanwhile, an extended Modified Logvinovich Model (MLM) is presented to predict the slamming forces due to the large-amplitude incident waves. The numerical predictions of the hydroelastic responses of a 6750-TEU containership and S175 in regular waves are presented. The effects of wave steepness and sailing speed as well as the variation characteristics of load components against with wave direction are studied. The numerical and experimental results are in acceptable agreement which proves the accuracy of the proposed method.

Second-order hydroelastic analysis of a flexible floating structure under spatially inhomogeneous waves

In this study, a second-order hydroelastic analysis method for moored flexible floating structures under spatially inhomogeneous, long-crested waves is developed, based on the beam-connected-discrete-modules (BCDM) hydroelastic method. The continuous structure is first discretized into rigid modules connected by elastic beams to evaluate the first-order hydroelastic responses. The inhomogeneous wave field is spatially discretized by different isolated homogeneous waves. Based on the first-order motions, the mean drift force on each module is obtained in the frequency-domain. According to the discrete inhomogeneous wave field, the second-order slowly-varying wave drift forces by Newman’s approximation with the drift forces are calculated using the corresponding wave spectra. Finally, the second-order hydroelastic equation in inhomogeneous waves is established in the time-domain using Cummins’ equation. The mean drift force of flexible structures and the nonlinear time histories of motions under homogeneous waves using the proposed method are verified against the results obtained by general commercial software. Then, a limited case study using the proposed method is conducted by a flexible barge and an inhomogeneous wave condition, both of which are exaggerated. The results show that slowly-varying drift forces dominate the horizontal displacements and mooring forces, and the spatial inhomogeneity of waves has a significant effect on the second-order hydroelastic responses, especially in beam seas.

Modeling Nonlinear Hydroelastic Response for the Endwall of the Plane Channel Due to Its Upper-Wall Vibrations

A mathematical model for studying the nonlinear response of the endwall of a narrow channel filled with a viscous fluid to the vibration of the channel’s upper wall was formulated. The channel, formed by two parallel, rigid walls, was investigated. The right end-channel wall was supported by a nonlinear spring. At the end of the left channel, the fluid flowed into a cavity with constant pressure. The upper channel wall oscillated according to a given law. As a result of the interaction between the endwall and the upper wall via a viscous fluid, the forced, nonlinear oscillations of the channel endwall arose. The fluid motion was considered in terms of the hydrodynamic lubrication theory. The endwall was studied as a spring-mass system with a nonlinear cubic restoring force. The coupled hydroelasticity problem was formulated, and it was shown that the problem under consideration was reduced to a single equation in the form of the Duffing equation. The nonlinear hydroelastic response of the end wall was determined by means of the harmonic balance method. The results of numerical experiments on nonlinear hydroelastic response behavior and a comparison with the case when the support spring is linear were presented. The obtained results are of a fundamental nature and can be used in modeling various devices and systems that have narrow channels filled with viscous fluid and are subjected to vibrations on one side of the channel. For example, coolant pipes are subjected to vibrations from the engine. Of particular interest is the application of the presented solution to the mathematical modeling of nano- and micro-spacecraft systems with fluids since the proposed decision allows for the consideration of some boundary effects, which is important for nano- and micro-spacecraft due to their small size.

Bend-twist coupling effects on the cavitation behavior and hydroelastic response of composite hydrofoils

The objective of this work is to investigate the influence of bend-twist coupling effects on the cavitation behavior and hydroelastic response of four hydrofoils with identical geometry: one rigid hydrofoil and three composite hydrofoils of carbon-fiber reinforced plastic (CFRP) with the ply angles of 45°, 0°, and -45°, respectively. A synchronized measurement system consisting of a high-speed camera, a hydrodynamic load cell, and a Laser Doppler Vibrometer is applied to obtain the cavity dynamics and hydroelastic response simultaneously. The theoretical cross-sectional effective bending stiffness, twisting stiffness, and bend-twist coupling stiffness are calculated based on a chordwise-rigid laminated plate model. In addition to the primary cavity shedding frequency and natural frequencies, additional modulated frequencies are excited for all the hydrofoils due to time-varying added mass effects, which broadens the frequency response of the hydrofoil. Re-entrant jet and shockwave mechanisms are identified as instabilities causing periodic cloud cavity shedding. The cloud cavity shedding frequency is found to be independent of cavitation number in certain conditions induced by the shockwave mechanism. Bubbly shock induces a secondary shedding of cloud cavities, causing the reduction of the cavity shedding frequency. The CFRP 45 hydrofoil with a positive bend-twist coupling effect promotes the bending and twisting deformations of the hydrofoil, increasing lift and moment, decreasing the cloud cavity shedding frequency due to the increase in cavity length, and accelerating the cavitation regime transition by reducing cavitation number. And the opposite is true for the CFRP -45 hydrofoil with a negative bend-twist coupling effect.

Complex wave loads and hydroelastic responses of a very large tourism floating platform ‘Ocean Diamond'

Potential fluid In order to meet the needs of tourism development in the South China Sea, this paper designs a heart-shaped very large floating tourism platform, which integrates the comprehensive functions of cruise ship docking and entertainment. The platform has the characteristics of low flexibility, discontinuous multi-free-surface water and complex phenomena such as water resonance. Firstly, the three-dimensional finite element model of the platform is established, and the complex vibration modes in vacuum are analyzed. It is found that the heart-shaped platform has complex vertical and torsional coupled vibration modes. Secondly, the irregular frequency and water resonance of the elastic structure of the platform are studied by using the three-dimensional frequency domain hydroelastic method based on the extended boundary integral method. The irregular frequencies of the floating body are removed through one rigid lid effectively, and the resonant points of internal free water is clearly revealed. Then, the laws of motions, elastic deformation responses and structural sectional loads are analyzed, and the effects of water resonance point and structural elastic resonance point on motions, structural deformation responses and sectional loads are investigated. Finally, the extreme load prediction is carried out, and the dangerous load conditions and dangerous structural areas are identified.

Hydroelastic responses of an elastic cylinder impacting on the free surface by MPS-FEM coupled method

Hydroelastic responses of an elastic cylinder impacting on the free surface by MPS-FEM coupled method

Motion and elastic response of wind-tracing floating offshore wind turbines

A multi-unit floating offshore wind turbine concept, the wind-tracing floating offshore wind turbine, is introduced. In this concept, the floating structure is a triangular platform that hosts three 5 MW wind turbines and is moored to the seabed with a turret-bearing mooring system. This mooring system allows the structure to rotate about the turret such that the total yaw moment by the environmental load on the turret is minimized. In this study, the optimum properties of the mooring lines and the location of the turret are determined. To identify the preferred location of the turret, the responses of the structure to combined co-directional and misaligned wind and wave loads are computed. The motions of the structure are obtained with a frequency-domain numerical model integrated with structural finite-element method for hydroelastic and aeroelastic analyses. The hydrodynamic and aerodynamic loads are obtained by wave diffraction theory and steady blade element momentum method, respectively. Finally, with the optimum configuration of the mooring system, the motion and aero- and hydroelastic responses of the fully flexible wind-tracing floating offshore wind turbines to combined waves and wind loads are determined and discussed.

Hydroelastic Response to Oblique Wave Incidence on a Floating Plate with a Submerged Perforated Base

A hydroelastic model is developed of a floating flexible structure in the presence of a submerged perforated base connected with mooring lines under oblique wave action. Using the velocity decomposition method, the analytical solution of the referred model is obtained in finite water depth. The convergence of the analytical solution for different oblique wave incidences is examined, and the present results of deflection amplitude are compared with experimental datasets and the numerical results available in the literature. The effects of oblique wave incidence, along with various design parameters on the reflection, transmission, and dissipation coefficients, as well as structural displacements, are analysed through hydroelastic analysis. Further, the effect of oblique incidence angle on the free oscillation hydroelastic waves in two wave modes is investigated by deriving the free motion velocity potential in a wave basin.

WRF-CFD/CSD analytical method of hydroelastic responses of ultra-large floating body on maritime airport under typhoon-wave-current coupling effect

Accurate prediction of dynamic responses of the ultra-large floating body on maritime airportto extreme marine environments like typhoons is the key premise to guaranteeing its safety performances. In this study, a WRF-CFD/CSD analytical method of hydroelastic responses of the ultra-large floating body on Maritime airport based on mesoscale and small-scale nesting is proposed. Firstly, WRF-SWAN-FVCOM coupling simulation is realized based on the secondary development of the MCT platform. The typhoon-wave-current spatial-temporal evolution laws when the “Meranti” went through the ultra-large floating body on maritime airport are analyzed. Hydrodynamic and aerodynamic forces are calculated by the medium/small scaled flow field nesting technology, which is brought into the structural dynamic equation for two-way iterative decoupling solving. Finally, hydroelastic responses of the floating bodies under typhoon-wave-current coupling effect and influencing mechanism are analyzed by case study. Research conclusions demonstrate that the proposed method can predict hydroelastic responses of the maritime airport under the action of typhoons, waves, and currents effectively. The typhoon field will significantly increase the vertical displacement plus deformation of the structure and the extreme negative pressure of the superstructure, which can provide a reference for the design and construction of maritime airports under typhoon conditions.

Frequency-domain approach of hydroelastic response for offshore bottom-mounted slender structure

ABSTRACT Bottom-mounted slender structures are often flexible enough to induce hydroelastic effect which could change their dynamic properties and affects the resultant response. This study presents a frequency-domain approach to characterise the hydroelastic response as well as wet dynamic characteristics for offshore bottom-mounted structure with arbitrary geometry. The boundary element method associated with Euler-Bernoulli theory is integrated and coupled. The approaches for wet fundamental period, wet damping ratio and hydroelastic response are validated against a benchmark experiment. Results show that surrounding water has obvious influence on the structural period and damping ratio, particularly on damping ratio. The distribution of added mass grows quickly with depth to about z/d = 0.2 and then gradually decreases whereas added damping mainly concentrates near free surface and drops rapidly with depth. If hydroelastic effect is not considered, it could cause considerable frequency-to-frequency errors from −26% to 52% in responses.

Frequency-domain approach of aero-hydro-elastic response for offshore bottom-mounted slender structures under wind and wave

Bottom-mounted slender structures in offshore regions, e.g., high-rising bridge pylon and monopile foundation are often flexible enough to induce considerable aero-hydro-elastic effect which could change their dynamic properties and obviously affects the resultant response against wind and free surface waves. This study presents an efficient frequency-domain approach to characterize the aero-hydro-elastic response as well as the dynamic properties for offshore bottom-mounted structure. A specific attention is drawn to the hydrodynamic mass and damping modeling. The boundary element method associated with Euler-Bernoulli theory is coupled in frequency domain to bridge the gap between the deformable boundary condition on discretized panels and the elastic structure. The quasi-stationary approach considering instantaneous structural motion is employed to calculate the aero-elastic effect. The proposed approach is well validated against a benchmark experiment. Results of a prototype structure subject to wind and wave emphasize the significant influence of aero- and hydro-elastic effect on the structural responses as well as dynamic properties, particularly on damping ratio. If the aero-hydro-elastic effect is not properly considered, it could obviously overestimate the structural responses by 6.8%–32.6%. It is necessary to investigate the aero- and hydro-elastic response in a coupled scheme, otherwise it may induce errors up to 8.5% in structural responses.

Effect of Considering Wave Angles on the Motion Response of Oversized Floating Bodies in Offshore Airports under Irregular Wind and Wave Loads

Most existing studies are based on the hydroelastic response of oversized floating bodies under regular waves, ignoring the effect of wave conditions and incident wave angle on the vibration response of oversized floating bodies in real sea conditions. In this paper, the stability performance of a single module of a mega-floating body at an offshore airport was investigated by using STAR-CCM + numerical simulations based on a specific model, with the introduction of parameters under extreme random wind and wave combined sea conditions. By comparing and analyzing the distribution characteristics of the single module floating body under the action of regular waves and irregular waves, wind, and wave load, and by considering the wave angle along the flow direction, as well as the spreading direction and vertical displacement value under the action of mooring, the overall displacement amplitude of rigid displacement, elastic deformation displacement and considering the influence of both, and the distribution law of motion response along the length of the floating body, we summarize the influence of the wave angle on the dynamic response of an oversized floating body of an offshore airport. The results show that the maximum value of the hydro-elastic response tends to appear at the head and tail of the floating body, the rigid body vertical displacement dominates the role, the amplitude of all displacements of the floating body under the action of the cross wave is larger, and the stress area along the floating body is larger when the wave angle is between 15–30°. The floating body stress value is smaller with angles of 30–65°, and the ability to bear the load is stronger. The hydroelastic response under irregular wave conditions is more sensitive to the wave direction angle, and the elastic deformation has less influence.

Hydroelastic responses of a Truss Pontoon Mobile Offshore Base – An experimental investigation

The present study focuses on the details of 1:150 scaled model tests conducted on a Semisubmersible type Very Large Floating Structures (VLFS), namely Truss Pontoon Mobile Offshore Base (TP-MOB) under regular waves. The laboratory tests were performed in a deep water wave basin at the Indian Institute of Technology Madras (IIT Madras). The model, governed by Froude and Cauchy scaling laws, was constructed of thermoplastic polyurethane (TPU) sheet with internal steel stiffener plates. The top deck was constructed using thicker TPU to bring in the elasticity effect. The responses of the model were measured for regular waves with different wave frequencies at a constant wave height of 0.04 m and various heading angles. Three different accelerometers recorded the responses at three different locations for different heading angles. From the recorded time series Response Amplitude Operators (RAOs) of vertical displacements, roll, and pitch are obtained. The characteristics frequency response curve obtained from the experiments are consistent with that reported in the literature by Suzuki et al. (1996). The results indicate the presence of resonant elastic modes in the wave excitation ranges. The uncertainty analysis carried out indicated that the uncertainty in the frequency responses is generally small for kd values greater than 0.2.