VIV Response and Fatigue Damage of Flexible Cylinders With Time-Varying Axial Tension

2018 ◽  
Author(s):  
Yuan Yuchao ◽  
Xue Hongxiang ◽  
Tang Wenyong ◽  
Liu Jun
Keyword(s):  
Fatigue Damage ◽  
Structural Response ◽  
Design Stage ◽  
Small Scale ◽  
Time Varying ◽  
Flexible Cylinder ◽  
Time Step ◽  
Response Characteristics ◽  
Axial Tension ◽  
Tension Time

The time-varying effect of axial tension has recently attracted increasing focus when investigating vortex-induced vibration (VIV) for flexible cylinders. This paper applies an alternative time domain force–decomposition model to predict VIV response, in which the structural stiffness will be updated at each time step to take the tension variation into account. Firstly, the adopted numerical model is compared against the latest published experimental results of a small-scale cylinder with constant and time-varying tensions. Then, extensive cases of a long flexible cylinder are designed to investigate the tension time-varying effect on structural response and fatigue damage respectively. Several new response characteristics different from the constant tension case are analyzed from the VIV mechanism level. Fatigue analysis also reveals the influence laws of the amplitude and frequency of varying tension. Mathieu-type resonance between VIV and time-varying tension excitation is captured, under which structural response as well as fatigue damage will enlarge significantly. Some conclusions drawn by this research can provide reference at the engineering design stage of marine slender structures.

Multiloop state-dependent nonlinear time-varying sliding mode control of unmanned small-scale helicopter

2019 ◽  
Vol 234 (3) ◽  
pp. 585-606 ◽  
Author(s):  
Sinan Ozcan ◽  
Metin U Salamci ◽  
Volkan Nalbantoglu
Keyword(s):  
Sliding Mode Control ◽  
Sliding Mode ◽  
Operating Conditions ◽  
Small Scale ◽  
Time Varying ◽  
Parameter Uncertainties ◽  
Time Step ◽  
Mode Control ◽  
State Dependent ◽  
Improved Performance

Time delays, parameter uncertainties, and disturbances are the fundamental problems that hinder the stability and reduce dramatically the tracking performance of dynamical systems. In this paper, a new state-dependent nonlinear time-varying sliding mode control autopilot structure is proposed to cope with these dynamical and environmental complexities for an unmanned helicopter. The presented technique is based on freezing the nonlinear system equations on each time step and designing a controller using the frozen system model at this time step. The proposed method offers an improved performance in the presence of major disturbances and parameter uncertainties by adapting itself to possible dynamical varieties without a need of trimming the system on different operating conditions. Unlike the existing linear cascade autopilot structure, this study also proposes a nonlinear cascade state-dependent coefficient helicopter autopilot structure consisting of four separate nonlinear sub-systems. The proposed method is tested through the real time and PC-based simulations. To show the performance of the proposed robust method, it is also bench-marked against a linear sliding control control in PC-based simulations.

A Time Domain Prediction Method for Vortex-Induced Vibrations of a Flexible Pipe With Time-Varying Tension

2018 ◽  
Author(s):  
Mengmeng Zhang ◽  
Shixiao Fu ◽  
Haojie Ren ◽  
Runpei Li ◽  
Leijian Song
Keyword(s):  
Time Domain ◽  
Method Comparison ◽  
Prediction Method ◽  
Hydrodynamic Forces ◽  
Experimental Results ◽  
Force Model ◽  
Time Varying ◽  
Flexible Pipe ◽  
Time Step ◽  
Axial Tension

Prediction of vortex induced vibration (VIV) for a long-flexible pipe has always been an important concern for the design of risers. Currently, VIV prediction methods are mainly based on the linear beam theory, where the axial tension is treated as time-independent, and the couples between VIV and axial tension are totally ignored. However, experimental results have illustrated strong couples between the axial tension and VIV [1–2]. The purpose of this paper is to develop a time domain VIV prediction model. This model consists of pipe’s structural non-linearity, couplings between axial force, cross-flow/in-line (CF/IL) VIV responses, and the hydrodynamic forces. The hydrodynamic forces are further divided into vortex-induced force in CF and IL directions, and drag force in IL direction. The former one is determined via empirical force model based on forced oscillation test of rigid cylinders. The IL drag coefficients model considering the effects of VIV developed by Song [3] is adopted. VIV responses under these hydrodynamic forces at each time step are solved by Newton-Raphson method. Comparison between present method and the experimental results under uniform flows and shear flows are conducted, which verified the feasibility and reliability of the proposed method. In addition, by comparing the results under constant tension and time-varying tension, it is proved that the time-varying tension has a significant effect on VIV responses, especially under the case of high flow velocity and high vibration mode.

The effect of time-varying axial tension on VIV suppression for a flexible cylinder attached with helical strakes

2021 ◽  
Vol 241 ◽  
pp. 109981
Author(s):  
Yexuan Ma ◽  
Wanhai Xu ◽  
Huanan Ai ◽  
Yingying Wang ◽  
Kun Jia
Keyword(s):  
Time Varying ◽  
Flexible Cylinder ◽  
Axial Tension ◽  
Helical Strakes ◽  
Viv Suppression

Time Domain Structural Fatigue Analysis of Floating Offshore Platforms: A Response Based Technique

2020 ◽  
Author(s):  
Johyun Kyoung ◽  
Sagar Samaria ◽  
Jang Whan Kim
Keyword(s):  
Spectral Method ◽  
Fatigue Damage ◽  
Time Domain ◽  
Structural Response ◽  
Fatigue Analysis ◽  
Offshore Platform ◽  
Offshore Platforms ◽  
Time Step ◽  
Structural Fatigue ◽  
Non Linear

Abstract This paper presents a response-based, time-domain structural fatigue analysis of a floating offshore platform. The conventional technique for structural fatigue assessments of offshore platforms uses a linear, frequency-domain analysis based on the spectral method. Although this conventional method is computationally efficient, there is a room for improving accuracy and reducing uncertainties because it cannot accurately address non-linear loadings on the offshore platform. Such non-linear loads arise from the wave, wind, and current as well as from the riser and mooring systems; these non-linearities necessitate large factors of safety that lead to conservative design and frequent inspection. As an extension of previous work (Kyoung et al.[12]), this study presents the development of a time-domain, structural fatigue analysis that explicitly addresses non-linear loading on the platform. The external load time-histories are directly mapped onto the structure at every time interval to create a stress-based response with the varying environment. In each time step, the load mapping accurately captures the phase relationship between the external loading and hull inertial response. Therefore, present method reduces uncertainties in the fatigue damage computation and overcomes the assumptions of spectral method. Present load component-based approach is applied onto a finite element structural model, which provides unit structural response at locations of interest. Time history of structural response is obtained by synthesizing the obtained unit stress-based structural response with environmental loading and platform motion response. Fatigue damage can be computed from the obtained time series of structural response using rain-flow counting. As an application, a conventional semisubmersible platform is used to evaluate structural fatigue damage for a given wave scatter diagram. A comparison between results from this response-based time-domain approach and the conventional spectral method is presented.

Structural Response Analysis of Tension Leg Platforms

1984 ◽  
Vol 106 (1) ◽  
pp. 10-17 ◽  
Author(s):  
K. Yoshida ◽  
M. Ozaki ◽  
N. Oka
Keyword(s):  
Equations Of Motion ◽  
Structural Response ◽  
Structural Member ◽  
Small Scale ◽  
Response Analysis ◽  
Test Results ◽  
Analysis Method ◽  
Regular Waves ◽  
Response Characteristics ◽  
Structural Responses

A linear response analysis method of the tension leg platform (TLP) subjected to regular waves is proposed. In this analysis method, flexibility of the superstructure can be taken into account in the equations of motion; response motions, tension variations of tendons and structural member forces are solved simultaneously. The applicability of this method is confirmed by comparison with the test results on two kinds of small-scale TLP models. The structural responses obtained from these calculations and their effects on tension variation of tendons are studied. Finally, several kinds of structural response characteristics are conclusively discussed.

scholarly journals A broadbanded pressure differential wave energy converter based on dielectric elastomer generators

2021 ◽  
Author(s):  
Michele Righi ◽  
Giacomo Moretti ◽  
David Forehand ◽  
Lorenzo Agostini ◽  
Rocco Vertechy ◽  
...  
Keyword(s):  
Wave Energy ◽  
Large Deformations ◽  
Dielectric Elastomer ◽  
Wave Energy Converter ◽  
Pressure Differential ◽  
Design Stage ◽  
Small Scale ◽  
Energy Converter ◽  
Wide Range ◽  
The Waves

AbstractDielectric elastomer generators (DEGs) are a promising option for the implementation of affordable and reliable sea wave energy converters (WECs), as they show considerable promise in replacing expensive and inefficient power take-off systems with cheap direct-drive generators. This paper introduces a concept of a pressure differential wave energy converter, equipped with a DEG power take-off operating in direct contact with sea water. The device consists of a closed submerged air chamber, with a fluid-directing duct and a deformable DEG power take-off mounted on its top surface. The DEG is cyclically deformed by wave-induced pressure, thus acting both as the power take-off and as a deformable interface with the waves. This layout allows the partial balancing of the stiffness due to the DEG’s elasticity with the negative hydrostatic stiffness contribution associated with the displacement of the water column on top of the DEG. This feature makes it possible to design devices in which the DEG exhibits large deformations over a wide range of excitation frequencies, potentially achieving large power capture in a wide range of sea states. We propose a modelling approach for the system that relies on potential-flow theory and electroelasticity theory. This model makes it possible to predict the system dynamic response in different operational conditions and it is computationally efficient to perform iterative and repeated simulations, which are required at the design stage of a new WEC. We performed tests on a small-scale prototype in a wave tank with the aim of investigating the fluid–structure interaction between the DEG membrane and the waves in dynamical conditions and validating the numerical model. The experimental results proved that the device exhibits large deformations of the DEG power take-off over a broad range of monochromatic and panchromatic sea states. The proposed model demonstrates good agreement with the experimental data, hence proving its suitability and effectiveness as a design and prediction tool.

scholarly journals A two-dimensional glacier–fjord coupled model applied to estimate submarine melt rates and front position changes of Hansbreen, Svalbard

2018 ◽  
Vol 64 (247) ◽  
pp. 745-758 ◽  
Author(s):  
E. DE ANDRÉS ◽  
J. OTERO ◽  
F. NAVARRO ◽  
A. PROMIŃSKA ◽  
J. LAPAZARAN ◽  
...  
Keyword(s):  
Coupled Model ◽  
Small Scale ◽  
Two Dimensional ◽  
Time Step ◽  
Software Packages ◽  
Front Position ◽  
Circulation Patterns ◽  
Glacier Front ◽  
Order Of Magnitude ◽  
Frontal Ablation

ABSTRACTWe have developed a two-dimensional coupled glacier–fjord model, which runs automatically using Elmer/Ice and MITgcm software packages, to investigate the magnitude of submarine melting along a vertical glacier front and its potential influence on glacier calving and front position changes. We apply this model to simulate the Hansbreen glacier–Hansbukta proglacial–fjord system, Southwestern Svalbard, during the summer of 2010. The limited size of this system allows us to resolve some of the small-scale processes occurring at the ice–ocean interface in the fjord model, using a 0.5 s time step and a 1 m grid resolution near the glacier front. We use a rich set of field data spanning the period April–August 2010 to constrain, calibrate and validate the model. We adjust circulation patterns in the fjord by tuning subglacial discharge inputs that best match observed temperature while maintaining a compromise with observed salinity, suggesting a convectively driven circulation in Hansbukta. The results of our model simulations suggest that both submarine melting and crevasse hydrofracturing exert important controls on seasonal frontal ablation, with submarine melting alone not being sufficient for reproducing the observed patterns of seasonal retreat. Both submarine melt and calving rates accumulated along the entire simulation period are of the same order of magnitude, ~100 m. The model results also indicate that changes in submarine melting lag meltwater production by 4–5 weeks, which suggests that it may take up to a month for meltwater to traverse the englacial and subglacial drainage network.

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