Efficient Time-Domain Model for Frequency-Dependent Added Mass and Damping

2004 ◽  
Author(s):  
Karl Erik Kaasen ◽  
Knut Mo
Keyword(s):  
Differential Equation ◽  
Time Domain ◽  
Mass Function ◽  
Added Mass ◽  
Domain Model ◽  
Frequency Dependent ◽  
Induced Motion ◽  
Linear Differential ◽  
Wave Induced ◽  
Velocity History

In simulation of 1st-order wave-induced motion of vessels it is sometimes necessary to express frequency-dependent added mass and damping in time-domain formulation. One way to do this is to transform the frequency dependence into retardation functions. During simulation these are convolved with the velocity history, which is time-consuming and impractical. To get a more efficient model a method for expressing the retardation function as a linear differential equation has been developed. The method calculates the coefficients of the differential equation from the damping function only, avoiding the uncertain added mass function.

A Numerical Method for Representing Retardation Functions With Complex Exponentials

2017 ◽  
Author(s):  
Fushun Liu ◽  
Lei Jin ◽  
Jiefeng Chen ◽  
Wei Li
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Degrees Of Freedom ◽  
Added Mass ◽  
Memory Effects ◽  
Floating Structure ◽  
Frequency Dependent ◽  
Laplace Domain ◽  
The Time Domain ◽  
Frequency Domain Techniques

Numerical time- or frequency-domain techniques can be used to analyze motion responses of a floating structure in waves. Time-domain simulations of a linear transient or nonlinear system usually involve a convolution terms and are computationally demanding, and frequency-domain models are usually limited to steady-state responses. Recent research efforts have focused on improving model efficiency by approximating and replacing the convolution term in the time domain simulation. Contrary to existed techniques, this paper will utilize and extend a more novel method to the frequency response estimation of floating structures. This approach represents the convolution terms, which are associated with fluid memory effects, with a series of poles and corresponding residues in Laplace domain, based on the estimated frequency-dependent added mass and damping of the structure. The advantage of this approach is that the frequency-dependent motion equations in the time domain can then be transformed into Laplace domain without requiring Laplace-domain expressions of the added mass and damping. Two examples are employed to investigate the approach: The first is an analytical added mass and damping, which satisfies all the properties of convolution terms in time and frequency domains simultaneously. This demonstrates the accuracy of the new form of the retardation functions; secondly, a numerical six degrees of freedom model is employed to study its application to estimate the response of a floating structure. The key conclusions are: (1) the proposed pole-residue form can be used to consider the fluid memory effects; and (2) responses are in good agreement with traditional frequency-domain techniques.

scholarly journals A Wiener-Laguerre Model of VIV Forces Given Recent Cylinder Velocities

2011 ◽  
Vol 2011 ◽  
pp. 1-43 ◽  
Author(s):  
Philippe Mainçon
Keyword(s):  
Cross Section ◽  
Vortex Shedding ◽  
Time Domain ◽  
Structural Design ◽  
Laguerre Polynomials ◽  
Cross Flow ◽  
Domain Model ◽  
Interpolation Function ◽  
Chaotic Nature ◽  
Velocity History

Slender structures immersed in a cross flow can experience vibrations induced by vortex shedding (VIV), which cause fatigue damage and other problems. VIV models that are used in structural design today tend to assume harmonic oscillations in some way or other. A time domain model would allow to capture the chaotic nature of VIV and to model interactions with other loads and nonlinearities. Such a model was developed in the present work: for each cross section, recent velocity history is compressed using Laguerre polynomials. The compressed information is used to enter an interpolation function to predict the instantaneous force, allowing to step the dynamic analysis. An offshore riser was modeled in this way: some analyses provided an unusually fine level of realism, while in other analyses, the riser fell into an unphysical pattern of vibration. It is concluded that the concept is promising, yet that more work is needed to understand orbit stability and related issues, in order to produce an engineering tool.

An Investigation Into the Limitations of the Panel Method and the Gap Effect for a Fixed and a Floating Structure Subject to Waves

2016 ◽  
Author(s):  
Blanca Peña ◽  
Aaron McDougall
Keyword(s):  
Time Domain ◽  
Real Life ◽  
Added Mass ◽  
Panel Method ◽  
Gap Effect ◽  
Wave Radiation ◽  
Literature Study ◽  
Floating Structure ◽  
Cfd Analyses ◽  
Wave Induced

The wave-induced motions of vessels moored next to a fixed object and open to the sea impact the operability of many offshore operations, and should be assessed in order to avoid accidents and catastrophes. When analysing vessels moored by a fixed object (e.g. quay-side or platform), time domain simulations have shown numeric instabilities resulting in unreliable outcomes. The origin of the numerical instability might lie in the hydrodynamic added mass and wave radiation damping. This is typically calculated using potential flow methods and influenced by the existence of standing waves in the gap between the two bodies. For certain frequencies, these give negative values, potentially causing instabilities in non-linear (coupled) time domain simulations. In these cases, the vessel can behave unexpectedly, generating energy rather than dissipating it. As such, certain simulations have been disregarded as they are unlikely to accurately represent real-life scenarios. This paper investigates and compares added mass and damping using two different tools and studies the gap effect when conducting diffraction analysis using 3D panel methods. The work covers a literature study into potential theory, multibody analysis, Computational Fluid Dynamics (CFD) and lid techniques. This is followed by a study conducted using both panel method and CFD analyses. The results from both approaches have been compared, showing interesting information and the necessity of researching more into the problem addressed in this paper.

Atmosphere–Ocean Coupling in the Northern Gulf of St. Lawrence: Frequency-Dependent Wind-Forced Variations in Nearshore Sea Temperatures and Currents

1988 ◽  
Vol 45 (7) ◽  
pp. 1222-1233 ◽  
Author(s):  
G. A. Rose ◽  
W. C. Leggett
Keyword(s):  
Time Domain ◽  
Domain Model ◽  
Fish Migration ◽  
Biological Processes ◽  
Frequency Dependent ◽  
Sea Temperature ◽  
Temperature Changes ◽  
North Shore ◽  
The North ◽  
Ocean Coupling

We found nearshore sea temperatures and currents on the north shore of the Gulf of St. Lawrence to be linked to wind-forced upwellings and downwellings. Multiple coherence of alongshore and cross-shore wind stresses with sea temperature (f > 1/d removed) was significant at periods > 3 d (maximum K2 = 0.90) and at 1.8 d. Partial coherences were frequency dependent. At periods > 3 d, which contained most of the variance alongshore forces dominated. Cross-shore winds were significant only at 1.8 d. A time domain model using lagged wind stress and cumulative air temperature as predictors explained 95% of the variance in post-stratification nearshore temperature. Temperature changes were virtually synchronous along 150 km of coastline. Alongshore currents were coherent with alongshore winds at periods > 2 d. Cross-shore currents were coherent with cross-shore winds at periods of 1–2 d. These results are compared with Csanady's models of wind-forced coastal thermocline oscillation. We conclude that alongshore winds force major upwellings and downwellings in this region. Cross-shore forces are important at f > 0.5/d. These dynamics regulate local primary biological processes and the transport of energy through their influence on fish migration.

Time Domain Methodology for Vortex-Induced Motion Analysis in Monocolumn Platform

2009 ◽  
Author(s):  
Thiago Aˆngelo Gonc¸alves de Lacerda ◽  
Gilberto Bruno Ellwanger ◽  
Marcos Queija de Siqueira ◽  
Elizabeth Frauches Netto Siqueira
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Low Frequency ◽  
Cylindrical Shape ◽  
Floating Structures ◽  
Induced Motion ◽  
Six Degree Of Freedom ◽  
Offshore Oil ◽  
Wave Induced ◽  
Force Calculation

The offshore oil exploration in Brazil has been, traditionally, made by semi-submersible and moored ship-based units. The need for more restricted wave-induced motions has demanded new conceptions of floating structures, in which the mono-column concept distinguishes itself. Due to its cylindrical shape hull, this floating unit could present a significant low frequency vibratory movement caused by the vortex shedding phenomenon. This kind of phenomenon on huge structures like platforms is usually known as VIM (Vortex Induced Motions). The main objective of this work is to evaluate a time domain methodology applied in VIM problems. This methodology uses a Van der Pol equation to represent the vortex shedding phenomenon. The force calculation schemes presented in this work are applied in physical examples and its results will be compared to model test data. The analyses were performed in a non linear dynamic analysis program, using a six degree of freedom model, developed for this study.

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