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.

Towards a Time-Domain Finite Element Analysis of Vortex Induced Vibrations

2011 ◽  
Author(s):  
Philippe Mainc¸on ◽  
Carl M. Larsen
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Laguerre Polynomials ◽  
Cross Flow ◽  
Domain Model ◽  
Time Step ◽  
Element Analysis ◽  
Vortex Induced Vibrations ◽  
Frequency Components ◽  
Chaotic Nature

Slender structures immersed in a cross flow can experience vibrations induced by vortex shedding (VIV), which cause fatigue damage and other problems. Engineering VIV models tend to operate in the frequency domain. A time domain model would allow to capture effects beyond the scope of today’s frequency domain empirical codes: interaction between in-line and cross-flow vibrations, higher order frequency components, structural non-linearities, simultaneous actions from other loads like waves and forced motions at boundaries. There is also the potential to capture the chaotic nature of VIV. Such a model was formulated in the present work: for each cross section and at each time step, the recent velocity history is described as a combination of Laguerre polynomials. The coefficients of that combination are 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 very promising, yet that more work is needed to understand trajectory stability and related issues, in order to further progress towards an engineering tool.

Simulating High Mode Vortex Induced Vibration of Riser in Linear Sheared Current Using an Empirical Time Domain Model

2020 ◽  
Author(s):  
Sang Woo Kim ◽  
Svein Sævik ◽  
Jie Wu
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Structural Model ◽  
Cross Flow ◽  
Domain Model ◽  
Vortex Induced Vibrations ◽  
The One ◽  
The Time Domain ◽  
Empirical Coefficients

Abstract This paper addresses the performance evaluation of an empirical time domain Vortex Induced Vibrations (VIV) model which has been developed for several years at NTNU. Unlike the frequency domain which is the existing VIV analysis method, the time domain model introduces new vortex shedding force terms to the well known Morison equation. The extra load terms are based on the relative velocity, a synchronization model and additional empirical coefficients that describe the hydrodynamic forces due to cross-flow (CF) and In-line (IL) vortex shedding. These hydrodynamic coefficients have been tuned to fit experimental data and by considering the results from the one of existing frequency domain VIV programs, VIVANA, which is widely used for industrial design. The feature of the time domain model is that it enables to include the structural non-linearity, such as variable tension, and time-varying flow. The robustness of the new model’s features has been validated by comparing the test results in previous researches. However, the riser used in experiments has a relatively small length/diameter (L/D) ratio. It implies that there is a need for more validation to make it applicable to real riser design. In this study, the time domain VIV model is applied to perform correlation studies against the Hanøytangen experiment data for the case of linear sheared current at a large L/D ratio. The main comparison has been made with respect to the maximum fatigue damage and dominating frequency for each test condition. The results show the time domain model showed reasonable accuracy with respect to the experimental and VIVANA. The discrepancy with regard to experiment results needs to be further studied with a non-linear structural model.

On Vessel Motion Induced Vortex-Induced Vibrations of a Steel Lazy Wave Riser

2021 ◽  
Author(s):  
Decao Yin
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Oscillatory Flow ◽  
Domain Model ◽  
Time Varying ◽  
Structure Interaction ◽  
Response Characteristics ◽  
Vortex Induced Vibrations ◽  
The Time Domain ◽  
Vessel Motion

Abstract Deepwater steel lazy wave risers (SLWR) subject to vessel motion will be exposed to time-varying oscillatory flow, vortices could be generated and the cyclic vortex shedding force causes the structure vibrate, such fluid-structure interaction is called vortex-induced vibrations (VIV). To investigate VIV on a riser with non-linear structures under vessel motion and oscillatory flows, time domain approaches are needed. In this study, a time-domain approach is used to simulate a full-scale SLWR. Two cases with simplified riser top motions are simulated numerically. By using default input parameters to the time domain approach, the key oscillatory flow induced VIV response characteristics such as response frequency, curvature and displacements are examined and discussed. More accurate VIV prediction could be achieved by using realistic hydrodynamic inputs into the time domain model.

Investigation on Fluidelastic Instability Accompanied by Wake Shedding With a Time-Domain Model

2019 ◽  
Author(s):  
Kai Guo ◽  
Yipeng Wang ◽  
Tong Su ◽  
Liyan Liu ◽  
Zhanbin Jia ◽  
...  
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Domain Model ◽  
Elastic Instability ◽  
Flow Induced Vibration ◽  
Fluid Forces ◽  
Double Peaks ◽  
Critical Instability ◽  
Periodic Characteristic ◽  
The Time Domain

Abstract As the most dangerous flow-induced vibration (FIV) mechanism, fluid-elastic instability is always accompanied by the wake shedding. If both of the two FIV mechanisms are considered, fluid forces in this condition can be quite complex. In this paper, a time-domain model based on unsteady flow theory was used to investigate the fluid-elastic instability in a rotated triangular tube array. The vortex shedding forces were simplified as harmonic forces. Computational fluid dynamics (CFD) was used to get the fluid force coefficients with vortex shedding. The model was established by a finite element code with MATLAB software and simulation results agreed with the experiment results. The results showed the critical instability velocity can be influenced by vortex shedding forces, and double peaks can be found in the frequency spectrum of displacements of tubes. The time-domain displacements showed the phases had been impacted by the shedding and periodic characteristic was found in the displacements results. The model can also be adopted in fluid-elastic instability analysis in other tube arrangements and flow conditions.

Simulating Riser VIV in Current and Waves Using an Empirical Time Domain Model

2017 ◽  
Author(s):  
Mats J. Thorsen ◽  
Svein Sævik
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Theoretical Background ◽  
Domain Model ◽  
Wave Period ◽  
Time Variability ◽  
Analysis Tool ◽  
Load Model ◽  
The Time Domain

The theoretical background of an empirical model for time domain simulation of VIV is reviewed. This model allows the surrounding flow to be time varying, which is in contrast to the traditional frequency domain tools. The hydrodynamic load model consists of Morison’s equation plus an additional term representing the oscillating effect of vortex shedding. The magnitude of the vortex shedding force is given by a dimensionless coefficient, and this force is assumed to act perpendicular to the relative velocity between the cylinder and the fluid. The time variability of the vortex shedding force is described by a synchronization model, which captures how the instantaneous frequency reacts to cylinder motion. The parameters in the time domain load model are calibrated against a commonly used frequency domain VIV analysis tool, VIVANA. To do this, a finite element model of a vertical tensioned riser is established, and the structure is exposed to a linearly sheared flow. Key results such as cross-flow displacements along the riser, frequency content, r.m.s. of bending stresses and mean in-line displacements are compared, and it is shown that the frequency and time domain methods are close to equivalent in this simple case with stationary flow. Finally, the time domain model is utilized to study VIV of a riser subjected to regular waves. The characteristics of wave induced VIV are discussed in light of the simulation results. It is seen that VIV is excited in the zone close to the surface, and the energy is transported downwards as traveling waves. The vibrations typically build up as the horizontal water particle velocity is high, and die out as the velocity decreases. The effect of varying the wave amplitude and period is investigated, and it is found that the dominating frequency, mode and r.m.s. stresses increase together with the wave height. The effect of the wave period is however more complicated. For example, reducing the wave period increases the dominating mode but decreases the displacements. Hence the stress may increase or decrease, depending on which of these effects are strongest.

Part II: Spanwise Correlation and Loading

1981 ◽  
Vol 32 (2) ◽  
pp. 111-125 ◽  
Author(s):  
R.H. Wilkinson
Keyword(s):  
Cross Section ◽  
Vortex Shedding ◽  
Cross Flow ◽  
Correlation Lengths ◽  
Pressure Distributions

SummaryThe influence of spanwise correlation of fluctuating pressures due to vortex shedding on the total oscillatory loading of a long bluff cylinder in cross-flow is considered. Experimentally determined pressure correlation lengths on a stationary and vibrating cylinder of square cross section are then combined with previously obtained chordwise pressure distributions to demonstrate the amplification of the oscillatory lift on a long bluff cylinder during cross-flow vibrations.

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.

In-Line Vibrations of Flexible Pipes

2017 ◽  
Author(s):  
Jan V. Ulveseter ◽  
Svein Sævik
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Linear Time ◽  
Domain Model ◽  
Integration Scheme ◽  
Force Model ◽  
Prediction Tool ◽  
Flexible Pipe ◽  
Non Linear

A semi-empirical prediction tool for pure in-line vortex-induced vibrations is under development. The long-term goal is to be able to realistically model the dynamic behavior of free spanning pipelines exposed to arbitrary time dependent external flows at low velocities. Most VIV programs operate in frequency domain, where only steady currents and linear structural models can be simulated. In contrast, the proposed model predicts hydrodynamic forces as function of time, enabling a time integration scheme to solve the equation of motion. Non-linear time domain simulations allow for modelling of excitation from non-steady currents. In addition, non-linear effects such as soil-pipe interaction, varying tension, and response dependent material, stiffness and damping properties may be included in the analysis, when combining the hydrodynamic force model with a structural non-linear finite element model. Hydrodynamically, the proposed prediction tool consists of the general Morison equation plus two vortex shedding forcing terms. The latter two are able to synchronize with the structural motion for a given frequency band, to induce vibrations in lock-in regimes. In this paper, the proposed pure in-line VIV model is compared to the frequency domain model VIVANA and DNV Recommended Practice, simulating experiments with a model-scale flexible pipe exposed to current velocities at which cross-flow vibrations have not yet developed. A few experimental data points are included in verifying the performance of the newly developed time domain model. The effect of changing empirical coefficients in the vortex shedding forcing terms, and allowing only one of the terms to excite structural vibrations during a simulation, is numerically investigated. A goal is to obtain increased understanding of how the proposed time domain model performs when simulating VIV of a flexible pipe, which is more complex than that of an elastically mounted rigid cylinder since several natural frequencies and corresponding modes might be excited.

Time Domain Simulation of Vortex-Induced Vibrations Based on Phase-Coupled Oscillator Synchronization

2015 ◽  
Author(s):  
Mats J. Thorsen ◽  
Svein Sævik ◽  
Carl M. Larsen
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Hydrodynamic Force ◽  
Circular Cross Section ◽  
Cross Flow ◽  
Force Model ◽  
Time Domain Simulation ◽  
Local Flow ◽  
Hydrodynamic Damping ◽  
Vortex Induced Vibrations

Since 2012, there has been ongoing development of a simplified hydrodynamic force model at the Norwegian University of Science and Technology which enables time domain simulation of vortex-induced vibrations (VIV). Time domain simulation has a number of advantages compared to frequency domain. More specifically, having a time domain formulation of the hydrodynamic force which is efficient and reliable, will allow designers to include any relevant non-linear effects in their simulations, thereby increasing the level of realism and confidence in the results. The present model computes the dynamic cross-flow and in-line fluid force on a circular cross-section based on the incoming local flow velocity and the motion of the cylinder section. The most important difference between this and other existing models is the way synchronization between the vortex shedding and cylinder motion is taken into account. In contrast to the traditional VIV prediction tools, the vortex shedding frequency is in this model free to vary within a specified range, and changes according to the instantaneous phase difference between the cylinder velocity and the vortex shedding process itself. Hence, the oscillating lift and drag forces continuously update their frequencies, trying to lock on to the frequency of vibration. Combined with a simple hydrodynamic damping model and a constant added mass, it has previously been shown that highly realistic results can be obtained. In this paper, the theoretical background is reviewed, and the capabilities of the model are tested against new cases. These are: i) High mode VIV of tension-dominated riser in sheared flow, and ii) Low mode VIV of a pipeline with high bending stiffness. Both cross-flow and in-line vibrations are considered, and comparison with experimental observations is given. Based on the results, strengths and weaknesses of the model is discussed, and an outline of future developments is given.

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