Experimental Investigation of Vortex-Induced Vibrations on Yawed and Inclined Flexible Cylinders

2021 ◽  
pp. 1-16
Author(s):  
Daniel P. Vieira ◽  
Guilherme R. Franzini ◽  
Fredi Cenci ◽  
Andre Fujarra
Keyword(s):  
A Priori ◽  
Flexible Structures ◽  
Resonance Region ◽  
Offshore Structures ◽  
Natural Mode ◽  
Flexible Cylinder ◽  
Decomposition Scheme ◽  
Independence Principle ◽  
Modal Amplitude ◽  
Time Histories

Abstract An experimental setup was built to investigate the Vortex-Induced Vibration (VIV) phenomenon on yawed and inclined flexible cylinders, in which five yaw angles θ = 0°, 10°, 20°, 30° and 45° and five azimuth angles ß = 0°, 45°, 90°, 135°, and 180° were combined. The experiments were carried out in a towing tank facility at Reynolds numbers from 1800 to 18000, comprising vibrations up to the eighth natural mode. Time histories of displacements were recorded using a submerged optical system that tracks 17 reflective targets. A modal decomposition scheme based on Galerkin's method was applied, aiming multimodal behavior investigations. Such an approach allowed the analysis of the modal amplitude throughout time, revealing interesting results for such a class of VIV tests. The flexible cylinder total response is generally a combination of two or more modes. Only for azimuths 0°, 90°, and 180°, a unimodal response was observed for the two first lock-in regimes. The frequency response showed that, when the response was multimodal, non-dominant modes can follow the vibration frequency of the dominant one. Assuming a priori the Independence Principle (IP) valid to define the reduced velocities (Vr), it was observed that the resonance region was restricted to 3 <= Vr <= 8 for the tested cases, indicating that the IP can be at least partially applied for flexible structures. As the literature scarcely explores the simultaneous yawed and inclined configurations, the present work may contribute to further code validation and improvements regarding the design of slender offshore structures.

Seismic Analysis of a Double-Hinged Articulated Offshore Tower

2008 ◽  
Author(s):  
Syed Danish Hasan ◽  
Nazrul Islam ◽  
Khalid Moin
Keyword(s):  
Seismic Analysis ◽  
Offshore Structures ◽  
Random Wave ◽  
Wave Forces ◽  
Offshore Structure ◽  
Concentrated Mass ◽  
Energy Principle ◽  
Rotational Angle ◽  
Sea Bed ◽  
Time Histories

The response of offshore structures under seismic excitation in deep water conditions is an extremely complex phenomenon. Under such harsh environmental conditions, special offshore structures called articulated structures are feasible owing to reduced structural weight. Whereas, conventional offshore structure requires huge physical dimensions to meet the desired strength and stability criteria, therefore, are uneconomical. Articulated offshore towers are among the compliant offshore structures. These structures consist of a ballast chamber near the bottom hinge and a buoyancy chamber just below the mean sea level, imparting controlled movement against the environmental loads (wave, currents, and wind/earthquake). The present study deals with the seismic compliance of a double-hinged articulated offshore tower to three real earthquakes by solving the governing equations of motion in time domain using Newmark’s-β technique. For this purpose Elcentro 1940, Taft 1952 and Northridge 1994 earthquake time histories are considered. The tower is modeled as an upright flexible pendulum supported to the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air (a concentrated mass above water level). The computation of seismic and hydrodynamic loads are performed by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed to be a broadband stationary process. Effects of horizontal ground motions are considered in the present study. Monte Carlo simulation technique is used to model long crested random wave forces. Effect of sea-bed shaking on hydrodynamic modeling is considered. The dynamic equation of motion is formulated using Lagrangian approach, which is based on energy principle. Nonlinearities due to variable submergence and buoyancy, added mass associated with the geometrical non-linearities of the system are considered. The results are expressed in the form of time-histories and PSDFs of deck displacement, rotational angle, base and hinge shear, and the bending moment. The outcome of the response establishes that seismic sea environment is an important design consideration for successful performance of hinges, particularly, if these structures are situated in seismically active zones of the world’s ocean.

Regular and Irregular Vibrations of a Non-Ideal Autoparametric System

Volume 2 ◽  
2004 ◽  
Author(s):  
Danuta Sado ◽  
Maciej Kot
Keyword(s):  
Energy Source ◽  
Degrees Of Freedom ◽  
Equations Of Motion ◽  
Resonance Region ◽  
The Body ◽  
Damping Force ◽  
Power Spectral ◽  
Power Spectral Densities ◽  
Time Histories ◽  
Limited Power

This paper studies the regular and irregular vibrations of two degrees of freedom autoparametrical system, when the excitation is made by an electric motor (with unbalanced mass), which works with limited power supply. The investigated system consists of a pendulum of the length l and mass m, and a body of mass M suspended on the flexible element. It was assumed that the damping force acting on the body of mass M and resistive moment acting on the pedulum are non-linear. In this case, the excitation has to be expressed as an equation describing how the energy source supplies the energy to the system. The non-ideal source of power adds one degree of freedom, and then the system has three degrees of freedom. The system has been researched for known characteristic of the energy source (DC motor). The equations of motion have been solved numerically what permit to enrich the investigations and to examine not only small and steady state oscillations but also large-amplitude oscillations in transient states. The influence of motor’s speed on the phenomenon of energy transfer has been researched. Near the internal and external resonance region, except different kind of periodic vibration, the chaotic vibration has been observed. For characterizing an irregular chaotic response bifurcation diagrams and time histories, power spectral densities, Poincare´ maps and maximal exponents of Lyapunov have been constructed.

scholarly journals Systematizing and addressing theory uncertainties of unitarization with the Inverse Amplitude Method

2021 ◽  
Vol 11 (2) ◽  
Author(s):  
Alexandre Salas-Bernárdez ◽  
Felipe Llanes-Estrada ◽  
Juan Escudero-Pedrosa ◽  
José Antonio Oller
Keyword(s):  
Perturbation Theory ◽  
Partial Wave ◽  
Chiral Perturbation Theory ◽  
A Priori ◽  
Resonance Region ◽  
New Physics ◽  
Effective Field ◽  
Chiral Perturbation ◽  
Inverse Amplitude Method ◽  
Inverse Amplitude

Effective Field Theories (EFTs) constructed as derivative expansions in powers of momentum, in the spirit of Chiral Perturbation Theory (ChPT), are a controllable approximation to strong dynamics as long as the energy of the interacting particles remains small, as they do not respect exact elastic unitarity. This limits their predictive power towards new physics at a higher scale if small separations from the Standard Model are found at the LHC or elsewhere. Unitarized chiral perturbation theory techniques have been devised to extend the reach of the EFT to regimes where partial waves are saturating unitarity, but their uncertainties have hitherto not been addressed thoroughly. Here we take one of the best known of them, the Inverse Amplitude Method (IAM), and carefully following its derivation, we quantify the uncertainty introduced at each step. We compare its hadron ChPT and its electroweak sector Higgs EFT applications. We find that the relative theoretical uncertainty of the IAM at the mass of the first resonance encountered in a partial-wave is of the same order in the counting as the starting uncertainty of the EFT at near-threshold energies, so that its unitarized extension should a priori be expected to be reasonably successful. This is so provided a check for zeroes of the partial wave amplitude is carried out and, if they appear near the resonance region, we show how to modify adequately the IAM to take them into account.

Tolerance Optimization: A Decomposition Scheme, Variance Reduction and Fractional Approximation

2001 ◽  
Author(s):  
Mohamed H. Gadallah
Keyword(s):  
Variance Reduction ◽  
A Priori ◽  
Statistical Design ◽  
Global Optimum ◽  
Constrained Systems ◽  
Statistical Design Of Experiments ◽  
System Decomposition ◽  
Decomposition Scheme ◽  
Statistical Measures ◽  
Original Objective

Abstract Development of involved optimization algorithms is not an easy task for several reasons: First, every analyst is interested in a specific problem; Second, the capabilities of these methods may not be fully understood a priori; Third, coding of multi-purpose and more involved algorithms is not an easy job. In this paper, the optimization problem employing the near to global optimum algorithm is studied (Gadallah, M.H., 2000). The focus is to exploit 2 ideas: First, the algorithm can be modified to act as a variance reduction technique; Second, the algorithm can be modified to tackle the problem of system decomposition. Both ideas are novel within the context of statistical design of experiments. The first, if fully proved experimentally could yield the simultaneous integration of nominal and variance optimization possible. The second, can be extended to deal with multi-dimensional highly constrained systems with ease. These two ideas are explained wife the use of a simple example to illustrate the idea. An algorithm is developed that deal with the problem in several stages according to a predetermined decomposition scheme. The original objective and constraint functions are dealt with to suit each stage. Accordingly, all NP hard problems can ideally be transformed into NP complete ones with a consequence on the number of stages resulting from decomposition. Several decomposition scenarios are used and their results are compared numerically. Two orthogonal arrays and four composite arrays are used to plan experimentation; these are L27OA and L54OA and their subfamilies. These arrays are compared with respect to their statistical measures. The algorithm as such, is very promising optimization tool, especially for coupling system decomposition and variance reduction. Past work focused on either decomposition or statistical optimization. This work offers both capabilities. Several studies are reviewed and conclusions are drawn.

Estimating Extreme Response of Drag Dominated Offshore Structures From Simulated Time Series of Structural Response

2007 ◽  
Author(s):  
A. Naess ◽  
O. Gaidai ◽  
S. Haver
Keyword(s):  
Response Time ◽  
Structural Response ◽  
Weather Conditions ◽  
Response Prediction ◽  
Offshore Structures ◽  
Rate Function ◽  
Extreme Response ◽  
Time Histories ◽  
Short Time

The paper presents a study of extreme response statistics of drag dominated offshore structures, showing a pronounced dynamic behaviour when subjected to harsh weather conditions. The key quantity for extreme response prediction is the mean up-crossing rate function, which can be simply extracted from simulated stationary response time histories. Present practise for obtaining adequate extremes for design purposes requires a number — say 20 or more — of 3-hour time domain analyses for several extreme sea states. For early phase considerations, it would be convenient if extremes of a reasonable accuracy could be obtained based on shorter and fewer simulations. It is therefore of interest to develop specific methods which make it possible to extract the necessary information from relatively short time histories. The method proposed in this paper opens up the possibility to predict simply and efficiently long-term extreme response statistics, which is an important issue for the design of offshore structures. A short description of this is given, but in the present paper the emphasis is on short-term analyses. The results presented are based on extensive simulation results for the Kvitebjo̸rn jacket structure, in operation on the Norwegian Continental Shelf. Specifically, deck response time histories for different sea states simulated from a MDOF model were used as the basis for our analyses.

scholarly journals Near Fault Effect on the Response of Single Hinged Compliant Offshore Tower

2018 ◽  
Vol 203 ◽  
pp. 01015
Author(s):  
Syed Yusuf Javed
Keyword(s):  
Offshore Structures ◽  
Dynamic Responses ◽  
Simultaneous Action ◽  
Integration Scheme ◽  
Random Waves ◽  
Sea Waves ◽  
Offshore Structure ◽  
Wave Condition ◽  
Near Fault ◽  
Time Histories

The response of compliant offshore structure under simultaneous action of random waves and earthquake loading has been analyzed. Since earthquake forces play a significant role in affecting the response of these offshore structures, comparative studies have been carried out considering near fault and far fault seismic excitations in the presence of moderate random sea waves. The offshore tower is modeled as an inverted pendulum with a cylindrical shaft connected by an articulated joint at the base. Seismic forces are evaluated by dividing the tower shaft into finite elements with masses lumped at the nodes. The nonlinearities associated with the system owing to variable submergence, drag force, variable buoyancy along with the geometry are considered in the analysis. The nonlinear dynamic equation of motion is formulated considering Lagrangian approach, which is solved in time domain by the Newmark-beta integration scheme. The sea state conditions, more precisely the water particle kinematics are evaluated using Airy’s wave theory along with the stretching modifications, as suggested by Chakrabarti. To minimize the dynamic responses, emphasis has to be given to the variations in height and position of the buoyancy chamber in extreme wave condition. The results are expressed in the form of time histories of deck displacement, hinge rotation, hinge shear and the bending moment. Parameters like maximum, minimum, mean and standard deviation are also determined by statistical analysis of response time histories of the dynamic responses at articulated joint.

Effect of Different Synthetic Sport Surfaces on Ground Reaction Forces at Landing in Netball

1988 ◽  
Vol 4 (2) ◽  
pp. 130-145 ◽  
Author(s):  
Julie R. Steele ◽  
Peter D. Milburn
Keyword(s):  
A Priori ◽  
Reaction Force ◽  
Movement Pattern ◽  
Ground Reaction Forces ◽  
Maximum Peak ◽  
Time To Peak ◽  
Time Histories ◽  
Reaction Forces ◽  
Force Time ◽  
Initial Peak

This study examined the influence of 12 different synthetic sport surfaces (bitumen, concrete, 3 samples of synthetic grass, and 7 samples of rubber surfaces) on ground reaction forces at landing in netball. Ground reaction force data were obtained for 10 skilled netball players at landing after performing a typical attacking netball movement pattern. Force–time histories of the maximum peak vertical ground reaction forces (VGRF), the initial peak VGRF, and peak braking forces were determined for each trial. Results of the a priori planned comparison analysis indicated that subjects demonstrated significantly longer time to maximum peak VGRF and initial peak VGRF when landing on grass, higher peak braking forces when landing on bitumen and concrete combined, and a significantly shorter time to peak braking force when landing on grass in comparison to other samples tested. It was concluded that the rubber surfaces tested demonstrated the potential for being the most suitable playing surface for minimization of injuries in netball.

Extreme Response Statistics of Fixed Offshore Structures Subjected to Ringing Loads

2014 ◽  
Vol 136 (3) ◽  
Author(s):  
Oleg Gaidai ◽  
Jørgen Krokstad
Keyword(s):  
Structural Response ◽  
Offshore Structures ◽  
Wave Impact ◽  
Structural Support ◽  
Random Seas ◽  
Wave Basin ◽  
Extreme Response ◽  
Time Histories ◽  
Short Time ◽  
Diffraction Effects

This paper describes an efficient Monte Carlo based method for prediction of extreme response statistics of fixed offshore structures subjected to random seas. The nonlinear structural response known as “ringing” is studied, which is caused by the wave impact force on structural support units. Common challenge for design of such structures is a sound estimate of the hydrodynamic load including diffraction effects. The aim of the work was to develop specific methods which make it possible to extract the necessary information about the extreme response from relatively short time histories. The method proposed in this paper opens up the possibility to predict simply and efficiently both short-term and long-term extreme response statistics. The results presented are based on extensive simulation results for the large fixed platform operating on the Norwegian continental shelf. Structural response time histories, measured in MARINTEK (MT) wave basin lab, were used to validate numerical results.

Formulation of a Basic Building Block Model for Interaction of High Speed Vehicles on Flexible Structures

1989 ◽  
Vol 56 (2) ◽  
pp. 451-458 ◽  
Author(s):  
L. Vu-Quoc ◽  
M. Olsson
Keyword(s):  
Nonlinear Equations ◽  
Building Block ◽  
High Speed ◽  
Traditional Approach ◽  
Equations Of Motion ◽  
A Priori ◽  
Flexible Structures ◽  
Structural Deformation ◽  
Block Model ◽  
Nonlinear Equations Of Motion

In traditional analyses of vehicle/structure interaction, especially when there are constraints between vehicle masses and the structure, vehicle nominal motion is prescribed a priori, and therefore unaffected by the structure flexibility. In this paper, a concept of nominal motion is defined, and a methodology is proposed in which the above restriction is removed, allowing the vehicle nominal motion to become unknown, and encompassing the traditional approach as a particular case. General nonlinear equations of motion of a building block model, applicable to both wheel-on-rail and magnetically levitated vehicles, are derived. These equations are simplified to a set of mildly nonlinear equations upon introducing additional assumptions — essentially on small structural deformation. An example is given to illustrate the present formulation.

An Assessment of Linear Spectral Analysis Method for Offshore Structures via Random Sea Simulation

1982 ◽  
Vol 104 (1) ◽  
pp. 39-46 ◽  
Author(s):  
S. Kao
Keyword(s):  
Spectral Analysis ◽  
Spectral Method ◽  
Drag Force ◽  
Flexible Structures ◽  
Offshore Structures ◽  
Analysis Method ◽  
Corrective Measures ◽  
Spectral Analysis Method

With random sea simulation, application of linear spectral analysis method to offshore structures with moderate drag force has been assessed. Findings indicate overprediction of response for short natural periods and underprediction for very long periods. Tentative corrective measures are recommended. Significant force and response reductions have been calculated for flexible structures which are not adequately predicted by the linear spectral method.

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