Research on Resonant Fluid Behaviors and Induced Violent Slamming Effects in a Stepped Moonpool Under Wave Excitations With Ship Forward Speed

2020 ◽  
Author(s):  
Hongguan Lyu ◽  
Jiayang Gu ◽  
Yanwu Tao ◽  
Zhongyu Zhang
Keyword(s):  
Natural Frequencies ◽  
Structural Strength ◽  
Nonlinear Effects ◽  
Hydrodynamic Performance ◽  
Linear Potential ◽  
Water Effects ◽  
Practical Engineering ◽  
Moon Pool ◽  
Piston Mode ◽  
External Wave

Abstract Moonpools on drilling vessels are structures located at the mid-ship position to facilitate drilling and other marine operations. The existence of the moonpools is prone to result in intense resonant fluid behaviors even other unexpected violent nonlinear impact on the flow system under external excitations, especially for those types with one or more recesses. The present study focuses on the resonant fluid behaviors and the induced violent nonlinear effects so-called slamming in a moonpool with a recess within the frameworks of both theoretical and numerical investigations. The natural frequencies of the piston mode and the first-three modes of the studied moonpool are approximated based on the theoretical formulas extracted from the linear potential flow theory. Furthermore, the calculated frequency of piston mode are carried out as the external wave excitations combined with varying current velocity to investigate the slamming effects at the typical locations such as the wall of the moonpool and the bottom of the recess. Finally, the characteristics of the slamming pressure both in space and time are discussed in detail based on the RANS-based numerical simulations. The result shows that the resonant fluid behaviors under the wave-current interactions are greatly different from the wave-only excitations. Moreover, intense slamming occur in a moon-pool with a recess under some particular wave-current interactions due to the shallow water effects in the recess region and the phenomenon of energy transfer. The phenomenon of slamming must be treated with special cautions in practical engineering because of the fact that some induced adverse effects could weaken the hydrodynamic performance of the drilling vessel, as well as the structural strength of the moonpool.

scholarly journals THE INTERACTION OF OBLIQUE WAVES WITH A PARTIALLY IMMERSED WAVE ABSORBING BREAKWATER

2011 ◽  
Vol 1 (32) ◽  
pp. 56 ◽  
Author(s):  
Yong Liu ◽  
Yucheng Li
Keyword(s):  
Hydrodynamic Performance ◽  
Design Stage ◽  
Wave Forces ◽  
Linear Potential ◽  
Transmission Coefficients ◽  
Present Solution ◽  
Preliminary Design Stage ◽  
Practical Engineering ◽  
Linear Potential Theory ◽  
Incident Waves

By considering obliquely incident waves, the hydrodynamic performance of a partially immersed wave absorbing breakwater is examined in this study. The breakwater consists of a perforated front barrier and a solid rear barrier. The two barriers are both partially immersed with the same draft. An analytical solution based on the linear potential theory is developed to calculate the reflection and transmission coefficients of the breakwater and the wave forces acting on the barriers. Some useful results are presented according to numerical examples. The present solution may be used at a preliminary design stage in practical engineering.

Numerical Analysis of a Floating Body With Two-Dimensional Moonpool Including Piston Mode

2010 ◽  
Author(s):  
Jaekyung Heo ◽  
Jong-Chun Park ◽  
Moo-Hyun Kim ◽  
Weon-Cheol Koo
Keyword(s):  
Free Surface ◽  
Viscous Flow ◽  
Experimental Results ◽  
Floating Body ◽  
Navier Stokes ◽  
Linear Potential ◽  
Navier Stokes Equation ◽  
Nonlinear Potential ◽  
Heave Motion ◽  
Piston Mode

In this paper, the potential and viscous flows are simulated numerically around a 2-D floating body with a moonpool (or a small gap) with particular emphasis on the piston mode. The floating body with moonpool is forced to heave in time domain. Linear potential code is known to give overestimated free-surface heights inside the moonpool. Therefore, a free-surface lid in the gap or similar treatments are widely employed to suppress the exaggerated phenomenon by potential theory. On the other hand, Navier-Stokes equation solvers based on a FVM can be used to take account of viscosity. Wave height and phase shift inside and outside the moon-pool are computed and compared with experimental results by Faltinsen et al. (2007) over various heaving frequencies. Pressure and vorticity fields are investigated to better understand the mechanism of the sway force induced by the heave motion. Furthermore, a nonlinear potential code is utilized to compare with the viscous flow. The viscosity effects are investigated in more detail by solving Euler equations. It is found that the viscous flow simulations agree very well with the experimental results without any numerical treatment.

Effect of Fiber Orientation on Vibration Characteristic of Composite Laminated Plates

2014 ◽  
Vol 670-671 ◽  
pp. 158-163 ◽  
Author(s):  
Hui Fen Peng ◽  
Cheng Wang ◽  
Peng Wang
Keyword(s):  
Fiber Orientation ◽  
Natural Frequencies ◽  
Structure Design ◽  
Laminated Plates ◽  
Mode Shapes ◽  
Vibration Characteristic ◽  
Composite Laminated Plates ◽  
Practical Engineering ◽  
Lamination Theory ◽  
Theoretical Results

To describe vibration characteristic of composite laminated plates with various fiber orientations, a composite laminated finite element, which follows classical lamination theory, was constructed. In each ply of rectangular composite laminated plates, the fiber orientation changes with respect to the horizontal coordinate. Natural frequencies and mode shapes of composite laminated plates were studied. The first six natural frequencies and mode shapes of composite laminated plates with various fiber orientations are obtained. The accuracy of this composite laminated element is verified by comparing numerical and theoretical results. The results show that the changes of fiber orientation bring a greater degree of flexibility for structure design of composite laminated plates, which can be used to adjust frequencies and mode shapes of composite laminated plates according to practical engineering need.

Characteristics of OWC Type WEC Dampers Installed on a Very Large Floating Structure

2020 ◽  
Author(s):  
Tomoki Ikoma ◽  
Shoichiro Furuya ◽  
Yasuhiro Aida ◽  
Koichi Masuda ◽  
Hiroaki Eto
Keyword(s):  
Wave Energy ◽  
Sea Water ◽  
Prediction Method ◽  
Hydrodynamic Performance ◽  
Floating Body ◽  
Hydrodynamic Characteristics ◽  
Linear Potential ◽  
Floating Structure ◽  
Combined System ◽  
Floating System

Abstract Oscillating water column (OWC) type wave energy converters (WECs) have been researched and developed. OWC WECs are relatively friendly to maintain them in operation because all of mechanical units are set above a sea water surface. In addition, a feature of an OWC device is similar to an air dumper system. Thus, it should be possible not only to harvest wave energy but also to reduce motion of a floating system at the same time. As well as WEC system should be used with other ocean renewable energies as a combined system. This paper describes hydrodynamic characteristics of OWC devices and wave fields around them of multi-OWC devices equipped large floating structures. For this research, the linear potential theory based in-house programme code was applied to calculate hydrodynamic performance of OWC regions and elastic motion behaviours of the structures. Besides, calculation results were compared with some experimental results of characteristics of OWC devices on reference papers published. Then we proved validity of the calculation method. We have quantitatively summarized how much the reduction effect can be seen according to the aircushion placement and the number of aircushions on the floating body. the paper investigated arrangement of OWC devices on the floating structure with several variations. Using the prediction method, effects of arrangement of OWC devices on the performances are investigated.

Free vibration analysis of partially connected parallel beams with elastically restrained ends

2016 ◽  
Vol 230 (16) ◽  
pp. 2851-2864 ◽  
Author(s):  
Alborz Mirzabeigy ◽  
Reza Madoliat
Keyword(s):  
Free Vibration ◽  
Natural Frequencies ◽  
Flexural Rigidity ◽  
Free Vibration Analysis ◽  
Mode Shapes ◽  
Computational Stability ◽  
Transform Method ◽  
Practical Engineering ◽  
Elastically Restrained ◽  
Connection Length

In the present paper, the problem of transverse free vibration of two parallel beams partially connected to each other by a Winkler-type elastic layer is investigated. Euler–Bernoulli beam hypothesis has been applied, and translational and rotational elastic springs in each end considered as support. The motion of the system is described by coupled, piece-wise differential equations. The differential transform method (DTM) is employed to derive natural frequencies and mode shapes. DTM is a semi-analytical approach based on Taylor expansion series which does not require any admissible functions and yields rapid convergence and computational stability. After validation of the DTM results with results reported by well-known references and finite elements solution, the influences of the inner layer connection length, boundary conditions, the coefficient of elastic inner layer and ratio of beam’s flexural rigidity on natural frequencies as well as influences of the inner layer connection length on mode shapes are discussed. This problem is treated for the first time, and results are completely new which candidate them to being considered for practical engineering applications.

Nonlinear Vibration of Rotating Thin Disks

2000 ◽  
Vol 122 (4) ◽  
pp. 376-383 ◽  
Author(s):  
Albert C. J. Luo ◽  
C. D. Mote,
Keyword(s):  
Nonlinear Vibration ◽  
Natural Frequencies ◽  
Plate Theory ◽  
Nonlinear Effects ◽  
Linear Vibration ◽  
Rotating Disks ◽  
Nodal Diameter ◽  
Von Karman ◽  
Thin Disks ◽  
Von Kármán

The response and natural frequencies for the linear and nonlinear vibrations of rotating disks are given analytically through the new plate theory proposed by Luo in 1999. The results for the nonlinear vibration can reduce to the ones for the linear vibration when the nonlinear effects vanish and for the von Karman model when the nonlinear effects are modified. They are applicable to disks experiencing large-amplitude displacement or initial flatness and waviness. The natural frequencies for symmetric and asymmetric responses of a 3.5-inch diameter computer memory disk as an example are predicted through the linear theory, the von Karman theory and the new plate theory. The hardening of rotating disks occurs when nodal-diameter numbers are small and the softening of rotating disks occurs when nodal-diameter numbers become larger. The critical speeds of the softening disks decrease with increasing deflection amplitudes. [S0739-3717(00)02004-3]

Non-Linear Fluid-Structure Interaction in Cylindrical Shells

2002 ◽  
Author(s):  
M. H. Toorani ◽  
A. A. Lakis
Keyword(s):  
Natural Frequencies ◽  
Cylindrical Shells ◽  
Axial Flow ◽  
Present Theory ◽  
Nuclear Plant ◽  
Nuclear Industry ◽  
Linear Potential ◽  
Critical Flow Velocity ◽  
Fluid Structure ◽  
Non Linear

Nuclear plant reliability depends directly on its component performance. The higher heat transfer performance of nuclear plant components often requires higher flow velocities through the shell and tube heat exchangers. So, these cylindrical structures are subjected to either axial or cross flow, while the excessive flow-induced vibrations, (which are a major cause of machinery downtime; fatigue failure and high noise), limit the performance of these structures. On the other hand, these shell components often experience large amplitude vibrations that are greater than the shell thickness. Therefore, the evaluation of complex vibrational behavior of these structures is highly desirable in the nuclear industry. A semi-analytical approach has been developed in the present theory to predict the geometrical non-linearity influence on the natural frequencies of anisotropic cylindrical shells conveying axial flow. Particular important in this study is to obtain the natural frequencies of the coupled system of the fluid-structure, taking into account the geometrical non-linearity of the structure, and also estimating the critical flow velocity at which the structure loses its stability. The displacement functions, mass and stiffness matrices, linear and non-linear ones, of the structure are obtained by exact analytical integration over a hybrid element developed in this work. Linear potential flow theory is applied to describe the fluid effect that leads to the inertial, centrifugal and Coriolis forces. Numerical results are given and compared with those of experiment and other theories to demonstrate the practical application of the present method.

Restoration of the linear potential in the Sturm-Liouville problem

2017 ◽  
Vol 12 (2) ◽  
pp. 152-156 ◽  
Author(s):  
A.M. Akhtyamov ◽  
I.M. Utyashev
Keyword(s):  
Boundary Value Problems ◽  
Variable Coefficient ◽  
Natural Frequencies ◽  
Boundary Value ◽  
Liouville Problem ◽  
Frequency Equation ◽  
Linear Potential ◽  
Linearly Independent ◽  
Sturm Liouville ◽  
Coefficient Of Elasticity

The problem of identifying the variable coefficient of elasticity of a medium with respect to natural frequencies of a string oscillating in this medium is considered. A method for solving the problem is found, based on the representation of linearly independent solutions of the differential equation in the form of Taylor series with respect to two variables, substituting them into the frequency equation, and determining the unknown coefficients of the linear function from this frequency equation. An analytical method has also been developed that allows one to prove the uniqueness or nonuniqueness of the restored polynomial coefficient of elasticity of a medium by a finite number of natural frequencies of oscillations of a string, and also to find a class of isospectral problems, that is, boundary value problems for which the eigenvalue spectra coincide. The latter is based on the method of variation of an arbitrary constant. We consider examples of finding isospectral classes, and also unique boundary value problems having a given spectrum.

An Analytical Solution for the Nonlinear Vibration of Rotating, Thin Disks

1999 ◽  
Author(s):  
Albert C. J. Luo ◽  
C. D. Mote
Keyword(s):  
Nonlinear Vibration ◽  
Nonlinear Vibrations ◽  
Natural Frequencies ◽  
Plate Theory ◽  
Nonlinear Effects ◽  
Computer Memory ◽  
Linear Vibration ◽  
Rotating Disks ◽  
Nodal Diameter ◽  
Thin Disks

Abstract The response, natural frequencies for the linear and nonlinear vibrations of rotating disks are given analytically through the Luo and Mote’s plate theory of 1998. The results for the nonlinear vibration can reduce to the ones for the linear vibration when the nonlinear effects vanish, and they are applicable to disks experiencing large-amplitude displacement or initial flatness and waviness. The natural frequencies for symmetric and asymmetric responses of a 3.5-inch diameter computer memory disk as an example are predicted through the linear theory, the von Karman theory and the new plate theory. The hardening of rotating disks occurs when nodal-diameter numbers are small and the softening of rotating disks occurs when nodal-diameter numbers becomes larger. The critical speeds of softening disks decrease with increasing deflection amplitudes.

scholarly journals Analytical Study on an Oscillating Buoy Wave Energy Converter Integrated into a Fixed Box-Type Breakwater

2017 ◽  
Vol 2017 ◽  
pp. 1-9 ◽  
Author(s):  
Xuanlie Zhao ◽  
Dezhi Ning ◽  
Chongwei Zhang ◽  
Yingyi Liu ◽  
Haigui Kang
Keyword(s):  
Wave Energy ◽  
Wave Energy Converter ◽  
Hydrodynamic Performance ◽  
Maximum Efficiency ◽  
Width Ratio ◽  
Geometrical Parameters ◽  
Linear Potential ◽  
Energy Converter ◽  
Transmission Coefficients ◽  
Incident Waves

An oscillating buoy wave energy converter (WEC) integrated to an existing box-type breakwater is introduced in this study. The buoy is installed on the existing breakwater and designed to be much smaller than the breakwater in scale, aiming to reduce the construction cost of the WEC. The oscillating buoy works as a heave-type WEC in front of the breakwater towards the incident waves. A power take-off (PTO) system is installed on the topside of the breakwater to harvest the kinetic energy (in heave mode) of the floating buoy. The hydrodynamic performance of this system is studied analytically based on linear potential-flow theory. Effects of the geometrical parameters on the reflection and transmission coefficients and the capture width ratio (CWR) of the system are investigated. Results show that the maximum efficiency of the energy extraction can reach 80% or even higher. Compared with the isolated box-type breakwater, the reflection coefficient can be effectively decreased by using this oscillating buoy WEC, with unchanged transmission coefficient. Thus, the possibility of capturing the wave energy with the oscillating buoy WEC integrated into breakwaters is shown.

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