scholarly journals Sloshing: From Theory to Offshore Operations

2012 ◽  
Author(s):  
Günther F. Clauss ◽  
Florian Sprenger ◽  
Matthias Dudek ◽  
Daniel Testa
Keyword(s):  
Added Mass ◽  
Liquefied Natural Gas ◽  
Roll Motion ◽  
Free Fluid ◽  
Natural Modes ◽  
Filling Height ◽  
Resonance Frequencies ◽  
Longitudinal Bow ◽  
Water Filling ◽  
Loading Procedure

The current demand in liquefied natural gas (LNG) from remote marine locations drives the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred to shuttle carriers (LNGC). During the loading procedure, which takes about 18–24 hours for a standard sized LNGC, free fluid surfaces and varying filling levels occur inside the internal cargo tanks. This condition is critical since the seakeeping behavior of the LNGC — especially the roll motion — is strongly influenced and varying. In order to estimate and forecast the LNGC motions, numerical methods based on potential theory are the most efficient and appropriate method. The selected approach is validated by model tests at 30% water filling height inside four prismatic tanks. In-depth analyses, including force and moment measurements between tanks and hull, revealed a discrepancy between the analytical natural modes of a prismatic tank and the resonance frequencies for four prismatic tanks mounted to a LNGC hull. This effect is caused by the ratio of rigid to added mass of the system as well as the fact that the tanks are mounted to a standard hull shape featuring a longitudinal bow-stern asymmetry. In order to investigate this phenomenon systematically, surface elevations inside the tanks and natural modes for a symmetric cuboid hull are compared to results for a standard LNGC hull, both with the same main dimensions. The influence of the tank positions is also considered by comparing the original (longitudinally asymmetric) LNGC tank positions on the cuboid hull to an exactly symmetric arrangement.

Natural Frequency Shift in a Centrifugal Compressor Impeller for High-Density Gas Applications

2011 ◽  
Author(s):  
Yohei Magara ◽  
Kazuyuki Yamaguchi ◽  
Haruo Miura ◽  
Naohiko Takahashi ◽  
Mitsuhiro Narita
Keyword(s):  
Centrifugal Compressor ◽  
Natural Frequencies ◽  
Mass Density ◽  
Mass Effect ◽  
Added Mass ◽  
High Density ◽  
Experimental Results ◽  
Centrifugal Compressors ◽  
Resonance Frequencies ◽  
Discharge Gas

In designing an impeller for centrifugal compressors, it is important to predict the natural frequencies accurately in order to avoid resonance caused by pressure fluctuations due to rotorstator interaction. However, the natural frequencies of an impeller change under high-density fluid conditions. The natural frequencies of pump impellers are lower in water than in air because of the added mass effect of water, and in high-pressure compressors the mass density of the discharge gas can be about one-third that of water. So to predict the natural frequencies of centrifugal compressor impellers, the influence of the gas must be considered. We previously found in the non-rotating case that some natural frequencies of an impeller decreased under high-density gas conditions but others increased and that the increase of natural frequencies is caused by fluid-structure interaction, not only the added mass effect but also effect of the stiffness of the gas. In order to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications, this paper presents experimental results obtained using a variable-speed centrifugal compressor with vaned diffusers. The maximum mass density of its discharge gas is approximately 300 kg/m3. The vibration stress on an impeller when the compressor was speeding up or slowing down was measured by strain gages, and the natural frequencies were determined by resonance frequencies. The results indicate that for high-density centrifugal compressors, some natural frequencies of an impeller increased in high-density gas. To predict this behavior, we developed a calculation method based on the theoretical analysis of a rotating disc. Its predictions are in good agreement with experimental results.

scholarly journals Dynamic response of a wedge through asymmetric free fall in 2 degrees of freedom

2017 ◽  
Vol 233 (1) ◽  
pp. 229-250 ◽  
Author(s):  
Parviz Ghadimi ◽  
Sasan Tavakoli ◽  
Abbas Dashtimanesh ◽  
Pouria Taghikhani
Keyword(s):  
Experimental Data ◽  
Dynamic Response ◽  
Degrees Of Freedom ◽  
Time Derivative ◽  
Free Fall ◽  
Added Mass ◽  
Roll Motion ◽  
Physical Parameters ◽  
Two Dimensional ◽  
Roll Speed

In this article, a mathematical model is presented for simulation of the coupled roll and heave motions of the asymmetric impact of a two-dimensional wedge body. This model is developed based on the added mass theory and momentum variation. To this end, new formulations are introduced which are related to the added mass caused by heave and roll motions of the wedge. These relations are developed by including the asymmetrical effects and roll speed. In addition, by considering the roll speed, a particular method is presented for the time derivative of half-wetted beam of an asymmetric wedge. Furthermore, two equations are derived for the roll and heave motions in which damping terms appear. Validity of the proposed method is verified by comparing the predicted results against available experimental data in two conditions of roll motion and no roll motion. Favorable agreement is observed between the predicted results and experimental data. The pressure and hydrodynamic load are computed, and the differences between the results associated with the considered conditions are explored. Subsequently, the effects of different physical parameters including deadrise angle, initial roll angle, and initial velocity on the dynamic response of a two-dimensional wedge section are investigated. Ultimately, time histories of hydrodynamic coefficients are determined in order to provide a better understanding of the derived equations.

Natural Frequency Shift in a Centrifugal Compressor Impeller for High-Density Gas Applications

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Yohei Magara ◽  
Kazuyuki Yamaguchi ◽  
Haruo Miura ◽  
Naohiko Takahashi ◽  
Mitsuhiro Narita
Keyword(s):  
Centrifugal Compressor ◽  
Natural Frequencies ◽  
Mass Density ◽  
Mass Effect ◽  
Added Mass ◽  
High Density ◽  
Experimental Results ◽  
Centrifugal Compressors ◽  
Resonance Frequencies ◽  
Discharge Gas

In designing an impeller for centrifugal compressors, it is important to predict the natural frequencies accurately in order to avoid resonance caused by pressure fluctuations due to rotor-stator interaction. However, the natural frequencies of an impeller change under high-density fluid conditions. The natural frequencies of pump impellers are lower in water than in air because of the added mass effect of water, and in high-pressure compressors the mass density of the discharge gas can be about one-third that of water. So to predict the natural frequencies of centrifugal compressor impellers, the influence of the gas must be considered. We previously found in the nonrotating case that some natural frequencies of an impeller decreased under high-density gas conditions but others increased and that the increase of natural frequencies is caused by fluid-structure interaction, not only the added mass effect but also effect of the stiffness of the gas. In order to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications, this paper presents experimental results obtained using a variable-speed centrifugal compressor with vaned diffusers. The maximum mass density of its discharge gas is approximately 300 kg/m3. The vibration stress on an impeller when the compressor was speeding up or slowing down was measured by strain gauges, and the natural frequencies were determined by resonance frequencies. The results indicate that for high-density centrifugal compressors, some natural frequencies of an impeller increased in high-density gas. To predict this behavior, we developed a calculation method based on the theoretical analysis of a rotating disk. Its predictions are in good agreement with experimental results.

scholarly journals Analysis of the Scattering from a Two Stacked Thin Resistive Disks Resonator by Means of the Helmholtz–Galerkin Regularizing Technique

2021 ◽  
Vol 11 (17) ◽  
pp. 8173
Author(s):  
Mario Lucido
Keyword(s):  
Cross Section ◽  
Near Field ◽  
Surface Current ◽  
Coefficient Matrix ◽  
Hankel Transform ◽  
General Purpose ◽  
Analytical Technique ◽  
Planar Surfaces ◽  
Natural Modes ◽  
Resonance Frequencies

In this paper, the scattering of a plane wave from a lossy Fabry–Perót resonator, realized with two equiaxial thin resistive disks with the same radius, is analyzed by means of the generalization of the Helmholtz–Galerkin regularizing technique recently developed by the author. The disks are modelled as 2-D planar surfaces described in terms of generalized boundary conditions. Taking advantage of the revolution symmetry, the problem is equivalently formulated as a set of independent systems of 1-D equations in the vector Hankel transform domain for the cylindrical harmonics of the effective surface current densities. The Helmholtz decomposition of the unknowns, combined with a suitable choice of the expansion functions in a Galerkin scheme, lead to a fast-converging Fredholm second-kind matrix operator equation. Moreover, an analytical technique specifically devised to efficiently evaluate the integrals of the coefficient matrix is adopted. As shown in the numerical results section, near-field and far-field parameters are accurately and efficiently reconstructed even at the resonance frequencies of the natural modes, which are searched for the peaks of the total scattering cross-section and the absorption cross-section. Moreover, the proposed method drastically outperforms the general-purpose commercial software CST Microwave Studio in terms of both CPU time and memory occupation.

Selection of Vibro-Characteristics for Monitoring Flange Integrity in the Field Conditions

2016 ◽  
Author(s):  
Vladimir Palmov ◽  
Len Malinin
Keyword(s):  
Transfer Functions ◽  
Reliable Data ◽  
Harsh Environment ◽  
Elastic Parameters ◽  
Natural Modes ◽  
Resonance Frequencies ◽  
Longitudinal And Shear Waves ◽  
Processing Procedure ◽  
Speed Of Propagation ◽  
Selection Of

Monitoring of flange integrity in the field traditionally has been based on evaluating bolt tension and deriving pressure on the gasket from these data. Multiple techniques of evaluating bolt tension based on measuring speed of propagation of a longitudinal wave (time of flight), or ratio of speeds of longitudinal and shear waves (“L+S method”), demonstrate 5–10% accuracy in the controlled laboratory conditions [1]. However, accuracy in the field, on the flanges exposed to harsh environment, is often worse than 20%, which makes it difficult to evaluate flange integrity and predict a leak. The need for knowing acousto-elastic parameters of steel (in case of L+S method) may also present a hurdle. Tools based on measuring a shift of the resonance frequencies (RF) are commercially available, though RF are relatively robust to tension, and require a reliable data processing procedure to discern the proper peaks. Evaluation of tension based on natural modes is more sensitive, but relatively complex and costly. Use of transfer functions (TF) offers several advantages over both RF and natural modes, as TFs depend on both RF frequencies and natural modes, and a properly selected parameter of TF can offer greater sensitivity.

A Simple Method for Calculating the Bending Modal Frequencies of an Underwater Simply Supported Beam with Hydrodynamic Study

2012 ◽  
Vol 625 ◽  
pp. 7-11
Author(s):  
Rui Tang ◽  
De Jiang Shang ◽  
Qi Li
Keyword(s):  
Approximate Formula ◽  
Added Mass ◽  
Water Medium ◽  
Simple Method ◽  
Simply Supported Beam ◽  
Modal Frequency ◽  
Simply Supported ◽  
Resonance Frequencies ◽  
Added Water ◽  
Hydrodynamic Study

A theoretical model of an added water mass representation for calculating the bending modal frequency of a simply supported beam in water medium is presented. To accomplish this, the explicit expressions for computing the hydrodynamic interaction pressure and forced transversal displacements are first derived, and the bending modal frequencies can be accurately obtained by searching the corresponding resonance frequencies. The validity of the analytical process is checked by the finite element method. As the complicated interaction between the structures and water can be approximated to added mass, if the beam is sufficiently slender. Using the added mass curves, a simple method for evaluating the bending modal frequency of an underwater simply supported beam is proposed, and an approximate formula is found subsequently. The comparisons between the analytical solution and the simple method show that the simple method is adequately accurate, with errors less than 1%, and both of the added mass curves and the approximate formula have nothing to do with different material attributes.

Experimental Analysis of Vibration of Micromachined Resonators

2008 ◽  
Author(s):  
Pezhman A. Hassanpour ◽  
Ebrahim Esmailzadeh ◽  
William L. Cleghorn ◽  
James K. Mills
Keyword(s):  
Analytical Model ◽  
Close Agreement ◽  
Large Range ◽  
Added Mass ◽  
Model Parameters ◽  
Mass System ◽  
Resonance Frequencies ◽  
Comb Drives ◽  
Theory And Experiment ◽  
Structural Layer

Among many different mechanisms that are used for excitation and detection of vibration of micro-beam resonators, electrostatic comb-drives have the benefit of simplicity and large range of linear operation. The disadvantage of using comb-drives is the effect of added mass to the beam; however, the analytical model of the beam-mass system predicts that this shortcoming can be overcome by proper adjustment of the mass, rotary inertia, and location of the comb-drive. In addition, the analytical model can predict the effect of the axial force of the beam on the resonance frequencies. In this paper, the results of the experiments on two resonators are presented. These results are used to verify the validity of the analytical model and finding its parameters. Very close agreement between the theory and experiment is observed. The residual stress of the MEMS structural layer is measured using the calibrated analytical model parameters.

Effects of Vertical Motions on Roll of Planing Hulls

2021 ◽  
Vol 143 (4) ◽  
Author(s):  
Sasan Tavakoli ◽  
Abbas Dashtimanesh ◽  
Simone Mancini ◽  
Javad A. Mehr ◽  
Stefano Milanesi
Keyword(s):  
Dynamic Response ◽  
Vertical Plane ◽  
Hydrodynamic Pressure ◽  
Added Mass ◽  
Quantitative Agreement ◽  
Roll Motion ◽  
Published Data ◽  
High Speeds ◽  
Planing Vessel ◽  
The Stability

Abstract Roll motion of a planing hull can be easily triggered at high speeds, causing a significant change in hydrodynamic pressure pattern, which can threaten the stability of the vessel. Modeling and investigating roll motion of a planing vessel may require a strong coupling between motions in vertical and transverse planes. In the present paper, we have used a mathematical model to analyze the roll of a planing hull by coupling surge, heave, pitch, and roll motions using 2D + T theory to study the effects of roll-induced vertical motions on roll coefficients and response. Mathematically computed forces and moments as well as roll dynamic response of the vessel are seen to be in fair quantitative agreement with experimentally measured values of previously published data. Using the 2D + T method, it has been shown that to model the roll of a planing hull at high speeds, we need to consider the effects of heave, pitch, and surge motions. Through our mathematical modeling, it is found that freedom in vertical motions increases time-dependent roll damping and added mass coefficients, especially at early planing speeds. The results of dynamic response simulations suggest that freedom in the vertical plane can decrease the roll response.

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