Added Mass Method Research Articles

Feasibility study of new hybrid piled concrete foundation for offshore wind turbine

Offshore wind turbine energy has recently attracted significant attention and many researchers have attempted to develop cost-efficient offshore wind turbine support structures to reduce capital costs. This paper proposes a new hybrid support structure using driven piles that eliminates the disadvantages of conventional support structures for offshore wind turbines. The hybrid support structure proposed here is a piled concrete foundation (PCF) structure, which combines a concrete base with a steel shaft and is supported not only by gravity type foundations but also by driven piles. The soil was modeled as an elastic foundation, and the added mass was calculated using the Added mass method to determine the effects of water surrounding the support structure. To evaluate the feasibility of the PCF, quasi-static analysis, natural frequency analysis and seismic analysis were performed using environmental conditions at the Southwest Coast of Korea. In quasi-static analysis, the lateral displacement did not exceed the allowable displacement, and natural frequency analysis confirmed the superiority of the dynamic behavior of the PCF. Seismic evaluation through response spectrum analysis and time-history analysis demonstrated that the PCF is safe. The proposed PCF was confirmed to have sufficient structural stability for applications in real-site conditions in Korea. Optimization of the structure using the preliminary design resulted in an efficient structure that reasonably reduces fabrication costs.

Seismic fragility analyses of sea-crossing cable-stayed bridges subjected to multi-support ground motions on offshore sites

As key components in the transportation networks at coastal areas, sea-crossing cable-stayed bridges play a very important role in the development of regional economy. These bridges may be subjected to severe earthquakes during their life-cycles. Owing to the lack of actual seafloor earthquake recordings and approaches in synthesizing offshore seismic motions, the onshore seismic motions are commonly utilized as inputs in the seismic design of sea-crossing cable-stayed bridges. However, this approach may lead to erroneous structural response predictions since the characteristics of onshore and offshore seismic motions are different. In this paper, the seismic performance of a sea-crossing cable-stayed bridge is comprehensively evaluated based on the fragility function methodology. A novel approach is presented to theoretically calculate the ground motion transfer function at any location within an offshore site and stochastically synthesize the offshore multi-support ground motions at different depths (MGMDDs). The OpenSees analysis platform is employed to develop the three-dimensional finite element model of the example bridge, in which the p-y, t-z and q-z elements are installed at the pile nodes to simulate the interaction between the bridge piles and surrounding soils. Moreover, the effect of seawater on the bridge seismic responses is modeled using the hydrodynamic added mass method. The seismic fragility curves of the example bridge are generated by using the synthesized MGMDDs as inputs. The influences of spatial and depth varying offshore seismic motions, soil-structure interaction (SSI) and seawater added mass on the bridge component and system fragilities are investigated and discussed. Numerical results show that the seismic fragility of the example sea-crossing cable-stayed bridge is affected by the above mentioned influencing factors with different extents. The proposed approach can rationally and effectively assess the seismic fragilities of sea-crossing cable-stayed bridges.

Study on added mass of a circular curved membrane vibrating in still air

It is widely known that added mass has a significant influence on the natural frequency of membrane structures. Previously, some experimental studies on the added mass of flat membranes have been performed. However, the added mass of curved membrane vibrating in still air is unclear. In this study, first, vibration tests of a circular curved membrane in still air with various air pressures are conducted, and the natural frequencies of the circular curved membrane are identified with the Hilbert–Huang transform (HHT). An approximate estimation of the added mass coefficient of the circular curved membrane is given as 0.65 by fitting results of tests. Then, the efficiencies of two added mass models, one as simplified added mass method and the other as numerically analysis method established by using the Boundary Element Method (BEM), are verified by the circular curved membrane tests. At last, the Modal Assurance Criterion (MAC) values indicate that for uniform mass distribution of the circular curved membrane, there is little difference between the mode shapes of the membrane vibrating in vacuum and in air.

Investigation of the added mass method for seismic design of lock gates

During an earthquake, lock gates are subjected to additional pressure since the water contained in the chamber is put into motion by the earthquake. It is difficult to assess the level of this pressure because the system is affected by a fluid–structure interaction. The gate deformations have an effect on the water pressure, which in turn affects the gate vibrations. A common approach, referred to as the added mass method, consists of simulating the fluid action by distributing lumped masses over the gate. However, this method has been questioned, since the calculation of the lumped masses is usually based on the Westergaard formula, which was derived assuming a perfectly rigid structure. Consequently, fluid–structure interactions may not be captured correctly. This paper proposes to investigate the validity of this approach for such problems and to explain why it might not be conservative. The numerical solutions of an added mass model and a fluid–structure interaction model are confronted. The results indicate that the added mass method may eventually lead to conservative results depending on the type of damping used in the model. Based on these observations, some recommendations are suggested to improve the design of lock gates subjected to earthquakes.

Nonlinear analysis of the crashworthy component of an eagle wave energy converter in rotating-collision

The Eagle wave energy converter (WEC) is novel device and has high power generation efficiency demonstrated by the small-scale model in the tank tests. During the sea trials of 10kW full-scale prototype performed from 2012 to 2014, a design problem occurred that the crashworthy component (namely End Beam) was crashed up by the rotor. In order to investigate the damage mechanism of End Beam and improve the design of the Eagle WEC, an impact analysis was conducted utilizing ANSYS/LS-DYNA. In the simulation, structures were modeled as full-scale parameters and assumed to be elastic–plastic. Meanwhile, the bottoms of End Beam were presumed to be standstill. Moreover, the influences of water surrounding the WEC structures were considered using added mass method. Through the simulation, the stress and strain distributions, collision force and the energy absorption of the system were obtained. The results fit well with the actual damage situation. Furthermore, three improvement schemes were proposed including enhancing the structure of End Beam, increasing the upward angle of the rotor and adding energy accumulator. The simulation results demonstrated that the third method can effectively absorb the kinetic energy of the rotor and prevent the WEC from destruction in the rough sea states.

A comparative analysis of the fluid-structure interaction method and the constant added mass method for ice-structure collisions

The constant added mass (CAM) method and the fluid-structure interaction (FSI) method are widely used to simulate ship-ship and ship-ice collisions. In the CAM method, the hydrodynamic effect of the surrounding water is treated as a constant added mass, whereas in the FSI method the surrounding fluid flow is explicitly modelled. The objective of the paper is to compare the two methods and to explain the causes of the differences in the results. We considered collision between a freshwater ice mass and a floating steel structure. For both methods, the numerical simulations were performed with the LS-DYNA software. The behaviour of the ice mass was modelled using an elliptic yield criterion and a strain-based pressure-dependent failure criterion. To ensure realistic ice behaviour, the ice model was calibrated using general trends found in laboratory and in-situ indentation tests with focus on the laboratory-grown ice and the fluid model in the LS-DYNA was verified by comparing the added mass coefficients for a spherical body and a rectangular block with the corresponding WADAM results. To validate and benchmark the numerical simulations, experimental data on ice-structure interactions in water were used, including the acceleration of the floater wall measured with the dynamic motion unit (DMU), the relative velocity between the ice mass and the floater before the impact and some images extracted from video recording of the test. The comparisons indicated that the FSI method yields better results for the motion of the floater, i.e., the acceleration of the floater wall caused by the ice mass’s impact and the relative velocity were in reasonably good agreement with experimental measurements. It was also found that the CAM method was faster but predicted a higher peak contact force and more dissipated energy in the ice mass than in the FSI method.

Seismic Analysis for Multi-pile Concrete Foundationin 5MW Class Offshore Wind Turbine

Recently, Wind-turbine electronic generator become popular. Wind-Turbine is free to cost for purchase and noise problem. For this reason, trend is shifting from Wind-turbine on land to offshore. Research and Development for offshore Wind-turbine has been conducted by various research institution. However, There is no solid design code for offshore Wind-turbine even in domestic as well as foreign. In this paper, conduct seismic analysis and compare results using design codes Korea Bridge Design Codes, Korea Harbor and Marina Design Codes, and DNV OS. Time-History analysis conducted for checking time dependent effect. The Added-Mass Method applied to consider water-structure effects and compared for w/ water and w/o water condition. 최근 신재생 에너지 중 하나인 풍력발전에 대한 관심이 증가하고 있다. 풍력발전은 토지구입비, 소음문제에 자유로운 해상풍력으로 추세가 옮겨가고 있으며 이를 위한 연구개발이 전 세계적으로 활발히 이루어지고 있다. 그러나 해상에 위치한 풍력발전을 위한 설계기준은 국내, 국외 모두 없는 실정이다. 이 점을 고려하여 국내, 국외의 구조설계기준인 도로교 설계기준, 항만 및 어항 설계기준, DNV OS를 참고하여 다중 파일기초 콘크리트 지지구조물(MCF)의 내진해석을 수행하여 결과를 비교하였다. 또한 시간에 의한 효과를 확인하기 위하여 시간이력 해석 또한 수행되었다. 부가질량법(Added-mass method)을 사용하여 물과 구조의 상호작용을 고려하였고 물의 유무에 따라 구조물의 반응을 비교하였다.

Parallel Dynamic Analysis of a Large-Scale Water Conveyance Tunnel under Seismic Excitation Using ALE Finite-Element Method

Parallel analyses about the dynamic responses of a large-scale water conveyance tunnel under seismic excitation are presented in this paper. A full three-dimensional numerical model considering the water-tunnel-soil coupling is established and adopted to investigate the tunnel’s dynamic responses. The movement and sloshing of the internal water are simulated using the multi-material Arbitrary Lagrangian Eulerian (ALE) method. Nonlinear fluid–structure interaction (FSI) between tunnel and inner water is treated by using the penalty method. Nonlinear soil-structure interaction (SSI) between soil and tunnel is dealt with by using the surface to surface contact algorithm. To overcome computing power limitations and to deal with such a large-scale calculation, a parallel algorithm based on the modified recursive coordinate bisection (MRCB) considering the balance of SSI and FSI loads is proposed and used. The whole simulation is accomplished on Dawning 5000 A using the proposed MRCB based parallel algorithm optimized to run on supercomputers. The simulation model and the proposed approaches are validated by comparison with the added mass method. Dynamic responses of the tunnel are analyzed and the parallelism is discussed. Besides, factors affecting the dynamic responses are investigated. Better speedup and parallel efficiency show the scalability of the parallel method and the analysis results can be used to aid in the design of water conveyance tunnels.

SEISMIC BEHAVIOR OF ANCHORAGE IN DIVERSE LIQUID STORAGE STEEL TANKS BY ADDED-MASS METHOD

Deformation of liquid storage tanks and the interaction between fluid and structure result in a variety of possible failure mechanisms during earthquakes. Among all failure modes, base-anchor failure is this paper’s focus. Three cylindrical steel tanks with different H/D were selected to investigate dynamic loadings on the tank seismic responses. The added-mass method was used in the finite element modeling of the steel tanks and fluid, and numerical analyses were performed. The added-mass method results were compared to conventional method outcomes using two or more lumped-mass and equivalent springs for tank-liquid simulation (Housner method). It was found that the added-mass method results in greater forces on the anchors in comparison to the lumped-mass method.

Hydraulic calculation and dynamic analysis of columnar reversing gate

According to the hydraulic calculation principles of the orifice outflow, the discharge capacity of the columnar reversing gate under the partial opening condition was calculated and checked. Using ANSYS, a large finite element analysis software, the discharge process was simulated. The distribution rule of the velocities in the gate chamber and downstream channel was obtained. An FEM model of the columnar reversing gate was built, and the natural vibration properties of the gate were analyzed. Based on the Westergaard added mass method, the added mass caused by the fluid-structure coupling motion was taken into account, and the effects of the coupling interaction were discussed. The results show that the size of the small gates meets the demand for discharge capacity, the current in the gate chamber is quite turbulent, the trunnion and arms are obviously impacted by flow, and the effects of water on vibration characteristics are remarkable. The study provides a reference for the design and calculation of gates of the same type.

Mode Analysis and Study of Sunjiagou Aqueduct

This paper mainly adopts the finite element analysis software ANSYS to calculate and analyze the dynamic characteristic of aqueduct structure,using added mass method to deal with interaction between water and aqueduct wall, and get the natural vibration frequency and main vibration mode of U-shaped aqueduct in different water depth. The calculation result shows that water has a small effect on the vibration mode of aqueduct structure, but has a greater influence on the natural vibration frequency of aqueduct structure; natural vibration frequency of aqueduct structure decreases with the depth deepening. The research results provide the basis for the seismic design and protection of Sunjiagou aqueduct.

A new added mass method for fluid-structure interaction analysis of deep-water bridge

In order to calculate hydrodynamic pressure of piers with arbitrary cross-sections under earthquake, a new added mass method is presented. To accomplish this, the relation between fundamental frequency reduction rate and the ratio of added mass to structural mass per unit length is deduced. The relation is validated by using added mass from Morison equation. The fundamental frequency reduction rates of arbitrary section piers are available by utilizing the fluid element method in ANSYS, added masses of any piers are achieved according to the relation. Based on added mass data, the expressions of which can be proposed by curve fitting. The added mass expressions for two kinds of piers, circular and square, are suggested and compared with other two methods. Results show that the new method is accurate in both dynamic property calculation and harmonic response calculation. Practical application reveals that the new method is an accurate, efficient and adaptable way to evaluate hydrodynamic pressure under earthquake.

Dynamic buckling and seismic fragility of anchored steel tanks by the added mass method

SUMMARYBuckling plays a fundamental role in the design of steel tanks because of the small thicknesses of the walls of this class of structures. The first part of the paper presents a review of this phenomenon for liquid‐containing circular cylindrical steel tanks that are fully anchored at the base, considering the different buckling modes and especially the secondary buckling occurring in the top part of the tank.A case study based on a cylindrical tank is then introduced in order to investigate various aspects of dynamic buckling. The finite element model of the case study tank is set‐up using the added mass method for fluid modelling. The influence of pre‐stress states caused by hydrostatic pressure and self‐weight on the natural periods of the structure is first studied and it is found that this influence is very small as far as the global behaviour of the tanks is considered, while it is important for local, shell‐type, vibration modes.In the following, the efficiency and sufficiency of different ground motion intensity measures is analysed by means of cloud analysis with a set of 40 recorded accelerograms. In particular, the peak ground displacement has been found being the most efficient and sufficient intensity measure so far as the maximum relative displacement of the tank walls is concerned.Finally, incremental nonlinear time‐history analyses are performed considering the case study structure under recorded earthquake ground motions in order to identify the critical buckling loads and to derive fragility curves for the buckling limit state. Copyright © 2013 John Wiley & Sons, Ltd.

Estimation of Tension Force in Short Hangers Using Added Mass Method

For short hangers, the influence of flexural rigidities and boundary conditions on tension measuring cannot be neglected. But there has no effective method to precisely calculate hanger tension considering the two parameters. This paper proposes the Added Mass Method (AMM) to simultaneously identify the tension, flexural rigidity and boundary condition of short hangers. In this method, the effect of mass distribution of structure on natural frequencies has been studied to add extra calculation conditions for establishing the eigenvalue equation set. The method of Genetic Algorithm (GA) with adaptive crossover and mutation rate is used to solve the equation set quickly. The results provides that the AMM is a high-accurate field testing technology in tension measurement for short hanger of arch bridge.

Atomic force microscope cantilever calibration using a focused ion beam

A calibration method is presented for determining the spring constant of atomic force microscope (AFM) cantilevers, which is a modification of the established Cleveland added mass technique. A focused ion beam (FIB) is used to remove a well-defined volume from a cantilever with known density, substantially reducing the uncertainty usually present in the added mass method. The technique can be applied to any type of AFM cantilever; but for the lowest uncertainty it is best applied to silicon cantilevers with spring constants above 0.7 N m−1, where uncertainty is demonstrated to be typically between 7 and 10%. Despite the removal of mass from the cantilever, the calibration method presented does not impair the probes’ ability to acquire data. The technique has been extensively tested in order to verify the underlying assumptions in the method. This method was compared to a number of other calibration methods and practical improvements to some of these techniques were developed, as well as important insights into the behavior of FIB modified cantilevers. These results will prove useful to research groups concerned with the application of microcantilevers to nanoscience, in particular for cases where maintaining pristine AFM tip condition is critical.

Fluid-structure coupled analysis of underwater cylindrical shells

Underwater cylindrical shell structures have been found a wide of application in many engineering fields, such as the element of marine, oil platforms, etc. The coupled vibration analysis is a hot issue for these underwater structures. The vibration characteristics of underwater structures are influenced not only by hydrodynamic pressure but also by hydrostatic pressure corresponding to different water depths. In this study, an acoustic finite element method was used to evaluate the underwater structures. Taken the hydrostatic pressure into account in terms of initial stress stiffness, an acoustical fluid-structure coupled analysis of underwater cylindrical shells has been made to study the effect of hydrodynamic pressures on natural frequency and sound radiation. By comparing with the frequencies obtained by the acoustic finite element method and by the added mass method based on the Bessel function, the validity of present analysis was checked. Finally, test samples of the sound radiation of stiffened cylindrical shells were acquired by a harmonic acoustic analysis. The results showed that hydrostatic pressure plays an important role in determining a large submerged body motion, and the characteristics of sound radiation change with water depth. Furthermore, the analysis methods and the results are of significant reference value for studies of other complicated submarine structures.

Optimal design of resonant piezoelectric buzzer from a perspective of vibration-absorber theory

In this paper, an optimization technique is presented for the design of piezoelectric buzzers. This design technique aims at finding the optimal configuration of the coupled cavity and diaphragm structure to maximize the sound pressure output. Instead of measuring the material constants of the piezoelectric ceramic and the metal diaphragm, an "added-mass method" is developed to estimate the equivalent electromechanical parameters of the system on which an analogous circuit can be established. The electrical impedance and on-axis sound pressure level of the piezoelectric buzzer can be simulated by solving the loop equations of the electromechanoacoustical analogous circuit. An interesting finding of this research is that the nature of the piezoelectric buzzer bears remarkable resemblance to that in the dynamic vibration absorber theory. Much physical insight can be gained by exploiting this resemblance in search of the optimal configuration. According to the system characteristic equation, a design chart was devised to "lock" the critical frequency at which the system delivers the maximal output. On the basis of the analogous circuit and the vibration absorber theory, an optimal design was found with constrained optimization formalism. Experiments were conducted to justify the optimal design. The results showed that the performance was significantly improved using the optimal design over the original design. Design guidelines for the piezoelectric buzzers are summarized.

Nondestructive and noncontact method for determining the spring constant of rectangular cantilevers

We present here an experimental setup and suggest an extension to the long existing added-mass method for the calibration of the spring constant of atomic force microscope cantilevers. Instead of measuring the resonance frequency shift that results from attaching particles of known masses to the end of cantilevers, we load them with water microdrops generated by a commercial inkjet dispenser. Such a device is capable of generating drops, and thus masses, of extremely reproducible size. This makes it an ideal tool for calibration tasks. Moreover, the major advantage of water microdrops is that they allow for a nearly contactless calibration: no mechanical micromanipulation of particles on cantilevers is required, neither for their deposition nor for removal. After some seconds the water drop is completely evaporated, and no residues are left on the cantilever surface or tip. We present two variants: we vary the size of the drops and deposit them at the free end of the cantilever, or we keep the size of the drops constant and vary their position along the cantilever. For the second variant, we implemented also numerical simulations. Spring constants measured by this method are comparable to results obtained by the thermal noise method, as we demonstrate for six different cantilevers.

A unified approach for the free vibration analysis of an elastically supported immersed uniform beam carrying an eccentric tip mass with rotary inertia

Although an immersed beam with elastic support and fixed support are usually tackled, separately, to simplify the procedures of analysis, a unified approach will be more convenient for the computer programming in the computer method. The purpose of this paper is to use a unified approach to determine the “exact” lowest several natural frequencies and the associated mode shapes of the (partially or fully) immersed beam in both the elastic- and fixed-support conditions. Furthermore, by modeling the distributed added mass along the immersed part of the beam with a number of concentrated added masses, a point added mass method (PAM) incorporated with the mode-superposition approach is also presented to determine the “approximate” lowest several natural frequencies and the associated mode shapes of the last two types of immersed beam. It is unlike most of the vibrating systems with their boundary conditions independent on the eigenvalues that the boundary conditions of the current immersed beam are frequency dependent due to the existence of frequency-dependent boundary inertial forces and moments. Since the last frequency-dependent boundary conditions significantly affect the orthogonal condition of the mode shapes and so do the applicability of the mode-superposition approach for the vibration analysis, the theory regarding the orthogonal condition for the mode shapes of the current vibrating system will be one of the key points in this paper. The numerical results have been compared with the existing information or the results of finite element method and a good agreement is achieved. Furthermore, to check the last theoretical results, several model tests are also carried out on the scale models of the fixed and elastically supported towers and reasonable agreement is obtained.

Euler–Lagrange coupling with damping effects: Application to slamming problems

During a high velocity impact of a structure on a nearly incompressible fluid, impulse loads with high-pressure peaks occur. This physical phenomenon called ‘slamming’ is a concern in shipbuilding industry because of the possibility of hull damage. Shipbuilding companies have carried out several studies on slamming modeling using FEM software with added mass techniques to represent fluid effects. In the added mass method inertia effects of the fluid are not taken into account and are only valid when the deadrise angle is small. This paper presents the prediction of the local high pressure load on a rigid wedge impacting a free surface, where the fluid is represented by solving Navier–Stokes equations with an Eulerian or ALE formulation. The fluid–structure interaction is simulated using a coupling algorithm; the fluid is treated on a fixed or moving mesh using an ALE formulation and the structure on a deformable mesh using a Lagrangian formulation. A new coupling algorithm is developed in the paper. The coupling algorithm computes the coupling forces at the fluid–structure interface. These forces are added to the fluid and structure nodal forces, where fluid and structure are solved using an explicit finite element formulation. Predicting the local pressure peak on the structure requires an accurate fluid–structure interaction algorithm. The Euler–Lagrange coupling algorithm presented in this paper uses a penalty based formulation similar to penalty contact in Lagrangian analyses. Both penalty coupling and penalty contact can generate high frequency oscillations due to the nearly incompressible nature of the fluid. In this paper, a damping force based on the relative velocity of the fluid and the structure is introduced to smooth out non-physical high frequency oscillations induced by the penalty springs in the coupling algorithm.