Showing papers on "Added mass published in 2022"

Hydrodynamic Modeling and Parameter Identification of a Bionic Underwater Vehicle: RobDact

Abstract: In this paper, the hydrodynamic modeling and parameter identification of the RobDact, a bionic underwater vehicle inspired by Dactylopteridae, are carried out based on computational fluid dynamics (CFD) and force measurement experiment. Firstly, the paper briefly describes the RobDact, then establishes the kinematics model and rigid body dynamics model of the RobDact according to the hydrodynamic force and moment equations. Through CFD simulations, the hydrodynamic force of the RobDact at different speeds is obtained, and then, the hydrodynamic model parameters are identified. Furthermore, the measurement platform is developed to obtain the relationship between the thrust generated by the RobDact and the input fluctuation parameters. Finally, by combining the rigid body dynamics model and the fin thrust mapping model, the hydrodynamic model of the RobDact at different motion states is constructed.

Viscous fluid–structure interaction of micro-resonators in the beam–plate transition

Abstract: We numerically investigate the fluid–structure interaction of thin elastic cantilever micro-structures in viscous fluids. The Kirchhoff plate equation describes the dynamics of the structure, and a boundary integral formulation represents the fluid flow. We show how the displacement spectrum of the structures changes as the geometry is altered from a narrow beam to a wide plate in a liquid. For narrow beams, the displacement spectrum exhibits only a few resonance frequencies, which correspond to the vibrational modes described by the Euler–Bernoulli equation (Euler–Bernoulli modes). The spectrum of wide plates exhibits several additional resonance frequencies associated with the plate’s torsional and higher-order vibrational modes. Wide plates in Euler–Bernoulli modes exhibit higher damping coefficients, but due to an increased added-mass effect, also higher Q-factors than slender beams. An investigation into the fluid flow reveals that for the Euler–Bernoulli modes of wider plates, the fluid flow and energy dissipation near the plate’s edges increase, resulting in increased damping coefficients. Concomitantly, a region of minimal viscous dissipation near the plate’s center appears for wider plates, resulting in an increased added-mass effect. Higher-order modes of wider plates exhibit lower Q-factor than the Euler–Bernoulli modes due to a decreased fluid flow at the plate’s edges caused by the appearance of circulation zones on both sides of the plate. This decreased flow at the edge reduces the damping and the added-mass effect, yielding lower Q-factors. We anticipate that the results presented here will play a vital role in conceiving novel MEMS resonators for operation in viscous fluids.

Optimization of the Wake Oscillator for Transversal VIV

Abstract: Vibrations of slender structures associated with the external flow present a design challenge for the energy production systems placed in the marine environment. The current study explores the accuracy of the semi-empirical wake oscillator models for vortex-induced vibrations (VIV) based on the optimization of (a) the damping term and (b) empirical coefficients in the fluid equation. This work investigates the effect of ten fluid damping variations, from the classic van der Pol to more sophisticated fifth-order terms, and prediction of the simplified case of the VIV of transversally oscillating rigid structures provides an opportunity for an extended, comprehensive comparison of the performance of tuned models. A constrained nonlinear minimization algorithm in MATLAB is applied to calibrate considered models using the published experimental data, and the weighted objective function is formulated for three different mass ratios. Comparison with several sources of published experimental data for cross-flow oscillations confirms the model accuracy in the mass ratio range. The study indicates the advantageous performance of the models tuned with the medium mass ratio data and highlights some advantages of the Krenk–Nielsen wake oscillator.

A tuned liquid mass damper implemented in a deep liquid storage tank for seismic vibration control of short period structures

Abstract: Deep liquid storage tanks cannot be implemented as tuned liquid damper (TLD) as the depth ratios often exceed the limit (0.3) for nonlinear sloshing and wave breaking. This study exploits the impulsive liquid mass to implement a tuned liquid mass damper (TLMD) by flexibly attaching the tank to the structure. The flexible mounting allows tuning of vibration of the impulsive mass to short period structures. The small, suboptimally tuned sloshing mass may also have minor contribute in dissipation. The performance of the TLMD is demonstrated by simulation and shake table experiments. The simulation involves solution of the governing equations for the structure‐TLMD system, in which the vibration of liquid is described by the Housner model. The optimal parameters for tuning are obtained through optimization. The dynamic response evaluation employs a suite of far field, recorded ground motions of varying hazard levels to demonstrate robustness. The simulation and the experiment results corroborate well. Comparative assessments with TMD and TLD with identical mass ratios show comparable efficiency. Given that the stored liquid is employed for the functional purpose (unlike the added mass in TMD), the proposed TLMD may be a preferred alternative to conventional tuned mass damper (TMD) or TLD.

Influence of internal blade-interactions on the added mass and added damping of a prototype Kaplan turbine runner

Abstract: Different from other types of hydraulic turbines, the fluid–structure coupling vibration behavior of Kaplan turbine runners has still been studied limited before. One problem is that their blades can rotate according to load changes, and the internal interactions among the blades via the flow field may produce an important influence on the added mass and added damping of the runner, particularly when the blade angle is small. In this paper, the influence of internal blade-interactions on the added mass and added damping of a prototype Kaplan turbine runner has been studied numerically. An isolated stage model from the end of the stay vanes to the bottom of the hub with six blades was considered for simulation. The Acoustic Fluid-Structure Interaction (FSI) technology based on the Finite Element Method was used to investigate the added mass effect first and to provide the modal shapes and initial frequencies for the following one-way FSI analysis based on the Finite Volume Method. The natural frequencies predicted by the Acoustic FSI were compared with those from the one-way FSI analysis to validate the simulations. Then, the influence of internal blade-interactions on the added mass and added damping, as well as the mechanisms, were analyzed.

Modal analysis of an arch dam combining ambient vibration measurements, advanced fluid‐element method and modified engineering approach

Abstract: Modal analysis, aiming at estimating modal characteristics such as natural frequencies and mode shapes, is fundamental for studying the dynamic behaviours of a structure. This paper presents a modal analysis of an arch dam using three different techniques. The reference one is based on the statistical analysis of the ambient vibration data collected from the dam crest. The two other approaches are both numerical but with different methods (fluid‐element and Westergaard) for the modelling of the reservoir and its interactions with the dam. By applying the three techniques to the studied dam and comparing their results, it is demonstrated that: (1) analysing the ambient vibration data through an operational modal analysis method is able to extract the dam modal characteristics; (2) the fluid‐element method is effective for arch dams since the first 10 natural frequencies can be accurately predicted once the material parameters are calibrated on the first three modes; and (3) the Westergaard method, a technique with only additional masses, produces significantly under‐estimated frequencies for the first few modes if same parameters are used as the fluid‐element method; the underestimation can be corrected for several modes by using a higher stiffness parameter but the required value is unrealistic for the case study. Furthermore, a modified Westergaard method is introduced in this paper by using a reduced added‐mass coefficient. This method, once the coefficient is calibrated on the 1st mode, is able to well predict the partially coupled modes as illustrated with the case study of the Saint‐Guérin dam.

FSI—vibrations of immersed cylinders. Simulations with the engineering open-source code TrioCFD. Test cases and experimental comparisons

Abstract: In this paper, we assess the capabilities of the Arbitrary Lagrangian-Eulerian method implemented in the open-source code TrioCFD to tackle down two fluid-structure interaction problems involving moving boundaries. To test the code, we first consider the bi-dimensional case of two coaxial cylinders moving in a viscous fluid. We show that the two fluid forces acting on the cylinders are in phase opposition, with amplitude and phase that only depend on the Stokes number, the dimensionless separation distance and the Keulegan-Carpenter number. Throughout a detailed parametric study, we show that the self (resp. cross) added mass and damping coefficients decrease (resp. increase) with the Stokes number and the separation distance. Our numerical results are in perfect agreement with the theoretical predictions of the literature, thereby validating the robustness of the ALE method implemented in TrioCFD. Then, we challenge the code by considering the case of a vibrating cylinder located in the central position of a square tube bundle. In parallel to the numerical investigations, we also present a new experimental setup for the measurement of the added coefficient, using the direct method introduced by Tanaka. The numerical predictions for the self-added coefficients are shown to be in very good agreement with a theoretical estimation used as a reference by engineers. A good agreement with the experimental results is also obtained for moderate and large Stokes numbers, whereas an important deviation due to parasitic frequencies in the experimental setup appears for low Stokes number. Still, this study clearly confirms that the ALE method implemented in TrioCFD is particularly efficient in solving fluid-structure interaction problems. As an open-source code, and given its ease of use and its flex-ibility, we believe that TrioCFD is thus perfectly adapted to engineers who need simple numerical tools to tackle down complex industrial problems.

Passive suppression of flow-induced vibrations of a cantilevered pipe discharging fluid using non-linear vibration absorbers

Abstract: This paper addresses the problem of passive suppression of vibrations induced by flow in a cantilevered pipe discharging fluid. The suppressor utilized, called rotative non-linear vibration absorber (NVA, for short), is composed of a mass connected to the extremity of a rigid bar linked to the pipe by means of a hinge and a rotational damper. The NVA is defined by its mass, radius and damping coefficient. The suppressor may affect the pipe dynamics in two different ways, which are the inherent mass addition to the system, and the fact that the device can rotate and thus can dissipate energy through its damper. The former is characterized by its mass and position along the pipe while the latter is also a function of other design parameters, such as damping coefficient and radius. It is shown that while mass additions play an important role in the pipe stability, the energy dissipation associated with the relative motion between the device and the pipe is capable of alleviating the pipe vibrations. Considering a fixed set of parameters for the pipe, a parametric study is carried out with the objective of identifying a proper set of parameters for the NVA, which leads to a robust and effective vibration mitigation. It is shown that the methodology adopted herein allows for such identification to be made and that the suppression achieved with the selected NVA is one of the best among the results presented, even if supercritical internal flow velocities are considered.

Influence of internal blade-interactions on the added mass and added damping of a prototype Kaplan turbine runner

Abstract: Different from other types of hydraulic turbines, the fluid–structure coupling vibration behavior of Kaplan turbine runners has still been studied limited before. One problem is that their blades can rotate according to load changes, and the internal interactions among the blades via the flow field may produce an important influence on the added mass and added damping of the runner, particularly when the blade angle is small. In this paper, the influence of internal blade-interactions on the added mass and added damping of a prototype Kaplan turbine runner has been studied numerically. An isolated stage model from the end of the stay vanes to the bottom of the hub with six blades was considered for simulation. The Acoustic Fluid-Structure Interaction (FSI) technology based on the Finite Element Method was used to investigate the added mass effect first and to provide the modal shapes and initial frequencies for the following one-way FSI analysis based on the Finite Volume Method. The natural frequencies predicted by the Acoustic FSI were compared with those from the one-way FSI analysis to validate the simulations. Then, the influence of internal blade-interactions on the added mass and added damping, as well as the mechanisms, were analyzed.

Irregular vortex-induced vibrations of a two-dimensional circular cylinder at a low Reynolds number

Abstract: Irregular vortex-induced vibrations (VIV) of a two-dimensional circular cylinder, characterized by the time-varying oscillation amplitude and frequency, were observed in both experiments and numerical simulations. This paper attempts to elucidate the phenomena by numerical simulations of a circular cylinder in VIV, with mass ratios of 2, 10 and 50, at a Reynolds number of 150. The lift force acting on the cylinder is assumed to be a sum of two components associated with added mass and hydrodynamic damping. The time-dependent added-mass and hydrodynamic damping coefficients are determined by the least-squares fit of lift from the cylinder displacement. The effective natural frequency varies with time due to the added mass, and the cylinder oscillation amplitudes are influenced by the hydrodynamic damping force. Consequently, VIV irregularities arise from the complex evolution of the added-mass and hydrodynamic damping coefficients. The irregular oscillations are associated with the interaction of vortices in the near wake, which results in the intermittent appearance of disorganized Karman vortex street. Moreover, the vortex interactions involve a rapid reduction of the hydrodynamic damping coefficients. Besides, the irregular oscillation region is identified in terms of the reduced velocity, and smaller mass ratio tends to have larger width of the region.

An equivalent spring-based model to couple the motion of visco-hyperelastic dielectric elastomer with the confined compressible fluid/air mass

Abstract: Soft materials exhibiting large voltage-induced deformation under external stimuli have gained increasing attention in the recent past owing to their potential applications in soft transducers. Dielectric elastomer actuators (DEAs) coupled with a fluid/air mass have proved to undergo considerable voltage-induced deformation. This article presents a framework to couple the motion of an inflated Dielectric elastomer (DE) membrane with a confined compressible fluid/air mass and develops a nonlinear dynamic analytical model to predict the polar region deflection of an inflated DE membrane attached to an airtight chamber (namely bulging actuators) stimulated by an electric field. In order to derive the differential equations which determine the dynamic behavior of the membrane coupled to an airtight chamber, the Euler–Lagrange equation is employed, which takes into account the effects of membrane pre-stretch, initial inflation pressure, membrane viscoelastic behavior, and transient electric loading. A hypothesis of pseudo-air-spring is incorporated to consider the effect of the volume of the confined fluid/air mass. The experimental investigation conducted for various geometrical and loading conditions in both depressurized and pressurized states of actuation shows that the developed nonlinear model satisfactorily predicts the influence of fluid/air mass on the dynamic response of the considered DE membrane undergoing small voltage-induced deformation. Finally, the proposed framework is incorporated for building insights on several effecting parameters such as inflation pressure, quality, and quantity of the confined fluid mass, etc., on the electrodynamic behavior of the actuator. The results reveal that the volume of the air column appreciably alters the nonlinear dynamic behavior of the bulging actuator. Poincaré plots along with phase diagrams are presented for analyzing the periodicity of the nonlinear oscillations of the actuator. The underlying theoretical framework and the obtained inferences of the present article can be implemented effectively in the design of a futuristic class of structures whose motion is coupled with compressible fluid/ air mass subjected to time-dependent actuation.

Modal analysis of offshore monopile wind turbine: An analytical solution

Abstract: An analytical solution of the dynamic response of offshore wind turbines under wave load with nonlinear Stokes's wave theory and wave-structure and soil-foundation interactions is developed. Natural frequencies and the corresponding modes are obtained. The effect of the wave-structure interaction, the added mass, the foundation stiffness, and the nacelle translational and rotational inertia on the motion of the structure is investigated. The nonlinear loading provided by the drag term of Morison's equation is successfully handled. A parametric study to examine the effect of the structural parameters on the dynamic response is conducted and the results of the proposed analytical solution are compared to numerical ones. The proposed method has the following advantages: a) it is accurate and straightforward because of its analytical nature, b) it does not ignore the drag term in the wave loading by keeping its nonlinearity nature, c) the structure of the wind turbine is modeled as a continuous system, d) it takes into account the effect of the rotational and translational inertia of the nacelle on the dynamic response, e) it provides an interpretation of the effect of the sea level variation in changing the natural frequencies.