Showing papers on "Added mass published in 1999"

Adaptive control of flow-induced oscillations including vortex effects

Abstract: Flow-induced vibrations constitute important design criteria for most offshore structures as well as for many other structures subjected to flow-induced forces. Both the main structural elements as well as supporting structural members such as guyed cables must be designed to withstand such oscillations. It is well established that in the process of vibrations induced by flow, vortices form around the body which initiate oscillations in a direction transverse to the general direction of motion. In this paper, Morison’s equation is used to represent the interaction between the flow field and the structure, complemented by terms including the vortex dynamics effect. In order to mitigate against extreme vibration conditions, here, the implementation of active control is proposed to stabilize the motion of a structure immersed in a flow field. Specifically, a tuned mass damper is attached to the structure and adaptive control is utilized for moving the mass along a particular path while allowing for uncertainty in the various hydrodynamic coefficients. The proposed procedure have two distinct beneficial results. The first one being to control the vibration of the structure, and the second one is the estimation of the hydrodynamic coefficients and the validation/calibration of Morison’s equation model for the flow-induced forces.

Radiation and Diffraction Analysis of the McIver Toroid

Abstract: A hydrodynamic analysis is performed of a special toroidal body which is known to have a nontrivial solution of the homogeneous linearized free-surface boundary-value problem with oscillatory time- dependence. This solution corresponds physically to unbounded resonant motion of the fluid in the 'moon pool' at the center of the toroid. The added mass, damping, and elevation of the free surface in the moon pool are computed for a range of wavenumbers, with singular results in the resonant regime.

A self-adaptative oscillator

Abstract: The dynamics of a system where a mass is free to slide on a vibrating string is investigated as the excitation frequency is varied. One degree of freedom is thus added to the system studied by Helmholtz in which a mass was fixed on a vibrating string. This new system exhibits a specific dynamics characterized by the existence of a self- adaptative behaviour. When the driving frequency falls into wide and well defined frequency bands, a long transient is observed by which the mass adjusts its position so that the whole system becomes resonant. In the gaps between these bands, bifurcations give other equilibrium positions. A theoretical model is proposed. It accounts for all the experimental results. In the case where two masses are present on the string, two degrees of freedom are added and the set of equilibrium positions would be expected to be infinite. However, in the experiment, the two masses are observed to go to positions where they are symmetrical with respect to the middle of the string. A selection mechanism due to the string stretching is pointed out.

Hydroelastic Vibration of Free-Edge Annular Plates

Abstract: This paper is concerned with the virtual mass effect due to the presence of water on the natural frequencies of free-edge annular plates resting on free surface or completely submerged, which has never been studied theoretically Experiments were carried out for free-edge annular plates to find the so-called nondimensionalized added virtual mass incrementalfactors In this paper, theoretical nondimensional added virtual mass incremental factors are obtained by employing the Hankel transformation technique in conjunction with the Fourier-Bessel series approach It is found that the theoretical nondimensionalized added virtual mass incremental factors for free-edge annular plates resting on free-surface agree well with experimental ones The proposed method can be applied to different boundary conditions of plates

On the motion of dispersed balls in a potential flow: a kinetic description of the added mass effect

Abstract: In order to describe the motion of a large number of bubbles in a three-dimensional flow and to rigorously put into evidence the added mass effect, Russo (SIAM J. Appl. Math., 56 (1996), pp. 327--357) and Smereka ( J. Fluid Mech., 248 (1993), pp. 79--112) considered the following very idealized situation. The bubbles are assumed to be spherical particles of radius a. The external fluid is simply represented as a dipole approximation of a potential flow. The motion of the bubbles is created only by the pressure forces on the boundary of each bubble. In this paper we go further, showing that the exact kinetic equation, of Vlasov type, for the dispersed phase can be exactly derived and no further approximation is needed to write the mean field equation. We also prove that this model preserves the energetic structure of the departing dynamical system. This has effects on the solution computed by numerical simulation, and we show that the behavior of the particles can be different if a truncation is used.

Modeling and dynamic behavior of rotating blades carrying a tip mass and incorporating adaptive capabilities

Abstract: The problems of the mathematical modeling and dynamical behavior of rotating blades carrying a tip mass and incorporating adaptive capabilities are considered. The blade is modeled as a thinwalled beam incorporating non-classical features such as anisotropy, transverse shear, secondary warping, and includes the centrifugal and Coriolis force fields. For non-adaptive rotating blades, a thorough validation of the structural model and solution methodology is accomplished. The adaptive capabilities provided by a system of piezoactuators bonded or embedded into the structure are also implemented. Based on the converse piezoelectric effect, the piezoactuators produce a localized strain field in response to an applied voltage, and, as a result, an adaptive change of the dynamic response characteristics is obtained. A combined feedback control law relating the piezoelectrically induced bending moment at the beam tip with the kinematical response quantities appropriately selected is used, and its beneficial effects upon the closed-loop eigenvibration characteristics are highlighted.

Experimental determination of the hydrodynamic coefficients of an underwater manipulator

Abstract: A single degree-of-freedom (DOF) manipulator arm with a rectangular cross section is built to investigate the hydrodynamic effects, including drag force, added mass, and the moments induced by these forces. The drag–velocity relationships (linear and angular) are experimentally established and the drag force/moment coefficients are deduced. The added mass and the added moment of inertia are determined for the first time through the relationship between the added mass of the manipulator and its acceleration. These data are very useful for developing the dynamic model and therefore the optimum control of underwater manipulators. ©1999 John Wiley & Sons, Inc.

The Measurement of Fluidelastic Effects at Low Reduced Velocities Using Piezoelectric Actuators

Abstract: Fluidelastic effects which are responsible for fluidelastic instabilities may be indirectly measured through the analysis of the vibrating motion of system under flow. In this paper, piezoelectric actuators are used to increase the vibratory level when buffeting forces which excite tube vibration are low, and to improve the measurement of fluidelastic forces. The proposed method based on an added excitation allows the study of the added mass and provides a better accuracy on the measurement of the vibrating characteristics and, thereby, of the fluidelastic forces. This added excitation method is compared with a standard indirect approach on a tube underwater crossflow. The influence of the level of piezoelectric excitation forces is discussed, as well as the range of application of this technique.

Achieving Maximum Tip Displacement during Resonant Excitation of Piezoelectrically Actuated Beams

Abstract: A linear, lumped-parameter, single-degree-of-freedom model is formulated to describe the dynamic response of the first mode of a piezoelectrically-actuated cantilevered beam. The modeled system consists of two identical, symmetrically disposed piezoelectric patch actuators bonded to a uniform beam with a tip mass. Specifically, the first mode of the system is described by an equivalent mass-spring-damper system where the equivalent mass, stiffness, damping, and force are functions of the parameters of the piezoelectrically-actuated system. Experimental data are used to verify model damping and displacement. The model is then used to show that the dynamic response of the system depends upon four independent non-dimensional geometric variables. Numerical simulations are performed to graphically demonstrate this dependence and to indicate an optimal configuration for achieving a maximum deflection at the beam's free end during resonant excitation.

Spectral analysis of offshore structures under combined wave and earthquake loadings

Abstract: A spectral analysis procedure is presented in general for steel offshore structures, which are subjected to simultaneously random wave and earthquake loading processes. The 3D modeling of the structure and the deep-water condition of the wave model are used. The random wave Is represented by a surface elevation spectrum, P.M or JONSWAP, and the random earthquake is represented by the spectrum of the ground acceleration, a modified well-known Kanai-Tajiml spectrum. It is formulated that the earthquake introduces not only inertia forces on the structural system but also fluid forces on the submerged members due to the ground motion. A different linearization process is applied to the force components derived from the modified Morison's equation so that the linearization of the drag force term does not equally affect the force components. It is implicitly stated that the earthquake has also effect on the added mass and the hydrodynamic damping ratio by means of the related linearization coefficients. It is demonstrated by a simple example that the earthquake forces are considerably influential at the natural frequencies of the structure whereas the wave forces are also effective at the peak wave frequency.

Added Fluid Mass for Bellows Expansion Joints in Axial Vibrations

Abstract: This paper presents the results of an analysis of fluid added mass in bellows expansion joints during bending vibrations. The added mass is shown to consist of two parts, one due to transverse rigid body motion and the other due to distortion of the convolution during bending. The latter component, neglected in previous analyses, is shown to be important for relatively short bellows, as are commonly used for expansion joints, and to become increasingly important for higher vibration modes. The distortion component has been determined using finite element analysis and the results are presented in graphical form for a typical range of bellows geometries. The total added mass is given in a form suitable for hand calculations.

Acceleration Performance of High-Speed Planing Craft from Rest

Abstract: In this study, the performance of a high-speed planing craft during rapid acceleration from rest is investigated, following series studies on the performances in steady running condition including ship motions due to instability in calm water.Measurement of running attitude of a model of planing craft during acceleration from rest is carried out to compare with that in running at constant speed. The results show the force component which is proportional to acceleration is important as well as a conventional displacement ship. Measurements of hydrodynamic forces acting on it during acceleration suggest, however, that the added mass component does not play an important role because it is much smaller than the inertia force and the steady drag components which are proportional to square of advanced speed.A computer simulation program to predict this performance is developed using a motion equation with experimental and theoretical hydrodynamic coefficients. The results of the simulation are in fairly good agreement with the experimental ones. It is shown that the procedure can simulate unsteady motions like porpoising in accelerating condition as well as in steady running condition.

柔軟で薄いシートの高速気流中における挙動に関する研究 : (第2報, 低質量比領域における実験結果および予測される挙動)

Abstract: Flutter behaviors of flexible thin sheets of such as papers and plastic films blown in wind, whose mass ratios are very low, tend to deviate far from those for ordinarily stiff materials. A theoretical prediction method of the flutter occurrence developed by the authors was applied to the low mass ratio region, and the results explained well the tendencies found in the experimental data. The predicted results have shown clearly the effects of both the fluid friction and the added mass effect by surrounding fluids in the region for the low mass ratios, compared with those for higher mass ratios whose behaviors are dominated mainly by elasticity and inertia force, and fluid pressure. The flutter limits and the behaviors in a region for very low mass ratios, which has not yet been observed experimentally, were predicted by the method and given in this report.

Effect of Memory on the Stability of Spread Mooring Systems

Abstract: The importance of including the hydrodynamic memory effect in modeling and analysis of spread mooring systems (SMS) is assessed based on the design methodology for mooring systems developed at the University of Michigan. The memory effect is modeled by the hydrodynamic radiation forces expressed in terms of added mass at infinite frequency and convolution integrals of impulse response functions. The convolution integrals, which are explicit functions of time, are converted to autonomous excitation by the method of extended dynamics. For a given SMS configuration, nonlinear stability and bifurcation theory are used to produce catastrophe sets in the parametric design space separating regions of qualitatively different system dynamics. This approach reveals the complete picture of nonlinear phenomena associated with system dynamics and eliminates the need for extensive simulations. Catastrophe sets are developed in several parametric design spaces, providing fundamental understanding of the memory effects on SMS nonlinear dynamics. The mathematical model is based on the slow-motion maneuvering equations in the horizontal plane, including hydrodynamic memory effect and third-order quasi-steady hydrodynamic forces. Mooring lines are modeled by synthetic fiber ropes attached to surface terminals and deep-water catenary chains with touchdown and nonlinear drag. Environmental loads consist of time-independent current, wind, and mean wave drift forces.

Time-domain motion computations of twin- and mono­ hull ships in head seas

Abstract: Wave exciting forces on a restrained body with steady forward speed in the time-domain were computed by the authors (1995) As one step further, radiation forces and ship motions in regular waves are computed in the present paper With impulse response functions, the integral-differential equations of motions are solved in the time-domain The same method is also applied to compute the radiation forces of a catamaran with forward speed The results show that the time-domain computation for ship motions is particularly meaningful in the cases where the forward speed effect is considered Added mass and damping coefficient of a catamaran with the Lewis-form demi-hull are computed for Froude numbers 015 and 030 Good agreements with experimental results for heave and pitch motions are obtained A Wigley hull and a Series 60, C B = 0 hull are also adopted for the motion computation The computed results for Froude number 020 are compared with experimental results

Base isolation structure

Abstract: PROBLEM TO BE SOLVED: To provide a base isolation structure which can be easily installed while a building is used, and can reduce the vibration response of the building's main body without being affected by the periodic component of an earthquake wave, the characteristics and position of a base isolation mechanism and the like. SOLUTION: A base isolation mechanism 1 is comprised of base isolation units 6, each of the unit 6 comprising added mass bodies provided on a building' s main body 3 in a horizontal direction thereto in such a way that relative displacement is possible, and an elastic body provided between the added mass body side and the side of the building's main body 3 for elastically controlling the relative displacement of the added mass bodies with respect to the building's main body 3. These added mass bodies are coupled to each other by means of coupling rods 24,... in such a way that the relative displacement of added mass bodies can be integrally conducted with respect to the building's main body 3.

An adaptive mass damper for self-excited oscillations

Abstract: An adaptive tuned mass damper aiming to optimally increase the critical flow velocity of transverse galloping oscillations is presented. The structure, simply modeled as an elastically supported bluff body exposed to a steady flow, has been connected to an added mass by a spring and a dashpot varying according to an adaptive control algorithm. Based on the knowledge of a reference model, describing the optimal conditions for the system, the paper presents two procedures to design a controller that allows to identify the uncertain mechanical parameters relying on a direct and an indirect approach. In the direct case, by assuring the convergency to zero of the error between the system and the reference model variables, in the indirect case, by using a wavelet based approach for on-line system identification. The analytical developments are validated by numerical investigations on the nonlinear system in order to assess the feasibility of the proposed control system.