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Single Degree-of-Freedom Undamped Vibration
Published in Haym Benaroya, Mark Nagurka, Seon Han, Mechanical Vibration, 2017
Haym Benaroya, Mark Nagurka, Seon Han
If the structure is displaced from its equilibrium position, then an imbalance of forces leads to a structural oscillation about the equilibrium. Such an oscillation is affected significantly by the water surrounding the structure. When the structure oscillates, it causes motion in the surrounding fluid. In general, the fluid dampens the structural motion as a result of drag between it and the structure, and the structure entrains or pulls with it some of the surrounding fluid as it moves. This means that the structure has an effective mass that is larger than its actual value. Therefore, it is necessary to include this added mass in any dynamic analysis. The fluid motion past the structure also creates a drag force that is a function of friction and the shape of the structure. If the fluid flow is accelerating, then in addition to the added mass, an inertial force is exerted by the fluid on the submerged structure.
Swimming
Published in Malcolm S. Gordon, Reinhard Blickhan, John O. Dabiri, John J. Videler, Animal Locomotion, 2017
John O. Dabiri, Malcolm S. Gordon
Physically, the added-mass of a body is the mass of fluid surrounding the body that, due to the pressure field on the surface of the body, is set into motion along with the body. The concept is subtle in that whereas the added-mass is a constant fraction of the fluid mass displaced by the body, the added-mass is not comprised of the same set of fluid particles at all times. This is illustrated in Figure 3.28, which shows the trajectories of fluid particles surrounding a sphere in inviscid fluid (see also Section 3.3). Individual fluid particles are constantly entrained by the sphere and subsequently released. However, the total mass of fluid in motion at any time remains constant.
Mode-Summation Procedures for Continuous Systems
Published in William T. Thomson, Theory of Vibration with Applications, 2018
When a structure is altered by the addition of a mass or a spring, we refer to it as a constrained structure. For example, a spring tends to act as a constraint on the motion of the structure at the point of its application, and possibly increases the natural frequencies of the system. An added mass, on the other hand, can decrease the natural frequencies of the system. Such problems can be formulated in terms of generalized coordinates and the mode-summation technique.
Surge and heave hydrodynamic coefficients for a combination of a porous and a rigid cylinder in motion in finite ocean depth
Published in Waves in Random and Complex Media, 2021
Abhijit Sarkar, Swaroop Nandan Bora
Solving radiation problems for ocean waves yields the important hydrodynamic coefficients, namely, added mass and damping coefficients. These coefficients arise as the real and imaginary parts of the hydrodynamic reaction loads on the body due to the prescribed body motions. In physical sense, the added mass is the weight added to a system in a fluid due to the fact that an accelerating or decelerating body must move some volume of surrounding fluid with it as it moves. Damping is an influence within or upon an oscillatory system that has the effect of reducing, restricting or preventing its oscillations. The hydrodynamic forces in the x- and z-directions (i.e. for surge and heave motions) due to the motion of the cylinder in modes m = 1, 2 can be found out by integrating the corresponding pressure over the cylinder. For this configuration, only surge motion is considered. The following explains why heave motion is not considered for this problem:
Added mass variation effect on vortex-induced vibration for flexible risers based on force-decomposition model
Published in Ships and Offshore Structures, 2018
Yu-Chao Yuan, Hong-Xiang Xue, Wen-Yong Tang
Added mass not only directly determines the inertia force, but also and more importantly, affects structural natural frequency. As the natural frequency closest to fr = 0.17 is in fact VIV dominant frequency to furtherly determine the hydrodynamic forces, the calculation deviation of natural frequency under predicted and assumed Ca has crucial impact on final prediction results. Therefore, added mass variation effect is very significant when investigating VIV response. Since the calculation deviation of natural frequency is relatively larger for small-scale risers and high excited modes, the advantage of the proposed model over Ca = 1.0 on VIV prediction tends to become more obvious under such situations. For deepwater flexible risers in the actual marine environment, VIV usually excites quite high modes up to potentially 30th (Vandiver et al. 2006). And calculation error of natural frequency for Ca = 1.0 will gradually enlarge with the increase of excited modal order. In that case, added mass variation effect on structural response is surely necessary to be considered in VIV simulations.
Free vibration analysis of variable stiffness composite laminate (VSCL) plates coupled with fluid
Published in Mechanics of Advanced Materials and Structures, 2021
Amine Bendahmane, Sidi Mohammed Hamza-Cherif, Mohammed Nabil Ouissi
When a structure is in contact with the fluid at rest or in flow, its dynamic behavior undergoes considerable changes. The significant drop in frequency observed during the fluid/structure interaction may even affect the dynamic stability of the system. The analysis of dynamic behavior of the submerged plate in a fluid medium is presented in this article. The plate immersion in the fluid creates interactions phenomena, which can be translated into a mass increase of the system, considered as an added mass. In this investigation, the fluid is assumed nonviscous, incompressible and irrotational.