Showing papers on "Added mass published in 1969"

Lateral motion of a slender body between two parallel walls

Abstract: The method of matched asymptotic expansions is used to determine the lateral flow of an ideal fluid past a slender body, when the flow is constrained by a pair of closely spaced walls parallel to the long axis of the body. In the absence of walls, the flow field would be nearly two-dimensional in the cross-flow plane normal to the body axis, but the walls introduce an effective blockage in the cross-flow plane, which causes the flow field to become three-dimensional. Part of the flow is diverted around the body ends, and part flows past the body in the inner cross-flow plane with a reduced 'inner stream velocity'. An integro-differential equation of identical form to Prandtl's lifting-line equation is derived for the determination of this unknown inner stream velocity in the cross-flow plane. Approximate solutions are applied to determine the added mass and moment of inertia for a accelerated body motions and the lift force and moment acting on a wing of low aspect ratio. It is found that the walls generally increase these forces and moments, but that the effect is significant only when the clearance between the body and the walls is very small.

Fluid-Free-Surface Proximity Effect on a Sphere Vertically Accelerated from Rest

Abstract: Theory is developed to estimate the effect of free surface proximity on the initial added mass of a sphere accelerated vertically upward from rest in an ideal fluid. It is assumed that the acceleration regime is sufficiently brief that inertial forces predominate and gravitational effects may be neglected. Results of tests in water indicate that while there are slight viscous and gravitational effects over the acceleration regime, the agreement between theory and experiment is good. It is concluded that over briefer acceleration regimes these effects would decrease and the agreement would improve.

Equivalent Added Mass of Ship in the Collision

Abstract: To calculate the absorbed energy by the flacture of ship's structure in case of collision, it is necessary to estimate accurately the added mass of striken ships.Minorsky has proposed in his famous paper to use 0.4 for the added mass coefficient of striken ships. In this paper, the authors tried to obtain an exact expression of the hydrodynamic force which will act on ships during a collision by theoretical calculations and series of model experiments on the first atomic-powered ship of Japan were conducted.As a result, the authors have shown that the ratio of external force to the acceleration of the striken ship varies with the time elapsed during the collision : i. e., the added mass of the ship is not constant during the collision. From this result, the authors introduced an equivalent added mass which, dividing the external force by it, will give an acceleration equal to the exact value of acceleration at the end of the collision.It was also shown that this equivalent added mass changes its value with the “duration” of the collision. If the duration is infinitely small, the equivalent added mass is equal to the added mass for infinite high frequency. This agrees with Minorsky's assumption. However, if the duration is finite, the equivalent added mass becomes larger as the duration increases.Since the duration is only related to the initial speed of the striking ship and the amount of penetration, and lower the initial speed or deeper the penetration the longer the duration, the equivalent added mass coefficient will be much greater than 0.4 in the case when a soft structured ship is striken by a low speed ship resulting in a considerable amount of penetration.

Added Mass for both Vertical and Horizontal Vibration of Two-Dimensional Cylinders of Curvilinear-Element Sections with Chines in a Free Surface

Abstract: Added masses of two-dimensional cylinders of curvilinear-element sections with chines, which are similar to marked V character ship sections with either single or double chines, oscillating at high frequency in a free surface of an ideal fluid are calculated for both vertical and horizontal vibration by employing two particular two-parameter families of the conformal transformation. The numerical results are graphically presented in the forms of added mass coefficient curves in terms of the sectional area coefficient and the half beam-draft ratio together with the section contours derived with the employed transformations, and discussed in comparision with those of the Lewis forms and of straightline-element sections with single chine for vertical vibration, and, for horizontal vibration, with those of the Lewis forms.