Virtual Mass and Acceleration in Fluids

The “effective inertia” of a body is greater in air than in a vacuum and is much greater in a liquid. This effective inertia, which is greater than the “quantity of matter” or mass (ratio of weight to the acceleration due to gravity) of a body, is termed “virtual mass.” The excess of virtual mass over the quantity of matter is the added mass caused by the surrounding fluid. Measurements were made on objects as they were immersed in water and accelerated in oscillatory motion. The added masses, which depend on the size and shape of the body, the direction of acceleration, and the density of the fluid, were determined from a mass-frequency relationship for the supporting beam. The experimental results are reasonable, consistent, and agree with previous analytical studies of potential flow.