Added mass, density stratification and buoyancy oscillations

International audienceAdded mass represents the additional inertia experienced by a body as it moves through a fluid, owing to the flow that this motion generates within the fluid. When waves are present, added mass becomes complex at any given frequency, with a real part associated with inertia and an imaginary part with wave damping. We consider the way internal waves affect the added mass of oscillating bodies in uniformly stratified fluids. As for potential flow, the energy and impulse of the fluid, the dipole strength of the body and the forces exerted on it may all be expressed in terms of added mass, which then provides direct access to the wave energy, of primary importance for internal tides (Garrett & Kunze, 2007), and to the forces governing the dynamics of oceanographic floats (Goodman and Levine, 1990; D’Asaro, 2018). By Fourier inversion in time, added mass appears as a new memory force acting in addition to the Basset force associated with viscous effects.Added mass is calculated for the translational motion of an elliptic cylinder and a spheroid, based on their earlier study using boundary integrals (Voisin, 2021). Particular attention is paid to the oscillations of a sphere displaced from its neutral buoyancy level then released (Larsen, 1969; Levitskii and Chashechkin, 1999), to the oscillations of a Cartesian diver (Le Gal et al., 2022), and to the measurement of added mass by the impulse response method (Brouzet et al., 2017)