arXiv2025
We consider the motion of a rigid body immersed in an inviscid incompressible fluid. In 2D, an important physical effect associated with this system is the famous Kutta-Joukowski effect. In the present paper, we identify a similar effect in the 3D case. For this, we first recast the Newtonian dynamics of the rigid body as a first-order nonlinear ODE for the $6$-component body velocity, in the body frame. Then, we focus on the particular case where the rigid body occupies a slender tubular domain with a smooth closed curve as the centerline and a circular cross-section, in the limit where the radius goes to zero, with fixed inertia and circulation around the curve. We establish that the dynamics of the limit massive rigid loop are given by a first-order nonlinear ODE with coefficients that depend only on the inertia, on the fluid vorticity, and on the limit curve through two $3$D vectors, which are involved in a skewsymmetric $6 \times 6$ matrix that appears in the limit force and torque, a structure which is reminiscent of the 2D Kutta-Joukowski effect. We also identify the limit fluid dynamics as, where, as in the case of the Euler equation alone, the vorticity evolves according to the usual transport equation with stretching, but with a velocity field that is due not only to the fluid vorticity but also to a vorticity filament associated with the circulation around the limit rigid loop. This result is in stark contrast with the case where the filament is made of fluid, with non-zero circulation, since in the latter, the filament velocity becomes infinite in the zero-radius limit. However, considering the inertia scaling that corresponds to a fixed density, we prove that there are solutions for which the solid velocity and its displacement tend to infinity over a time interval of size $\mathcal{O}(1)$.