Shigeo Kida

Turbulence driven by precession in spherical and slightly elongated spheroidal cavities

Motivated by the fascinating fact that strong turbulence can be sustained in a weakly precessing container, we conducted a series of laboratory experiments on the flow in a precessing spherical cavity, and in a slightly elongated prolate spheroidal cavity with a minor-to-major axis ratio of 0.9. In order to determine the conditions required to sustain turbulence in these cavities, and to investigate the statistics of the sustained turbulence, we developed an experimental technique to conduct high-quality flow visualizations as well as measurements via particle image velocimetry on a turntable and by using an intense laser. In general, flows in a precessing cavity are controlled by two non-dimensional parameters: the Reynolds number Re (or its reciprocal, the Ekman number) which is defined by the cavity size, spin angular velocity, and the kinematic viscosity of the confined fluid, and the Poincare number Po, which is defined by the ratio of the magnitude of the precession angular velocity to that of the s...

A turbulent ring and dynamo in a precessing sphere

A new ring structure of high activity, both in vorticity and magnetic flux density, is observed in MHD turbulence in a precessing sphere of which the spin and precession axes are orthogonal. This ring is fixed to the precession frame being localized near a great circle whose normal is inclined slightly from the spin axis. Both the velocity and magnetic fields are activated near the cross sections of the ring with the equatorial plane, and their fluctuations make a prograde motion.

Motion of a small flat plate in a viscous flow

A small flat plate of arbitrary shape moving in a viscous flow is investigated analytically under the Stokes approximation. It is shown that the unit normal n(t) of the plate at position x rotates, irrespective of the plate shape, with angular velocity Ω(t)xa0=xa0−(nxa0×xa0∇)(u·n), where u(x,t) is the velocity field at time t. This angular velocity is identical to that of material surface element at the same position and time. In contrast, the angular velocity of the spinning motion (around n(t)) of the plate depends on the shape. A circular disc spins with the same angular velocity of the local fluid. The translational velocity of the centre of gravity of the plate coincides with the local fluid velocity. This result explains the perfect reproduction by direct-numerical simulation using a flat-plate model of the bright pattern observed experimentally with mica particles for the flow in a precessing spherical cavity (Goto and Kida 2011 J. Fluid Mech. 683 417).

Turbulence visualization using reflective flakes

The physical mechanism of flow visualizations using reflective flakes is investigated. First, we derive theoretically the governing equations of flake motion based on the assumption that flakes are infinitely thin elliptic disks. Secondly, we verify numerically and experimentally that these equations describe the flake behavior excellently. An important indication of these equations is that the temporal evolution of flake orientations, which determine the intensity in a visualized image, is identical to that of the infinitesimal material surface elements. Since the orientation of material surface elements is governed by velocity gradients, and since the velocity gradient field of turbulent flows is accompanied by coherent vortical structures at the Kolmogorov length, it is expected that such coherent structures in turbulence may be visualized by reflective flakes. It is numerically demonstrated that a flake visualization with appropriate light thickness, indeed, captures the clusters of the coherent structures in isotropic turbulence.