Makoto Iima

Flow transitions in the surface switching of rotating fluid

We study ‘surface switching’ quantitatively in flows driven by the constant rotation of the endwall of an open cylindrical vessel reported by Suzuki, Iima & Hayase ( Phys. Fluids , vol. 18, 2006, p. 101701): the deformed free surface switches between axisymmetric and non-axisymmetric shapes accompanied by irregular vertical oscillation. Detailed simultaneous measurements showed that the magnitude of the velocity fluctuations (turbulent intensity) temporally varies greatly and are strongly correlated with the surface height, suggesting that dynamic switching between laminar and turbulent states is accompanied by vessel-scale surface shape changes. The study also identified clear hysteresis in the turbulent intensity arising from changes in the Reynolds number; the bifurcation diagram consists of two overlapping branches representing a high-intensity (turbulent) state and a low-intensity (laminar) state. Based on the results, a switching mechanism is suggested.

Modeling and emergence of flapping flight of butterfly based on experimental measurements

The objective of this paper is to clarify the principle of stabilization in flapping-of-wing flight of a butterfly, which is a rhythmic and cyclic motion. For this purpose, a dynamics model of a butterfly is derived by Lagranges method, where the butterfly is considered as a rigid multi-body system. For the aerodynamic forces, a panel method is applied. Validity of the mathematical models is shown by an agreement of the numerical result with the measured data. Then, periodic orbits of flapping-of-wing flights are searched in order to fly the butterfly models. Almost periodic orbits are obtained, but the model in the searched flapping-of-wing flight is unstable. This research, then, studies how the wake-induced flow and the flexibly torsional wings effect on the flight stability. Numerical simulations demonstrate that both the wake-induced flow and the flexible torsion reduces the flight instability. Because the obtained periodic flapping-of-wing flight is unstable, a feedback control system is designed, and a stable flight is realized.

Propulsion hydrodynamics of a butterfly micro-swimmer

Propulsion motion of a simple mechanical model at low Reynolds numbers is considered. The model consists of two spheroids (wings) connected by a hinge. Its non-reciprocal operation cycles represent combinations of flapping motions of the wings and of their rotations, resembling conformational motions characteristic for real protein machines and similar to the propulsion pattern of a butterfly. The net generated velocity and the net stall force, exhibited by an immobilized machine on its support, are calculated and their dependence on the model parameters is discussed.

Rotating flow transition related to surface switching

We have investigated quantitatively the recently reported phenomena called surface switching that occurs in the flow driven by a rotating disk in an open cylindrical vessel [1]. The deformed free surface abruptly changes from an axisymmetric to a non-axisymmetric shape accompanying with a vertical oscillation at irregular intervals. We have studied the flow transition quantitatively using ultrasonic velocity profiling (UVP). The turbulent intensity shows a transition at the same Reynolds number as that for the surface switching. We find that the fluid-air interface becomes unstable at a smaller Reynolds number than the critical Reynolds number for the surface switching. Effects of the material of the rotating disk on the surface switching is discussed.

Response to "Comment on 'Surface switching of rotating fluid in a cylinder"' [Phys. Fluids 19, 069101 (2007)]

In our Letter [T. Suzuki, M. Iima, and Y. Hayase, “Surface switching of rotating fluid in a cylinder,” Phys. Fluids 18, 101701 (2006)], we reported a surface switching phenomenon of fluid in a cylinder driven by the endwall rotation. In the Comment by Vatistas [G. Vatistas, “Comment on ‘Surface switching of rotating fluid in a cylinder,” Phys. Fluids 19, 069101 (2007)], the author claims that a periodic sloshing phenomenon reported by Vatistas in 1990 is the same phenomenon as the surface switching. We clarify the difference between these phenomena in terms of the parameters, the phenomena, the boundary condition, and the physical mechanism.