Trees are, though they are playing important roles for soil conservation, often triggering land-slides and mud avalanches, if they fall down due to strong winds. Relatively small number of research papers are available regarding with the magnitudes of wind forces acting on trees and wind hydrodynamical drag coefficients. Mayhead proposed an evaluation system of this problem. This paper presents the recent advancements on this problem including wind tunnel tests carried out by Johnson and others, and field tests by Shi-igai and Maruyama.

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Chaotic mixing in two-dimensional square cavity flows with time-periodic lid velocity was numerically simulated. The mixing in the cavity was quantified by measuring the pseudo-Lyapunov exponent λ. It was found that λ depends on the period of lid velocity (T) and neither the Reynolds number nor amplitude of lid velocity affects the value of T which gives the maximum λ. In addition we considered the efficient mixing process in the cavity flows related to the pattern of particle paths and analytically solved the particle motion near the corner of the cavity.

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Separated flows around a rotating circular cylinder and flows around an airfoil with a rotating cylinder are investigated by a vortex method applied to moving walls. The random walk method is used to simulate the diffusion of vorticity and linear vortex distribution is used to represent a body.
Computations are performed for the unsteady flow past a circular cylinder which starts translating and rotating impulsively from rest at Reynolds number Re =1000 and velocity ratios U_c/U_∞=0.5 and 2.0. Good agreements are obtained between numerical results and the flow visualization by Badr et al. (1990). The lift coefficient obtained here, which is much smaller than that obtained by the potential theory shows that the viscous effects for the circulation around the rotating cylinder are correctly computed by this method.
Numerical solutions for flows around an impulsively started airfoil with a rotating cylinder at different speed ratios U_c/U_∞=0.0, 2.0, 4.0, Re=10000 and incidence α=24° are obtained. The Kutta condition is satisfied automatically at the trailing edge immediately after an impulsive start, and the starting vortex is observed. As U_c/U_∞ increases, the separated region is reduced in size and an increase in circulation is observed. Leading edge separation is controlled at U_c/U_∞ = 2.0 and 4.0. Numerical results suggest that inverse circulation shed from the rotating cylinder cancels the circulation at the separated region and suppresses separation.

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In order to clarify the mechanism of wall turbulence generation, we have been studying a subcritical transition in a flat-plate boundary layer experimentally, through observing its nonlinear response to energetic hairpin eddies acoustically excited near the leading edge of boundary-layer plate. In this paper, our recent results on the development of wall turbulence structure in this transition (upto R_x =1.2 × 10⁵ where the log-low velocity distribution appears) is presented. We also examined the effect of riblets on the development of wall turbulence and found that just as observed in the developed turbulent boundary layers, riblets cause the spanwise spacing of the near-wall stream wise vortices to increase and the intensity of the associated velocity fluctuation near the wall to decrease. Even in the manipulated boundary layer, the wall turbulence occurs through the subcritical transition.

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