Hideo Taniguchi

Factors of Three-Dimensional Laminar Boundary Layers Affecting the Characteristics of Cross-Flow Instability

The correlations between the characteristic values of the three dimensional boundary layer and the characteristics of the cross-flow instability have studied by a linear stability analysis of the Falkner-Skan-Cooke boundary layers. Several parameters to predict the critical point of the cross-flow instability and prediction functions of the critical Reynolds number are suggested. It has been found that new parameter defined in this study is available to predict the critical point of the cross-flow instability. It is also shown that the shape factor obtained from the velocity profile of the cross-flow velocity component and the non-dimensional cross-flow velocity at the inflection point of the cross-flow velocity profile are available to predict the critical Reynolds number. The wave length of the critical disturbance in the cross-flow direction has been found to be about 3-4 times as long as the 1% boundary layer thickness of the cross-flow velocity profile.

The Effect of Sphere Rotation on Vortex Shedding from a Sphere in Uniform Flow.

Vortex shedding from a rotating sphere in uniform flow is investigated experimentally. Two hot wires are used to examine the three-dimensional behaviour of vortex shedding and to measure the vortex shedding frequency. It is shown that, when the sphere is rotating, both high and low modes of vortex shedding similar to those of a nonrotating sphere exist. However, it is shown that when the sphere is rotating, the high mode can be found at Reynolds numbers higher than that of the nonrotating sphere case. The phases of velocity fluctuations are the same in the rotating direction for the high mode, but phases for the low mode differ when the sphere is in rotation. The phase difference for the low mode is greater than the angular rotational speed of the sphere. In the rotating sphere case, a new mode with its frequency about twice that of the low mode is found. The direction of the phase velocity of this new mode is found to be opposite to that of the low mode.

The Stability of Boundary Layer on a Rotating Sphere in Still Fluid.

Theoretical study of three-dimensional boundary layer transition on a sphere rotating in still fluid has been carried out by a linear stability analysis. It was determined from experimental results that the relative location of spiral vortices which appear in the transition region is fixed against the rotating sphere surface when the rotating effect is large. However as the rotating effect decreases, these vortices begin to move. So far, this phenomenon has not been explained theoretically. In this study, the boundary layer instability and the characteristics of spiral vortices are theoretically investigated by calculating the behavior of the small disturbances and the critical Reynolds number of the boundary layer on a rotating sphere using the linear stability theory.