Kie Okabayashi

Direct numerical simulation of turbulent flow above zigzag riblets

We investigate the drag-reducing performance of a zigzag riblet inspired by the “Miura fold” and its influence on the streamwise vortices above a wall-bounded turbulent flow through direct numerical simulations of channel flows. The employed channel wall geometries are a flat plate, straight riblet, and zigzag riblet. The drag-reducing performance of various zigzag riblets with adjusted configuration parameters is superior or at least comparable to that of the straight riblet. We assume an analogy between the drag reduction mechanisms of spatially periodic forcing and the zigzag riblet, because both methods induce similar sinusoidal velocity profiles. To investigate the characteristics of the zigzag riblets, we apply a conditional sampling technique to the near-wall streamwise vortices that is also applied to the wall-bounded flow under spatially periodic forcing. The flow toward the wall of the rotating motion of a vortex becomes relatively small, as the main flow levitates at the tips of the zigzag riblet. Thus, the ejection increases and the sweep decreases. In the phase where the direction of the induced spanwise velocity coincides with the vortical motion, the latter is weakened because the vortex tilts in the spanwise direction at the phase; then, vortex stretching is suppressed. The zigzag riblet can reduce the drag via a mechanism analogous to active control, such as periodic oscillation. However, the effect of the upward flow is dominant above the zigzag riblet, and the analogous mechanisms are less remarkable. The considered zigzag riblet has potential application as a kind of active feedback control.

二次元直線翼列において旋回する部分キャビテーションの一般性(流体工学,流体機械)

Numerical simulations of 2-dimensional (2D) unsteady cavitating flows were carried out under various conditions of the number of blades, incidence angles and cavitation numbers. When the incidence angle increased or the cavitation number decreased, the steady balanced cavitation transited to unsteady and non-uniform patterns. Typical patterns reported in the previous studies such as rotating, asymmetric and alternating for 3and 4-blades were successfully reproduced. In this study, cascades of the larger number of blades were dealt with to consider the generality of unsteadiness by reducing the influence of periodicity. The cavitation is basically triggered in the backward next section. However, the period of time for growing causes complexity in the discrimination of propagation. In most cases of rotating partial cavitation, except for 4-blades, the cavity develops in the second passage of backward direction after the decay of largest cavity. In case of many blades, multiple cavities rotate simultaneously and the particular patterns observed in cascades of small even numbers of blades attenuate.