Abstract
There are two types of separation in the boundary layer of an airflow. One of them is laminar separation and the other is turbulent. The problem of laminar separation has long been studied, and the condition of the separation or the mechanism of the transition from the laminar separation to the turbulent boundary layer have been made clear. On the other hand, in the study of the turbulent separation, which was started at the same time, the problem of the transition from the turbulent separation to a more turbulent boundary layer has hardly been dealt with. As shown in this report, however, the formation of turbulent separation or its dissolution can be seen frequently in the meteorological phenomena. For instance, there is a local wind called“Oroshi”in Japan. There is an airflow like“Föhn”blowing severely down the slope of a mountain and occasional ly bringing about as great disasters as a typhoon. In this phenomenon, it is found that if the general wind is not so strong, the airflow forms turbulent separation in the leeside, causing a reverse flow near the ground surface. But, when the intensity of the turbulence increase with the growth of the general wind, the reverse flow changes into a strong descending flow. This closely resembles the process of the transition from the laminar separation to the turbulent boundary layer with the increase of the Reynolds number. CAT in the leeside of Mt. Fuji which has recently been much discussed in relation to the great aircraft accident, seems to have something to do with turbulent separation. In basin-type topography or on the leeside of a mountain the air will become stagnant and be polluted. This phenomenon is also closely related to the turbulent separation. It may be said from the above considerations that the phenomenon of turbulent separation can never be disregarded when the effects of topography on airflows are in question.
