An Experiment Concerning Partly Closed Cavities Behind a Surface-Piercing Rod

Abstract

Thomsen in Ref. (1) emphasizes three different states of ventilation occurring at surface-piercing rods, as observed by Hay (2). Hay has towed cylindrical rods which intersect the water surface, at different speeds and at different depths of submergence. From his photographic records, the following observations have been made. At relatively low speeds, an air filled cavity is formed in the wake of the rod, which is open to atmosphere and which extends downwards to a point above the base of the rod. With increasing speed, this point moves downward towards the base of the rod. This form of cavity has been called the "Pre-Base Ventilation State". After the cavity has reached the base, the state was referred to as the "Base Ventilation State", which is found to persist with further increased speed. After the speed was increased over a certain level, the size of the cavity was observed to decrease again forming a cavity closed at the water surface. In this case the state was called the "Post-Base Ventilation State". Since this last phase in the development of the air-cavity has not been observed in the Free Surface Water Tunnel at the California Institute of Technology, it was thought desirable to perform some experiments in order to find the conditions under which this Post-Base Ventilation State can occur. The present results are to be regarded as preliminary. A full explanation of the ventilation phenomena described in the references above and the text of this report to follow is not yet available. Nevertheless, since the results of the present work are at variance with published work, it was thought worthwhile to present them now. Hopefully a more thorough understanding of the ventilation phenomena will be obtained in the not too distant future.

Files

Additional details