Investigations on a Mechanism to Induce Free-Stream Turbulence in a Water Channel by Controlled Injection of Air Bubbles

Abstract

In this paper we investigate a new approach to influence the free-stream turbulence (FST) in a water channel. The mechanism induces air bubbles into the mean flow, which naturally rise to the water surface. The air bubbles and their wakes are capable of increasing the low turbulence intensity in the facility’s test section by more than one order of magnitude. It is possible to control the FST by varying the air pressure of the generator. The resulting turbulence intensity increases rapidly for the lowest setting and shows an asymptotic behaviour for higher working pressures. By installing an additional flow control screen we are capable of reducing the overall turbulence intensities and hence increasing the controllability of the FST. Besides the analysis of the flow-field with hot-film sensors, we are able to show streaky structures in the laminar boundary-layer on a flat plate.