The Starting Process of a Pulsed Jet as seen by Schlieren Measurements

Abstract

Pulsed jets are amongst the most common measures of active flow control. For example applied to airfoils, stall effects can be dampened or even suppressed, and, hence, the aerodynamic performance can be improved. Although the implementation is more complex, pulsed jets often prove to be a superior alternative to their continuous counterparts. The current jets were designed with respect to the dynamic stall behavior of helicopter rotor blades. Dynamic stall is a well-known limit for forward flight speeds or maneuver loads that can be addressed using flow control. The current work investigates the starting process of a pulsed jet by means of the Background Oriented Schlieren Technique (BOS), using both ‘standard’ and ‘laser-speckle’ background patterns. In agreement with PIV measurements, the results reveal that the initial structure of the start-up process consists of a vortex ring with increasing diameter but constant convection velocity. The vortex is followed by a turbulent wake and a steady but unadapted supersonic jet with corresponding cell structure. The significance of the results is increased by a volumetric reconstruction of the density data, offering further details of the spatial structures. The experimental setup of BOS is not only more convenient in comparison to a standard PIV setup, but also reveals very different phenomena in a single exposure, without additional considerations regarding the follow-up behavior of tracer particles, double-frame time shift, etc.