The Time-Resolved Flow within an Unsteady Ejector Jonathan J H Heffer, Robert J Miller and Chris Freeman Whittle Laboratory, Department of Engineering, University of Cambridge, United Kingdom The flow field within an unsteady ejector has been investigated using experimental and computational techniques. The experimental results show a peak thrust augmentation of 1.4; numerical simulation gives a value of 1.37. It is shown that the vortex ring dominates the flow field. At optimal thrust augmentation the vortex ring acts like a fluid piston accelerating the fluid inside the ejector. A model is proposed for the operation of unsteady ejectors, based on the vortex ring acting like a fluid piston. Control volume analysis is presented showing that mass entrainment is responsible for thrust augmentation. It is proposed that the spacing of successive vortex rings determines the mass entrainment and therefore thrust augmentation. The efficiency of unsteady ejectors was found to vary between 28% and 32% depending on the L/D ratio of the unsteady jet source. I. Introduction Ejectors are passive devices used to increase the thrust produced by a jet. When placed downstream of a jet, ambient fluid is entrained into the ejector, producing an aerodynamic force on the ejector. This force is directed towards the jet; conservation of momentum states that the total momentum of the jet is increased by an amount equal to the force on the ejector. Ejectors can be used with steady jets (steady ejectors) or unsteady jets (unsteady ejectors). This paper concentrates on those driven by unsteady ejectors. Figure 1 Schematic Representation of an Ejector Thrust augmentation is the ratio of the total thrust produced by the jet and ejector to the thrust produced by the jet alone as defined in equation 1. imary Ejector imary imary Total F F F F F Pr Pr Pr + = = φ (1) Previous research into unsteady ejectors has shown that the thrust augmentation can be as large as 2, Lockwood, 1959. By comparison ejectors driven by steady flows have a maximum thrust augmentation of ~1.2. Unsteady ejectors can also be smaller than steady ejectors, which is an advantage in practical applications. These advantages can only be exploited if an unsteady jet is available, Mason and Miller, 2006. 46th AIAA Aerospace Sciences Meeting and Exhibit 7 - 10 January 2008, Reno, Nevada AIAA 2008-117 Copyright © 2008 by J H Heffer. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.