EFFECTS OF A NEEDLE ON HARTMANN-SPRENGER TUBE FLOWS Guoping Xia * , Ding Li † and Charles L. Merkle ‡ The University of Tennessee 411 B. H. Goethert Parkway Tullahoma, TN 37388, USA ABSTRACT A computational analysis of the effects of a needle on a Hartmann-Sprenger tube driven by a Mach two jet is presented. Solutions in the absence of a resonance tube indicate that the ‘ideally’ expanded jet contains a complex, but weak, Mach diamond pattern that is essentially unaffected by a needle. The resonant modes, however, are strongly impacted by a needle. When a needle is not present, a 4 kHz screech mode is generated with an oscillating shock in front of the tube. When a needle is present, the high frequency modes are suppressed and a quarter-wave (~550 Hz), regurgitant oscillation is observed. Lengthening the needle causes the high frequencies to decrease gradually and the quarter wave to increase. Needles longer than the nozzle tend to reduce the amplitude. The results are in quantitative agreement with available measurements and the trends are consistent with results for underexpanded converging nozzles. INTRODUCTION A Hartmann-Sprenger tube is created by aligning a blind tube with the axis of a jet so that the on-coming flow is directed into its open end. This configuration generates a variety of resonant conditions that have attracted the attention of researchers for more than three-quarters of a century. High frequency oscillations * PhD Student; Department of Mechanical Engineering, tudent Member AIAA. S † Research Associate Professor; Department of Mechanical Engineering, Computational Fluids Dynamics Research roup; Member AIAA. G ‡ Professor, Department of Mechanical Engineering; H. H. Arnold Chair of Excellence in Computational Mechanics; enior Member AIAA. S Copyright © 2003 The American Institute of Aeronautics and Astronautics, Inc. All rights reserved. were first observed in a pitot tube by Hartmann [1] while traversing the Mach diamond region of a supersonic jet from an underexpanded convergent nozzle. In later investigations, he reported that the fluctuations were produced in regions of positive pressure gradient by the oscillations of a strong normal shock in front of the tube [2]. Sprenger noted that a similar configuration could generate intense heating at the closed end of the tube through the dissipation generated by shocks inside the tube [3]. In addition, he noted that even subsonic jets could produce heating if a needle were placed along the axis or across the open end of the tube. Since these initial investigations, the mechanics of Hartmann-Sprenger tubes have been the subject of numerous experimental and analytical studies. Most have used the non-uniform Mach diamond flowfield downstream of an underexpanded convergent nozzle [4- 7], but resonance has also been generated in subsonic jets [3, 8] and in ideally expanded supersonic jets [5]. In addition to these experiments in jet flowfields, oscillations have been observed in tubes placed in uniform subsonic [8, 9] and supersonic [9] flows. Hartmann-Sprenger tubes have also been considered for specific applications such as utilizing their heat- generation capabilities for rocket engine ignition, [10]; or using them as fluid actuators for flow control [11- 13]. REVIEW OF PREVIOUS WORK In reviewing the literature it is important to distinguish between Hartmann-Sprenger tubes that make use of a needle or other trip device and those that do not. Many of above-mentioned experiments were conducted without such a device, while others [5, 14] reported that removing the needle caused the oscillations to decay to zero. In the present paper, we look at the role of a needle on the oscillation mechanisms behind a divergent supersonic nozzle of a design used in recent experiments [15]. American Institute of Aeronautics and Astronautics 1