Technical Notes Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet Rajan Kumar, * Anjaneyulu Krothapalli, † and Isaac Choutapalli ‡ Florida State University, Tallahassee, Florida 32310 DOI: 10.2514/1.J050026 I. Introduction P ULSED jets have a variety of applications ranging form pulsing blood ﬂow in the human heart (Gharib et al. [1]) to pulsed- detonation engines (PDEs). The ﬁrst application of a pulsed-jet engine was during the World War II for propelling a ﬂying/buzz bomb (Manganiello et al. [2]). Pulsed-jet engines are known to yield higher levels of thrust, light in weight and simple in design, and therefore have found interest in variety of applications such as radio controlled small aircraft to the vertical takeoff and landing aircraft. The pulsation is known to play an important role on the engine thrust performance. Theoretical analysis by Seikman [3] on a two- dimensional planar and Weihs [4] on an axisymmetric nozzle show that for the same nozzle exit mass ﬂow, the average thrust of a pulsed jet is much higher than the equivalent steady jet. The present experiment is conducted in the context of a study on pulsed-jet ejectors that are shown to produce thrust augmentation ratio (total thrust/primary pulsed-jet thrust) of about 2 (Choutapalli et al. [5]). In a pulsed jet, the ﬂuid ejected from the nozzle rolls up into a vortex ring, which in turn moves downstream (Fig. 1). When a jet is pulsed continuously, a sequence of large-scale structures or vortex rings is produced at regular intervals with jetlike ﬂows in between. In the context of jet mixing and far-ﬁeld noise studies, many investigations have been carried out on acoustically exited jets with a focus on dynamics of the resultant coherent structures. A notable study by Crow and Champagne [6] generated small pulsation by forcing an axisymmetric steady jet using a loud speaker to produce coherent large eddies. The exit velocity of the jet was relatively constant and the pulsation was simply a perturbation to the ﬂow. Hence, it may be considered as the perturbed jet. Here, the pulsed jet is referred to as one in which variation with time of the exit velocity is large and of the same order of magnitude as the mean exit velocity. Even though, the pulsed jet shows vortex rings similar to those of perturbed jets, they are much stronger resulting in a jet that has very different characteristics than those of a corresponding steady jet [7]. Most of the previous investigations on pulsed jets have been focused on the trust augmentation and associated turbulent vortex structures including the recent detailed investigation by Choutapalli et al. [5] and the references there in. Choutapalli et al. [5] provided a detailed understanding of the ﬂow physics responsible for the increased thrust. Detailed ﬂowﬁeld analysis using particle image velocimetry (PIV) showed the evolution of vortices and their role in enhanced mass entrainment and mixing. The connection between the large-scale structures and the far-ﬁeld noise of a jet has been a subject of many studies and a summary can be found in Arakeri et al. [8]. Near-ﬁeld microphone measurements, for example by Mollo-Christensen [9], have shown that pressure ﬂuctuations arrive in rather well-deﬁned wave packets, suggesting them to be associated with the well organized structures existing within the jet. It is also known that the presence of coherent vortical structures in supersonic jets operating at off design conditions (e.g. screeching jets) show a signiﬁcant increase in the near-ﬁeld sound pressure levels. Alkislar et al. [10] have shown that the near-ﬁeld screech amplitude is related to the coherent vorticity strength. With these observations in mind, it is suggested that vortex rings in a pulsed jet will generate signiﬁcant near-ﬁeld pressure ﬂuctuations which will result in increased overall sound pressure level (OASPL). Hooker and Rumble [11] have experimentally studied the noise of the rotating valve simulator to simulate the exhaust cycle of a pneumatic rock drill and reported the dependence of air supply pressure on the measured sound pressure levels. He and Karagozian [12] performed numerical simulations (quasi-1-D computations) on a number of nozzle shapes to study the effect of geometry on the performance and noise characteristics of a PDE. Most of their noise estimates were made inside the nozzle and in the jet centerline. In general, their estimates indicate that noise levels produced by convergent or divergent nozzles are slightly less than straight tubes. Shaw et al. [13] carried out acoustic measurements on a PDE being developed at the U.S. Air Force Research Laboratory’ s Propulsion Directorate. The tests were conducted with one of the four tubes of 1 in. diameter and the engine was ﬁred at a pulsing frequency of 20 Hz. The measurements included a microphone array located at 13 diameters from the nozzle exit and the OASPL measured were in the range of 147 to 159 dB, depending upon the location of the microphone. However, little information exists in the literature on the near-ﬁeld acoustic characteristics of a pulsed jet, with the thoroughness required to provide some guidance in estimating the increased OASPL over a corresponding steady jet. Hence, this study attempts to investigate the role of pulsation frequency and jet exit velocity on the near-ﬁeld noise. The mea- surements were made over a range of subsonic jet Mach numbers and pulsing frequencies. Many of the parameters chosen here are consistent with those of Choutapalli [7]. II. Experimental Techniques A. Test Facility The experiments were carried out in the pulsed-jet facility (Fig. 2) of the Advanced Aero Propulsion Laboratory located at the Florida State University. The facility was designed to produce pulsed jets at high subsonic Mach numbers with a stagnation temperature up to 800 K. High-pressure compressed air (15 MPa) is stored in large storage tanks (10 m 3 ) and is used to drive the facility. The stagnation pressure and temperature in the settling chamber was maintained steady by a dome regulator combined with low pressure pneumatic valve and an inline ﬂow through electrical heater with suitable controllers. The pressure and temperature were maintained to set conditions within a variation of 2 kPa and 0:5K, respectively. The settling chamber assembly housed a rotating disk used for chopping the ﬂow. The rotational speed of the rotating disk was controlled by a DC motor (Baldor Smart Motor, model CSM3615T- 2) through a shaft. The rotating disk (D  475 mm) has six equally spaced circular holes with a nominal diameter of 76 mm. The pulsing frequency can be varied by changing the rotational speed of the Received 24 June 2009; revision received 25 October 2009; accepted for publication 31 January 2010. Copyright © 2010 by Rajan Kumar, Anjaneyulu Krothapalli, and Isaac Choutapalli. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0001-1452/10 and $10.00 in correspondence with the CCC. * Research Scientist, College of Engineering, Department of Mechanical Engineering. Senior Member AIAA. † Eminent Scholar and Professor, College of Engineering, Department of Mechanical Engineering. ‡ Currently Assistant Professor, Department of Mechanical Engineering, University of Texas—Pan American. Member AIAA. AIAA JOURNAL Vol. 48, No. 6, June 2010 1256