Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet

P ULSED jets have a variety of applications ranging form pulsing blood flow in the human heart (Gharib et al. [1]) to pulseddetonation 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 twodimensional 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 inwhich 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 ofmany 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 inmind, it is suggested that vortex rings in a pulsed jetwill generate significant near-fieldpressurefluctuationswhichwill result in increasedoverall soundpressure 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 wasfired at apulsing frequencyof20Hz.Themeasurements included a microphone array located at 13 diameters from the nozzle exit and the OASPLmeasured 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 measurements weremade over a range of subsonic jetMach numbers and pulsing frequencies. Many of the parameters chosen here are consistent with those of Choutapalli [7].