The flow and acoustic fields of an ideally expanded Mach 1.3 axisymmetric jet with delta tabs were examined to explore the effects of the tabs on noise sources. This work continues research that was performed on a baseline (no-tab) jet. Noise measurements were made at an angle of 30t o the downstream jet axis to allow a direct comparison to previous work, and to relate the sound generation mechanisms to the large structures that were visualized with temporally resolved flow visualization.

more »

... onal acoustic measurements were made at 60˚ and 90˚ locations. Three cases were examined: a baseline jet, a single delta tab jet, and a dual delta tab jet. Both tab jets were operated at the same pressure ratio as the baseline jet, which was ideally expanded. Power spectra and average acoustic waveform measurements were made for a variety of azimuthal locations; apparent noise origins were estimated with a 3-D microphone array; and temporally resolved flow visualization was used to examine the dynamic flow structure of the jet's mixinglayer. The results confirm that the tabs generate strong streamwise vortices that have a significant effect on both the flow and acoustic fields of the jet. The tabs cause significant deformation in the cross-stream plane of the mixing-layer, as well as regulating the formation and roll-up of vortices due to Kelvin Helmholtz instability. With the addition of tabs, the noise field becomes azimuthally dependent and the region of noise generation moves dramatically upstream. It appears that the tabs are directly responsible for an increase in noise over a range of Strouhal numbers between 0.8 and 2.5 through generated streamwise vortices and they are indirectly responsible for the modification of the noise generating mechanisms at Strouhal numbers below 0.6 through the induced spanwise vortex roll-ups. ACRONYMS AND SYMBOLS D Jet diameter, equivalent to 1 inch (0.025 m) σ Standard deviation of acoustic pressure St D Strouhal number based on jet diameter SPL Sound pressure level (given in dB) U Jet exit velocity, equivalent to 380 m/s U c Convective velocity for the jet, equivalent to 270 m/s aeroacoustics volume 2 · number 1 · 2003 -pages 35 -63 35 eighth power of its exit velocity. By utilizing larger by-pass ratio engines, the effective velocity of the exhaust has been significantly reduced, and the noise levels have dropped. However, additional reductions in noise levels are required and the exhaust velocities of today's engines cannot be lowered much further. As such, different approaches are required for noise reduction. This work is a continuation of research that has focused on how large turbulence structures create noise [2] [3] . It is expected that more effective noise control strategies could be developed with such knowledge. The specific focus of this work is how vortex-generating tabs modify the noise generation mechanisms of an ideally expanded, high-speed jet. The noise from an ideally expanded jet has been traditionally thought to have two principal components. One component radiates preferentially in the downstream direction and is associated with the large-scale turbulence structures within the jet's mixing-layer. The other component comes from the small-scale turbulence structures and radiates uniformly in all directions. The exact nature of the mechanisms for the two components is not well understood, but their difference is apparent when sound spectra from various measurement angles are compared. For the baseline (no-tab), Mach 1.3 jet of this study, the sound radiation over a region that is normal to the jet axis is uniform over most frequencies; however, at shallow angles (around 30˚, as measured from the downstream jet centerline), the acoustic spectrum has a broadband peak between 2 and 4 kHz (St D = 0.13 to 0.27) [2]. This low frequency, large amplitude peak has been associated with the large-scale structures of the flow. Since such structures are the easiest to measure and ultimately to manipulate, the current jet aeroacoustics work at the Gas Dynamics and Turbulence Laboratory has focused on sound measurements made at the 30˚ location. The main objective of this long-term study is to relate the sound generation to time-dependent processes of these structures. In a baseline jet, the large amplitude turbulent mixing noise originates mostly from a region surrounding the end of the potential core [3] [4] [5] [6] [7] . The higher frequency noise appears to emanate from nearer the jet exit, while lower frequency noise comes from regions of the flow that are further downstream [8] [9] [10] . In studies of low-speed jets and mixing-layers, the noise creation process has been linked to vortex pairing [11] [12] [13] . However, Bridges and Hussain [14] argue that for vortex pairing to be the dominant noise generation mechanism in low-speed jets, the pairing needs to be asymmetric. A direct numerical simulation of a low Reynolds number, Mach 0.9 jet showed the farfield noise originates from a region where the computed Lighthill noise sources were both strong and rapidly changing in time [15] . Sarohia and Massier [16] performed experiments using high-speed schlieren motion pictures that were synchronized with 36 Effects of vortex generating tabs on noise sources in an ideally expanded mach 1.3 jet aeroacoustics volume 2 · number 1 · 2003