Physics and Control of Flow and Acoustics in Low Aspect Ratio Supersonic Rectangular Twin Jets

All dates for this event occur in the past.

Scott Lab E525
201 W. 19th Avenue Columbus, OH 43210
Columbus, OH 43210
United States

Name: Ata Esfahani

Abstract:
The propulsion/airframe integration benefits of non-axisymmetric nozzles have led to renewed interest in their integration into future generations of aircraft design. Rectangular nozzles can offer significant benefits in terms of drag reduction, improved mixing for heat signature reduction and ease of implementing thrust vectoring. Aircraft with high- and low-aspect-ratio rectangular nozzles have already been operational for years and the recent interest in developing manned and unmanned platforms with such nozzles integrated with the airframe underscores the need for further development and understanding the physics of flow in such geometries. Jet noise emitted from the hot, high-speed jet plumes of high-performance tactical fighters severely affects the crew and communities exposed to it. Furthermore, interaction and coupling of jet plumes in twin-engine tactical aircraft has the potential to cause structural fatigue and failure due to elevated pressure fluctuations. This work seeks to address these issues by studying the physics of flow and acoustics in low aspect ratio rectangular twin jets (RTJs) and implementing active flow control to alleviate near-field pressure fluctuations and far-field noise. One of the major contributions of the present work is the extensive characterization of baseline RTJs in a wide range of operation conditions (jet Mach number, Mj, or nozzle pressure ration, NPR) to better understand the underlying processes that drive flow and acoustic behavior of these jets. The second contribution of this work is to implement active flow control with localized arc filament plasma actuators (LAFPAs), the control authority of which has been demonstrated in a wide range of high-speed flows. In the present work, LAFPAs have been used to manipulate the growth and development of large-scale structures (LSS) in jet shear layers and thus affect the flow-field and acoustics of RTJs. The twin jet setup studied in this work consists of two military-style, sharp throat, rectangular nozzles with an aspect ratio (AR) of 2, placed side-by-side along the major axis of the jets with a center-to-center spacing of 2.25 nozzle equivalent diameter (De). The design Mach number (Md) of the jets is 1.5 and the baseline jets have been operated and characterized between Mj = 1.20 and 1.90. Various diagnostics techniques used in this work include near- and far-field measurements using free-field microphones and flow-field imaging with high-speed schlieren. Results indicate that the baseline jets tend to adopt an anti-symmetric screech mode throughout the wide range of Mjs considered in this work. The RTJs show intermittent out-of-phase coupling followed by steady out-of-phase coupling for the Mjs that are close to Md in the overexpanded regime. As the Mj is increased, the jets show strong, in-phase coupling in the underexpanded regime. The screech tone amplitude shows a strong dependence to the strength of the screech loop and significant downstream and upstream directivity. Ambient conditions, mainly relative humidity (RH) and temperature (ambient and jet stagnation) have shown to affect the jet screech amplitude and coupling mode of the jets in the overexpanded regime, respectively. Far-field noise of the jets showed strong directivity, with OASPL and screech tone amplitude both being higher in the minor axis plane of the jets. Jet shielding was also observed in the far-field of the jets with its effect being more pronounced at shallow downstream angles but persisting at polar angles of up to 75°. The flow-field of the baseline jets showed the presence of standing waves at both overexpanded and underexpanded regimes in the minor axis plane of the jets, but no sign of standing waves was found in the major axis plane. An empirical screech and coupling closure mode developed during the present work showed great utility in predicting the coupling mode of RTJs and was used in the excited flow experiments to intelligently select the excitation frequency (Ste). LAFPAs demonstrated good control authority throughout the overexpanded regime and for a considerable range of underexpanded range of Mj investigated. It is possible to change the screech/ coupling mode of RTJs and strengthen the coupling loop or decouple the jets using excitation. The most viable strategy for decoupling RTJs was found to be excitation at Ste > Sts but within jet column mode (Sts < Ste < 0.6). Doing so removes energy from the screech loop and diverts it establish the new screech loop, which is not as strong as the baseline screech loop, but not a coupling loop. The flow-field of the jets showed increased mixing if the jets were excited at Ste = Sts. This is mainly due to strengthening of LSS that are effective at transporting momentum across the shear layer. Forcing the jets to couple out-of-phase via excitation at Ste = Ste was found to be effective in reducing the near-field pressure fluctuations in the symmetric plane of the RTJ assembly and close to the nozzle lips by up to 9 dB. This was found to be due to the destructive interference between the upstream propagating acoustic feedback waves from the adjacent jets. Other regions in the near-field of the jets (mainly further downstream), did not show a dependence of pressure fluctuations on the coupling mode of the jets. Finally, the effects of excitation on the far-field noise level of RTJs were investigated. De-toned OASPL and OASPL levels of considered cases in major and minor axis planes of RTJs did not show a strong dependence on coupling phase of the jets. Given that the far-field noise of RTJs is not dependent on coupling phase of the jets, changing the coupling phase is not a viable strategy for far-field noise reduction. Rather, it was found that a combination of high frequency excitation (0.6 < Ste < ~ 1, upper limit dictated by pulsers’ maximum frequency) and promoting three-dimensionality in the jet shear layers can lead to far-field noise reductions of up to 2 dB in the shallow downstream angles. This was due to a reduction in coherence of LSS, the dynamics of which are responsible for emitting noise to these angles. The practical applications of flow control effects achieved in this work for heat signature management, reducing acoustic loads structures, and far-field noise reduction of high-performance tactical aircraft are also discussed.

Zoom Link (or alternative) - if available
https://osu.zoom.us/j/98251960301?pwd=N0JRZVRSOUU2a2tteEJ2WmRZUFFvdz09 Meeting ID: 982 5196 0301 Password: 0173

Committee Members
Professor Mo Samimy
Professor Lian Duan
Professor Datta Gaitonde
Dr. Nathan Webb
 

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