Multistream Twin Supersonic Low Aspect-Ratio Rectangular Jets: Flow and Acoustics Characterization and Passive Noise Mitigation Technology
Abstract
Modern exhaust systems of military aircraft will potentially have a geometry thatresembles that of the F22 with two rectangular engine exhaust nozzles adjacent to each other. Rectangular nozzles have several advantages over a circular ones since they enable easier adjustment of the jet geometry, simpler design of a thrust direction control, produce better mixing, and are simpler to design and fabricate. However, rectangular jets are more susceptible to instabilities such as flapping mode and axis switching. When two rectangular jets are placed next to each other these unstable modes interact with each other. This interaction may resultin fatigue failures of flaps in engine nozzles caused by excessive pressure fluctuations on the inter-nozzle region of closely-spaced two engines. Measurements of pressure level showed that in the two-nozzle case, fluctuations were greater than expected from simply doubling the pressure level induced by a single jet, especially at the screech frequency. The results suggestedthat the interaction between the two engine jets enhanced the amplitude of the flapping instability mode of these jets hence elevating the pressure fluctuations level. It was also shown that nozzle spacing and their Mach number affect the interaction between the jets and the dynamic pressure fluctuations. Small spacing had an opposite effect than a moderate and large spacings.This proposal aims to investigate the physical mechanisms of this interaction and their impact on adjacent structures. The parameters that will be investigated are the impact of the jets??? aspect ratio (ratio between the long and short dimensions of the rectangle), spacing between the jets, differences between the jets??? Mach numbers, nozzles Mach number and nozzle???s temperature. The proposed work will include constructing a laboratory scale twin rectangular jet configuration. The Twin-Jet nozzle has many design parameters that can be varied to produce noise and performance differences. In the first step the flow field of the jets and thenoise generating by them will be studied. The second step will be to add internal secondary flow inside the nozzle to determine its effect on the flow and noise patterns. The final step will be to study the impact of adjacent surfaces on the flow and noise. One configuration will be a ramp that will extend from the nozzles??? lip and will simulate the effect of the aircraft wing when the engines are embedded in the airframe. Another configuration is when the surface is placed in the proximity of the jet, simulating the impact of the ground surface when the aircraft is takingoff.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812582
Entities
People
- Ephraim Gutmark
Organizations
- Office of Naval Research
- United States Navy
- University of Cincinnati