Ph.D. Proposal

Jackson Larisch

(Advisor: Prof. Krish Ahuja)

Noise Reduction Mechanisms and Sources of Noise in Jets Issuing from Lobed Mixer Nozzles

Friday, May 23 

12:00 p.m. 

Montgomery Knight Building 317

Abstract

With the return of SST on the horizon, future supersonic civilian airliners will need to meet stringent noise criteria for certification. As a result of this, such aircraft are likely to employ internally mixed exhaust systems with forced mixers which demonstrate the ability to improve thrust and reduce jet noise during LTO regimes. In such a configuration, the core and bypass exhaust gases are mixed within the engine before being expelled through a final exhaust nozzle. Improved mixing of these streams reduces the average velocity and temperature of exhaust gasses, helping to combat increased levels of thrust-specific jet noise introduced through the use of low-to-moderate bypass jet engines more efficient at higher cruise speeds. One popular jet mixing enhancement device is the so-called lobed mixer or lobed suppressor nozzle. These forced mixer nozzles have petal-shaped corrugations on the nozzle trailing edge which greatly increase the contact perimeter between the jet and ambient (or core stream and bypass stream if used internally) and introduce streamwise vorticity to rapidly inject quiescent gas into the jet. However, these two mechanisms paint an incomplete picture and are insufficient in describing the complex processes by which lobed nozzles enhance mixing and reduce jet noise. As with most jet mixing-enhancement devices, lobed nozzles typically result in decreased levels of jet noise at low frequencies - especially at observation angles close to the jet axis - and substantially increased levels of jet noise at higher frequencies. The former is typically ascribed to the inhibited formation of large-scale turbulent structures within the jet, while the latter is chalked up as mixing noise due to increased levels of fine-scale turbulence. This too fails to provide a comprehensive explanation of the effects of lobed nozzles on jet aeroacoustics. The proposed thesis seeks to provide a detailed explanation of the precise mechanisms by which lobed nozzles combat jet noise through complementary acoustic and flow analyses. This is achieved through various experimental tests involving two novel lobed mixer nozzles of equivalent exit area and a circular, axisymmetric baseline nozzle. The findings of this work are expected to inform the design of future noise suppression devices including nozzles and acoustic liners, as well as provide models to predict jet noise reduction and mixing noise levels. 

Committee 

• Prof. Krishan K. Ahuja – School of Aerospace Engineering (advisor) 

• Prof. Lakshmi Sankar – School of Aerospace Engineering 

• Assistant Prof. Beckett Zhou – School of Aerospace Engineering 

• Dr. Joseph Gavin – Preliminary Design, Gulfstream Aerospace Corporation

• Dr. Donald Nance – Senior Scientist, Harris Miller Miller & Hanson Inc