Numerical simulations were performed to investigate the transient compressible flow through lobed mixers inside the internal mixing nozzles of turbofan engines. The Lattice Boltzmann Method (LBM) was used. A detailed model of the mixer and the nozzle was created to capture both the internal flow through the mixer and the external jet plume. The mean flow characteristics were obtained both inside the nozzle and within the jet plume. The transient behavior of the streamwise vortices at the nozzle exit was visualized and quantified. A confluent mixer was selected as a baseline to investigate the performance of both the low and high penetration lobed mixers. The goal was to better understand the detailed noise reduction mechanisms of lobed mixers, as well as thrust enhancements. The Reynolds number based on nozzle exit diameter was 1.36×10 6 and the peak Mach number reached 0.5. The low-Mach setting is to abide by the constraints of the 19-stage LBM algorithm used in this study. The sub grid scales were modeled using the renormalization group (RNG) forms of the standard k-e equations. Far-field sound was computed using the porous Ffwocs William-Hawkings (FWH) surface integral method. The results suggested an increase in thrust coefficient as expected for the lobed mixers. The far-field sound analysis showed considerable low-frequency noise reduction (i.e. ~4-5 dB) for the lobed mixers, as well as about 3dB reduction in the overall sound pressure level (OASPL) in comparison with the confluent nozzle. Both near field and far field results and trends were as expected based on available subsonic experimental data.

http://www.researchgate.net/profile/Alireza_Najafi-Yazdi/publication/263925478_Numerical_Simulations_of_Sound_radiated_from_Internal_Mixing_Nozzles_with_Forced_Mixers_using_the_Lattice_Boltzmann_Method/links/00b4953c54a1bd7542000000.pdf

Prediction of the Sound radiated from Low-Mach Internal Mixing Nozzles with Forced Mixers using the Lattice Boltzmann Method

A comprehensive database for the acoustic and aerodynamic characteristics of several model-scale lobe mixers of bypass ratio 5 to 6 has been created for mixed jet speeds up to 1080 ft/s at typical take-off (TO) conditions of small-to-medium turbofan engines. The flight effect was simulated for Mach numbers up to 0.3. The static thrust performance and plume data were also obtained at typical TO and cruise conditions. The tests were done at NASA Lewis anechoic dome and ASK's FluiDyne Laboratories. The effect of several lobe mixer and nozzle parameters, such as, lobe scalloping, lobe count, lobe penetration and nozzle length was examined in terms of flyover noise at constant altitude. Sound in the nozzle reference frame was analyzed to understand the source characteristics. Several new concepts, mechanisms and methods are reported for such lobed mixers, such as, "boomerang" scallops, "tongue" mixer, detection of "excess" internal noise sources, and extrapolation of flyover noise data from one flight speed to different flight speeds. Noise reduction of as much as 3 EPNdB was found with a deeply scalloped mixer compared to annular nozzle at net thrust levels of 9500 lb for a 29 in. diameter nozzle after optimizing the nozzle length.

Lobed Mixer Design for Noise Suppression Acoustic and Aerodynamic Test Data Analysis

Acoustic evaluation of a forced mixer on a high bypass ratio engine

Acoustic test results are presented for 1/4th-scaled nozzles with internal lobed mixers used for reduction of subsonic jet noise of turbofan engines with bypass ratio above 5 and jet speeds up to 830 ft/s. One coaxial and three forced lobe mixers were tested with variations in lobe penetration, cut-outs in lobe-sidewall, lobe number and nozzle-length. Measured exit flow profiles and thrusts are used to assist the inferences from acoustic data. It is observed that lobed mixers reduce the low-frequency noise due to more uniformly mixed exit flow; but they may also increase the high-frequency noise at peak perceived noise (PNL) angle and angles upstream of it due to enhanced mixing inside the nozzle. Cut-outs and low lobe penetration reduce the annoying portion of the spectrum but lead to less uniform exit flow. Due to the dominance of internal duct noise in unscalloped, high-penetration mixers their noise is not reduced as much with increase in free-jet speed as that of coaxial or cut-out lobed mixers. The latter two mixers also show no change in PNL over the wide range of nozzle-lengths tested because most of their noise sources are outside the nozzle; whereas, the former show an increase in noise with decrease in nozzle-length.

/pdf/noise-reduction-with-lobed-mixers-nozzle-length-and-free-jet-5b5jdmc6k3.pdf

Noise reduction with lobed mixers - Nozzle-length and free-jet speed effects

Numerical simulation of the transient compressible flow through lobed mixers inside turbofan engine nozzles was performed. A novel entropic Lattice Boltzmann (LB) scheme was used for the computational studies. The very large eddy simulation scheme (LBMVLES) was used to capture sub-grid scale flow structures. The sound generation process was investigated using the porous Ffwocs William-Hawkings (FW-H) surface integral method. A contoured dual-stream nozzle with a standard 12-lobed mixer (12CL) was selected as a test case to validate the LBM simulation. A detailed model of the mixer and the nozzle was concurrently created to simulate both the internal flow through the mixer and the external jet plume. All operating boundary conditions were imposed based on available measured data for the same mixer geometry. The transient and mean flow characteristics of the flow field were obtained both inside the nozzle and within the jet plume. The transient behavior of the streamwise vortices at the nozzle exit was visualized and quantified. The Reynolds number based on nozzle exit diameter was 2.02×10 6 and the acoustic Mach number reached 0.94. Both near field and far field results and trends were in good agreement with experimental data. Specifically the predicted averaged velocity field was within the range of measure values. The overall radiated sound pressure was within 2-3dB of measured sound at all directional angles.

A. Barta

Papers

Development of an advanced exhaust mixer for a high bypass ratio turbofan engine