Jet Noise Analysis of a Mixed Turbofan Engine Jens TRÜMNER 1 ; Christian MUNDT 2 1,2 Institute for Thermodynamics, UniBw München, Germany ABSTRACT Due to constantly increasing flight traffic the aircraft industry is faced with major challenges. Essentially these are reduction of fuel consumption and noise emission. The latter plays an outstanding role in the vicinity of airports where residents are subjected to increased noise disturbance. Strict regulations protect these citizens and force airlines to decrease the noise emission of their fleet to stay cost efficient. Noise is generated by diverse mechanisms at several parts of the aircraft. During take-off jet noise is the most important source since the engines operate at maximum thrust condition. The development of turbofan engines drastically reduced jet velocities and thus sound emission. A further improvement could be achieved by adding forced mixers and expanding both, bypass and core flow through a common nozzle. These mixers increase the energy transport from hot to cold flow and thereby further decrease the maximum jet velocity. In this work such a mixer is aeroacoustically studied using a hybrid approach to determine the noise level at distant observer points. Keywords: Jet noise, DES, Ffowcs-Williams & Hawkings Analogy 1. INTRODUCTION With the development of civil jet engines in the early 50 th the importance of aerodynamically generated noise became obvious. The work of Lighthill(1,2) laid the foundation for a new field of research known as aeroacoustics. Lighthill derived an exact formulation resembling wave equations from classical acoustical problems with a source term depending on the flow field. From these findings he could show that the sound intensity scales with the 8 th power of the jet velocity. Lighthill's analogy was later extended to shear flows by Lilley(3), solid objects in the source region by Curle(4) and objects in motion by Ffowcs-Williams and Hawkings (FW-H)(5). Initially acoustic analogies helped to better understand the fundamental mechanisms of sound generation while engineering problems were investigated experimentally. With increasing computational resources it became possible to numerically integrate the wave equation with its sources and thereby determine the sound pressure level (SPL) at arbitrary observer points. Experiments by Seiner et al.(6) show that noise generation is not only determined by the jet velocity, but also by temperature gradients. Lighthill's 8 th -power law fails on hot jets because he neglected the dipole term for density fluctuations. Tam et al.(7) proved analytically that temperature gradients also increase the instability of Kelvin-Helmholtz waves and thus the growth of turbulent shear layers. The latter finding does not directly affect the source terms in acoustic analogies but leads to an under prediction of Reynolds stresses in most turbulence models and therefore indirectly of Lighthill's stress tensor. To determine the noise level of aircraft engine jets at certain observer locations different approaches are used. Empirical and semi-empirical models such as ISVR/Purdue by Tester et al.(8) are very well validated with scale and full-scale exhaust systems. Theoretically one could directly compute the transient jet and the wave propagation to the observer in a single simulation. However, this would lead to a huge computational domain and by far exceed arguable calculating capacity. Even a transient simulation of a domain which only covers the free jet is not practicable today. Therefore different alternatives have been developed. Tam et al.(9) proposed an adjoint approach to determine the spectral density of the sound pressure field 1 [email protected]2 [email protected]INTER-NOISE 2016 760
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Jet Noise Analysis of a Mixed Turbofan Engine
Jens TRÜMNER1; Christian MUNDT2
1,2 Institute for Thermodynamics, UniBw München, Germany
ABSTRACT
Due to constantly increasing flight traffic the aircraft industry is faced with major challenges. Essentially
these are reduction of fuel consumption and noise emission. The latter plays an outstanding role in the
vicinity of airports where residents are subjected to increased noise disturbance. Strict regulations protect
these citizens and force airlines to decrease the noise emission of their fleet to stay cost efficient.
Noise is generated by diverse mechanisms at several parts of the aircraft. During take-off jet noise is the most
important source since the engines operate at maximum thrust condition. The development of turbofan
engines drastically reduced jet velocities and thus sound emission. A further improvement could be achieved
by adding forced mixers and expanding both, bypass and core flow through a common nozzle. These mixers
increase the energy transport from hot to cold flow and thereby further decrease the maximum jet velocity.
In this work such a mixer is aeroacoustically studied using a hybrid approach to determine the noise level at