Download Contributions regarding the development of a highly

Contributions regarding the development of a highly
aerodynamically efficient chevron for use in turbofan
noise mitigation – the rhino chevron
Author: Eng.Valeriu Drãgan
Affiliation: the Faculty of Aerospace Engineering- “Politehnica” University of
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Contributions regarding the development of a highly
aerodynamically efficient chevron for use in turbofan
noise mitigation – the rhino chevron
Abstract
Current non-regional airliners rely on turbofan technology for propulsion. The main
concerns regarding noise pollution with regard to these engines is the fan flow interaction noise.
The phenomenon that causes this noise has been analyzed and modeled mathematically since Sir
Cyril Lighthill’s eight power law. Mitigation of the interaction noise has numerous technical
solutions, of which chevron technology stands out. However, airlines are hesitant to adopt these
solutions because of their poor aerodynamic performance. For this reason the last few years have
seen the development of retractable chevron technologies, such as mechanical retraction and
shape memory alloy chevrons. Such technologies are far from solving the efficiency issue,
because both require constant maintenance.
This paper tries a different approach: by studying the way chevrons interact with the flow
field, we could determine the mechanisms that lead to aerodynamic losses and try to eliminate
them.
Aerodynamic losses have been found in regular chevrons to be caused by a pair of
parasitic vortices on the top side of the chevron. By placing a divider between them we were able
to eliminate them almost completely. Preliminary results are encouraging for this low cost high
aerodynamically efficient chevron.
1. Introduction
This paper tries to investigate wais to reduce turbofan noise by understanding the
aerodynamics of turbofan noise generation and also seek to provide with a low cost solution for
mitigating the generating mechanisms.
The main noise output of the turbofan is due to the Kelvin-Helmholtz phenomenon which
occurs whenever two fluids in contact have different velocities. This phenomenon was first
described in a mathematical way by Sir Cyril Lighthill who gave the so called “eight power law”
Eq (1).
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Since then, various refinements have been made to insure a closer link to the natural
phenomenon. In this paper we will use Proudman Eq.(2) and Lilley’s Eq.(3) equations for
calculating both the Sound Pressure Level (SPL) and Sound Intensity Levels (SIL).
In order to mitigate jet interaction noise a series of proposals have been made including
chevrons and fluid chevrons. This paper will investigate how chevrons interact with the flow
from the turbofan and also compare conventional chevrons to an original new chevron,
optimized for reducing drag and parasitic turbulence: the rhino chevron.
As it will become apparent, virtually any chevron –and in fact all noise reduction
devices- will induce a different, lower noise as it mitigates the main one.
Using the two above mentioned formalisms we will also compare the noise made by
conventional chevron interaction and the proposed rhino chevrons.
V8  L
I ( x)   0 5  
c x
2
(1)
Where: I(x) is the sound intensity observed at the distance x from the source
V is the flow velocity difference
c is the velocity of sound in the flow
L is the diameter of the nozzle
(2)
Where:
(2.1)
(2.2)
(2.3)
Ф=0.42
(2.4)
(3)
Where:
(3.1)
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(3.2)
P
SPL  10 log 10 
P
 ref




(4)




(5)
Where Pref is the reference pressure
 I
SIL  10 log 10 
I
 ref
Where I ref is the reference intensity
1.1 The state of the art
In their 1964 US patent Ref. [1], Young et al. describe what is considered to be the first
chevron devices-along with various other quite interesting solutions. Since then the main
conceirn of chevron designers was to control the immersation, i.e. retract the chevrons after the
take-off and re-immerse them for the landing procedures.
Mechanically actuated chevrons are described in Ref.[2], however these chevrons were
never applied to a commercial turbofan. Further development of the technology introduced shape
memory alloys, described in Ref [3]. The Nitinol strips inside the composite chevron are handled
by means of a resistive heater that commands the shape of the metallic insertion and varies the
angle of immersion of the chevron. A downside of this technique is that it is fairly expensive to
buid and requires constant “programming” of the shape memory alloy every 40-60 cycles.
Other devices such as the so called “fluid chevrons” were studied in Ref [4] in both
stationary and pulsating mode showing better results in the latter, however the devices used are
cumbersome and, thus far, too heavy for any practical application. Also, the mass flow of the air
used for generating the pulses is fairly large 4-7% hence adding more stress on the turbine engine
that provides it.
It can be stated that for the moment, actuated chevrons are not practical since their use
requires complications that outweigh the benefits.
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It is therefore the purpouse of this paper to investigate a new type of chevron that is
inherently more aerodynamic and thus does not require its retraction during level flight.
2.The use of chevrons in fan flow interaction noise mitigation
. The general accepted theory regarding chevron noise mitigation properties is that, by
generating controlled vortices along the circumference of the nozzle, the boundary between the
slow and the fast flowing flux will become less abrupt, hence lessening the downstream KelvinHelmholtz instability.
It is also important to point out that the noise generating region will occur downstream
from the nozzle somewhere between 10-30 times the diameter of the nozzle fig.1.
Fig.1 The noise generating region
Judging the efficiency and effectiveness of the chevrons we will take into account three
different aspects, namely: ability to generate vortices along the axis of the main flow-critical to
achieving the main goal of “defusing” the Kelvin-Helmholtz instability, the turbulent kinetic
energy of the displaced flow- as a measure of aerodynamic efficiency and also the SPL and SIL
associated with the chevron itself, the self-noise of the chevrons, an important aspect for further
improvement of the technology.
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2.1. Vortex generation of the chevrons-comparative study
In order to better judge the effectiveness of the chevrons, trajectory plots have been
drawn to show exactly how the vortices are generated. Theory states that for an effective mixing,
two counter rotating axial vortices must be created downstream of the chevron.
The parameters of these vortices depend on the flow conditions and on the geometric
parameters of the chevrons themselves, by which we understand also the angle of immersion.
The conditions for both of the simulations were :
Pressure=1 atm
Velocity of the fast stream=300 m/s
Temperature= 300 K
Angle of immersion = 15°
viscosity model: k-epsilon standard
The rhino chevron was developed as a result of trajectory studies on the conventional
chevrons. It has been found that on conventional chevrons two parasitic counter rotating vortices
arise perpendicular to the upper surface. By placing a dividing wall between them the two
vortices are prevented from taking place.
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Fig. 2 Frontal view trajectory plot of the normal chevron (left) and rhino chevron (right)
Comparing the two trajectory plots we can observe quite easily how, on a normal
chevron, parasitic turbulence is created right on top of the chevron surface. These parasitic
vortices are energy consuming – as we can extrapolate from the lowering of the flow velocity –
and also play an important role in lessening the ability of the conventional chevron to produce
the desired axial vortex pair.
By adding the separation wall, the rhino chevron is able to reduce the parasitic turbulence
on the chevron, consuming less energy from the flow whilst improving the intensity of the
desired axial vortex pair.
Fig.3 Top view of velocity trajectory plots for regular chevrons (top) and rhino chevrons (bottom)
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2.2 Turbulent kinetic energy
As it became apparent in chapter 2.1, there are significant differences between the flows
around the two considered chevron types. The main difference being the two parasitic counter
rotating vortices that rise perpendicular to the chevron surface.
Studying the turbulent kinetic energy (TKE) we are able to determine exactly how much
energy is lost by this undesired phenomenon and compare it with the rhino chevrons.
Figure 4 shows a top view of the two chevrons with TKE plots.
The mean value or the Turbulent Kinetic Energy lost for the normal chevron is
approximately 1600 J/kg whilst for the rhino chevron it is of approximately 700 J/kg, less than
half.
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Fig.4 Side view of TKE trajectory plots for regular chevrons (top) and rhino chevrons (bottom) note the trajectory
differences between the desired longitudinal vortices of the two chevrons
2.3. Noise associated with the chevrons themselves
Due to the purpose of the chevrons i.e. to reduce jet interaction noise, it is also important
to consider the effects of the turbulences induced by their presence.
Although significantly less nosy than the main phenomenon of flow interactions, chevron
induced noise may become a comfort factor for passengers. This is especially true for the current
fixed position chevrons that cannot be retracted after take off and landing.
Figure 5 shows a SPL plot comparison between the two chevrons considered in this
paper, the plane of the cut plot is parallel to the side plane and slices the chevron in half.
Significant differences can be observed here as well, mainly due to the extreme parasitic
turbulence observed on the regular chevron and the lack of such parasitic turbulence on the rhino
chevron.
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Fig.5 Side view of Sound Pressure Level in dB cut plots for regular chevrons (left) and rhino chevrons (right)
3. Conclusions
The paper researches the way current turbofan noise mitigation devices operate, with a
focus on chevrons, namely uncovering the aerodynamic mechanisms at work in their usage.
Also, this paper offers a comparative study of an aerodynamically improved chevron,
named “rhino chevron” providing both explanations regarding it’s construction and relevant
comparison parameters.
Turbulence CFD studies have been carried out with encouraging results, yielding a
reduction in turbulent losses from 1600J/kg of a conventional chevron to 700J/kg of the proposed
rhino chevron.
In the end, the paper provides a noise plot test to compare the noise generated by the
chevrons themselves based on the Proudman formalism. Also in this case, the rhino chevron
shows better results, with lower Sound Pressure Levels.
4.References
[1] Young et al. US3153319
[2] US6813877 (Rolls Royce)
[3] Benchtop Demonstration of an Adaptive Chevron Completed Using a New HighTemperature Shape-Memory Alloy Dr. Ronald D. Noebe, Dr. Todd R. Quackenbush, and Dr.
Santo A. Padula II NASA Glenn Research Center
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[4] Barton et al. US7159383
[5] Balzer US6612106
[6] Reba et al. US20070033922
[7] US6718752 (G.E.) 2004
[8] 2002I. Proudman, “The Generation of Noise by Isotropic Turbulence”, Proc. Roy. Soc.
London, A214, 119 (1952)
[9] Design, fabrication, and testing of a SMA hybrid composite jet engine chevron Turner et. al.
[10] Seiner US20070246293
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