U.S. patent application number 12/998115 was filed with the patent office on 2011-07-14 for bluff body noise control.
Invention is credited to David Angland, Leung Choi Chow, Michael Goodyer, Xin Zhang.
Application Number | 20110168483 12/998115 |
Document ID | / |
Family ID | 40097839 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110168483 |
Kind Code |
A1 |
Chow; Leung Choi ; et
al. |
July 14, 2011 |
BLUFF BODY NOISE CONTROL
Abstract
The present invention provides a method of and apparatus for,
reducing noise caused by the interaction of airflow between two
bluff bodies which are in a generally tandem arrangement, the
method comprising providing flow control such that the peak
turbulence is at least partially displaced from, or reduced at, the
surface of the downstream body. The invention also provides an
aircraft noise reduction device, and a method of using such a
device, comprising a flow control apparatus (2) arranged to be
positioned downstream of a flow-facing element (1), wherein the
flow control apparatus is arranged, in use, to reduce noise induced
by unsteady flow downstream of the flow-facing element, and wherein
the flow control apparatus (2) is arranged to be moveable in
relation to the flow-facing element (1) between a stowed position
and a deployed position.
Inventors: |
Chow; Leung Choi; (Bristol,
GB) ; Angland; David; (Southampton, GB) ;
Zhang; Xin; (Southampton, GB) ; Goodyer; Michael;
(Southampton, GB) |
Family ID: |
40097839 |
Appl. No.: |
12/998115 |
Filed: |
September 28, 2009 |
PCT Filed: |
September 28, 2009 |
PCT NO: |
PCT/GB2009/051268 |
371 Date: |
March 18, 2011 |
Current U.S.
Class: |
181/226 |
Current CPC
Class: |
B64C 2230/14 20130101;
B64C 25/001 20130101; B64C 2025/003 20130101; Y02T 50/10 20130101;
B64C 7/00 20130101 |
Class at
Publication: |
181/226 |
International
Class: |
F01N 1/08 20060101
F01N001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2008 |
GB |
0819366.6 |
Oct 23, 2008 |
GB |
PCT/GB2008/050983 |
Claims
1. A method of reducing noise caused by the interaction of airflow
between two bluff bodies which are in a generally tandem
arrangement, the method comprising providing flow control such that
the peak turbulence is at least partially displaced from, or
reduced at, the surface of the downstream body.
2. A method of reducing noise as claimed in claim 1, wherein the
flow control is provided by a flow control device on the upstream
or downstream body.
3. A method of reducing noise as claimed in claim 1, wherein the
flow control comprises the use of blowing.
4. A method of reducing noise as claimed in claim 3, wherein the
flow control device is arranged to provide distributed blowing
through the surface of one of the bodies.
5. A method of reducing noise as claimed in claim 3, wherein the
blowing is applied to both sides of the body from 60 degrees to 150
degrees as measured from the leading-edge of the body.
6. A method of reducing noise as claimed in claim 4, wherein the
flow control device comprises a series of plenum chambers
distributed around one of the bluff bodies.
7. A method of reducing noise as claimed in claim 6, wherein the
plenum chambers are covered by an air-permeable plate.
8. A noise reduction apparatus for use on a bluff body which, in
use, is arranged in a generally tandem arrangement with a further
bluff body, the apparatus comprising a flow control device arranged
such that the peak turbulence is at least partially displaced from,
or reduced at, the surface of the downstream body.
9. An aircraft noise reduction device comprising a flow control
apparatus arranged to be positioned downstream of a flow-facing
element, wherein the flow control apparatus is arranged, in use, to
reduce noise induced by unsteady flow downstream of the flow-facing
element, and wherein the flow control apparatus is arranged to be
moveable in relation to the flow-facing element between a stowed
position and a deployed position.
10. An aircraft noise reduction device as claimed in claim 9,
wherein the flow control apparatus comprises a splitter plate
arranged to extend downstream of the flow-facing element when in
the deployed position.
11. An aircraft noise reduction device as claimed in claim 10,
wherein, when in the deployed position, the splitter plate is
substantially aligned with the free stream airflow.
12. An aircraft noise reduction device as claimed in claim 10,
wherein, when in the stowed position, the splitter plate is
orientated to at least partly align with the flow-facing
element.
13. An aircraft noise reduction device as claimed in claim 9,
wherein the flow control apparatus is made from a flexible material
such that the flow control apparatus is deformable between the
stowed and deployed positions.
14. An aircraft noise reduction device as claimed in claim 13,
wherein the flow control apparatus is made from a resiliently
flexible material such that the flow control apparatus naturally
assumes the deployed position.
15. An aircraft noise reduction device as claimed in claim 9,
wherein the flow control apparatus is made from rubber.
16. An aircraft noise reduction device as claimed in claim 9,
wherein the flow control apparatus comprises brushes with
resiliently deformable bristles.
17. An aircraft noise reduction device as claimed in claim 9,
wherein the flow control apparatus is arranged to be automatically
moved between the stowed and deployed positions when the
flow-facing element is moved from its stowed and deployed
positions.
18. An aircraft noise reduction device as claimed in claim 9,
wherein the flow-facing element comprises an aircraft structural
element.
19. An aircraft noise reduction device as claimed in claim 18
wherein the structural element comprises a component of an aircraft
landing gear.
20. An aircraft noise reduction device as claimed in claim 19,
wherein the flow control apparatus is arranged to be automatically
moved between the stowed and deployed positions by a force applied
by an element of the landing gear when the landing gear is moved
from its stowed and deployed positions.
21. An aircraft landing gear comprising a noise reduction apparatus
as claimed in claim 8 or an aircraft noise reduction device.
22. An aircraft comprising a noise reduction apparatus as claimed
in claim 8.
23. A method of reducing noise caused by landing gear on an
aircraft including the steps of identifying a part of the landing
gear that contributes to the noise generated by the landing gear
when in flight, and providing a noise reduction apparatus as
claimed in claim 8.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to noise-reduction apparatus
for use on an aircraft.
[0002] More particularly, but not exclusively, the invention also
relates to a method of, and apparatus for, reducing noise generated
by the interaction of the landing gear or parts thereof and the air
flowing past it during flight, take-off and/or landing.
[0003] The invention also relates to an aircraft noise reduction
device comprising a flow control apparatus arranged to be
positioned downstream of a flow-facing element, possibly on a
landing gear and a method of reducing noise caused by landing gear
on an aircraft.
[0004] Flow around bodies generates noise, which is detrimental in
particular aerodynamic applications, for example where low noise
emissions are a design requirement. One such application where the
level of noise emissions is important is in the design of
commercial aircraft. Over the past decades engine noise has been
significantly reduced, for example by the introduction of
high-bypass ratio turbofan engines. However, maintaining the
minimum engine ground clearance with such high-bypass ratio turbo
fan engines results in longer landing gear.
[0005] Thus, landing gear on commercial aircraft have been
identified as major noise contributors during approach and landing.
The design of a landing gear is primarily based on its structural
and dynamic function. These stringent requirements make it
extremely difficult to have a totally aerodynamic gear. This
complex geometric design gives rise to unsteady flow which leads to
unwanted noise generation.
[0006] WO2009/053745 A1 discloses the use of a splitter plate to
reduce noise induced by unsteady flow downstream of a flow-facing
element. This document also discloses preliminary experiments which
were carried out to evaluate the effectiveness of using a splitter
plate.
[0007] Fairings have been proposed as a means of reducing landing
gear noise. For example, a noise reduction fairing for an aircraft
landing gear is disclosed in WO 01/04003A1. Such noise reduction
fairings at least partially shield downstream components such as
strut stays and actuators from high-speed flow.
[0008] The present invention seeks to mitigate the above-mentioned
problems and/or to provide an improved noise-reduction method or
apparatus.
SUMMARY OF THE INVENTION
[0009] According to a first aspect the present invention provides a
method of reducing noise caused by the interaction of airflow
between two bluff bodies which are in a generally tandem
arrangement, the method comprising providing flow control such that
the peak turbulence is at least partially displaced from, or
reduced at, the surface of the downstream body. In this context a
tandem arrangement is an arrangement of two bluff bodies in series
in the streamwise direction. The two bluff bodies do not have to be
directly upstream/downstream of each other (i.e. aligned with each
other in the streamwise direction) but can be at an oblique angle
to each other (i.e. not aligned with each other in the streamwise
direction).
[0010] Peak turbulence can be examined by looking at the Reynolds
stresses or perturbed velocities. In the present invention, the use
of flow control causes the position where peak turbulence occurs
(seen as peak perturbed velocities and peak Reynolds stresses) to
move away from the centerline of the downstream body such that the
peak turbulence is displaced from the surface of the downstream
body.
[0011] The applicants have recognised that when an unsteady wake
from an upstream body impinges on a downstream body, the resultant
interaction noise can be significant.
[0012] The skilled person will appreciate that a bluff body may be
generally characterised as any body where there is significant flow
separation and a generally unsteady wake.
[0013] The flow control may be arranged to break down large flow
structures in the flow between, and/or around, the bluff
bodies.
[0014] The flow control may be provided by a flow control device on
either the upstream or downstream body.
[0015] The flow control may comprise the use of blowing. For
example, the flow control device may be arranged to provide
distributed blowing through the surface of one of the bodies.
[0016] The flow control device may form part of the bluff body. In
other words, the flow control device may be contained within the
bluff body.
[0017] The flow control device may comprise a series of plenum
chambers distributed around one of the bluff bodies. For example,
plenum chambers may be distributed at a variety of angular
positions around the body. This gives a more uniform distribution
of blowing.
[0018] Blowing may be applied to both sides of the body from 60
degrees to 150 degrees as measured from the leading-edge of the
body.
[0019] The positions where blowing is applied can be optimised
depending on the airflow speed and the geometry configuration of
the bluff bodies.
[0020] Each plenum chamber may, for example, be fed by a
cross-drilled pipe.
[0021] The plenum chambers may be covered by air-permeable plate.
This allows air to pass through the plate. For example, a
perforated steel plate or a sintered bronze plate.
[0022] The air-permeable plate may have a small pore size. This may
enable a large pressure loss coefficient and/or less variation in
permeation velocity across it and/or a higher pressure differential
across the perforated plate for a given permeation velocity. A
small pore size also produces a lower volume of high frequency
sound (hiss) from the air being blown through the pores.
[0023] The porosity of the plate may be approximately 30%. Porosity
is defined as the ratio of open area of the plate to total area of
the plate.
[0024] The pore size and porosity may be optimised for each airflow
speed and geometry configuration of the bluff bodies.
[0025] An embodiment of the first aspect of the invention concerns
a method of reducing noise caused by an airborne aircraft resulting
from the interaction of airflow between an upstream bluff body and
a downstream bluff body positioned in a generally tandem
arrangement in the streamwise direction, wherein the method
comprises the step of blowing a plurality of jets of air from at
least one of the upstream bluff body and the downstream bluff body,
the jets of air being blown from different positions distributed
over the surface of the bluff body. Such an arrangement of
distributed blowing may act to provide air flow control such that
the peak turbulence downstream of the upstream body, caused by air
flowing past the upstream body, is reduced and/or displaced to a
position further from the downstream bluff body. Such reduction or
displacement in the peak turbulence downstream of the upstream
bluff body acts to reduce noise, that would otherwise be caused by
the interaction of the airflow between the upstream bluff body and
the downstream bluff body.
[0026] According to a second aspect of the invention there is also
provided a noise reduction apparatus for use on a bluff body which,
in use, is arranged in a generally tandem arrangement with a
further bluff body, the apparatus comprising a flow control device
arranged such that the peak turbulence is at least partially
displaced from, or reduced at, the surface of the downstream body.
In this context a tandem arrangement is an arrangement of two bluff
bodies in series in the streamwise direction. The two bluff bodies
do not have to be directly upstream/downstream of each other (i.e.
aligned with each other in the streamwise direction) but can be at
an oblique angle to each other (i.e. not aligned with each other in
the streamwise direction).
[0027] An embodiment of the second aspect of the invention concerns
an aircraft including a first bluff body and a second bluff body
positioned in a generally tandem arrangement such that when the
aircraft is airborne the first bluff body is upstream of the second
bluff body, wherein the aircraft includes a flow control device on
at least one of the first bluff body and the second bluff body, the
flow control device being arranged to blow a plurality of jets of
air from different positions distributed over the surface of the
bluff body.
[0028] It will be appreciated that any features described with
reference to the method according to the first aspect of the
invention are equally applicable to the noise reduction apparatus
of the second aspect of the invention.
[0029] According to a third aspect of the invention there is also
provided an aircraft noise reduction device comprising a flow
control apparatus arranged to be positioned downstream of a
flow-facing element, wherein the flow control apparatus is
arranged, in use, to reduce noise induced by unsteady flow
downstream of the flow-facing element, and wherein the flow control
apparatus is arranged to be moveable in relation to the flow-facing
element between a stowed position and a deployed position.
[0030] The applicants have found that unsteady flow, around and in
the wake of a flow-facing element can cause a significant
contribution to creation of broadband noise. In particular, it has
been noted that unsteady velocity fluctuations and/or net lift
forces generated in the flow may be a key noise generating
mechanism. As such, embodiments of the invention utilise a flow
control apparatus downstream of the flow-facing element to reduce
broadband noise. In particular, the flow control apparatus may be
arranged to reduce the flow fluctuations due to relatively large
scale flow structures in the wake. For example, the flow control
apparatus may be arranged to suppress vortex shedding downstream of
the flow-facing element. Allowing the flow control apparatus to be
stowed while not in use is advantageous as, for example, when used
on a landing gear, it allows deflection and articulation of the
landing gear. It also allows stowage/retraction of the landing gear
while the flow control apparatus is installed thereon. In other
words, movement of the landing gear is not impeded by a flow
control apparatus. It also means there is more space available in
the very limited stowage space of a landing gear bay.
[0031] The flow control apparatus may be a passive flow control
apparatus. A passive flow control apparatus may be optimised to
provide the desired flow control in a particular phase of flight.
For example the flow control apparatus may be optimised to provide
the maximum noise reduction in flow conditions that would occur
during approach and landing.
[0032] The flow control apparatus may comprise a splitter plate
arranged to extend downstream of the flow-facing element when in
the deployed position. Splitter plates (alternatively referred to
as split plates) are a known means of aerodynamic flow control and
have been primarily used to modify the separated wake behind bluff
bodies. Splitter plates generally extend from the centre-line of
the downstream face of the bluff body.
[0033] When in the deployed position, the splitter plate may extend
in a substantially radial direction with respect to the flow-facing
element. It may be substantially aligned with the free stream
airflow.
[0034] When in the stowed position, the splitter plate may be
orientated to at least partly align with the flow-facing element.
This allows the effective space taken up by the flow control
apparatus to be reduced. When on a landing gear, this allows the
landing gear to be retracted more easily.
[0035] The flow control apparatus may be made from a flexible
material such that the flow control apparatus is deformable between
the stowed and deployed positions.
[0036] The flow control apparatus may be made from a resiliently
flexible material such that the flow control apparatus naturally
assumes the deployed position. This means that no active control is
required to deploy the flow control apparatus.
[0037] The flow control apparatus may be made from a material with
a sufficient stiffness to be maintained in the deployed position
during use.
[0038] The flow control apparatus may be made from rubber.
[0039] The flow control apparatus may comprise brushes with
resiliently deformable bristles.
[0040] The bristles may be arranged to be mounted on a downstream
side of the flow-facing element such that the bristles extend
downstream of the flow-facing element when the flow control
apparatus is in the deployed position.
[0041] The flow control apparatus may be arranged to be
automatically moved between the stowed and deployed positions when
the flow-facing element is moved from its stowed and deployed
positions. This means that no active control (other than control of
the stowing/deployment of the flow-facing element) is required to
cause stowing/deployment of the flow control apparatus.
[0042] The flow-facing element may comprise an aircraft structural
element. For example the flow-facing element may be a strut.
Alternatively, the flow-facing element may comprise a fairing, for
locating upstream of a structural element such that, in use,
airflow is at least partially diverted away from the structural
element, and the flow control apparatus may be provided between the
fairing and the structural element. Such an arrangement may help
reduce self-noise which may otherwise be produced by the
fairing.
[0043] Where the flow-facing element is a fairing, the flow control
apparatus may be arranged to reduce re-circulating flow between the
fairing and the structural element. In some embodiments a splitter
plate may be arranged such that it is also adapted to secure the
fairing to the structural member. The structural element may
comprise a component of an aircraft landing gear.
[0044] The flow control apparatus may be arranged to be
automatically moved between the stowed and deployed positions by a
force applied by an element of the landing gear when the landing
gear is moved from its stowed and deployed positions. This means
that the flow control apparatus can be urged into the stowed
position by elements of the landing gear moving into the landing
gear stowed position.
[0045] The flow control apparatus may be arranged to suppress
vortex shedding downstream of the flow-facing element.
[0046] The splitter plate may have a length which is less than,
equal to or greater than the streamwise length of the structural
element.
[0047] The invention also provides an aircraft landing gear
comprising a noise reduction apparatus or an aircraft noise
reduction device as described above.
[0048] The invention also provides an aircraft comprising a noise
reduction apparatus or an aircraft noise reduction device or an
aircraft landing gear as described above.
[0049] According to a fourth aspect of the invention there is also
provided a method of reducing noise caused by landing gear on an
aircraft including the steps of identifying a part of the landing
gear that contributes to the noise generated by the landing gear
when in flight, and providing a noise reduction apparatus or an
aircraft noise reduction device according as described above for
reducing the noise generated by said part.
[0050] It will of course be appreciated that features described in
relation to one aspect of the present invention may be incorporated
into other aspects of the present invention. For example, the
method of the invention may incorporate any of the features
described with reference to the apparatus of the invention and vice
versa.
DESCRIPTION OF THE DRAWINGS
[0051] Embodiments of the present invention will now be described
by way of example only with reference to the accompanying schematic
drawings of which:
[0052] FIG. 1a shows, in plan view, a noise reduction apparatus in
accordance with an embodiment of the first aspect of the invention,
which utilises distributed blowing on a cylinder upstream of an
H-beam;
[0053] FIG. 1b shows, in plan view, a noise reduction apparatus in
accordance with an embodiment of the first aspect of the invention,
which utilises distributed blowing on a cylinder downstream of an
H-beam;
[0054] FIG. 2 shows an anechoic chamber, used for measuring noise
levels;
[0055] FIG. 3 shows a cut away section of a cylinder model, showing
3 plenum chambers;
[0056] FIG. 4 shows, schematically, a perspective view of a
cylinder model, showing cross-drilled pipes for feeding air to the
plenum chambers;
[0057] FIG. 5 is a graph showing mean velocity profiles in the wake
at x=0.079 m;
[0058] FIGS. 6a to 6f are graphical representations of the Reynolds
stresses, comparing the use of blowing to no blowing;
[0059] FIGS. 7a to 7c are graphs showing the profiles of Reynolds
stresses across the wake;
[0060] FIG. 8 is a graph of sound pressure levels (SPL) against
frequency for different flow rates of blowing air for the cylinder
upstream of the H-beam;
[0061] FIG. 9 is a graph of sound pressure levels (SPL) against
frequency for the cylinder upstream of the H-beam;
[0062] FIG. 10a shows, in plan view, a noise reduction device in
accordance with an embodiment of the invention which utilises a
stowable splitter plate, shown in its deployed position;
[0063] FIG. 10b shows, in plan view, a noise reduction device
of
[0064] FIG. 11 shows, in plan view, a noise reduction device in
accordance with an embodiment of the invention which utilises a
stowable splitter plate made from brushes, shown in its deployed
position; and
[0065] FIG. 12 shows, in plan view, a noise reduction device in
accordance with an embodiment of the invention which utilises a
noise reduction fairing and stowable splitter plate, shown in its
deployed position.
DETAILED DESCRIPTION
[0066] While previous work has focused on drag reduction or tonal
noise reduction, this work investigates controlling broadband
interaction noise between bluff body components. An experimental
investigation was conducted to determine how the application of
blowing to the cylinder modified the aerodynamics and acoustics of
two bluff bodies in a directly tandem arrangement in the streamwise
direction.
[0067] The configurations investigation were an H-beam 6 and a
cylinder 7 in tandem, one with the cylinder 7 directly upstream
(FIG. 1a) and the second with the cylinder 7 directly downstream of
the H-beam 6 (FIG. 1b).
[0068] Preliminary experiments were carried out to evaluate the
effectiveness of embodiments of the invention. An example will now
be described to illustrate the effectiveness of providing a flow
control apparatus between the fairing and the structural element
and to demonstrate that embodiments of the invention are suitable
for use as an aircraft noise-reduction apparatus.
[0069] In the following description, the following nomenclature and
symbols will be used: [0070] A Blowing area, m.sup.2 [0071] C.mu.
Blowing coefficient={dot over (m)}V.sub.j/qS (where V.sub.j is the
velocity of the jet, in m/s, and q is the dynamic pressure, in
kg/ms.sup.2) [0072] {dot over (m)} Mass flow rate, kg/s [0073] {dot
over (q)} Flow rate, m.sup.3/s [0074] S Planform area, m.sup.2
[0075] Component separation distance, m [0076] u, v, w Cartesian
components of velocity vector, m/s [0077] u'u' Component of
Reynolds stress tensor, m.sup.2/s.sup.2 [0078] V Magnitude of
velocity, m/s [0079] x, y, z Cartesian coordinates (x positive
downstream, y positive to port, z positive down) [0080] .phi.
Cylinder diameter, m
[0081] The use of distributed blowing to reduce this source of
noise was investigated in a series of experiments. The two bluff
bodies in tandem were a cylinder 7 and an H-beam 6. Two
configurations were tested, one with the cylinder 7 directly
upstream of the H-beam 6 and the other with the H-beam 6 directly
upstream of the cylinder 7. This modelled the interaction noise due
to large perturbations in the wake generated by an upstream
component inducing unsteady pressure fluctuations on a downstream
component. In both cases distributed blowing was applied to the
cylinder 7. The objective of the test was to reduce the interaction
noise of the two components by the use of blowing. Blowing was used
to break down the large flow structures and to displace the peak
turbulence away from the surface of the downstream component.
[0082] Microphone measurements were made in an anechoic chamber 8,
shown in FIG. 2. The loudest configuration was the cylinder 7
upstream of the H-beam 6. Blowing produced a reduction of 16 dB at
a frequency of 70 Hz. There was a broadband reduction of up to 4 dB
up to a frequency of 10 kHz. Only a low pressure of blowing air is
needed. This is because, the air flowing over the cylinder 7 is
less than atmospheric pressure so only a low pressure jet is needed
to expel air from the cylinder 7. In addition, only a relatively
small blowing rate was required to provide the noise reduction. A
blowing coefficient of 1.4.times.10.sup.-3 was adequate to achieve
a noise reduction.
Apparatus and Procedure
Wind Tunnel
[0083] Particle image velocimetry experiments were conducted in the
University of Southampton's low speed 0.9 m.times.0.6 m wind
tunnel. The wind tunnel has a closed working section and is of an
open circuit design. Endplates were used to achieve a nominally
two-dimensional flow around the cylinder 7 and H-beam 6. Free field
acoustic measurements were conducted with an open jet wind tunnel
in an anechoic chamber 8 shown in FIG. 2. An arc 9 of 8 microphones
was placed over the model.
Model Design and Test Configuration
[0084] A cylinder model 7 was designed to allow distributed blowing
to be applied through the surface of the model. The model was made
from a carbon fibre cylinder 7 and contained 12 different plenum
chambers 10. The diameter of the cylinder 7 was 0.1 m. The angular
positions of the plenum chambers 10 were 60 to 90 deg., 90 to 120
deg. and 120 to 150 deg. as shown in FIG. 3.
[0085] The plenum chambers 10 were further divided at the half-span
of the model. There were holes on the cylinder 7 surface to allow
for the placement of on-surface microphones.
[0086] The wind tunnel model is shown in FIG. 4. The design of a
series of plenum chambers 10 ensured a more uniform distribution of
blowing. Blowing was applied to both sides of the cylinder 7 from
60 degrees to 150 degrees as measured from the leading-edge of the
cylinder 7. Each plenum chamber 10 was fed by a cross-drilled pipe
11.
[0087] The plenum chambers 10 were covered by a permeable plate 12
which allowed air to pass through it. Firstly, a perforated steel
plate was tested. Secondly, a sintered bronze plate was tested. The
sintered plate allowed a small pore size and therefore a large
pressure loss coefficient, which resulted in less variation in
permeation velocity across it. It also resulted in a higher
pressure differential across the perforated plate 12 for a given
permeation velocity. The porosity of both plates 12 was 30%. The
pore size for the perforated steel plate was 1 mm and for the
sintered bronze plate was 30 .mu.m. It has been found that using a
pore size of 12 .mu.m produces a low volume high frequency (hiss)
sound. Such "micro-perforated" plates were found to be beneficial
in reducing the far-field radiating noise that was due to the
blowing noise.
[0088] The dimensions of the H-beam 6 were 0.1 m.times.0.1 m and it
was extruded from aluminium. The default separation distance
between the two components (s) was 0.3 m (3.phi.). The Reynolds
number of this investigation was between 1.3.times.10.sup.5 and
2.6.times.10.sup.5.
[0089] The compressor used to supply the air was a double-flow side
channel compressor made by Rietschle. The maximum pressure
difference was .+-.17000 N/m.sup.2 (170 mbar). The maximum mass
flow rate was 0.078 kg/s. A sound absorption settling tank was used
for sound attenuation. The settling tank consisted of a large box
with two ducts offset from each other to minimise the direct
transmission of sound from the compressor. The chamber 8 was lined
with an open cell porous material for sound absorption.
Measurements
[0090] The system used for the particle image velocimetry (PIV)
measurements is known, for example as described in "Angland, D.,
Zhang, X., Chow, L. C., and Molin, N., "Measurements of Flow around
a Flap Side-Edge with Porous Edge Treatment," AIAA Paper 2006-0213,
2006".
[0091] The laser sheet was pointed in a horizontal plane through a
glass window, which makes up one wall of the working section. The
camera was placed above the transparent wind tunnel roof. An
adaptive cross correlation was performed on interrogation areas
measuring 16.times.16 pixels. The horizontal and vertical overlap
was 75%. A peak validation of 1.2 was used to reject spurious
vectors. These time averaged data were averaged over 500 images
sampled at 2 Hz. The image size was 110 mm.times.90 mm with a
spatial resolution of 0.35 mm in both directions.
[0092] The microphone measurements in the anechoic chamber 8 were
conducted using eight Behringer ECM8000 microphones mounted on an
arc. The frequency response of the microphones was from 15 Hz to 20
kHz. The microphones were powered by a DIGIMAX FS preamplifier.
These data were sampled at a frequency of 44.1 kHz with a block
size of 8192 averaged over 100 blocks. The 8 microphones were
spaced equally on the arc from 90 deg. to 157 deg. measured from
the freestream velocity vector.
Sample of Results
Particle Image Velocimetry
[0093] Particle image velocimetry (PIV) was used to determine the
time-averaged flowfield and the turbulence statistics. The time
averaged flowfield allowed the profiles in the wake to be
determined and an estimation of the momentum deficit or addition
due to the application of blowing. For comparison a solid wall
condition with no blowing was used. This provided a baseline
condition with which to compare the cylinder 7 with blowing
applied.
[0094] Mean velocity profiles in the wake at x=0.079 m are shown in
FIG. 5. The effect of blowing was most influential on the low
speed, low pressure separated flow behind the cylinder 7. The
blowing flow rate was insufficient to promote significant early
separation of the flow from the surface of the cylinder 7. However,
it did have an influence on the low speed flow in the wake
immediately aft of the cylinder 7. The velocity profiles showed a
widening of the wake to the sides of the cylinder 7 with blowing
applied. Another effect was an increase in the velocity gradient
across the shear layer. The use of blowing also produced a decrease
in the momentum deficit immediately aft of the cylinder 7. This was
an effect of blowing normal to the cylinder 7 surface. However, due
to the spreading of the wake the overall momentum deficit
increased.
[0095] Examining the Reynolds stresses helped determine how the
application of blowing changed the structures in the wake and
altered the shear layer. In two dimensional PIV three components of
the Reynolds stress tensor can be determined, i.e. u'u', u'w' and
w'w'. u' and w' are the perturbed velocities about the mean flow.
Therefore they are a measure of the magnitude of unsteady velocity
fluctuations in the wake. The unsteady wake impinging on the
downstream component is a source of significant additional noise.
To reduce the interaction noise the large perturbations in the wake
need to be reduced. These are shown in FIG. 6(a), (c) and (e) for
the solid wall configuration. The shear layers at the edge of the
wake behind the cylinder 7 showed significant spreading aft of the
cylinder 7.
[0096] The Reynolds stresses in the wake with blowing applied is
shown in FIG. 6(b), (d) and (f). With blowing, the magnitude of the
components of the stress tensor had reduced and their spatial
extent had also been reduced. The profiles of the Reynolds stresses
across the wake are shown in FIG. 7. The effect of applying blowing
through the perforated material was to reduce the magnitude of
Reynolds stress in the wake and shear layer and to displace the
peak of maximum stress further away from the centerline.
[0097] The effect of blowing on the upstream cylinder 7 was to
breakdown the large flow structures in the wake which was evidenced
by the reduction in the velocity fluctuations in the shear layer
and wake. This reduction in wake strength reduced the interaction
noise when the turbulent wake impinged on the downstream component.
To determine the acoustic reduction, free-field microphone
measurements were made in an anechoic chamber 8.
Acoustic Measurements
[0098] Acoustic measurements were made in an anechoic chamber 8
with an open jet wind tunnel. The changes in sound pressure levels
were averaged over the four microphones above the model in the
overhead position. These microphones recorded the loudest model
noise. No acoustic data could be obtained in the rearward arc due
to the wake of the model impinging on the microphones. The first
configuration tested was the cylinder 7 placed upstream of the
H-beam 6. The blowing was applied to the cylinder 7. A plot of
sound pressure level (SPL) versus frequency is shown in FIG. 8 with
different flow rates.
[0099] The maximum reduction was 16 dB centered around 70 Hz which
corresponded to a Strouhal number based on cylinder 7 diameter of
0.18 m. There was also a further broadband reduction from 800 Hz to
10 kHz of up to 4 dB. The largest flow rate tested produced
additional blowing noise at 12 kHz that was louder than the
baseline configuration. From the aerodynamic flowfield
investigation it was shown that blowing on the cylinder 7 reduced
the large velocity fluctuations in the wake due to breaking down
the large flow structures in the wake. This modified the wake that
impinged on the H-beam 6 downstream resulted in significantly less
noise been generated.
[0100] The second configuration tested was the H-beam 6 upstream of
the cylinder 7. The blowing was applied to the cylinder 7 to
displace the turbulent wake away from the downstream component. A
plot of SPL versus frequency is shown in FIG. 9. The peak reduction
was 20 dB at a frequency of 300 Hz which corresponded to a Strouhal
number of 0.75 based on the height of the H-beam 6. The flow rate
was 0.7.times.10.sup.-3 m.sup.3/s. There were broadband reductions
from 20 Hz up to 3 kHz.
[0101] Above this frequency the blowing noise was louder than the
noise generated by the wind tunnel model. This configuration of
H-beam 6 and cylinder 7 produced very little noise at and above
this frequency and all the energy was contained in the lower
frequencies. However, it is well known that high frequency noise
dissipates much quicker than lower frequency noise. Hence, when
distributed blowing is used on landing gear components, the
increase in high frequency noise would not be perceived on the
ground, depending on the pore size. This is because the additional
high frequency blowing noise is proportional to pore size.
CONCLUSION
[0102] Although, in the above example, the blowing is achieved by
the use of cross-drilled pipes 11 inside the cylinder 7 feeding air
to plenum chambers 10, the blowing mechanism may, alternatively,
simply comprise pipes attached to the element, each pipe having a
series of nozzles in form of simple holes or slots. The holes or
slots are distributed along length of the pipe (and therefore,
along the length of the element) to form an array. Pressurised air
is provided to the pipe to provide blowing from the nozzles in the
form of a relatively small jet downstream. As another alternative,
the pipes may be embedded in the cylinder 7 structure.
[0103] The use of blowing to reduce interaction noise between two
bluff body components was investigated. The tandem configuration
produced significantly more noise compared to the isolated
components. The application of blowing to the cylinder 7 produced a
noise reduction for both configurations. The noisiest configuration
was the cylinder 7 placed upstream of the H-beam 6. Blowing applied
to the cylinder 7 reduced the large velocity fluctuations in the
wake of the cylinder 7 thereby reducing the interaction noise. The
largest reduction was 16 dB at a freestream velocity of 40 m/s. The
largest flow rate produced additional high frequency noise at 12
kHz. Blowing on the cylinder 7 when it was downstream of the H-beam
6 also produced a broadband reduction in the interaction noise of
up to 20 dB. The frequency range over which the blowing was
successful in reducing the noise was from 20 Hz to 3 kHz.
Practical Application on an Aircraft
[0104] When used on a landing gear of an aircraft, it is envisaged
that the blowing air will be taken from a bleed air system of an
aircraft. Alternatively, the air can be taken from a fan output or
from an air scoop. The air will be supplied to plenum chambers
distributed about an element of the landing gear acting as a bluff
body.
[0105] FIG. 10a shows a noise reduction device in accordance with
an embodiment of the third aspect of the invention, in its deployed
position.
[0106] The noise reduction device comprises a structural element 1
which is exposed, in use, to an airflow V. In other words, the
structural element 1 is flow-facing. V. may be assumed to be the
free stream airflow. In the case where the structural element 1 is
a landing gear component it will be appreciated that it may be
deployable, such that it is only be exposed to the airflow
V.sub..infin. during take-off, landing and approach.
[0107] The structural element 1 is a bluff body, in this case a
H-beam 6. The structural element has a length in the streamwise
direction of W. The skilled person will appreciate that a bluff
body may be generally characterised as any body where there is
significant flow separation and a generally unsteady wake.
[0108] The noise reduction device further comprises a flow control
apparatus in the form of a splitter plate 2 of streamwise length L.
The splitter plate 2 is a resiliently flexible plate attached to
the downstream side of the structural element 1. When in the
deployed position, the splitter plate 2 extends perpendicularly
from the downstream surface of the structural element 1 and is
located on the centre line of the element.
[0109] The splitter plate 2 is made of a soft rubber material.
[0110] The splitter plate 2 is arranged such that when it is in the
deployed position, it is substantially aligned with the free stream
flow V. When mounted on an aircraft it may be convenient to simply
align the splitter plate 2 with the longitudinal axis of the
aircraft, since this is a reasonable approximation to the free
stream airflow during approach and landing.
[0111] The splitter plate 2 can be deformed into a stowed position
(shown in FIG. 10b) by a force exerted on it. In the case where the
structural element 1 is a landing gear component it will be
appreciated that the flow control apparatus may be deformed into
its stowed position by forces exerted on it by elements of the
landing gear (not shown) as they are moved into their stowed
position.
[0112] As is shown in FIG. 10b, the flow control apparatus 2, when
in the stowed position, is bent back towards the structural element
1 to a certain extent so that it takes up less effective space than
when in the deployed position. In other words, the flow control
apparatus 2 extends outwards from the structural element a length
less than its actual length L.
[0113] As an alternative, as shown in FIG. 11, the splitter plate 2
may comprise brushes with resiliently deformable bristles 5. The
bristles 5 are attached to the downstream side of the structural
element 1. When in the deployed position, the bristles 5 extend
perpendicularly from the downstream surface of the structural
element 1 and are located on the centre line of the element.
[0114] Preliminary experiments were carried out to evaluate the
effectiveness of using a splitter plate. An H-beam 6 was tested as
it is considered a good example of a simple bluff-body which
produces noise over a broad range of frequency spectrum. A splitter
plate 2 having a length L, measured in the streamwise direction,
was attached to the rear of the element 1 having a length W. A
selection of different splitter plate lengths (L/W=1, L/W=2 and
L/W=3), and a body without a splitter plate, were tested.
[0115] A comparison of flow visualisations with and without the
presence of the splitter plate showed that the presence of the
splitter plate blocked interaction between shear layers in the
vicinity of the body. The shear layers continued to converge
downstream leading to a longer and wider wake. The splitter plate 2
delays the roll-up of vortices behind the element 1 and interrupts
the interaction of shear layers.
[0116] The Coefficient of Drag for each arrangement was also
compared. The addition of the L/W=1 splitter plate resulted in a
drop in the coefficient of drag of C.sub.d=0.47. Increasing the
length of the splitter plate reduced the drag further by
C.sub.d=0.23 between L/W=1 and L/W=3.
[0117] Standard deviations of velocity plots were used to compare
the unsteady flow. The unsteadiness was concentrated around the
H-beam 6 with the highest velocity fluctuation just aft of it. In
the L/W=1 configuration the unsteadiness moved further downstream
and away from the model.
[0118] The narrowband spectra were measured in an anechoic chamber
8 and plots compared for the different configurations to show how
the noise signature of the model was affected. The L/W=0 case
showed a strong tonal peak at a Strouhal Number (based upon the
width of the body) of 0.125 and broadband noise "hump" centered
about a Strouhal Number of 0.75. In the cases of L/W=1, L/W=2 and
L/W=3 the tonal peak was suppressed and the noise was reduced over
the whole frequency range. The splitter plate configurations showed
very similar noise spectra up to a Strouhal Number of 17.5. Above
that frequency the L/W=2 configuration showed marginally lower
noise levels.
[0119] Source localization plots were used to identify where origin
of the noise reduction. The comparison between the plots showed
that the H-beam 6 is no longer the main noise source when the
splitter plate is used. Rather, the noise source is located towards
the trailing edge of the splitter plate.
[0120] FIG. 12 shows a further embodiment of the invention in which
the flow-facing component is a fairing 3, positioned upstream of a
structural element 1 and arranged to at least partially divert the
free stream airflow away from the element 1. Such fairings have
been proposed for noise reduction purposes. However, the applicants
have recognised that in some circumstances the noise-reduction
fairing 3 may itself contribute to the total broadband noise of the
aircraft. Thus, according to embodiments of the invention a
splitter plate 2 is provided in a cavity 4 defined between the
fairing 3 and the element 1. The splitter plate 2 may conveniently
be arranged to support the fairing 3 from the structural element
1.
[0121] The splitter plate 2 reduces or eliminates vortex shedding
from the fairing 3 and in turn reduces noise. As with the previous
embodiments this is due to the splitter plate 2 blocking the
interaction between opposing shear layers. The splitter plate 2
also reduced the interaction between the shear layers and the
downstream element 1.
[0122] It will be appreciated that the flexible splitter plate 2
may be attached to the downstream side of the fairing 3 or the
upstream side of the element 1 or to both. When the flow control
apparatus is moved to the stowed position (not shown), the
apparatus 2 can either be urged toward the fairing 3, the element 1
or both.
[0123] Preliminary experiments were carried out to evaluate the
effectiveness of using a splitter plate in the cavity between a
fairing and a structural element. Three different sizes of elements
1 were used to investigate the possibility of reducing the size of
the fairing 3 with respect to the element 1. Aerodynamic and
acoustic results were performed in wind tunnel and anechoic
facilities.
[0124] In the configurations without the splitter plate 2 a
re-circulating region of flow was observed in the cavity 4 between
the fairing 3 and the element 1 as the shear layer aft of the
fairings' trailing edge impinged on the element part, rolling up
inside the cavity 4. The element 1 was subjected to relatively
high-speed flow due to the shear layer interaction.
[0125] The application of the splitter plate for the two smaller
elements 1 blocked the interaction between the opposing shear
layers and inhibited the shear layer from interacting with the
element. As a result the re-circulating flow inside the cavity 4
was reduced considerably. The larger element 1 was large enough for
the shear layer to impinge on it, nevertheless the splitter plate 2
impeded the strong re-circulation flow within the cavity 4. Instead
a low velocity wake was observed aft of the element 1. The effect
of this change in flow structure had an impact on the noise
produced. The source strength around the apparatus was
significantly reduced as the magnitude of the velocities and the
unsteadiness around the fairing 3 and the element 1 were lower,
hence reducing the dipole strength attributed with the fluctuating
lift forces on the apparatus. The strong shedding produced a strong
tonal peak in the noise measurements, increasing the overall noise
signature. The splitter plate reduced or totally eliminated this
tone. The configurations involving the two smaller elements 1
reduced this tonal peak by about 14 dB, measured from the
1/3-octave band spectra. The larger element eliminated the tonal
peak completely although a second smaller tonal peak was observed
at a high frequency.
Practical Application on an Aircraft
[0126] When used on a landing gear of an aircraft, it is envisaged
that the splitter plate would be attached to the downstream side of
an element of the landing gear acting as a bluff body. When the
landing gear is retracted into its stowed position, elements of the
landing gear move in relation to each other. The splitter plate
will be moved/deflected by one or more of the landing gear elements
if the splitter plate is in the path of the moving landing gear
elements. The force from the elements acting on the splitter plate
will cause the splitter plate to deflect and allow the landing gear
to retract into its stowed position. Upon deployment of the landing
gear, the landing gear elements will move and any elements causing
the splitter plate to deflect will be moved out of the way allowing
the splitter plate to adopt its deployed position.
[0127] Whilst the present invention has been described and
illustrated with reference to particular embodiments, it will be
appreciated by those of ordinary skill in the art that the
invention lends itself to many different variations not
specifically illustrated herein. It will be appreciated that
various changes or modifications may be made without departing from
the scope of the invention.
[0128] Where in the foregoing description, integers or elements are
mentioned which have known, obvious or foreseeable equivalents,
then such equivalents are herein incorporated as if individually
set forth. Reference should be made to the claims for determining
the true scope of the present invention, which should be construed
so as to encompass any such equivalents. It will also be
appreciated by the reader that integers or features of the
invention that are described as preferable, advantageous,
convenient or the like are optional and do not limit the scope of
the independent claims. Moreover, it is to be understood that such
optional integers or features, whilst of possible benefit in some
embodiments of the invention, may not be desirable, and may
therefore be absent, in other embodiments.
* * * * *