U.S. patent application number 10/717672 was filed with the patent office on 2005-01-13 for apparatus for accelerating destruction of a vortex formed by a wing of an aircraft.
Invention is credited to Corjon, Alexandre, Laporte, Florent, Leweke, Thomas.
Application Number | 20050006533 10/717672 |
Document ID | / |
Family ID | 8860614 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006533 |
Kind Code |
A1 |
Corjon, Alexandre ; et
al. |
January 13, 2005 |
Apparatus for accelerating destruction of a vortex formed by a wing
of an aircraft
Abstract
An apparatus for accelerating a destruction of a vortex is
formed at the rear of a wing of an aircraft by a merging of first
and second co-rotating eddies. A perturbation device is disposed
adjacent of an area of creation of the first co-rotating eddy. The
device is configured to generate a periodic perturbation having a
wavelength capable of exciting at least on instability mode of the
first eddy.
Inventors: |
Corjon, Alexandre;
(Toulouse, FR) ; Leweke, Thomas; (Marseille,
FR) ; Laporte, Florent; (Villiers Sur Tholon,
FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
8860614 |
Appl. No.: |
10/717672 |
Filed: |
November 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10717672 |
Nov 21, 2003 |
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10080407 |
Feb 25, 2002 |
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6719246 |
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Current U.S.
Class: |
244/199.1 |
Current CPC
Class: |
Y02T 50/162 20130101;
Y02T 50/10 20130101; B64C 23/06 20130101 |
Class at
Publication: |
244/199 |
International
Class: |
B64C 023/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2001 |
FR |
01 02804 |
Claims
1. An apparatus for accelerating a destruction of a vortex formed
at a rear of a wing of an aircraft by a merging of first and second
co-rotating eddies, comprising: a perturbation device disposed
adjacent an area of creation of the first co-rotating eddy, the
device configured to generate a periodic perturbation having a
wavelength capable of exciting at least one instability mode of the
first eddy.
2. The apparatus according to claim 1, wherein the perturbation
device is disposed adjacent a flap of the aircraft.
3. The apparatus according to claim 2, wherein the device comprises
an unstreamed element.
4. The apparatus according to claim 3, wherein the unstreamed
element comprises a cylindrical cross section.
5. The apparatus according to claim 4, wherein the unstreamed
element comprises a circular cross section.
6. The apparatus according to claim 4, wherein the unstreamed
element comprises an elliptical cross section.
7. The apparatus according to claim 3, wherein the unstreamed
element is configured to be extended from and retracted into one of
the wing and the flap of the aircraft.
8. The apparatus according to claim 2, wherein the device comprises
a fluid jet.
9. The apparatus according to claim 8, wherein the fluid jet is
disposed within one of the wing and the flap of the aircraft.
10. An apparatus for accelerating a destruction of a vortex formed
at a rear of a wing of an aircraft by a merging of first and second
co-rotating eddies, comprising: means for generating a periodic
perturbation having a wavelength capable of exciting at least one
instability mode of the first eddy, the means disposed adjacent an
area of creation of the first co-rotating eddy.
11. The apparatus according to claim 10, wherein the perturbation
device is disposed adjacent a flap of the aircraft.
12. The apparatus according to claim 11, wherein the device
comprises an unstreamed element.
13. The apparatus according to claim 12, wherein the unstreamed
element is configured to be extended from and retracted into one of
the wing and the flap of the aircraft.
14. The apparatus according to claim 11, wherein the device
comprises a fluid jet.
15. The apparatus according to claim 8, wherein the fluid jet is
disposed within one of the wing and the flap of the aircraft.
16. An apparatus for accelerating a destruction of first and second
contra-rotating vortices formed at a rear of first and second wings
of an aircraft, the first contra-rotating vortex formed by a
merging of first and second co-rotating eddies, and the second
contra-rotating vortex formed by a merging of third and fourth
co-rotating eddies, the apparatus comprising: a first perturbation
device disposed adjacent an end of a first flap of the first wing
creating the first co-rotating eddy; and a second perturbation
device disposed adjacent an end of a second flap of the second wing
creating the third co-rotating eddy; wherein the first and second
perturbation devices are configured to generate periodic
perturbations having wavelengths capable of exciting instability
modes of the first and third eddies, such that diameters of the
first and second vortices are greater than a predetermined
proportion of a distance between the first and second vortices.
17. The apparatus according to claim 16, wherein the first and
second perturbation devices are configured to generate periodic
perturbations having wavelengths capable of exciting instability
modes of the first and third eddies, such that the diameters of the
first and second vortices are greater than about 30% of the
distance between the first and second vortices.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C. .sctn. 119
to French Patent Application 01 02804, filed on Mar. 1, 2001, and
under 35 U.S.C. .sctn. 120 to U.S. patent application Ser. No.
10/080,407 filed on Feb. 25, 2002, the disclosures of which are
incorporated by reference herein in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a process and device for
accelerating the destruction of at least two vortices in the wake
of a moving body.
[0003] The present invention is applied more particularly, but not
exclusively, to an aircraft, especially a transport aircraft.
[0004] In this case, the two (or more) vortices to which the
present invention is applied are generally created when the
aircraft is taking off or landing, and at least one flap,
particularly a wing flap, is deployed on each wing of the aircraft.
These two vortices are contra-rotating and one of them is created
in each of the two wakes of the said wings of the aircraft, each of
the said vortices being formed behind the corresponding wing by the
merging of at least two co-rotating eddies, one of which is created
by the wing tip and the other by the deployed flap. These
co-rotating eddies are caused by the pressure gradients existing at
the ends of the said aerofoils (wing tip, flap) between the lower
and upper surfaces of these aerofoils. The pressure gradient forces
the fluid to flow around the end of each aerofoil, creating a
rotary movement of the fluid which generates the eddy. In cruising
flight, there are also contra-rotating vortices which are generated
by the wing tips only, but a device for accelerating their
destruction would tend to impose excessive drag.
[0005] To avoid any confusion, the following terms will be used in
relation to the present invention:
[0006] "eddies", denoting the eddy phenomena existing before
merging, which will merge with each other; and
[0007] "vortices", denoting eddy phenomena caused by the merging of
at least two of these eddies.
[0008] The two contra-rotating vortices which are created in the
wake of the aircraft can be very energetic and can produce a
velocity field which is highly destabilizing for a following
aircraft (this velocity field is commonly called the "wake
turbulence"), and, in particular, may cause a powerful rolling
moment and a powerful downward motion of the air. Moreover, they
have a significant duration (several minutes), and therefore make
it necessary to maintain large separation intervals between
aircraft in the vicinity of airports.
[0009] A fixed separation grid, based on the weights of the
aircraft, is currently used to establish the separation intervals
between two aircraft at take-off and landing. Since air traffic is
constantly increasing, the frequency of take-offs and landings is
thus limited at many airports by excessively large separation
intervals.
[0010] The object of the present invention is, in particular, to
accelerate the destruction of this pair of contra-rotating vortices
created in the wake of an aircraft at take-off and landing.
DISCUSSION OF THE RELATED ART
[0011] There are various types of known device designed to act on
different wake eddies of aircraft.
[0012] In particular, the document U.S. Pat. No. 5,492,289
discloses a method for accelerating the destruction of an eddy
created in the wake of an aircraft by a wing tip or by a flap
carried by a wing. This known document proposes that the trailing
edge of the wing or flap be modified in such a way as to modify the
distribution of lift along the corresponding wing. Such a
modification of the lift causes a faster increase in the diameter
of the (wing-tip or flap) eddy and thus accelerates its
destruction. However, the efficacy of this method is questionable,
particularly as regards its effect on the actual acceleration of
the destruction. Moreover, this known solution requires a
modification of the wing geometry, which entails practical problems
of implementation.
[0013] The document U.S. Pat. No. 6,042,059 discloses another
system and method for destroying wake eddies of an aircraft more
rapidly. This known method entails the use of small aerodynamic
surfaces to generate a parasitic eddy designed to initiate the
process of destruction of the wake eddies. These small aerodynamic
surfaces are streamlined and arranged on the lifting surfaces of
the aircraft. However, they have the drawback of increasing the
drag.
[0014] Additionally, the document WO-99/00297 discloses an active
system for destroying the wake eddies of an aircraft. This known
system is based on the active excitation of the multiple
instabilities of eddies by the movable surfaces of the aircraft
wings, but without changing the internal structure of these eddies,
and without exciting an internal instability of the cores of these
eddies. This known active system is highly complex, since it
requires the use of a computer and synchronized commands for moving
the ailerons and spoilers in a controlled way during the flight.
Consequently, there may be effects on the manoeuvrability of the
aircraft, the control of its lift, the stress on its structure and
the comfort of passengers. Moreover, the efficacy of this known
system has not been proved.
BRIEF SUMMARY OF THE INVENTION
[0015] The object of the present invention is to overcome the
aforesaid drawbacks. It relates to a process for accelerating the
destruction of at least two contra-rotating vortices which are
generated in the wake of a moving body, particularly in the wake of
an aircraft when the flaps are deployed on the said aircraft, in
other words at the take-off and landing stages.
[0016] For this purpose, the said process for accelerating the
destruction of a pair of contra-rotating vortices which are created
in the wake of a moving body having at least two wings, where each
wing has at least two lifting surfaces, and which are separated
from each other by an inter-vortex distance, each of the said
contra-rotating vortices being formed at the rear of the
corresponding wing by the merging of at least two co-rotating
eddies which are created by the arrangement of the said lifting
surfaces of the wing, each of the said co-rotating eddies having a
plurality of core instability modes, is characterized in that a
periodic perturbation of the flow is generated on each of the said
wings, in the vicinity of the area of creation of at least one of
the said co-rotating eddies associated with the wing, and in that
each of the said periodic perturbations has a wavelength capable of
exciting at least one of the said instability modes of the
corresponding eddy in such a way as to increase the core of the
contra-rotating vortex which is created by the unstable merging of
this eddy with the other eddy, so that the diameter of the said
core becomes greater than a predetermined proportion of the said
inter-vortex distance.
[0017] Thus, owing to the periodic perturbations generated
according to the present invention, the ratios between the core
diameters of the two vortices on the one hand, and the inter-vortex
distance on the other hand, become greater than a predetermined
critical value, above which the two vortices begin to interact
strongly and are rapidly destroyed. This is because, as mentioned
above, when this critical value is reached there is an exchange of
fluid between the two contra-rotating vortices, with the creation
of smaller secondary eddies, which are perpendicular to the axes of
the said vortices. This exchange of fluid causes a rapid and marked
decrease in the circulation in each vortex, and consequently the
disintegration or destruction of the said pair of contra-rotating
vortices.
[0018] The process according to the present invention therefore
makes it possible to reduce the duration of the said
contra-rotating vortices and thus overcome the aforesaid
drawbacks.
[0019] The present invention is more particularly, but not
exclusively, applicable to an aircraft. In this case, the two
lifting surfaces of a wing, each of which generates one of the said
co-rotating eddies, are generally the surface of the wing tip on
the one hand and a deployed flap on the other hand. However, the
present invention may also be applied to other moving bodies,
particularly a submarine, which creates vortices in the wake of its
fins.
[0020] The said instability mode of the core to be excited is
advantageously determined from the size of the cores of the eddies
and the ratios between the sizes of the cores and the distance
between the eddies. The said instability mode is preferably
determined empirically. Generally, the wavelength of the
instability mode is essentially equal to the mean diameter of the
corresponding eddy core.
[0021] Additionally, and advantageously, the wavelength of a
perturbation to be generated is:
[0022] of the order of a divisor of the most unstable wavelength of
the instability mode which it is to excite, permitting a reduction
of the size of the means used to generate the said perturbation;
and/or
[0023] located within an instability range of each of the
co-rotating eddies of the corresponding wing.
[0024] The present invention also relates to a device for
accelerating the destruction of a pair of vortices such as those
described above.
[0025] According to the invention, the said device is characterized
in that it comprises, on each of the said wings, at least one
perturbation means which is positioned in the vicinity of the area
of creation of one of the said co-rotating eddies associated with
the wing, and in that each of the said perturbation means can
generate a periodic perturbation of the flow, which has a
wavelength capable of exciting at least one of the said instability
modes of the corresponding eddies in such a way as to increase the
core of the contra-rotating vortex which is created by the merging
of this eddy with the other co-rotating eddy in such a way that the
diameter of the said core becomes greater than a predetermined
proportion of the said inter-vortex distance.
[0026] Advantageously, at least one of the said perturbation means
comprises:
[0027] in a first embodiment, an unstreamlined element, for example
a cylinder, whose apparent diameter which is transverse with
respect to the flow depends on the wavelength of the periodic
perturbation to be generated; and
[0028] in a second embodiment, a means for producing a jet of fluid
emitted transversely with respect to the said flow. If the said jet
of fluid is emitted orthogonally to the flow, its velocity must be
at least equal to that of the moving body and its diameter must be
of the same order of magnitude as the apparent diameter of an
unstreamlined element which could be used in its place.
[0029] The device according to the invention is therefore easily
constructed and inexpensive. Moreover, it is passive and very
robust.
[0030] Additionally, at least one of the said perturbation means is
advantageously retractable. For this purpose, the said perturbation
means is preferably retractable into the wing or into fairings
fitted on the wing (the strut end fairing, for example) or into the
flap. This avoids increasing the drag and thus avoids degrading the
performance of the moving body, particularly in the absence of
contra-rotating vortices, especially in cruising flight in the case
of an aircraft.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The figures on the attached drawing clearly show a possible
embodiment of the invention. In these figures, identical references
indicate similar elements.
[0032] FIG. 1 shows in a partial and schematic way an aircraft to
which a device according to the invention is applied.
[0033] FIGS. 2 and 3 show, respectively, two different embodiments
of a perturbation means of a device according to the invention.
[0034] FIG. 4 is a synoptic diagram showing the principal stages of
the acceleration of the destruction of the vortices, due to the
application of the process according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The device 1 according to the invention, shown schematically
in FIG. 1, is fitted on an aircraft 2, particularly a transport
aircraft, of which only the two wings 3A and 3B and part of the
fuselage 4 are shown, to accelerate the destruction of at least two
contra-rotating vortices 5A and 5B which are created in the wakes
of the said wings 3A and 3B respectively, and which are separated
from each other by an inter-vortex distance D (the distance between
the centres of the cores of the said vortices 5A and 5B).
[0036] It is known that the vortex 5A is formed at the rear of the
wing 3A by the merging of at least two co-rotating eddies 7A and
8A, of which one 7A is created by the tip 9A of the wing 3A, and
the other 8A is created by the tip of the said deployed flap 6A,
each of the said co-rotating eddies 7A and 8A having a plurality of
core instability modes, as specified below. The said co-rotating
eddies 7A and 8A are maintained for a distance L1 before merging at
a distance L2. Similarly, the vortex 5B is formed at the rear of
the wing 3B by the merging of at least two co-rotating eddies 7B
and 8B, of which one 7B is created by the tip 9B of the wing 3B and
the other 8B is created by the tip of the deployed flap 6B.
[0037] These two contra-rotating vortices 5A and 5B, or more, which
are created in the wake of the aircraft 2, can be very energetic
and can produce a velocity field which is highly destabilizing for
a following aircraft, and, in particular, may cause a powerful
rolling moment and a powerful downward motion of the air. Moreover,
they have a significant duration (several minutes), and therefore
make it necessary to maintain large separation intervals between
aircraft in the vicinity of airports.
[0038] According to the invention, in order to accelerate the
destruction of the said vortices 5A and 5B, a periodic perturbation
of the flow is generated on each of the said wings 3A and 3B, in
the vicinity of the area 10A, 10B of creation of at least one 8A,
8B of the said co-rotating eddies 7A, 8A; 7B, 8B associated with
the wing 3A, 3B. Each of the said periodic perturbations has a
wavelength which can excite at least one, but preferably a
plurality, of the instability modes of the corresponding eddy 8A,
8B in such a way as to increase the core of the contra-rotating
vortex 5A, 5B which is created as a result of the unstable merging
of this eddy 8A, 8B with the other eddy 7A, 7B, in such a way that
the diameter dA, dB of the said core becomes greater than a
predetermined proportion (preferably equal to 0.3 at least) of the
said inter-vortex distance D. The term "unstable" is used to
emphasize that the merging, in the presence of core instabilities,
does not take place in the usual way.
[0039] To achieve this, the device 1 according to the invention
has, on each of the said wings 3A, 3B, at least one perturbation
means 11 or 12 specified below, which is positioned in the vicinity
of the said area 10A, 10B of creation of one of the said
co-rotating eddies 8A, 8B associated with the wing 3A, 3B, in other
words in the vicinity of the tip of the flap 6A, 6B in the example
of FIG. 1, and which can generate the said periodic perturbation of
the flow.
[0040] The device 1 according to the invention therefore generates
for each vortex 5A, 5B a perturbation which has a precise
wavelength which is capable of exciting a maximum of unstable
modes, particularly of what is known as the "elliptic" instability,
of the corresponding eddy, for example of the eddy 8A associated
with the tip of the flap 6A in the example of FIG. 1 for the wing
3A. This perturbation causes the core of the said eddy BA to
oscillate and makes it unstable. The eddy 8A therefore shows
perturbations of its internal structure.
[0041] As the merging of the eddy 8A with the eddy 7A (of the tip
9A of the wing 3A) progresses, the said highly perturbed eddy 8A
contaminates the eddy 7A. The instability of the eddy 8A modifies
the process of merging of the two eddies 7A and 8A. Consequently,
the diameter dA of the vortex 5A resulting from the unstable
merging of these eddies 7A and 8A is greater and its level of
internal turbulence is higher than in the absence of
instability.
[0042] This phenomenon is naturally the same for the vortex 5B
resulting from the unstable merging of the two eddies 7B and 8B
formed on the other wing 3B of the aircraft 2.
[0043] We are therefore left with two highly perturbed
contra-rotating vortices 5A and 5B having large diameters dA and
dB. As indicated above, provided that these diameters dA and dB are
sufficiently great (in other words, that the ratios dA/D and dB/D
are greater than a predetermined critical value (for example 0.3),
or that dA and dB become greater than a predetermined proportion of
the inter-vortex D, the said instabilities cause an exchange of
fluid between the two vortices 5A and 5B by the creation of small
scale secondary vortices (not shown), which are perpendicular to
the axes of the said principal vortices 5A and 5B. This situation
leads to a rapid disintegration of the pair of contra-rotating
vortices 5A and 5B and a rapid decrease of the circulation of each
vortex 5A, 5B. In other words, the vortices 5A and 5B are therefore
destroyed much more rapidly than in the absence of instability.
[0044] FIG. 4 shows the principal stages of the process described
above, leading to the accelerated destruction of the vortices 5A
and 5B of FIG. 1 by the action of the device 1 according to the
invention. This process comprises the following stages (the letter
A indicates a phenomenon or stage taking place in relation to the
wing 3A and the letter B indicates the same phenomenon taking place
in relation to the wing 3B):
[0045] in E1A and E1B, the aforesaid periodic perturbations
generated by the action of the device 1 are shown, each
corresponding to an instability known as a "Bnard-von Krmn
instability";
[0046] in E2A and E2B, three-dimensional core instabilities,
particularly "elliptic instabilities", present in the eddies 8A and
8B are increased by the periodic perturbations generated according
to the invention.
[0047] It is known that these three-dimensional instabilities
develop in an eddy subjected to stretching. This occurs in the case
of pairs of co-rotating (or contra-rotating) eddies, the stretch in
each eddy being induced by the presence of the other eddy, making
the said eddy unstable. The ensuing instability produces
perturbations of the internal structure of the eddy, with a
characteristic axial wavelength of the order of the diameter of the
eddy core.
[0048] Elliptic instability has been described:
[0049] in respect of contra-rotating eddies, by T. Leweke and C. H.
K. Williamson, in their article "Cooperative elliptic instability
of a vortex pair", published in J. Fluid Mech, vol. 360, pp.
85-119; and
[0050] in respect of co-rotating eddies, by P. Meunier, 35 T.
Leweke and M. Abid, in their article "Three-dimensional instability
of two merging vortices", published in "Advances in Turbulence
VIII", CIMNE, pp. 15-18.
[0051] The elliptic instability significantly modifies the
long-term development of the eddy pairs, in the case of both
co-rotating eddies and contra-rotating eddies.
[0052] The principal phenomenon in the interaction of co-rotating
eddies (7A and 8A on the one hand, 7B and 8B on the other hand) is
the merging of the two eddies into a single vortex 5A, 5B, as soon
as the cores of these co-rotating eddies, which increase over time
by viscous or turbulent diffusion of the vorticity, exceed a
critical fraction of the distance between the centres of the cores
of these eddies;
[0053] these mergings, between 7A and 8A on the one hand, and
between 7B and 8B on the other hand, take place at the stages E3A
and E3B respectively.
[0054] Because of the instabilities (which have been amplified
according to the invention) of the eddies 8A and 8B, the vortices
5A and 5B which are created by these unstable mergings have larger
diameters dA and dB and higher internal turbulence levels than in
the absence of amplification.
[0055] According to the invention, these diameters dA and dB are
greater than a given proportion (for example, 0.3) of the
inter-vortex D; and,
[0056] due to diameters dA and dB of this size the non-linear
regime of the instability between contra-rotating vortices causes
(stage E4) an exchange of fluid between the two vortices 5A and 5B
by the creation of small-scale secondary vortices, which are
perpendicular to the axes of the principal eddies. This situation
rapidly leads to a disintegration of the initial pair of vortices
5A and 5B due to a small-scale turbulent motion of the fluid and a
marked and rapid decrease of the circulation of each vortex. The
vortices 5A and 5B are therefore destroyed more rapidly.
[0057] According to the invention, the instability modes of the
co-rotating eddies 8A and 8B, which are to be excited, are
determined on the basis of the known theory of instability,
partially described above, which has been confirmed by simulation
and experiment. It will be noted that the parameters determining
the instability modes are essentially the sizes (diameters) of the
cores of the eddies 7A, 7B, 8A, 8B before merging and the ratio of
the core size to the distance between these eddies. The variation
of the Reynolds number (expressing the effect of the viscosity of
the fluid) has practically no effect in the case of application to
an aircraft.
[0058] Additionally, the most unstable wavelengths of the
instability modes can be determined by the theory of elliptic
instability, on the basis of the above parameters. However, these
predictions yield only an order of magnitude for the case in
question. The most unstable wavelengths are of the order of the
mean diameter of the corresponding eddy core. The precise values in
each case in question must be found empirically. The perturbations
to be generated must have wavelengths close to that of the
instability mode, or at least close to a divisor of this
wavelength.
[0059] Additionally, according to the invention, the wavelength of
a perturbation to be generated is located in an instability range
of each of the co-rotating eddies of the corresponding wing.
[0060] As indicated above, the device 1 according to the invention
has at least two perturbation means, each of which can be made in
different embodiments 11, 12.
[0061] In a first embodiment shown in FIG. 2, the said perturbation
means 11 has an unstreamlined element 13, whose "apparent" or
"effective" diameter, which is transverse with respect to the flow,
depends on the wavelength of the periodic perturbation to be
generated.
[0062] Preferably, this unstreamlined element 13 is a cylinder, of
circular section for example, as shown in FIG. 2. However, this
cylinder can also have an elliptical section or any other section.
The element can also be a flat plate with a high angle of
incidence, inclined at 45.degree. for example.
[0063] The "effective" transverse dimension (or width) of the
element 13 therefore determines the wavelength of the generated
perturbation. This "effective" width depends on the degree of
deviation of the stream lines by the element 13. It is not
necessarily identical to the real dimension of the element 13. For
example, a flat plate perpendicular to the flow deviates the stream
lines much more than a cylinder having the same diameter.
Additionally, the relation between the "effective" dimension and
the wavelength of the generated wake is known for a certain number
of elements on a purely empirical basis: there is no theoretical
result. The wavelength is of the order of several times the
"effective" width of the element. Thus it is possible to have an
approximate idea of the size of the element 13 in advance, but the
relation has to be newly determined on an empirical basis for each
shape used.
[0064] With respect to the present invention, the said element 13
can therefore be any (unstreamlined) element which serves to
generate the aforesaid periodic perturbation of the flow.
[0065] Additionally, this element 13 is retractable. For this
purpose, it is preferably retractable into the wing 3A, 3B or into
the flap 6A, 6B, or into existing fairings in the proximity of
which it is fitted, with the use of ordinary retraction means which
are not shown. This avoids an increase in drag and thus avoids
degradation of the performance of the aircraft 2, particularly in
cruising flight.
[0066] In a second embodiment shown schematically in FIG. 3, the
said perturbation means 12 has a means 14 of an ordinary type for
producing a jet of fluid 15, as indicated by the arrows 16. This
jet of fluid 15 is emitted transversely with respect to the flow E
in such a way as to generate the perturbation P according to the
invention, as shown in a partial way. If the said jet of fluid 15
is emitted orthogonally to the flow E, its velocity must be at
least equal to that of the aircraft 2 and its diameter must be of
the same order of magnitude as the apparent diameter of an
unstreamlined element 13 which could be used in its place.
[0067] Preferably, the said means 14 is positioned inside the
existing structure 17 in the proximity of the said creation area
10A, 10B, for example inside the flap 6A, 6B or the wing 3A, 3B. If
this is not the case, the said means 14 can also be
retractable.
[0068] As indicated above, the perturbation means 11, 12 must be
chosen in such a way as to produce a perturbation with a wavelength
which can excite a maximum of unstable modes of the elliptic
instability of the co-rotating eddies.
[0069] By way of illustration, an example of apparent or effective
(transverse) diameter (which may be either the apparent diameter of
the unstreamlined element 13 or the "apparent diameter" of the jet
of fluid 15) is of the order of 10 cm, generating a perturbation
having a wavelength of approximately 50 cm. A representative radius
of the wing tip or flap tip eddy before merging may be of the order
of 1 metre (m), giving a maximum wavelength of the elliptic
instability of the order of 3 m, to which many lower unstable
wavelengths are to be added. The perturbation generated by the
device 1 can excite the modes which have the same wavelength, but
also all those close to the multiples. In the example in question,
the perturbation created by the device 1 would excite the modes
having wavelengths of 50 cm, 1 m, etc., up to 3 m (above this
length, the eddies are stable), in other words a maximum of six
unstable modes.
[0070] It will be noted that the device 1 according to the
invention has at least one perturbation means 11, 12 for each
vortex 5A, 5B. Clearly, it may equally well have two (or more) of
these means, in which case one acts on the wing tip eddy and the
other on the flap tip eddy. If it only has one means for each
vortex, then preferably, but not exclusively, this perturbation
means acts on the eddy 8A, 8B associated with the tip of the flap
6A, 6B, which is more energetic, because the distribution of lift
is different and more favourable to eddy generation, than the eddy
7A, 7B associated with the tip 9A, 9B of the wing 3A, 3B.
[0071] Clearly, the said device 1 can be used to accelerate the
destruction of more than two vortices, if an appropriate number of
perturbation means 11, 12 is provided.
[0072] It will also be noted that the said device 1 according to
the invention is passive, simple, robust and inexpensive.
[0073] The present invention can be applied to any moving body
trailing vortices in the wake of its wings (in the widest sense of
the word), and in particular to a submarine in order to make its
detection, particularly by satellite, more difficult.
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