Note: Descriptions are shown in the official language in which they were submitted.
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Submerged object suspended from a towing cable optimized to
neutralize disrupting hydrodynamic forces
The present invention relates to an object intended to be towed in
a fluid, comprising means for neutralizing hydrodynamic forces generated by
the flow of fiuid around the object, for example induced by asymmetries of a
towing device. The invention finds a particular use in the field of underwater
acoustics, notably for towed active sonars.
Towed active sonars comprise an emission antenna integrated
into a submersible object also referred to as a towfish, and a receive
antenna, for example a linear array, also referred to as a streamer. When the
sonar is being used in dependent towing, the towfish and the streamer are
connected in succession to one and the same towing cable. Figure 1
illustrates a known configuration of an active sonar in dependent towing. The
deck of the ship 10 is equipped with a towing device comprising a motorized
winch 11 capable, via a fairlead 15, of towing a cable 12 to which are
connected on the one hand a towfish 13 and on the other hand a streamer
14.
During a sonar mission, the streamer and then the towfish are first
of ail launched into the water by the deck crew. The towfish and the streamer
are then towed by the ship at the desired depth of immersion, determined by
the length of the cable and the speed of the ship. The emission and receive
antennas are generally controlled from the ship, via electrical and/or optical
information transmitted via conductors integrated into the towing cable. At
the
end of the mission, the towfish and the streamer are brought back up and
placed on the deck.
ln the known way, the emission antenna emits, by means of one
or more transducers, an acoustic wave directed toward the seabed or into the
column of water. The wave reflected off the seabed or off an object situated
in the column of water is then detected by the acoustic receiver. A signal
processing device operating on the signal received then allows the seabed or
the detected object to be imaged. A precise measurement requires that the
towed elements achieve good stability, particularly the acoustic antenna, sa
as to allow stable angular coverage. For that, the towfish is generally made
of
a submersible body suspended from the towing cable by means of fixing
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arms, which under the effect of gravity are able to generate a moment that
tends to keep the towfish plumb.
This return moment generated by gravity may sometimes prove
insufficient to ensure good stability of the towed object. Specifically, the
flow
of water around the object generates hydrodynamic forces the intensity of
which increases with the speed of towing. These hydrodynamic forces may
generate a rolling moment about the transport axis of the object, a pitching
moment or even a yawing moment and unsettle the towed object by acting
against the gravity return moment.
These disrupting hydrodynamic forces are particularly significant in
instances in which the towed object has a shape that is asymmetric about
one of these axes. The applicant company's patent document published
under the reference FR2982579 and describing an articulated fairlead
intended to optimize the operations of launching and retrieving the towfish is
thus known. Such a towing device offers numerous advantages: it
guarantees a minimum bend radius for the towing cable and makes it easier
for the towed body to pass through the fairlead, making it possible to
dispense with one articulated arm for grabbing hold of the towed body when
raising the cable back up. The use of this articulated fairlead does, however,
require the fixing arm to pass laterally. For this reason, the towed body has
an architecture that is asymmetric. This physical asymmetry generates an
asymmetry in the lift which may, for high towing speeds, notably for speeds
corresponding to a Froude number (V/root (g*L), where V is speed in m/s, g
is the constant due to gravity, L is the length of the body) at least equal to
1.5
and potentially exceeding 3, cause the towed body to rotate about the towing
cable. At low speed, gravity is enough to stabilize the object.
Attempts are therefore being made to improve the stability of a
submersible body the towing means of which exhibit asymmetry. More
generally, it remains desirable to have available means for stabilizing the
orientation in roll, pitch and yaw, of a body towed in a fluid by a cable.
Te this end, the invention relates to an object intended to be towed
in a fluid by a cable along a substantially horizontal transport axis; the
object
comprising a body intended to be suspended under the effect of gravity from
the cable by means of a fixing arm, the object being characterized in that the
body comprises:
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- an exterior hydrodynamic surface that is symmetric with respect to a
vertical plane containing the transport axis so as to limit the lateral lift
of the body, and
- an opening passing through the body along a vertical axis, which
opening is intended to be occupied by the fluid so as to equalize the
pressures of fluid flowing along the exterior surface, so as to limit the
vertical lift of the body;
making it possible to limit the hydrodynamic forces that may be generated
perpendicular to the transport axis by the flow of fluid around the body and
that are liable to lead to a load in rotation about the transport axis by
opposing the effect of gravity. The object comprises at least one fixing arm.
Advantageously, the body is a single-shell monocoque.
Advantageously, the fixing arm has a shape that is asymmetric
with respect to the vertical plane containing the transport axis.
Advantageously, the fixing arm has the overall shape of a "C",
connected by a first end to the body and intended to be connected by a
second end to the cable.
Advantageously, the object comprises a ballast weight positioned
inside the body and configured so that the center of gravity of the object
maintained at zero speed in the fluid is positioned in the vertical plane
containing the transport axis.
Advantageously, the object comprises a ballast weight positioned
inside the body and configured so that the center of gravity of the object
kept
immersed in air at zero speed is positioned in the vertical plane containing
the transport axis.
Advantageously, the object comprises a ballast weight positioned
inside the body and configured so that the center of gravity of the object is
positioned in the vertical plane containing the transport axis both when the
object is kept submerged at zero speed in water and when the object is kept
immersed at zero speed in air.
Advantageously, the object comprises a set of deflectors secured
to the body, configured to generate a hydrodynamic force by the flow of fluid
around the object that neutralizes the hydrodynamic force induced by the
asymmetric shape of the fixing arm.
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Advantageously, the exterior surface comprises an upper surface
and a lower surface.
Advantageously, the lower surface and the upper surface are
symmetric with respect to one another about a horizontal plane so as to limit
the vertical lift of the body.
Advantageously, the upper surface and/or the lower surface is
configured so as to form in the vertical plane containing the transport axis a
thick NACA-type profile making it possible to limit the vertical lift of the
body.
Advantageously, the exterior surface is configured so as to form in
the horizontal plane a thick NACA-type profile making it possible to limit the
lateral lift of the body.
Advantageously, the exterior surface is configured to form, in a
vertical plane perpendicular to the transport axis, a curved profile,
comprising
a small vertical portion so as to limit the lateral lift of the body.
Advantageously, the body comprises a lateral opening passing
through the body along a horizontal axis perpendicular to the transport axis
and situated in a downstream part of the body so as to limit the lateral lift
of
the body.
Advantageously, the opening is intended to be occupied by the
fluid.
Advantageously, the exterior surface comprises an upper surface
and a lower surface, the upper surface and the lower surface being
configured so that the exterior surface forms:
- in the horizontal plane a thick NACA-type profile making it possible to
limit the lateral lift of the body;
- in a vertical plane containing the transport axis, a thick NACA-type
profile making it possible to limit the vertical lift of the body,
the opening opening on the one hand onto the upper surface and on the
other hand onto the lower surface.
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Advantageously, the body is wholly delimited by the exterior
surface.
Advantageously, the body comprises a lateral opening passing
through the body along a horizontal axis perpendicular to the transport axis
5 and situated in a downstream part of the body so as to limit the lateral
lift of
the body.
Advantageously, the object comprises a substantially vertical
aileron fixed on a downstream part of the body having a substantially
asymmetric hydrodynamic shape configured to stabilize the orientation of the
object by generating a moment about a vertical axis.
Advantageously, the object comprises a substantially horizontal
aileron fixed on a downstream part of the body having a substantially
asymmetric hydrodynamic shape configured to stabilize the orientation of the
object by generating a moment about the transport axis.
Advantageously, the object comprises two fixing arms configured
so as to connect, in first and second substantially vertical directions
respectively, the cable and, respectively, a first and a second end of the
body
along the transport axis, so that a force tending to separate the two fixing
arms makes it possible to stabilize the body about a vertical axis.
Advantageously, the object is intended for sonar detection in a
marine environment, and comprising an acoustic emission antenna fixed to
an internal structure of the body.
The invention also relates to an active sonar system intended to
be towed by a ship and comprising a towing cable that can be connected to
the ship and an object towed by the cable and having features as described
hereinabove.
Advantageously, the active sonar comprises a tau l containing a
longilinear body for receiving acoustic signais.
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The invention will be better understood and further advantages will
become apparent from reading the detailed description of one example of a
towed submersible object which is described in the following figures.
Figure 1, already introduced, illustrates the use of a towed active
sonar according to the known state of the art,
figures 2a, 2b, 2c and 2d depict one example of a towed object
according to the invention in, respectively, a first perspective view, a
second
perspective view, a face-on view and a rear view.
For the sake of clarity, the same elements will bear the same
references in the various figures.
In the example depicted in figures 2a, 2b, 2c and 2d, the towed
object 20 comprises a submersible body 21 intended to receive within its
structure an emission antenna of an active sonar, and connected to the
towing cable 12 via two fixing arms 22a and 22b. In a configuration similar to
the one depicted in figure 1, the cable 12 from which the body 21 is
suspended is connected by an end situated upstream of the body to a
motorized winch similar to the winch 11, and by another end situated
downstream of the body to a receive antenna 14. The towed object 20
therefore constitutes the towfish 13 of the active sonar in dependent towing
described previously. It should be understood that this particular embodiment
is not a limitation on the present invention which relates more broadly to any
type of body intended to be towed in any type of fluid by means of a towing
cable.
The body is significantly heavy in water, which means to say that it
has negative buoyancy. 'What is meant by significantly is that the upthrust of
the body is typically less than 80% of its weight in air.
The body is suspended from the towing cable by means of two
asymmetric fixing arms 22a and 22b which allow the fixing arms to pass
laterally through an articulated fairlead analogous to the one described in
the
patent application cited in the preamble. The invention is of particular
utility in
stabilizing such a body of which the design of the means that fix it to the
cable exhibits asymmetry inducing significant hydrodynamic forces liable to
generate a disrupting moment about the towing cable. This particular
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embodiment implies no limit on the invention which may also be applied to
stabilizing the towing of a body that does not have this asymmetry of the
fixing means.
The invention relates to a towed object comprising a body
suspended under the effect of gravity from a towing cable by means of at
least one fixing arm. The towing cable is intended to tow the object along a
substantially horizontal transport axis. Hereinafter, the towed object is thus
defined with respect to a vertical axis along which the force of gravity is
applied, and a substantially horizontal transport axis defining the main
direction of the flow of the fluid. Of course, the invention may also apply to
cases in which the object is towed in a direction that is not strictly
horizontal,
but slightly upward or slightly downward. What is meant by substantially
horizontal is an axis that preferably makes an angle smaller than 10 degrees
with the horizontal plane. This value does not constitute a limit on the
invention. The detailed definition of an object allowing, in the case of
movement that is not strictly horizontal, the effect of the hydrodynamic
forces
to be limited so as to stabilize the towing of the object, can be deduced
easily
from the example described hereinbelow.
In the remainder of the document, the towed object is described
with reference to a frame of reference made up of an axis of movement - or
axis of roll - referenced X in the figures, preferably aligned between the
fixing
arms with the longitudinal axis of the towing cable 12; of a yaw axis
referenced Z, corresponding to the vertical axis passing through the center of
gravity of the object; and of a pitch axis referenced Y, corresponding to the
horizontal axis perpendicular to the roll axis.
The general idea behind the present invention is that of reaching
equilibrium between the forces of gravity exerted on the towed object and the
hydrodynamic forces generated by the flow of fluid around the object as it is
being towed. The object uses various measures that contribute to achieving
this equilibrium and obtaining an architecture that is stable throughout the
range of operational speeds. These measures will now be described with
reference to figures 2a to 2d.
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The weight in water of the object suspended from the cable is the
key factor to stabilizing the system about its roll axis. This of course
assumes
that the center of gravity is offset from the axis of the supporting cable,
which
it is as the cable is fixed to the top of the body. In order to be effective,
it is
considered that the distance separating the center of gravity of the object
from the cable is at least equal to half the vertical thickness of the body 51
in
a vertical plane. The vertical thickness is defined later on. Increasing the
weight of the object in order to increase its stability is, however, limited
by the
capacity of the launch and recovery means. In practice, the weight of the
object is therefore defined as being a compromise between the improvement
in stability and the ease of object launch and recovery operations.
The length of the fixing arm is another factor in stabilizing the
object in roll. The magnitude of the return moment in roll generated by the
weight is in direct proportion to the distance separating the center of
gravity
of the object from the cable 12. Increasing the lever arm through the length
of
the fixing arms makes it possible to increase the return moment in roll.
However, long fixing arms reduce the pitch stabilization achieved
by fixing the two fixing arms to the cable. Specifically, the hydrodynamic
forces generated by the flow of fluid around the object are situated chiefly
in
the region of the body. Thus, increasing the lever arm for the return moment
in roll by increasing the length of the fixing arms aise increases the lever
arm
of the destabilizing effect in pitching. That aise increases the destabilizing
effect in roll when the object is at a yaw angle. In practice, an intermediate
length is adopted for the fixing arms, as in the example depicted in the
figures.
The body is suspended from the cable by means of the fixing arms
22a and 22b. Te allow lateral passage through the fairlead, the fixing arms
are in the overall shape of a "C" having asymmetry with respect to the
vertical
plane containing the transport axis X. This asymmetry of weight is likely to
cause an inclination at zero speed with respect to a vertical plane of
symmetry referenced Psv containing the transport axis X. In order to
compensate for the force of gravity exerted on the fixing arms and to
stabilize
the orientation of the body with respect to the vertical plane Psv, a ballast
weight is fixed inside the body. The parameters used to define this weight are
indicated in figure 2d. In this figure:
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= PB is the position of the center of gravity of the components exhibiting
asymmetry with respect to the vertical plane containing the transport axis. In
the example depicted, this is the center of gravity of the two fixing arms 22a
and 22b.
= PL is the position of the center of gravity of the ballast weight fixed
inside the body.
= Pc is the position of the center of gravity of the object as a whole,
including the fixing arms and the body fitted with its ballast weight.
To facilitate the operation of recovering the towed object in the air, the
desire is to position the center of gravity of the object in the vertical
plane of
symmetry Psv. The following relationship makes it possible to determine the
position of the ballast weight that will make it possible to cancel the
listing of
the object in the air:
ML * DL= MB * DB (1)
in which:
= mt_ is the mass of the ballast weight,
= DL is the distance from the center of gravity to the vertical symmetry
plane,
= mB is the mass of the asymmetric elements, and
= DB is the distance from the center of gravity to the vertical symmetry
plane.
It is also desired to obtain a zero listing when the object is immersed in
water at zero speed. To achieve that, the object needs to satisfy the
following
relationship:
ML * (dL ¨ dE) = MB* (dB - dE) (2)
in which:
= di_ is the density of the ballast weight,
= dB is the density of the asymmetric elements, and
= dE is the density of the water in which the object is moving.
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Solving equations (1) and (2) makes it possible to define the density
and position of the ballast weight in the body, so as to balance the object
kept at zero speed both in air and in water. That makes it possible to obtain
5 good acoustic measurements in the water and make it easier to recover the
object in the air. Figure 2d describes the positioning of the ballast weight
that
makes it possible to obtain a zero listing for the towed object 20. Note that
in
order flot to disrupt the flow of water around the body, the ballast weight is
advantageously positioned inside the structure, under the streamlining.
The return moment in roll for the object thus defined may be
expressed by a relationship of the type:
CR (Dc * mc ¨ Dv *V* dE) * g * sin (a) (3)
in which:
= Dc is the distance between the center of gravity and the cable,
= mc is the total mass of the object,
= Dv is the distance between the center of volume CdV of the object and
the cable,
= V is the total volume of the object,
= dE is the density of the water in which the object is moving,
= g is the acceleration due to gravity, and
= a is the angle of roll.
In order to stabilize the towed object it is appropriate to ensure that
this gravity return moment CR remains higher, throughout the envisioned
speed range, than the moments generated by the hydrodynamic forces about
the roll axis. There are various measures taken toward this.
In the example depicted in the figures, the body 21 comprises an
exterior surface 24 formed of an upper surface 25a and of a lower surface
25b which are delimited by an edge in common which is contained in a
horizontal plane PSH depicted in figure 2a. The common edge defines a left-
hand lateral profile referenced 70a (on the side of the positive values of the
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axis Y) and a right-hand lateral profile referenced 70b (on the side of the
negative values of the axis Y). The shape of the body 21 is also defined
hereinafter by means of a vertical plane referenced Psv containing the
transport axis X.
In order to reduce the vertical lift of the body, the upper 25a and
lower 25b surfaces are preferably substantially symmetric with respect to one
another about the horizontal plane PSH. Advantageously, each of the two
surfaces is also configured so as to form in a vertical plane parallel to the
transport axis, and notably in the plane Psv, a profile close to a
standardized
thick NACA profile; this profile on the one hand guaranteeing a vertical lift
that is limited and only slightly variable for slightly variable angles of
incidence and, on the other hand, limited drag.
What is meant by a profile that is thick in a vertical plane is a
profile of which the ratio between the vertical thickness (largest dimension
of
the profile in the vertical direction) and the chord (length separating the
leading edge and the trailing edge of the profile) is greater than 25%. It
should be noted that the rear part of the profile, which means to say the part
containing the trailing edge, may be truncated. The dimension of the body in
the vertical direction is the distance between the lower and upper surfaces.
As depicted in the figures, the body has an overall shape that is
substantially flattened along the vertical axis. In other words, the
dimensions
of the object along the vertical axis are less than the dimensions of the
object
in the horizontal plane. The body comprises a central opening 30 passing
through the body 21 along a vertical axis. The opening opens on the one
hand onto the upper surface 25a and on the other hand onto the lower
surface 25b. This opening 30, occupied by the fluid in which the object 20 is
immersed, makes it possible to equalize the pressures of the fluid flowing
along the upper surface and along the lower surface. This opening limits the
interior volume of the body that is available to house equipment such as the
emission antenna. However, in one advantageous embodiment of the
invention, what is envisioned is an acoustic emission antenna that is in the
overall shape of an annulus, positioned around the opening 30, so that the
reduction in the volume available in the body as a result of the opening 30 is
not a major limitation on the overall effectiveness of the sonar.
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The opening 30 passing through the body has, in a horizontal
plane, a surface area representing more than 15% of the adjoining horizontal
surface with no interior boundary formed from the contour of the projected
surface of the body on a plane that is horizontal but limited upstream and
downstream by the two vertical planes perpendicular to the transport axis
delimiting the extent of the opening 30 along the transport axis (X). That
makes it possible to guarantee a good limit on the vertical lift of the body.
In order to reduce the latere! lift on the body, the exterior surface
24 is substantially symmetric with respect to the vertical plane Psv
containing
the transport axis X. Advantageously, the exterior surface 24 is also
configured to form, in a horizontal plane and notably the plane PSH, two
profiles symmetric with respect to one another about the plane Psv which are
similar to a thick standardized NACA profile. In particular, the left-hand
lateral
profile 70a and the right-hand lateral profile 70b are advantageously similar
to a thick NACA-type profile; this profile on the one hand guaranteeing
lateral
lift that is limited and only slightly variable for angles of incidence that
are
slightly variable and, on the other hand, limited drag.
What is meant by a profile that is thick in a horizontal plane is a
profile of which the ratio between the horizontal thickness (the largest
dimension of the profile in the horizontal direction perpendicular to the
transport axis) and the chord (length separating the leading edge and the
trailing edge of the profile, is greater than 25%. It should be noted that the
rear part of the profile formed by the exterior surface, namely the part
containing the trailing edge, may be truncated as it is in the embodiment in
the figures. The body therefore does not comprise the trailing edge of the
profile formed by the exterior surface 24. The profile comprising the leading
edge and the trailing edge is obtained by extrapolating the exterior surface
24
as far as the trailing edge.
The exterior surface 24 is formed by joining the profiles formed in
the plane PSH and in the plane Psv on each side of each of these planes so
that it forms a hydrodynamic surface.
Advantageously, the body 21 overall has a teardrop shape (with
the opening 30 passing through it). The teardrop may be truncated so as not
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to comprise the trailing edge. The exterior surface is advantageously convex
overall.
Advantageously, the body is wholly delimited by the surface 24.
In order to further enhance the stabilization by limiting the effect of
lift, it is also envisioned to configure the exterior surface 24 so that, in a
vertical plane perpendicular to the transport axis X, it has a curved profile
comprising a small vertical portion. In other words, the lateral walls of the
body have a low proportion of substantially vertical surface so as to limit
the
possibility of them generating lateral lift. Note too that the body comprises
a
lateral opening 26 passing through the body along a horizontal axis
perpendicular to the transport axis X and situated in a downstream part of the
body making it possible further to reduce the lateral lift of the body. The
lateral opening is intended to be occupied by the fluid.
One example of a submersible body 21 having a hydrodynamic
shape optimized for reducing the lift of the suspended part of the object has
been described by means of the figures. The example depicted employs
various measures aimed at reducing, on the one hand, the vertical lift
(symmetry with respect to the horizontal plane PSH, NACA profiles in the
vertical plane, central opening 30) and, on the other hand, the lateral lift
(symmetry with respect to the vertical plane PSH, NACA profiles in the
horizontal plane, profiles comprising a small vertical portion, rear lateral
opening 26). Of course the invention also envisions a submersible body
retaining only some of these measures.
In a particularly advantageous simplified configuration, the body
comprises a hydrodynamic exterior surface that is substantially symmetric
about the vertical plane Psv containing the transport axis X, making it
possible to limit the lateral lift of the object, and comprises an open-ended
central opening passing through the body along a vertical axis making it
possible to limit the vertical lift. This unique configuration of the body
makes it
possible to limit the hydrodynamic forces that may be generated
perpendicular to the transport axis by the flow of fluid around the body and
which are liable to lead to a force in rotation about the transport axis
opposing the effect of gravity.
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The body 21 is of the single-shell monocoque type. In other words,
the body is not made up of multiple shells. It is flot possible to isolate a
portion that is an incomplete portion of the body, that forms a
hydrodynamically profiled independent elementary body.
The body 21 is wholly delimited by a shell 124.
This shell has a single substantially convex continuous front surface: in
other
words, the fluid impinges on the shell at a single point referred to as the
stagnation point (or over a very restricted zone), the object being a single-
shell monocoque.
The shell 124 comprises a streamlining or external surface of the
shell 124. The streamlining is the exterior surface 24. The streamlining 24
delimits a volume that forms a single hydrodynamically profiled cell. This
volume exhibits the shape of a single hydrodynamically profiled cell in any
vertical plane and in any horizontal plane.
Advantageously, the thickness of the body 21 corresponding to the
dimension of the body 21 along a vertical axis is substantially the same
around the entire periphery of the opening 30.
The present invention proposes forming a vertical through-opening
in a monocoque body, something which is unusual. The fact of proposing
a monocoque body means that it is possible to tow a body of significant
volume (and therefore to carry a sonar of significant volume) with limited
drag. The fact of forming a vertical through-opening through the body makes
25 it possible to limit its vertical lift. Now, a person skilled in the art
conventionally seeks to form a monocoque body that is as homogeneous as
possible without through-openings or protrusions, so as to limit its drag. Any
irregularity in the profile generates turbulence and therefore drag. Forming a
vertical through-opening in a monocoque body therefore impairs its drag
30 rating, and is counterintuitive.
In one particular embodiment, the body 21 comprises a leading
edge BA and is delimited, in any vertical plane perpendicular to the transport
axis situated between the leading edge BA and the opening 30, by a
streamlining that forms a convex or substantially convex curve delimiting a
CA 02963308 2017-03-31
single convex or substantially convex volume. In other words, the exterior
surface 24 delimits, in any vertical plane perpendicular to the transport axis
situated between the leading edge and the opening 30, a convex or
substantially convex single volume. What is meant by substantially convex is
5 that the exterior surface or streamline may contain elements of the
structure
of the body which project with respect to a convex curve.
In one particular embodiment, the volume is convex in any vertical
plane perpendicular to the transport axis. In other words, the exterior
surface
is, apart from the central opening 30, substantially convex. The body is a
10 monospace body. As an alternative, the volume is convex in any vertical
plane perpendicular to the transport axis situated between the leading edge
BA of the body 21 and a vertical plane perpendicular to the transport axis
situated at a distance from the trailing edge of the surface 24 that is equal
to
two-thirds of the distance separating the leading edge and the trailing edge
of
15 the surface 24.
Another measure aimed at stabilizing the towed object is to fix one
or more stabilizing ailerons on a downstream part of the body. In the example
depicted in the figures, the object comprises two substantially vertical
stabilizing ailerons 43 and 44 fixed on a downstream part of the body 21. The
dimensions of the ailerons are, on the one hand, small enough to generate lift
that is less than the weight of the object in water for the speed range
considered, and, on the other hand, large enough that their stabilizing effect
is effective. Furthermore, the use of a slightly asymmetric profile for the
ailerons 43 and 44 makes it possible to generate a return moment in yaw,
capable of at least partially neutralizing the asymmetry in lift generated by
the
C-shape of the fixing arms.
By the same principle, the towed object also comprises two
substantially horizontal stabilizing ailerons 41 and 42, fixed on a downstream
part of the body 21, respectively in the upper part and the lower part. Once
again, the dimensions of the ailerons need to be on the one hand sufficiently
small that they generate lift less than the weight of the object in water,
and,
on the other hand, high enough that their stabilizing effect is effective. A
slightly asymmetric configuration of the ailerons 41 and 42 makes it possible
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16
at least partially to neutralize the roll moment generated by the drag of the
fixing arms of the towed body suspended beneath the towing cable.
The example depicted in the figures illustrates the benefit of such
an asymmetric configuration of the ailerons. To compensate for the disruptive
moment induced by the asymmetry of the fixing arms 22a and 22b,
asymmetry is introduced into the horizontal ailerons by attaching two
deflectors 31a and 31b to the upper horizontal rear aileron 41. The
asymmetry of the fixing arms with respect to the vertical plane of symmetry
Psv, subjected to a nonzero speed in the water, generates a lateral force on
the towed object. This force produces a disruptive rolling moment
substantially proportional to the square of the speed. In simplified form,
this
disruptive moment can be expressed using the following relationship:
CP = *V2 (4)
in which:
= Ki is a coefficient defining the hydrodynamic disruption induced by the
asymmetry of the fixing arms, and
= V is the speed of the towed object.
In order to neutralize the effect of this disruptive torque CP on roll,
hydrodynamic asymmetry is created on the object, in this instance by means
of the two deflectors 31a and 31b. As before, the moment generated by this
set of deflectors is substantially proportional to the square of the speed. It
can be expressed in a simplified form of the type:
CA = K2 * V2 (5)
in which:
= 1<2 is a coefficient defining the hydrodynamic disruption induced by the
set of deflectors, and
= V is the speed of the towed object.
The set of deflectors is therefore designed to generate a moment CA
which opposes the disruptive moment CP induced by the asymmetry of the
fixing arms. In other words, solving equations (4) and (5) makes it possible
to
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determine the characteristics of the set of deflectors. By configuring the
deflectors in such a way as to satisfy the equality Ki= 42, the set of
deflectors allows the disruption induced by the asymmetry of the fixing arms
to be neutralized.
A final measure seeks to stabilize the towed object in terms of yaw.
This stabilization is essentially afforded by the towing force Tc exerted by
the
hydrodynamic drag that the set of towed elements downstream of the object
produces. For an active sonar in dependent towing, the movement of the
streamer 14 connected to the cable 12, downstream of the towed object,
generates hydrodynamic drag which applies to the object, via the cable, a
rearward load. The return moment generated by this rear towing force Tc can
be expressed through the following simplified relationship:
CL = Tc * L* sin ( [3 )
in which:
= L is the distance between the attachment points of the fixing arms 22a
and 22b, and
= [3 is the yaw angle with respect to the longitudinal axis X of the cable.
Thus increasing the distance L separating the two fixing arms
improves the yaw stability of the towed object. In the example depicted in the
figures, the fixing arms 22a and 22b are fixed at two opposite ends along the
axis X of the body 21, so as to maximize the yaw stabilization.
In other words, the two fixing arms 22a and 22b are configured so as
to connect, respectively in a first and a second substantially vertical
direction,
the cable 12 and, respectively, a first and a second end of the body along the
transport axis X so that a load tending to part the two fixing arms allows the
body 21 to be stabilized about the vertical axis Z.
The invention also relates to a towed system, for example a towed
system containing an active sonar, that may be towed by a ship. The system
comprises a towing cable and a submersible object as described
hereinabove. The object is towed by the ship by means of the towing cable.
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The system may be supplemented by a tail (of the flexible longilinear body
type) that may contain a streamer that receives the acoustic signais.
