EP3201073A1

EP3201073A1 - Submerged object suspended from a towing cable optimised to neutralise disrupting hydrodynamic forces

Info

Publication number: EP3201073A1
Application number: EP15775175.1A
Authority: EP
Inventors: Michaël JOURDAN; Christophe Borel
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2014-10-01
Filing date: 2015-10-01
Publication date: 2017-08-09
Anticipated expiration: 2035-10-01
Also published as: US10029765B2; EP3201073B1; FR3026714B1; FR3026714A1; CA2963308C; US20170291666A1; CA2963308A1; AU2015326850B2; SG11201702546PA; WO2016050936A1; AU2015326850A1

Abstract

The invention relates to an object (20) that can be towed in a fluid by a cable (12) according to a substantially horizontal transport axis (X); the object (20) including a body (21) suspended by gravity from the cable (12) by an attachment arm (22a, 22b), and including: an outer hydrodynamic surface (24) which is symmetrical relative to a vertical plane (P_sv) containing the transport axis (X), so as to limit the lateral lift of the body (21); and an opening (30) passing through the body (21) according to a vertical axis (Z), configured such as to balance the pressures of the fluid flowing along the outer surface (24), making it possible to limit the hydrodynamic forces that may be generated perpendicular to the transport axis (X) driving a rotating force around the transport axis (X) countering the effect of gravity.

Description

Submerged object suspended from an optimized towing cable to neutralize disruptive 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 fluid around the object, for example induced by asymmetries of a towing device. The invention finds particular utility in the field of underwater acoustics and in particular for towed active sonars.

Trailed active sonars include a transmitting antenna integrated in a submersible object, also called fish, and a receiving antenna for example linear, also called flute. When using the dependent towing sonar, the fish and the flute are successively connected to 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 1 1 capable of towing, through a fairlead 15, a cable 12 to which are connected on the one hand a fish 13 and on the other hand a flute 14.

During a sonar mission, the flute and the fish are first launched by the deck crew. The fish and flute are then towed by the vessel to the desired depth of immersion, determined by the length of the cable and the speed of the vessel. The transmitting and receiving antennas are generally controlled from the ship by means of electrical and / or optical information transmitted by conductors integrated in the traction cable. At the end of the mission, the fish and the flute are reassembled and stored on deck.

In known manner, the transmitting antenna emits by means of one or more transducers an acoustic wave directed towards the seabed or in the water column. The wave reflected on the seabed or on an object located in the water column is then detected by the acoustic receiver. A received signal processing device then makes it possible to image the seabed or the detected object. Accurate measurement requires good stability of the towed elements, including the acoustic antenna to allow stable angular coverage. For this, the fish generally consists of a submersible body suspended from the traction cable by means of arms of fixation, allowing the effect of gravity to generate a couple tending to maintain the fish plumb.

This return torque generated by the gravity may however be insufficient to ensure good stability of the towed object. Indeed, the flow of water around the object generates hydrodynamic forces whose intensity increases with the towing speed. These hydrodynamic forces can generate a torque in roll around the transport axis of the object, a pair in pitch or yaw, and unbalance the towed object by countering the restoring torque by gravity.

These disturbing hydrodynamic forces are particularly important in the case where the towed object has a non-symmetrical shape along one of these axes. The applicant is thus known from the patent document published under the reference FR2982579, describing an articulated fairlead intended to optimize the launching and removal of fish operations. Such a towing device has many advantages: it guarantees a minimum bending radius of the traction cable and facilitates the passage of the towed body through the fairlead, making it possible to do without an articulated arm to grip the towed body during the ascent. cable. The implementation of this articulated fairlead however involves a lateral passage of the attachment arm. For this reason, the towed body has an asymmetrical architecture. This physical dissymmetry generates a asymmetry of lift that can, for high towing speeds, especially for speeds corresponding to a Froude number (V / root (g ^* L), with V speed in m / s, g: constant of the gravity, L: body length) of at least 1 .5 and may exceed 3, rotate the towed body around the towing cable. At low speed, gravity is enough to stabilize the object.

It is therefore sought to improve the stability of a submersible body whose towing means have an asymmetry. In a more general manner, it remains desirable to have means for stabilizing the roll, pitch and yaw orientation of a body drawn into a fluid by a cable. For this purpose, 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 by gravity from the cable by means of a fixing arm, the object being characterized in that the body comprises:

an exterior hydrodynamic surface symmetrical 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 intended to be occupied by the fluid so as to balance the fluid pressures flowing along the outer surface, so as to limit the vertical lift of the body;

to limit hydrodynamic forces that can be generated perpendicularly to the transport axis by fluid flow around the body and likely to cause a force in rotation around the transport axis by opposing the effect of gravity . The object comprises at least one attachment arm.

Advantageously, the body is monocoque.

Advantageously, the attachment arm has an asymmetrical shape with respect to the vertical plane containing the transport axis.

Advantageously, the fixing arm has a general shape in "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 positioned inside the body and configured so that the center of gravity of the object held at zero speed in the fluid is positioned in the vertical plane containing the transport axis.

Advantageously, the object comprises a ballast positioned inside the body and configured so that the center of gravity of the object held immersed in the air at zero speed is positioned in the vertical plane containing the transport axis.

Advantageously, the object comprises a ballast 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 at a time when the object is kept immersed at zero speed in the water and when the object is kept immersed at zero speed in the air.

Advantageously, the object comprises a set of baffles integral with the body, configured to generate a hydrodynamic force by fluid flow around the object neutralizing the hydrodynamic force induced by the asymmetrical shape of the fixing arm.

Advantageously, the outer surface comprises an upper surface and a lower surface.

Advantageously, the lower surface and the upper surface are symmetrical to one another with respect to 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 NACA type profile of great thickness, to limit the vertical lift of the body. Advantageously, the outer surface is configured so as to form in the horizontal plane a NACA type profile of great thickness, to limit the lateral lift of the body.

Advantageously, the outer surface is configured so as 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 located 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 outer surface comprises an upper surface and a lower surface, the upper surface and the lower surface being configured so that the outer surface forms:

- In the horizontal plane a NACA-type profile of great thickness, to limit the lateral lift of the body;

in a vertical plane containing the transport axis, a NACA-type profile of great thickness making it possible to limit the vertical lift of the body,

the opening opening on the one hand on the upper surface and on the other hand on the lower surface.

Advantageously, the body is entirely delimited by the outer surface.

Advantageously, the body comprises a lateral opening through the body along a horizontal axis perpendicular to the transport axis, and located in a downstream part of the body so as to limit the lateral lift of the body.

Advantageously, the object comprises a substantially vertical fin fixed on a downstream part of the body, having a substantially asymmetrical hydrodynamic shape, configured to stabilize the orientation of the object by generating a torque around a vertical axis.

Advantageously, the object comprises a substantially horizontal fin fixed on a downstream portion of the body, having a substantially asymmetrical hydrodynamic shape, configured to stabilize the orientation of the object by generating a torque around the transport axis.

Advantageously, the object comprises two attachment arms configured so as to connect respectively in a first and a second substantially vertical directions, 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 around a vertical axis. Advantageously, the object is intended for sonar detection in a marine environment, and it comprises an acoustic emission antenna fixed on 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 traction cable connectable to the ship and a towed object by the cable and having characteristics as previously described. Advantageously, the active sonar comprises a tail containing an elongated body ensuring the reception of the acoustic signals.

The invention will be better understood and other advantages will appear on reading the detailed description of an example of a towed submersible object described in the following figures.

FIG. 1, already presented, illustrates the implementation of an active sonar towed according to the state of the art,

Figures 2a, 2b, 2c and 2d show an example of towed object according to the invention respectively in a first perspective view, a second perspective view, a front view and a rear view.

For the sake of clarity, the same elements will bear the same references in the different figures.

In the example shown in FIGS. 2a, 2b, 2c and 2d, the towed object 20 comprises a submersible body 21 intended to receive in its structure an antenna for transmitting an active sonar, and connected to the traction cable. 12 by means of two fixing arms 22a and 22b. In a configuration similar to that shown in Figure 1, the cable 12 to which the body 21 is suspended is connected by an end upstream of the body to a motorized winch similar to the winch January 1, and by another end downstream of the The towed object 20 then constitutes the fish 13 of the active sonar in dependent towing described. previously. It is understood that this particular implementation is not limiting of 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 traction cable.

The body is significantly heavy in the water, that is to say with negative buoyancy. By significantly it is meant that the archimedic thrust of the body is typically less than 80% of the weight in the air thereof.

The body is suspended from the traction cable by means of two asymmetric fixing arms 22a and 22b allowing lateral passage of the attachment arms in an articulated fairlead similar to that described in the patent application cited in the preamble. The invention finds particular utility for stabilizing such a body, the architecture of the cable attachment means has an asymmetry inducing significant hydrodynamic forces, capable of generating a disturbing torque around the traction cable. This particular embodiment is not limiting of the invention, which can also be applied to stabilize the towing of a body not having this asymmetry of the fastening means.

The invention relates to a towed object comprising a body suspended by the effect of gravity on a traction cable by means of at least one attachment arm. The traction cable is intended to tow the object along a substantially horizontal transport axis. In the following, the towed object is thus defined with respect to a vertical axis along which the force of gravity applies and a substantially horizontal transport axis defining the main direction of the flow of the fluid. It is understood that the invention can also apply to the case where the object is towed in a direction not strictly horizontal, but slightly upward or slightly downward. By substantially horizontal means an axis preferably having an angle less than ten degrees with the horizontal plane. This value does not constitute a limit to the invention. The detailed definition of an object allowing, in the case of a non-strictly horizontal displacement, to limit the effect of the hydrodynamic forces to stabilize the towing of the object, can be deduced simply from the example described below. In the remainder of the document, the towed object is described with respect to a mark consisting of a displacement axis - or roll axis - referenced X in the figures, preferably aligned between the attachment arms on the longitudinal axis of the cable. traction 12; a yaw axis referenced Z, corresponding to the vertical axis passing through the center of gravity of the object; and a pitch axis, referenced Y, corresponding to the horizontal axis perpendicular to the roll axis.

The general idea of the present invention is to find a balance between the gravitational forces exerted on the towed object and the hydrodynamic forces generated by the flow of fluid around the object during its towing. The object implements several measures contributing to finding this balance and obtaining a stable architecture over the entire operational speed range. We will now describe these measurements using Figures 2a to 2d.

The weight in the water of the object suspended from the cable is the main stabilizing factor of the system around its roll axis. This obviously assumes that the center of gravity is offset from the axis of the carrier cable, which is the case when we fix the cable on the upper body. To be effective, it is considered that the distance separating the center of gravity from the object of 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. The increase of the weight of the object to increase its stability is however limited by the capacity of the means of launching and recovery. In practice, the weight of the object is therefore defined as a compromise between the improvement of stability and the simplicity of launching operations and recovery of the object.

The length of the attachment arms is another stabilizing factor of the object in roll. The amplitude of the roll-resting torque generated by the weight is directly proportional to the distance separating the center of gravity from the object of the cable 12. Increasing the lever arm by the length of the attachment arms makes it possible to increase the torque booster in roll.

But long attachment arms reduce the pitch stabilization produced by attaching the two attachment arms to the cable. Indeed, Hydrodynamic forces generated by fluid flow around the object are located mainly at the body level. Thus, increasing the lever arm of the restoring torque in roll by the length of the attachment arms, also increases the lever arm of the pitch destabilizing effect. This also increases the destabilization in roll when the object has a yaw angle. In practice, an intermediate length is retained for the attachment arms, as in the example shown in the figures.

The body is suspended from the cable by means of the attachment arms 22a and 22b. To allow a lateral passage in the fairlead, the attachment arms have a general shape in "C" having an asymmetry with respect to the vertical plane containing the transport axis X. This asymmetry of the weight is likely to generate a tilt at zero speed with respect to a vertical plane of symmetry referenced P _S v containing the transport axis X. 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 P _S v, a ballast is fixed inside the body. In Figure 2d are shown the quantities to define this ballast. In this figure:

• P _B is the position of the center of gravity of the components having an asymmetry with respect to the vertical plane containing the axis of transport. In the example shown, it is the center of gravity of the two attachment arms 22a and 22b.

• P _L is the position of the center of gravity of the ballast fixed inside the body.

• P _c is the position of the center of gravity of the whole object, including the fixing arms and the body with its ballast.

To facilitate the air recovery operation of the towed object, it is sought to position the center of gravity of the object in the vertical plane of symmetry P _S v- The following relationship allows to determine the position of the ballast allowing cancel the lodging of the object in the air: m _L * DL = m _B * DB (1) in which:

· M _L is the mass of the ballast, • D _L is the distance from the center of gravity to the vertical plane of symmetry,

• m _B is the mass of the asymmetrical elements, and

• D _B is the distance from the center of gravity to the vertical plane of symmetry. We also seek to obtain a zero cottage when the object is immersed in water with zero speed. For this, the object must satisfy the following relation: mL * (dL - d _E ) = m _B * (dB - d _E ) (2) in which:

• d _L is the ballast density,

• d _B is the density of the asymmetrical elements, and

• d _E is the density of the water in which the object evolves.

The resolution of equations (1) and (2) makes it possible to define the density and the position of the ballast in the body, so as to balance the object maintained at zero speed both in the air and in the water. This makes it possible to obtain good acoustic measurements in the water and to facilitate the recovery of the object in the air. FIG. 2d describes the positioning of the ballast making it possible to obtain a zero deposit for the towed object 20. Note that in order not to disturb the flow of water around the body, the ballast is advantageously positioned inside the structure, under the fairing. The rolling restoring torque of the object thus defined can be expressed by a relation of the type:

C _R = (D _c ^* m _c - D _v ^* V ^* d _E ) ^* g ^* sin (a) (3) in which:

• D _c is the distance between the center of gravity and the cable,

• m _c is the total mass of the object,

• D _v is the distance between the volume center C _d V of the object and the cable,

• V is the total volume of the object,

· D _E is the density of the water in which the object evolves, • g is the acceleration of gravity, and

• a is the roll angle.

To stabilize the towed object, it must be ensured that this gravitational return torque C _R remains higher, over the entire speed range envisaged, at the torques generated by the hydrodynamic forces around the roll axis. Several measures are implemented for this purpose.

In the example shown in the figures, the body 21 comprises an outer surface 24 formed of an upper surface 25a and a lower surface 25b, delimited by a common edge contained in a horizontal plane P _S H shown in Figure 2a. The common edge defines a left lateral profile referenced 70a (on the side of the positive values of the Y axis) and a right lateral profile referenced 70b (on the side of the negative values of the Y axis). The shape of the body 21 is also defined in the following by means of a vertical plane referenced P _S v containing the transport axis X.

To reduce the vertical lift of the body, the upper 25a and lower 25b surfaces are preferably substantially symmetrical to each other with respect to the horizontal plane P _S H- Advantageously, each of the two surfaces is also configured so as to form in a vertical plane parallel to the transport axis, and in particular the plane P _S v, a profile close to a standardized profile NACA of great thickness; this profile guarantees on the one hand a limited vertical lift and slightly variable for low variable incidence angles, and on the other hand a limited drag.

By profile of great thickness in a vertical plane, is meant a profile whose ratio between the vertical thickness (largest dimension of the profile in the vertical direction) and the rope (length separating the leading edge and the trailing edge of the profile) is greater than 25%. It should be noted that the rear portion of the profile, i.e., the portion containing the trailing edge may be truncated. The body dimension in the vertical direction is the distance between the lower and upper surfaces.

As shown in the figures, the body has a general shape substantially flattened along the vertical axis. In other words, the dimensions of the object along the vertical axis are smaller 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 on the upper surface 25a and on the other on the lower surface 25b. This opening 30, occupied by the fluid in which the object 20 is immersed, makes it possible to balance the pressures of the fluid flowing along the upper surface and the lower surface. This opening limits the internal volume of the body available to receive equipment such as the transmitting antenna. However, in an advantageous implementation of the invention, it is envisaged an acoustic emission antenna having a generally ring shape, positioned around the opening 30, so that the reduction in the volume available in the body due to opening 30 is not a major limitation to the overall effectiveness of the sonar.

The opening 30 passing through the body has, in a horizontal plane, an area greater than 15% of the connected horizontal surface without internal boundary formed from the contour of the projected surface of the body on a horizontal plane but limited upstream and downstream. by the two vertical planes perpendicular to the transport axis defining the extent of the opening 30 along the transport axis (X) This ensures a good limitation of the vertical lift of the body.

To reduce the lateral lift of the body, the outer surface 24 is substantially symmetrical with respect to the vertical plane P _S v containing the transport axis X. Advantageously, the outer surface 24 is also configured so as to form in a horizontal plane, and in particular the plane P _S H, two profiles symmetrical to each other with respect to the plane P _S v close to a standardized profile NACA of great thickness. In particular, the left lateral profile 70a and the right lateral profile 70b are advantageously close to a NACA type profile of great thickness; this profile guarantees on the one hand a limited and slightly variable lateral lift for low variable incidence angles, and on the other hand a limited drag.

By thick profile in a horizontal plane, is meant a profile whose ratio between the horizontal thickness (largest dimension of the profile in the horizontal direction perpendicular to the transport axis) and the rope (length separating the edge of attack 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 outer surface, that is to say the portion containing the trailing edge can be truncated as is the case on the embodiment of the figures. The body therefore does not include the trailing edge of the profile formed by the outer surface 24. The profile comprising the leading edge and the trailing edge is obtained by extrapolating the outer surface 24 to the trailing edge.

The outer surface 24 is formed by joining the profiles formed in the plane P _S H and in the plane P _S v on either side of each of these planes so that it forms a hydrodynamic surface.

Advantageously, the body 21 generally has a form of water droplet (through which the opening 30). The drop of water can be truncated so as not to understand the trailing edge. The outer surface is advantageously generally convex.

Advantageously, the body is entirely delimited by the surface 24.

To further enhance the stabilization by limiting the lift effect, it is also envisaged to configure the outer surface 24 so that it has, in a vertical plane perpendicular to the transport axis X, a curved profile comprising a small portion vertical. In other words, the side walls of the body have a small proportion of substantially vertical surface, so as to limit the possibility of generating lateral lift. Note also that the body comprises a lateral opening 26, passing through the body along a horizontal axis perpendicular to the transport axis X, and located in a downstream part of the body to further reduce the lateral lift of the body. The lateral opening is intended to be occupied by the fluid.

We have described by means of the figures an example of submersible body 21 having a hydrodynamic shape optimized to reduce the lift of the suspended part of the object. The example shown implements several measures intended to reduce 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 by vertical PSH ratio, NACA profiles in the horizontal plane, profiles with a small vertical portion, lateral opening rear 26). It is understood that it is also envisaged by the invention a submersible body retaining only part of these measures.

In a particularly advantageous simplified configuration, the body comprises a hydrodynamic outer surface substantially symmetrical with respect to the vertical plane P _S v containing the transport axis X, making it possible to limit the lateral lift of the object, and comprises a central through-opening opening the body along a vertical axis, to limit the vertical lift. This unique configuration of the body makes it possible to limit hydrodynamic forces that can be generated perpendicularly to the transport axis by fluid flow around the body, and that can cause a force in rotation around the transport axis by opposing the the effect of gravity.

The body 21 is of the monocoque type. In other words, the body is not multihull. We can not isolate a portion, not complete of the body, forming an elementary body independent hydrodynamically profiled.

The body 21 is entirely delimited by a shell 124. This shell has a single substantially convex continuous front surface: in other words the fluid meets the shell at a single point called a stopping point (or a very limited area), the object being unibody

The shell 124 comprises a hull or outer surface of the hull 124. The hull is the outer surface 24. The hull 24 defines a volume forming a single hydrodynamically profiled cell. This volume has the shape of a single streamlined cell hydrodynamically 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 all around the opening 30.

The present invention proposes to form a through vertical opening 30 in a monocoque body, which is unusual. The fact of proposing a monocoque body makes it possible to tow a body of large volume (and thus to carry a sonar of large volume) with a limited drag. Forming a vertical through opening in the body allows to limit its vertical lift. However, those skilled in the art conventionally attaches to form a monocoque body as homogeneous as possible without through opening or protrusion so as to limit its drag. Any irregularity in the profile generates turbulence and therefore drag. The fact of forming a vertical opening through a monocoque body therefore degrades its trail which is unnatural.

In a particular embodiment, the body 21 comprises a leading edge BA and is delimited, in any vertical plane perpendicular to the transport axis located between the leading edge BA and the opening 30, by a hull forming a convex or substantially convex curve delimiting a single convex or substantially convex volume. In other words, the outer surface 24 defines, in any vertical plane perpendicular to the transport axis located between the leading edge and the opening 30 a single convex or substantially convex volume. By substantially convex is meant that the outer surface or hull may contain elements of the body structure that protrude from a convex curve.

In a particular embodiment, the volume is convex in any vertical plane perpendicular to the transport axis. In other words, the outer surface is, outside the central opening 30, substantially convex. The body is a single body. Alternatively, the volume is convex in any vertical plane perpendicular to the transport axis located between the leading edge BA of the body 21 and a vertical plane perpendicular to the transport axis located at a distance from the trailing edge of the surface 24 equal to two thirds of the distance between the leading edge and the trailing edge of the surface 24.

Another measure to stabilize the towed object is to fix on one downstream part of the body one or more stabilizing fins. In the example shown in the figures, the object comprises two stabilizing fins 43 and 44, substantially vertical, fixed on a downstream part of the body 21. The dimensions of the fins are, on the one hand, sufficiently small to generate a lift that is less than the weight of the object in the water for the speed range considered, and, on the other hand, sufficiently high for their stabilization effect to be effective. Moreover, the use of a weakly asymmetrical profile of the fins 43 and 44 makes it possible to generate a torque retraction device, capable of at least partially neutralizing the asymmetry of lift generated by the C-shape of the attachment arms.

On the same principle, the towed object also comprises two stabilizing fins 41 and 42, substantially horizontal, fixed on a downstream part of the body 21, respectively in the upper and lower part. Here again, the dimensions of the fins must be, on the one hand, sufficiently small to generate a lift that is less than the weight of the object in the water, and on the other hand sufficiently high for their stabilization effect to be effective. A weakly asymmetrical configuration of the fins 41 and 42 at least partially neutralizes the roll torque generated by the drag of the attachment arms of the towed body suspended under the towing cable.

The example shown in the figures makes it possible to illustrate the interest of such an asymmetrical configuration of the fins. To compensate for the disturbing torque induced by the dissymmetry of the attachment arms 22a and 22b, an asymmetry of the horizontal fins is introduced by fixing two deflectors 31a and 31b on the upper horizontal rear wing 41. The asymmetry of the attachment arms relative to the vertical plane of symmetry P _S v, subjected to a non-zero speed in the water, generates a lateral force on the towed object. This force produces a disturbing torque in roll substantially proportional to the square of the speed. In a simplified way, this disturbing couple can express itself by means of the following relation: in which :

K-i is a coefficient defining the hydrodynamic disturbance induced by the dissymmetry of the fixing arms, and

• V is the speed of the towed object.

To neutralize the effect of this disturbing torque C _P on the roll, a hydrodynamic dissymmetry is achieved on the object, here by means of the two deflectors 31a and 31b. As before, the torque generated by this set of baffles is substantially proportional to the square of the speed. It can be expressed in a simplified form such as: C _A = K ₂ ^* V ² (5) in which:

K ₂ is a coefficient defining the hydrodynamic disturbance induced by the set of deflectors, and

• V is the speed of the towed object.

The set of baffles is then designed to generate a torque C _A which opposes the disturbing torque C _P induced by the asymmetry of the fixing arms. In other words, the resolution of equations (4) and (5) makes it possible to determine the characteristics of the set of baffles. By configuring the deflectors so as to satisfy the equality Ki = -K ₂ , the set of deflectors makes it possible to neutralize the disruption induced by asymmetry of the fixing arms.

A final step is to stabilize the towed object in yaw. This stabilization is essentially ensured by the traction force T _c exerted by the hydrodynamic drag produced by all the elements towed downstream of the object. For an active sonar in dependent towing, the movement of the flute 14 connected to the cable 12, downstream of the towed object, generates a hydro dynamic drag which exerts on the object, via the cable, a force towards the 'back. The return torque generated by this rear traction force T _c can be expressed by means of the following simplified relationship:

C _L = T _c ^* L ^* sin (β) in which:

L is the distance between the hooking points of the attachment arms 22a and 22b, and

• β is the yaw angle with respect to the longitudinal axis X of the cable.

Thus, increasing the distance L separating the two attachment arms improves the yaw stability of the towed object. In the example shown in the figures, the fixing arms 22a and 22b are fixed in two opposite ends along the X axis of the body 21, so as to maximize the yaw stabilization.

In other words, the two attachment arms 22a and 22b are configured so as to connect, respectively in a first and a second direction substantially vertical, the cable 12 and respectively a first and a second end of the body along the transport axis X, so that a force tending to separate the two fixing arms makes it possible to stabilize the body 21 around the vertical axis Z. The invention also relates to a towed system, for example a towed system containing an active sonar, can be towed by a ship. The system comprises a traction cable and a submersible object as previously described. The object is towed by the vessel by means of the towing cable. The system can be completed by a tail (of elongated flexible body type) that can contain a flute for receiving acoustic signals.

Claims

1. Object (20) intended to be towed in a fluid by a cable (12) along a substantially horizontal transport axis (X); the object (20) comprising a body (21) and at least one attachment arm (22a, 22b), the body (21) being intended to be suspended by gravity from the cable (12) via said at least one attachment arm (22a, 22b),

the object being characterized in that the body (21) comprises:

an outer hydrodynamic surface (24) symmetrical with respect to a vertical plane (Psv) containing the transport axis (X) so as to limit the lateral lift of the body (21),

a central opening (30) passing through the body (21) along a vertical axis (Z) intended to be occupied by the fluid so as to balance the fluid pressures flowing along the outer surface (24), so as to limit the vertical lift of the body (21);

for limiting hydrodynamic forces which can be generated perpendicularly to the transport axis (X) by fluid flow around the body (21) and capable of causing a force in rotation about the transport axis (X) in s opposing the effect of gravity.

2. Object according to any one of the preceding claims, wherein the body is monocoque.

3. Object according to any one of the preceding claims, wherein the fixing arm (22a, 22b) has an asymmetrical shape relative to the vertical plane (PSV) containing the transport axis (X).

4. Object according to the preceding claim, wherein the fixing arm (22a, 22b) has a general shape "C", connected by a first end to the body (21) and intended to be connected by a second end to the cable (12). ).

5. Object according to any one of claims 3 to 4, comprising a ballast (PL) positioned inside the body and configured so that the center of gravity of the object (20) maintained at zero speed in the fluid is positioned in the vertical plane (PSV) containing the transport axis (X).

6. Object according to the preceding claim, comprising a ballast (PL) positioned inside the body and configured so that the center of gravity of the object (20) maintained immersed in the air at zero speed is positioned in the vertical plane (PSV) containing the transport axis (X).

7. Object according to any one of claims 1 to 5, comprising a ballast (PL) positioned inside the body and configured so that the center of gravity of the object (20) is positioned in the vertical plane ( PSV) containing the transport axis (X) both when the object (20) is kept immersed at zero speed in the water and when the object is kept immersed at zero speed in the air.

8. Object according to one of claims 4 to 7, comprising a set of baffles (31a, 31b) integral with the body (21), configured to generate a hydrodynamic force by fluid flow around the object (20) neutralizing the hydrodynamic force induced by the asymmetrical shape of the fixing arm (22a, 22b).

9. Object according to one of the preceding claims, wherein the outer surface (24) comprises an upper surface (25a) and a lower surface (25b) symmetrical to each other with respect to a horizontal plane (PSH) of so as to limit the vertical lift of the body (21).

10. Object according to one of the preceding claims, wherein the outer surface (24) comprises an upper surface (25a) and a lower surface (25b), the upper surface (25a) or the lower surface (25) is configured so to form in the vertical plane (PSV) containing the transport axis (X) a NACA type profile of great thickness, to limit the vertical lift of the body (21).

1 1. Object according to one of the preceding claims, the outer surface (24) is configured so as to form in the horizontal plane (PSH) a NACA-type profile of great thickness, for limiting the lateral lift of the body (21).

12. Object according to one of the preceding claims, wherein the outer surface (24) is configured to form in a vertical plane perpendicular to the transport axis (X) a curved profile, comprising a small vertical portion so as to limit the lateral lift of the body (21).

An article according to one of claims 1 to 8, wherein the outer surface (24) comprises an upper surface and a lower surface, the upper surface and the lower surface being configured so that the outer surface forms:

in the horizontal plane (PSH) a profile of the NACA type of great thickness, making it possible to limit the lateral lift of the body (21);

in a vertical plane containing the transport axis, a NACA type profile of great thickness making it possible to limit the vertical lift of the body (21),

14. Object according to the preceding claim, wherein the body is entirely delimited by the outer surface.

15. Object according to one of the preceding claims, wherein the body (21) comprises a lateral opening (26), passing through the body (21) along a horizontal axis perpendicular to the transport axis (X,) and located in a downstream part of the body (21) so as to limit the lateral lift of the body (21), the opening being intended to be occupied by the fluid.

6. Object according to one of the preceding claims, comprising a substantially vertical fin (43, 44) fixed on a downstream part of the body (21), having a substantially asymmetrical hydrodynamic shape, configured to stabilize the orientation of the object. (20) by generating a torque around a vertical axis (Z).

17. Object according to one of the preceding claims, comprising a substantially horizontal fin (41, 42) fixed on a downstream portion of the body

(21) having a substantially asymmetric hydrodynamic shape, configured to stabilize the orientation of the object (20) by generating a torque around the transport axis (X).

18. Object according to one of the preceding claims, comprising two fixing arms configured to connect respectively in a first and a second substantially vertical directions, the cable (12) and respectively a first and a second end of the body along the the transport axis (X), so that a force tending to move the two fixing arms allows to stabilize the body (21) about a vertical axis.

19. Object (20) according to one of the preceding claims, for sonar detection in a marine environment, and comprising an acoustic emission antenna fixed on an internal structure of the body (21).

20. Active sonar system, intended to be towed by a vessel

(10), comprising a pull cable (12) connectable to the ship (10) and an object (20) according to one of the preceding claims towed by the cable (12).

21. Active sonar system according to the preceding claim, comprising a tail containing an elongated body ensuring the reception of acoustic signals.