Pointed Bow Fluid Thruster Nozzle

Abstract

The invention relates to a nozzle (302) supporting the action of a fluid propellant (301) by acting directly on the fluid, comprising for a normal direction of propulsion: on the internal surface of its upstream portion, at least one surface portion diverging relative to the downstream (305)-upstream (304) direction of the current generated by the propellant; at least one thick nozzle profile (302), i.e. comprising a distinct external surface and an internal surface; at the end (310) of said profile through which the upstream current (307, 308) enters the nozzle, a projecting and angular apex, without this apex necessarily coinciding with the chord of said profile, so that the nozzle comprises at this location a projecting and angular edge, called "the tip of the bow". Figure for the abstract: [Fig 3].

Description

Title of the invention: FLUID PROPELLER NOZZLE WITH POINTED PROW

[0001] The present invention relates to the field of vehicle propulsion in fluids by propulsion devices comprising a nozzle arranged around a propeller such as for example a propeller or blade propeller. More particularly, the invention relates to a nozzle supporting the action of a fluid propellant by at least one pressure effect acting on the surface of said nozzle, and the end of which through which the upstream flow arrives at said nozzle comprises a projecting and angular edge.

[0002] In the field of propulsion in fluids, a person skilled in the art is known, in particular from patent document No. FR 3111324, to a fluid propellant nozzle, comprising on the internal surface of its upstream portion, for either direction of propulsion: • at least one portion of surface diverging from the downstream-upstream direction of the current generated by the thruster; • at least one inflection of the surface, that is to say a sudden change in the orientation of the surface (without there necessarily being a change of sign in the concavity), so that the nozzle has at least one edge on the internal surface of its upstream portion, and so that the portion just upstream of said inflection has a divergence from the downstream-upstream direction that is greater than the portion just downstream of said inflection.

[0003] Thus, when such a thruster operates in reverse (for example in reverse), said edge materializing the inflection of the surface allows a “dynamic stall” of the exiting fluid, thus removing the dynamic depression from the internal surface located downstream of the inflection and thereby inhibiting part of the parasitic thrust penalizing the reverse operation of the fluid thruster nozzles. Such characteristics can for example improve the capacity of a naval thruster to produce thrust in reverse.

[0004] However, an edge of the upstream internal surface of the nozzle positioned in the vicinity of the primary propellant (for example a propeller) significantly degrades the propulsion of normal operation (for example forward operation). Indeed, if said edge allows a dynamic stall in reverse operation, it necessarily induces a disturbance of the fluid entering the propellant in normal operation. The more intense this disturbance is when entering the primary propellant, the more the operation of the latter is degraded. For example, in the case where said primary propellant is a boat propeller, the areas of largest diameters of the propeller are at the same time the most important for propulsion due to their high speed, therefore the most sensitive to the stability of the incoming flow, and consequently the most exposed to a disturbance arising from such an edge located just upstream of the propeller, on the upstream internal surface of the nozzle.

[0005] It is specified that throughout this text, the following terms are understood to mean: • “upstream” the side of the propulsion device from which the propelled fluid comes, little matters the direction of operation of the thruster (e.g. forward or reverse); • “downstream” the side of the propulsion device opposite the upstream side; • “bow” of the nozzle: the upstream portion of the nozzle when the propellant operates in normal operation, that is to say the end starting from the leading edge (according to normal operation) to an intermediate plane located between the two ends of the nozzle; • “stern” of the nozzle: the downstream portion of the nozzle when the propellant operates in normal operation, that is to say the end starting from the trailing edge (according to normal operation) to an intermediate plane located between the two ends of the nozzle; • “nozzle axis”: the axis approximately collinear with the force resultant produced by the propulsion device including in particular the propellant and possibly its nozzle, oriented from downstream to upstream unless the orientation is specified otherwise (on a nozzle with axis symmetry, the axis of the nozzle is collinear with the axis of symmetry); • “directional” nozzle: a fluid propellant nozzle having a preferred direction of operation, characterized by a fluid inlet (in the preferred direction, i.e. on the bow side) with a larger cross-sectional area than that of the fluid outlet (on the stern side), said cross-section being produced by a plane whose normal is parallel to the axis of the nozzle and passing through the respective corresponding end (on the bow or stern side); • “bi-directional” nozzle: a fluid propellant nozzle having no preferred direction of operation, characterized by a fluid inlet with a sectional surface equivalent to that of the fluid outlet, said section being produced by a plane whose normal is parallel to the axis of the nozzle and passing through the respective corresponding end; • fluid “propellant”: the organ moving the fluid, a “propellant” primary” does not include its possibly associated nozzle; • “normal walking” or “normal direction” of propulsion: the direction of preferred operation of a fluid thruster (e.g., on a ship, forward motion); on a bi-directional nozzle, normal motion refers to either direction of operation; • “reverse gear” or “reverse direction” of propulsion: the direction of reverse operation to normal operation; • “profile” of a nozzle: the envelope defined by the section of the nozzle by a plane coinciding with two neighboring local normals included in the same plane, as well as with an axis parallel to the axis of the nozzle, • said cutting plane is called “profile plane” (on a nozzle with axis symmetry, the profile planes pass through the axis of symmetry); • “chord” of a profile: the longest rectilinear segment inscribed in a given nozzle profile; • “leading edge” of the nozzle: the possibly curvilinear segment further upstream of the propeller (depending on the direction of propulsion), and defined on a profile by the point of intersection between the chord and said profile.

[0006] It should be noted that the prior art in the field of thruster nozzles systematically presents on an upstream internal portion, at least one portion of surface diverging relative to the downstream-upstream direction of the current generated by the thruster. This (or these) surface(s) is (are) responsible for the thrust effect of the fluid thruster nozzles. The effect discussed in this paragraph is that of a local hydrodynamic depression applied to the upstream internal surface of the nozzle, caused by increased fluid speeds in this zone, the fluid being sucked in by the primary thruster (for example a propeller). The more the normal of said surface is oriented parallel to the axis of the nozzle, the greater the resulting force projected onto the axis of the nozzle and therefore supports the force of the primary thruster.The divergence of the upstream internal surface in the downstream-upstream direction is therefore of prime importance for the nozzle to withstand the force of a propellant. The effect described in this paragraph is called in this text the "conventional nozzle effect".

[0007] It should also be noted that, considering a propellant and its nozzle of the prior art in normal operation, the leading edge of such a nozzle links the upstream internal surface and the upstream external surface continuously, that is to say that its curvature evolves without sudden change in the orientation of said surface. The leading edge thus has a particularly effective surface for the conventional nozzle effect at low initial speed, since the normal of the surface at the leading edge is collinear with the propulsion direction.

[0008] A limitation of prior art nozzles operating in normal operation is that the conventional nozzle effect does not result in thrust supporting the propellant primary only if and only if the primary propellant of said nozzle operates at a minimum suction flow rate, said minimum flow rate increasing with the initial speed of the propelled fluid.

[0009] Note that in this text the term "initial speed" of a fluid is understood to mean: the speed of the fluid relative to the propeller that said propeller will move before the latter acts on said fluid in any way. The initial speed is positive when the fluid moves in the same direction as in normal operation. For example, on a boat this initial speed is sometimes approximated in absolute value by the forward speed of said boat.

[0010] Indeed, if the propellant does not reach this minimum suction flow rate, the initial velocity of the fluid is predominant in the upstream currents in the vicinity of the leading edge, which results in a plug phenomenon in the upstream zone of the nozzle. The fluid currents thus slowed then apply an overpressure on the upstream surface of the nozzle, particularly in the vicinity of its leading edge, consequently creating a parasitic thrust, the opposite of that of the primary propellant. The phenomenon described in this paragraph is called in this text the “saturation” of the nozzle. The saturation of a given nozzle of the prior art appears progressively, either as the initial speed increases for a given flow rate of the primary propellant, or as the primary propellant reduces its flow rate for a constant initial speed. A variation in initial speed at a given flow rate may, for example, be the consequence of a change in the mass of a vessel, or the launching of a fishing net significantly increasing the overall drag of the vessel. Before saturation, the nozzle is said to be propulsive, it supports the primary propellant. At the saturation point, the nozzle is neither propulsive nor braking, it is neutral. Once saturation is exceeded, the nozzle becomes braking, it is said to be "saturated".

[0011] Another limitation of the nozzles of the prior art is that, when they operate in reverse gear, the continuous aspect of the internal surface of the bow and in particular of the trailing edge (in reverse gear), without sudden change in the orientation of said surface, allows a progressive deviation of the expelled flow so that it is oriented at least partially along the tangent to the internal surface. Depending on the speeds and the shapes of the nozzles, this flow may finally be expelled radially to the axis of the nozzle, thus losing all reaction effect. Reverse gear may thus become inoperative. The phenomenon described in this paragraph is called in this text the "deviation of the expelled flow".

[0012] An object of the invention is to improve the thrust capacity and reversibility of fluid propellant nozzles, i.e. to improve their performance in both directions of propulsion.

[0013] To this end, the invention relates to a fluid propellant nozzle comprising, for a normal direction of propulsion (for example, forward movement on a ship): • on the internal surface of its upstream portion, at least one surface portion diverging from the downstream-upstream direction of the current generated by the thruster; • at least one thick nozzle profile, i.e. comprising a distinct outer surface and an inner surface; • at the end of said profile through which the upstream current enters the nozzle, a projecting and angular peak, without this peak necessarily coinciding with the chord of said profile, so that the nozzle comprises at this location a projecting and angular edge, called in this text “the tip of the prow”.

[0014] In all that follows, the tip of the bow delimits two surfaces of the nozzle called the external surface and the internal surface of the nozzle according to the invention. In the absence of a tip of the bow, the internal surface and the external surface are delimited by the chord of the profile corresponding to the location studied.

[0015] Thus, when the thruster operates in reverse operation, the current expelled by the primary thruster detaches from the surface of the nozzle at the latest at the tip of the bow, thereby minimizing the deviation of the expelled flow and therefore improving the performance of the nozzle in reverse operation.

[0016] In addition, a nozzle made according to the invention reduces the minimum suction flow rate necessary to be applied by the primary propellant to avoid the phenomenon of saturation of the nozzle in normal operation, at a given positive initial speed. Indeed, the upstream fluid flow will be better directed by the tip of the bow, between a path internal to the nozzle or a path external to the nozzle, thus resulting in a repulsion of the saturation phenomenon, which extends the range of use of the nozzle according to the invention.

[0017] Note that in the following, if the text does not explicitly refer to the direction of propulsion, it must be interpreted for a normal direction of propulsion, in particular with respect to the terms “upstream”, “downstream” or even “leading edge”.

[0018] According to one embodiment of the invention, the tip of the bow for at least one profile of the nozzle according to the invention is characterized by a radius of curvature less than 1% of the chord of said profile considered.

[0019] Thus, the effect of separating the current from the surface in reverse operation and the effect of directing the upstream current in normal operation will be effective, since the tip of the bow is sharper.

[0020] Note that, in order to maximize the increase in performance of a nozzle according to the latest embodiment of the invention, it is advantageous that the tip of the bow, for at least one profile of the nozzle according to the invention, is characterized by a radius of curvature as close to zero (0) as possible. Thus, the tip of the bow describes a sharp edge, further facilitating the separation of the expelled current in reverse operation, and the switching effect of the upstream current in normal operation. The performance of the nozzle in both directions of propulsion is consequently improved.

[0021] According to one embodiment of the invention, considering a normal direction of propulsion, the angle formed between: • the axis of the nozzle oriented in the upstream-downstream direction on the one hand and • the tangent of at least one nozzle profile at the internal surface upstream in the vicinity of the tip of the bow oriented in the direction of travel of the internal upstream-downstream profile on the other hand, hereinafter referred to in this text as the “entry angle”, is a maximum of 70°, so that the divergence of the upstream interior surface in the downstream-upstream direction is limited by this angle.

[0022] Thus, when the fluid expelled in reverse operation meets the tip of the bow, it will take an orientation close to the tangent to the downstream internal surface (according to the reverse direction of propulsion), of which at least one component is projectable onto the axis of the nozzle and supports the reverse operation of the primary propellant.

[0023] Indeed, in reverse operation, because of an effect similar to that known to those skilled in the art as the "Kelvin wake", the outgoing current encountering the tip of the bow will change orientation to incline towards the singularity represented by said tip of the bow, in the manner of a diffracted wave. The outgoing current therefore loses part of its reaction effect. In such an embodiment according to the invention, an entry angle of 70° is a maximum angle for the outgoing flow in reverse operation to produce a reverse operation effect.

[0024] On the other hand, by the introduction of this geometric characteristic, the nozzle according to said embodiment of the invention partially loses its static thrust performance in normal operation, that is to say its thrust when the initial speed is zero. In fact, the prow of the nozzle no longer offers a surface whose normal is parallel to the propulsion axis.

[0025] However, the nozzle according to said embodiment will be more efficient at higher initial speeds. Indeed, thanks to this inlet angle of at most 70°, the tip of the bow is thus better oriented to receive an upstream flow composed of the initial speed collinear with the propulsion axis, and the speed acquired by the primary propellant with a component essentially radial to the propulsion axis. The upstream flow and the tangency of the upstream internal surface of the nozzle according to said embodiment are consequently better aligned, thus deploying better suction performance by depression.

[0026] Note that, to optimize the increase in performance in reverse running and / or the increase in performance in normal running at a given initial speed, the entry angle can be adjusted and notably smaller, up to 5°. The optimal angle can be chosen by empirical iterative, semi-iterative approaches or even by finite element simulations.

[0027] According to one embodiment of the invention, for the normal direction of propulsion the angle formed between: • the tangent of at least one nozzle profile according to the invention at the level of the upstream external surface and in the vicinity of the tip of the bow, oriented in the direction of travel of the downstream-upstream external profile, on the one hand and • the tangent to said profile at the level of the upstream internal surface and in the vicinity of the tip of the bow, oriented in the direction of travel of the downstream-upstream internal profile on the other hand, hereinafter referred to in this text as the “point angle”, is less than 70°, so that the profile of the point of the bow forms an acute angle.

[0028] Thus, in normal operation the upstream fluid flow will be better directed by the tip of the bow, between a path internal to the nozzle or a path external to the nozzle and will not have the opportunity to slow down in the vicinity of a surface whose normal would be in opposition to the direction of the incoming fluid. Performance is therefore improved.

[0029] It should be noted that, to optimize the increase in performance in normal operation, the tip angle is as small as possible to minimize any resistance to the flow of the fluid. It is particularly advantageous for the tip angle of the nozzle according to the invention to be strictly less than 39°. Indeed, with such a characteristic the tip of the bow has an angle less than the wake angle known to those skilled in the art as "the Kelvin wake", and will therefore have the ability to not push the upstream fluid in a direction opposite to the direction of propulsion. The performance of the nozzle in normal operation according to the invention is thus improved.

[0030] According to one embodiment of the invention, for the normal direction of propulsion, the angle formed between: • the axis of the nozzle oriented in the upstream-downstream direction on the one hand and • the tangent of at least one nozzle profile according to the invention at the level of the upstream external surface and in the vicinity of the tip of the bow, oriented in the direction of travel of the upstream-downstream external profile, on the other hand, hereinafter called in this text “external angle” of the tip of the bow, is positive or zero so that the external surface of the portion downstream of the tip of the bow is cylindrical or convergent towards the axis of said nozzle in the upstream-downstream direction.

[0031] Such a feature minimizes the projected surface area of the nozzle on a plane orthogonal to the axis of the nozzle, thus minimizing the saturation effect of the nozzle according to the invention. This results in better performance during normal operation.

[0032] It should be noted that, in order to optimize the increase in performance during normal operation of the nozzle according to the embodiment of the invention, the external angle of the tip of the bow is positive, so that the external surface of the portion downstream of the tip of the bow is convergent towards the axis of said nozzle in the upstream-downstream direction. This convergence allows the flow of fluid directed towards the outside of the nozzle according to the invention to slow down in a zone where the increase in pressure is applied to a better oriented portion of the external surface of the nozzle, thus creating an additional positive thrust thereby increasing the performance of the nozzle according to the invention during normal operation.

[0033] According to a specific embodiment of the previous embodiment of the invention, the external angle of the tip of the bow is greater than the angle formed between: • the axis of the nozzle oriented in the upstream-downstream direction on the one hand and • the chord of the profile of the tip of the bow oriented in the upstream-downstream direction on the other hand, such that at least a portion of the external surface of the nozzle according to the embodiment of the invention is concave.

[0034] Thus, the outer surface of the nozzle according to the invention comprises at least one concave zone of better oriented surface, with a normal closer to the axis of the nozzle. The overpressure of the fluid directed to the outside of the nozzle thus generates, by slowing down, a stronger thrust, therefore increasing the performance of the nozzle according to said specific embodiment.

[0035] According to a specific embodiment of the previous embodiment of the invention, dedicated to static thrust or propulsion in a fluid of low initial speed, the external angle of the tip of the bow is equal to or greater than 90°, so that the tip of the bow takes the form of a horn protruding radially from the axis of the nozzle.

[0036] Thus, the upstream currents coming from a direction radial to the axis of the nozzle or coming from the external downstream zone at the tip of the bow encounter a tip of the bow optimally oriented for the routing of the currents. In addition, this characteristic provides a slowing surface for the external currents oriented in the direction of propulsion. The overpressure applied to this surface thus creates additional thrust. This results in an increase in the static thrust of such a nozzle according to said specific embodiment.

[0037] According to one embodiment of the invention, the stern is also produced according to one or more characteristics according to the invention previously cited but for the sense reverse propulsion. For the analysis of said characteristics and the effects produced by the present embodiment of the invention, when the terms of the present text are specific to the meaning (in particular, but not limited to: “preferred meaning” or even the “bow”), they are interpreted inversely. Thus, for example, “the bow” is interpreted as “the stern”. Similarly, the meaning of the angles described is reversed.

[0038] Thus the effects produced are similar in reverse operation, thereby improving the performance of the nozzle according to the embodiment of the invention in both directions of propulsion.

[0039] It should also be noted that in the present text, the nozzle according to the invention may have an evolving profile, that is to say that the profile is not identical for all possible profile planes. Thus, a nozzle with an evolving profile according to the invention is a nozzle for which at least one of the profiles of said nozzle comprises at least one of the characteristics of the present invention.

[0040] Other characteristics and advantages of the invention will appear on reading the detailed description of the non-limiting examples which follow, for the understanding of which reference will be made to the appended drawings, among which: • [Fig-1] is a cross-sectional view of an example of a marine nozzle of the prior art to the invention, illustrating the saturation state during normal operation; • [Fig.2] is a cross-sectional view of the nozzle of the prior art of [Fig.l], illustrating the phenomenon of deviation of the expelled flow in reverse operation; • [Fig.3] is a cross-sectional view of a marine nozzle according to one embodiment of the invention detailing the operation of the latter in normal operation; • [Fig.4] is a cross-sectional view of the nozzle according to an embodiment of the invention presented in [Fig.3], detailing the operation of the latter in reverse operation; • [Fig.5] is a cross-sectional view of an example of a marine nozzle according to an embodiment of the invention which would be dedicated to static thrust or low speeds, comprising a specific horn-shaped prow tip.

[0041] [Fig.l] illustrates an example of a nozzle (102) of a prior art naval thruster with axis symmetry (103), the primary thruster of which is a propeller (101) operating in normal operation, that is to say that it moves the fluid in the preferred direction of operation, from upstream (104) to downstream (105), producing a thrust force forward, that is to say upstream (104).

[0042] The primary propellant (101) not operating at a sufficient suction regime for the initial speed represented by the vectors (106), the initial speed of the fluid (water in this illustrated example) is predominant in the water stream just upstream (104) of the nozzle. This results in a plug effect: part of the upstream stream will be pushed outside the nozzle as illustrated by the lines representing the external stream (107), instead of being sucked inside as do the lines representing the internal stream (108).

[0043] As a result, the water currents are slowed down in the upstream zone of the nozzle (102), generating an overpressure in the zone (109) on the surface adjacent to the leading edge (110). The resultant of this overpressure on the entire upstream surface of the nozzle illustrated is a force opposite to the direction of propulsion of the propeller (101), the nozzle is therefore saturated and braking.

[0044] This saturation may for example be the result of poor selection of the propulsion unit for the corresponding ship. To reach its target speed, equal in value to the initial speed (106), said thruster (101) requires only a relatively low flow rate, not allowing the suction of all the upstream current flows (107, 108). The saturated nozzle (102) is therefore a brake, it degrades the efficiency of the propulsion unit.

[0045] [Fig.2] illustrates the primary propulsion unit (101) and its nozzle of the prior art (102) of [Fig.l] operating in reverse, i.e. in reverse. The propeller (101) moves the water from upstream (204), i.e. from the rear of the boat, to downstream (205), i.e. towards the front of the boat.

[0046] The lines (207) representing the flow of fluid expelled by the primary propellant (101) are bonded to the inner surface of the nozzle (210). The rounded shape of the nozzle prow (211) allows a progressive deflection of the expelled flow to a direction radial to the axis (103) of the nozzle, thus causing the fluid to lose any reaction effect. The nozzle (102) of the prior art illustrated therefore has poor performance in reverse.

[0047] [Fig.3] illustrates an example of a marine nozzle according to the invention (302) operating in normal operation. Said nozzle (302) is axially symmetrical (303) and surrounds a primary propellant (301), namely a propeller, moving the water from upstream (304), i.e. from the front of the ship on which the propellant is mounted, to downstream (305). In the example illustrated, the initial speed, represented by the vectors (306), corresponds to the forward speed of the ship. The propellant (301) and its nozzle (302) operate in an unsaturated configuration, i.e. the suction flow rate of the primary propellant is significantly greater than a flow rate simply due to the initial speed through a projected surface of the nozzle and its propellant on a plane orthogonal to the axis of the nozzle.

[0048] The nozzle (302) comprises on its upstream end a tip of the bow (310) whose tip angle (311) as defined in this text is 38°, allowing good switching of the upstream current represented by the lines (307, 308) arriving at the nozzle (302). The nozzle according to this exemplary embodiment of the invention does not offer a surface oriented upstream of the current lines (307, 308) on which the upstream current would otherwise be caused to slow down, generating a brake. In this illustrated example the tip angle (311) is therefore less than 39° and moreover the radius of curvature of said tip is zero, which maximizes the switching effect and consequently the performance of the example nozzle.

[0049] The nozzle illustrated in this example according to the invention further comprises an inlet angle (312) as defined in this text equal to 50°, so that the tip of the bow accommodates a current entering the nozzle having approximately this orientation. Said orientation of the current in the vicinity of the tip of the bow can be determined for example by finite element fluid studies. Thus, the example nozzle according to the invention deploys a thrust supporting the propeller (301) which is optimal at this regime, i.e. at this initial speed (306), and at this suction flow rate.

[0050] In this illustrated example the external angle (313) of the tip of the bow (310) as defined in this text is constrained by the entry angle (312) and the tip angle (311) of the bow, it is equal to 12°. This external angle makes it possible to minimize the total projected surface of the nozzle of the example on a plane orthogonal to the axis of the nozzle, thus minimizing the possible resistance to advancement.

[0051] The external angle (313) of the tip of the bow is greater than the angle formed between the axis of the nozzle (303) oriented in the upstream (304)-downstream (305) direction on the one hand and the chord of the profile studied oriented in the upstream-downstream direction. Thanks to this characteristic, the external surface of the nozzle according to the embodiment of the invention is concave in the vicinity of the tip of the bow, which orients said surface so that the external currents (307) slowing down in the vicinity of this surface create a thrust complementary to that of the propeller (301), thereby improving the performance of the example nozzle.

[0052] [Fig.4] illustrates the primary propulsion unit (301) and its marine nozzle according to the invention (302) of [Fig.3] operating in reverse, that is to say in reverse.

[0053] The water in the example illustrated in [Fig.4] is sucked in by the primary thruster (301) from upstream (404) to downstream (405), i.e. from the rear to the front of the ship. The expelled water current, represented by the streamlines (408), is stuck to the downstream internal surface, but detaches from it thanks to the tip of the bow. In the present example of a nozzle according to the invention, the “entry angle” (312) as defined in this text is 50°, which allows the expelled flow to have a component projectable on the axis of the nozzle (303), and oriented downstream (405). Consequently, the reaction effect of the expelled water flow allows a propulsion force oriented in the opposite direction to the flow, that is to say from downstream (405) to upstream (404). Therefore the example of nozzle according to the invention penalizes the reverse operation of the main propellant less than a nozzle of the prior art like that presented in [Fig.2].

[0054] [Fig.5] is a cross-sectional view of an example of a marine nozzle (502) according to a embodiment of the invention which would be dedicated to static thrust (i.e. zero initial speed) or low initial speeds. The nozzle (502) is symmetrical about the axis (503) and the primary propellant is a propeller (501). It is a directional nozzle, since it comprises a preferred direction of operation, characterized by a fluid inlet (504) (upstream in the preferred direction of operation) with a larger sectional area than that of the fluid outlet (505) (downstream in the preferred direction of operation), said section being produced by a plane whose normal is parallel to the axis of the nozzle (503), and passing through the respective corresponding end.

[0055] The example of this [Fig.5] has at its stern (505) characteristics according to the invention for reverse gear, in particular a stern tip, the entry angle (520) of which for reverse gear is 60°, and a zero external angle. Thus, the performance of said example nozzle in reverse gear is improved.

[0056] The nozzle according to the invention of the illustrated example also comprises a tip of the bow (521) taking the appearance of a horn. The external angle (513) of the tip of the bow is 95°, which gives the tip of the bow (521) a good orientation for directing the upstream current when the thruster is used in forward gear for static thrust. The external zone in the vicinity of the tip has a concave surface (522) on which the external flow, represented by the streamlines (523), can slow down, thus generating on this surface an overpressure and an additional thrust supporting the action of the propeller (501) in forward gear.

[0057] Thus the example of nozzle according to the invention of [Fig.5] optimizes its performance in both directions of operation, in particular for static thrust in forward motion.

[0058] Note that the examples illustrated by the figures are not limiting, and that the propellant nozzles presented can also be applied with other fluids, in particular air.

Claims

Claims

1. A fluid propellant nozzle (302, 502), characterized in that it comprises, for a normal direction of propulsion: • on the internal surface of its upstream portion (410), at least one surface portion diverging relative to the downstream (305)-upstream (304) direction of the current generated by the propellant; • at least one thick nozzle profile (302, 502), i.e. comprising a distinct external surface (522) and internal surface (410); • at the end (310, 521) of said profile through which the upstream current (307, 308) enters the nozzle, a projecting and angular apex, without this apex necessarily coinciding with the chord of said profile, so that the nozzle comprises at this location a projecting and angular edge, called "the tip of the bow".

2. A fluid propellant nozzle (302, 502) according to the preceding claim, characterized in that the tip of the prow (310, 521) for at least one profile of the nozzle according to the invention is itself characterized by a radius of curvature less than 1% of the chord of said profile considered.

3. A fluid propellant nozzle (302, 502) according to one of the preceding claims, characterized in that, for a normal direction of propulsion, the angle formed between: • the tangent of at least one nozzle profile (302, 502) according to the invention at the upstream external surface and in the vicinity of the tip of the bow (310, 521), oriented in the direction of travel of the downstream-upstream external profile, on the one hand and • the tangent to said profile at the upstream internal surface and in the vicinity of the tip of the bow, oriented in the direction of travel of the downstream-upstream internal profile on the other hand, called "tip angle" (311) is less than 70°, so that the profile of the tip of the bow forms an acute angle.

4. A fluid propellant nozzle (302, 502) according to one of the preceding claims, characterized in that, for a normal direction of propulsion, the angle formed between: • the axis of the nozzle (303, 503) oriented in the upstream (304)-downstream (305) direction on the one hand and • the tangent of at least one nozzle profile at the level of the upstream internal surface in the vicinity of the tip of the bow (310, 521) oriented in the direction of travel of the upstream-downstream internal profile on the other hand, called "entry angle" (312) is at most 70°, so that the divergence of the upstream internal surface in the downstream (305)-upstream (304) direction is limited by this angle.

5. A fluid propellant nozzle (302, 502) according to one of the preceding claims, characterized in that, for a normal direction of propulsion, the angle formed between: • the axis of the nozzle (303, 503) oriented in the upstream (304)-downstream (305) direction on the one hand and • the tangent of at least one nozzle profile (302, 502) according to the invention at the upstream external surface and in the vicinity of the tip of the bow (310, 521), oriented in the direction of travel of the upstream (304)-downstream (305) external profile, on the other hand, called the “external angle” (313) of the tip of the bow, is positive or zero so that the external surface of the portion downstream of the tip of the bow is cylindrical or convergent towards the axis of said nozzle in the direction upstream(304) downstream(305).

6. A fluid propellant nozzle (302, 502) according to the preceding claim, characterized in that the external angle (313) of the tip of the bow is greater than the angle formed between: • the axis of the nozzle (303, 503) oriented in the upstream (304)-downstream (305) direction on the one hand and • the chord of the profile of the tip of the bow oriented in the upstream-downstream direction on the other hand, so that at least a portion of the external surface (522) of the nozzle (302, 502) is concave.

7. A fluid propellant nozzle (502) according to the preceding claim excluding claim 4, characterized in that the external angle (513) of the tip of the bow (521) is equal to or greater than 90°, so that the tip of the bow takes the form of a horn protruding radially from the axis of the nozzle.

8. A fluid propellant nozzle (502) according to one of the preceding claims, characterized in that it also comprises, for a reverse direction of propulsion, a "stern tip", itself characterized as the bow tip according to the invention, but for a reverse direction of propulsion.

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