Towed acoustic source

A forward and aft open-ended, towed underwater acoustic source having a hollow with a plurality of wires disposed in parallel across one or both of the end openings. In operation, water flows around the wires and proceeds through the hollow body, exiting the aft open end of the body. The tensioned wires are situated normal to the direction of flow in order to cause production of Strouhal vibration frequencies due to the vortex shedding action of the water flow behind the wires. The source transmits broadband acoustic energy without requiring heavy transducers.

CROSS REFERENCE TO OTHER PATENT APPLICATIONS

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a towed, acoustic noise generating source.

(2) Description of the Prior Art

It is well known in the towed acoustic noise source art that there are a number of off-the-shelf, towable, underwater acoustic noisemakers that provide an easy and inexpensive way to generate in-water noise signals having reasonable bandwidth and source level characteristics. For example, one such well-known device is an underwater siren that includes a multi-bladed rotor with a plurality of holes in each rotor blade. These holes generate sound as the blades are turned by the water flow. Such devices however typically have the disadvantage that the generated noise waveform characteristics are not very controllable. These devices also have the drawback that the specific waveforms that they generate cannot be easily changed. What is needed is a way to overcome the present inherent lack of control over the generated sound waveforms that are produced by the prior art devices while at the same time keeping the noise-generating source simple and lightweight.

SUMMARY OF THE INVENTION

Accordingly, it is a general purpose and object of the present invention to transmit broadband acoustic energy from a towed acoustic source.

It is a further object that the acoustic source generates carefully preselected Strouhal vibration frequencies in order to create the desired broadband, high energy, acoustic signals.

Another object is to have the acoustic noise source be tension prior to or during deployment in order to produce a plurality of different desired Strouhal frequencies from a single noise source.

These and other objects are accomplished with the present invention by providing a lightweight, forward and aft open-ended, towed underwater acoustic noise generating source apparatus having a tapered exterior body shape with a plurality of vertical, controllably tensioned metal or synthetic fiber (e.g., carbon fiber, fiberglass or para-aramid synthetic fiber such as Kevlar® or the like) wires disposed in parallel across the forward end opening. In operation, water flows around the wires and proceeds through the tapered body, exiting the aft open end of the body. It is important that the forward and aft end openings of the body have cross sectional areas that are sufficiently close in size to avoid Venturi effects. The tensioned wires are situated normal to the direction of flow in order to cause Strouhal vibration frequencies to be produced due to the vortex shedding action of the water flow behind the wires. The source transmits broadband acoustic energy generated by the vortex shedding, thus not requiring that heavy, expensive noise generating transducers need to be included as part of the tow body in order to produce the desired noise signature.

Other objects and advantages of the present invention will be apparent from the ensuing description.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIGS. 1 and 2, there is shown one embodiment of the towed acoustic source10, built according to the teachings of the present invention. Source10includes a tapered, open-ended tow body12having a plurality of tensioned metal (e.g., steel) or synthetic fiber (e.g., carbon fiber, fiberglass or para-aramid synthetic fiber such as Kevlar® or the like) wires14disposed vertically or horizontally across the forward end of body12and oriented normal to the direction of water flow when towed. While this shows wires14disposed across the forward end of body12, it is understood that these wires could be disposed across either or both ends of body14. Tow body12further includes a forward opening16and an aft opening18, each opening selected to have a circular or oval shape, but both openings also having cross sectional areas that are sufficiently close in size to avoid venturi effects within body12while still permitting an increasing water flowrate through the successively tapering body12interior. For this purpose, forward opening16can be slightly larger than aft opening18or openings16and18can be substantially equal in size. Body12's exterior surface taper produces better hydrodynamic characteristics.

A tow cable20is fixedly attached to the exterior surface of the top of the front end of body12at a mount21. This provides the necessary tow force for the source10while also providing electrical power to source10. A plurality of stabilizing fins22, four are shown in the preferred embodiment, are attached to the aft end of tapered body12to permit a controlled tow to occur. WhileFIG. 1shows the fins22in vertical and horizontal orientations, it is understood that other fin arrangements such as diagonal or Y-shaped are possible. Dashed alignment lines24ofFIG. 1demonstrate that openings16and18each have the same cross sectional area. It is noted that the length of tow body12has no measurable effect on the signal characteristics.FIG. 2further shows a typical top end termination for one of wires14disposed inside tow body12, where the wire14is attached to a solenoid26that is powered by a lead28from cable20. While not shown inFIG. 2, it is understood that one each of a plurality of leads28and solenoids26are attached to the top end of a corresponding top of plurality of wires14. Other embodiments may feature multiple wires14joined to a single solenoid26, so that action of one solenoid26can reduce tension in the associated wires14.

The tensioned wires14have preselected diameters, tensions and chosen material properties that together produce the intended velocity induced vibrations at the desired Strouhal frequencies. The Strouhal vibration frequency fsgenerated by each wire due to flow induced vortex shedding is generally defined as:

fs=0.2⁢U⁢⁢sin⁢⁢θd(1)
where d is the wire diameter, fsis the Strouhal frequency, U is the tow speed, and Θ is the angle of the wires relative to the direction of water flow. For example, a synthetic fiber wire having a 1 mm diameter and being towed at 10 knots (−5 m/s) normal to the flow (i.e., Θ=90° generates sound at a frequency of 1000 Hz. Note that the tow body has a variable tapering diameter, leading to a variable flow speed along its length. This diameter narrowing can also occur internally and has a direct affect on the Strouhal frequency of the signal produced, but the internal taper should not be sufficiently large to induce Venturi effects.

In order to maximize the total source level produced, each wire is tuned to its Strouhal frequency by adjusting its tension. This tuning changes the transverse wave speed c according to the relationship:

c=Tm,(2)
where T is the tension force and m is the mass per unit length including the added mass of the displaced water. Kevlar® and many other synthetic fibers have a specific gravity approaching that of water, so the mass per unit length for a 1 mm diameter fiber is 7.85×10−4kg/m. The added mass for a cylinder accelerating in a direction transverse to its axis equals the mass of fluid displaced by the cylinder. Thus, the total mass is the sum of the two, or 1.57×10−3kg/m. A 1-meter long wire is exactly one wavelength at a tension of 1,570 Newtons or 353 lb of force. Metal wires have more mass per unit length for the same diameter, and thus would achieve a full wavelength at a 1-meter length at lower frequencies. In the preferred embodiment, the tension is preset before deployment so that the resonant frequency of each wire matches the vortex shedding frequency.

A tensioning device is also desired that can quickly remove the tension on each wire. The preferred way to do this involves attaching one end of each wire14to a corresponding solenoid-based actuator26, as shown inFIG. 3A, so that the wire can be quickly loosened and re-tightened. This allows the generated waveform to be started, stopped, or changed as needed by tightening or loosening the appropriate wires while under deployment.FIG. 3Bshows an alternate embodiment of the tensioning device. In this embodiment, solenoid-based actuator26′ is joined to a draw-bar30joined to multiple wires14. This embodiment uses fewer solenoids than that shown inFIG. 3A. Other tensioning devices can use electrical motors and winding drums to tension wires14.

A broadband waveform can be created with a plurality of wires of identical material at different tensions, so that each resonates at a different frequency. A second method would involve wires of different materials, or of different diameters. In this case, each wire would achieve a full wavelength at 1-meter at different frequencies even at the same tension. This would occur because the mass per unit length of each wire would be different.

The number of wires at the forward opening of the tow body should be kept as high as possible in order to maximize source sound level. However, when adjacent wires are spaced too close to each other, an individual wire's vortex shedding pattern begins to interfere with that of the neighboring wires and is altered due to vortex interactions. But, this only occurs when the wire separation distance “y” inFIG. 2is on the order of a few wire diameters d. Since the wire diameter is approximately one millimeter, a separation of several millimeters allows a high wire density, thereby maximizing the source sound level and bandwidth.

The primary advantages of the present invention are that because there is no need for sound producing transducers in the tow body, the body will be very light for easier deployment and recovery. The cost of using such transducers is also avoided. In addition, the ability to change or eliminate the wire tension gives greater control over the sound generated by apparatus10.

What has thus been described is a lightweight, forward and aft open-ended, towed underwater acoustic noise generating source having a tapered exterior body shape with a plurality of vertical, controllably tensioned metal or synthetic fiber wires disposed in parallel across the forward end opening. In operation, water flows around the wires and proceeds through the tapered body, exiting the body's aft open end. It is important that the forward and aft end openings of the body have cross sectional areas that are sufficiently close in size to avoid adverse hydrodynamic affects. The tensioned wires are situated normal to the direction of flow in order to cause Strouhal vibration frequencies to be produced due to the vortex shedding action of the water flow behind the wires. The source transmits broadband acoustic energy generated by the vortex shedding, thus not requiring that heavy, expensive noise generating transducers have to be included as part of the tow body in order to produce the desired noise signature.

Many modifications and variations of the present invention may become apparent in light of the above teachings. For example: the wire material may be metal or may be a synthetic fiber having sufficient tensile strength for the Strouhal frequencies desired; the number, diameter and spacing of the wires can be varied according to the teaching of the invention to produce the desired signal strength; the tension of individual wires can be preset or changed while deployed to many desired levels; the fins can be varied in number and orientation without deviating from the teachings of the present invention; and also the forward and aft opening cross sectional shape can be varied as long as the cross sectional area remains substantially equal.

In light of the above, it is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.