US3161167A - Method and means for reducing hydrodynamic noise - Google Patents

Description

Dec. 15, 1964 M. H. SILVERMAN METHOD AND MEANS FOR REDUCING HYDRODYNAMIC NOISE Filed Feb. 14, 1963 FREQUENCY IN KC/SEC Fig.3

HYDROFOI L TRAILING EDGE Fig.4

SURFACE PORTION O YDROF B T ETRE D FREQUENCY IN KC/SEC E o 1].] O. (n g m 2 a: v E i *Al -AFTER FIN TREATMENT 6 INVENTOR MURRAY H. SILVERMAN C7 5 A TTORm United States Patent Ofifice 3,161,167 METHOD AND MEANS FOR REDUCING HYDRODYNAMIC NOISE Murray H. Silver-man, Washington, D.C., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Feb. 14, 1963, Ser. No. 258,971 4 Claims. '(Cl. 114-20) The present invention relates in general to a method and means for eliminating, or materially reducing, hydrodynamic noise resulting from the passage of an object at relatively high speed through a fluid meduim. In a preferred embodiment, the invention relates to lowering the noise level produced by movement of an underwater torpedo so that the latter is rendered more difficult of detection by an enemy vessel toward which the torpedo has been launched.

The presence of hydrodynamic noise constitutes one of the major problems involved in the operation of modern high-speed underwater missiles. In many instances this noise is of sufiicient amplitude so that the missile is detectable by the enemy in time to allow evasive maneuvers to be successfully carried out. Consequently, it is desirable to eliminate to the maximum possible extent the external noise developed by such a missile in order that its combat efficiency may be increased.

One of the moreserious forms of hydrodynamic noise is singing noise and it is with this particular phenomenon that the present invention is concerned. Singing is of particular importance since it is tonal in nature and is usually very intense in water. Such a tone can significantly raise the detection level of an otherwise relative low noise energy spectrum. Singing is caused by the same physical phenomenon as the commonly noted vibration of iced telephone wires or car aerials in high winds when their axes are transverse to the wind. Although the flow ahead of the wire is relatively smooth, there is an unsteady fiow in its. wake caused by vortices periodically shed alternately above and below the wire. This unsteady flow is known as the Von Karman' vortex street and produces an oscillatory force resulting in singing of the wire. In other bodies, if the frequency of one of its natural modes coincides with the frequency of the street, singing will occur.

It has been found that this undesirable effect is due in large measure to the design of certain auxiliary fins located on the aft section of the torpedo just forward of the propellers. employed primarily to create a lift in the proper direction, which tends to reduce the attack angle resulting from the static turning moment inherent in the torpedo during its passage through the water. However, because of the initially severe stability requirements of a torpedo of the type being described, additional fins are desirable, and the latter are positioned at a 45 angle to the cruciform fins. Although-these added fins are designed for stability and hence possess a rather fiat profile, nevertheless they were found to. sing in such fashion that asubstantially constant pure tone was produced of a nature readily detectable by an enemy.

In a copending United States patent application of the present applicant, Serial No. 174,323 filed on February 19, 1962, and entitled Hydrodynamic Noise Reduction, there is disclosed a method and means for eliminating,

The usual cruciform fins on a torepdo are' 3,161,167 Patented Dec. 15, 1 964 or at least materially reducing, the over all noise output of the torpedo when it passes at relatively high speed through a fluid medium. The solution therein disclosed consists in modifying the profile of each 45 fin by removing a terminal portion of the fin material so as to form a wedge having a pretermined angle, and then extending the Wedge slope forwardly to intersect the respective fin surfaces at a pre-se-lected point. This results in the angle actually seen by the fluid stream lines being relatively small because of the symmetric configuration of the fin profile.

While the above expedient is in every way satisfactory insofar as noise reduction is concerned, it has now bee-n discovered that it is not necessary to actually alter the fin profile in order to achieve the desired results. Alternatively, it is feasible to treat the surface of the fin in the vicinity of its trailing edge so that, instead of being smooth, this particular fin surface area is rough. Con- Other objects and many of the attendant advantages of i this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when consider-ed in connection with the accompanying drawings wherein:

FIG. 1 is a side view of an underwater torpedo of the type to which the present invention is particularly applicable;

FIG. 2 is an aft view of the missile of FIG. 1, showing the angular relationship of the various fins with which the missile is provided;

FIGS. 3 and 6 are graphs illustrating the change in noise output of the missile of FIG. 1 when the 45 fins thereof have a surface portion treated in accordance with the invention;

FIG. 4 is a view showing the profile of 'one of the 45 fins illustrated in FIGS. 1 and 2; and

FIG. 5 is an enlarged perspective view of a portion of the fin of FIG. showing one manner in which a surface area of the fin may be treated in accordance with the principles of the present invention. I

Illustrated in FIGS. 1 and 2 of the drawings is an underwater missile or torpedo which is designed to be guided toward a target by control information supplied to 'the torpedo over a wire 8 which trails behind the .missile in the manner shown. The torpedo of FIGS. 1 and-2, thereforeyis formed with a body 10 which cona source of energy for carrying out its prescr'ibed'functions.

However, these'components form no part of the present invention, and'hence neither an illustration nor a detailed description-thereof isdeemed necessary. It need only be noted that the torpedo of FIGS. land 2 is driven by a pair of propellers 12, and, for purposes 3 of stability, is provided with a plurality of fins 14 which are spaced apart circumferentially about the longitudinal axis of the missile in a manner best brought out by HQ. 2. In addition to the usual cruciform fins 16, a plurality of auxiliary fins 18 are employed, each of these fins 18 being disposed at an angle of 45 to the principal cruciform fins 16 as clearly brought out by FIG. 2. It has been determined during actual operation of the torpedo being described that one of the principal sources of hydrodynamic noise is the singing of the fins 14 as the result of oscillatory forces produced by the unsteady fluid fiow past the fins 18, particularly when the missile is traveling at a relatively high speed. There is an interdependency among the effective depth of the fin trailing edge, fiuid velocity and oscillation frequency, and this relationship is designated by the Strouhal Number. This Strou'hal Number is defined as where f is the oscillation frequency, d is the effective depth of trailing edge of the fin (for cylinders and spheres a is the diameter) and v is the velocity of the fluid. As above mentioned, singing of the fins 18 is in the nature of a steady or constant tone, and, since this tone is very intense in water or other fluid, it is readily detectable by a device such as a hydrophone.

The interdependency of the factors involved permits the frequency of the exciting force to be changed by varying the thickness of the trailing edge of the fin and/ or its boundary layer, and, in the same manner, a change in the fins structural characteristics can also change its resonant frequency. In such situations, the quantity d is usually considered to be a function (the thickness) of the trailing edge of the fin.

Applying the above approach to the particular torpedo design set forth in FIGS. 1 and 2 of the drawings, the initial factor to be determined is the frequency at which the singing of the fins occurred. This information is readily derived, and, as shown in FIG. 3 of the drawings, a high noise peak is present in one particular region of the frequency spectrum. Utilizing the formula above given showed that this fin excitation occurred in the last .30 inch of each 45 fin and that, if excitation could be eliminated in this region, practically all of the objectionable singing would disappear. Following this avenue of approach, the terminal .30 inch on both surfaces of each of the 45 fins was modified as shown in FIGS. 4 and 5 by applying to each such surface a thin coating of an adhesive in which were dispersed minute particles of silica. It was found that these silica or sand particles disturbed the fluid flow over the trailing edge of each fin sufficiently to greatly reduce the noise level. Since a similar modification to each of the 90 cruciform fins produced no additional improvement, it was concluded that only the 45 fins were contributing to the hydrodynamic noise eifect. This discrepancy in the mode of vibration of the 45 and 90 fins might possibly be due in part to the fact that whereas all of the 45 fins Were 8.5 inches long in the example being given, the upper vertical fin, on the other hand, was 13.5 inches long and the lower vertical fin 48.5 inches long.

While the particular sand employed was ordinary silica sand, any sand of analogous size is'satisfactory. The

fin area to be treated is first covered with a thin coating of glue such as a rubber-based cement. The sand is then preferably sprinkled on while the glue is wet. The sand should be applied so that the particles are neither too densely distributed (each sand particle in direct contact with adjacent particles) nor too sparsely distributed (to result in wide streaks or open patches) but intermediate these extremes. Only one layer of sand is necessary. It has been found in practice that toothick, rough or dense a layer will cause cavitation and defeat the purpose of the invention. a

Extensive tests of a torpedo, the 45 fins of which were treated in accordance with this procedure, showed essentially no extreme peaks of oscillation, and the noise output throughout the frequency range was essentially uniform as brought out in FIG. 6 of the drawings. Consequently, detection of such a projectile by an enemy is rendered much more difficult than was the case prior to applicants investigations.

Since the purpose of sanding a portion of the fin surface area is to roughen the trailing edge of a hydrofoil, the same objective may be achieved by:

(l) Employing any solid particles of substantially homogeneous distribution and of a diameter comparable to silica sand, and utilizing any suitable adhesive which will preclude particle slippage or detachment during actual use, or

(2) Roughening the desired surface area of the fin by any known metal-forming or shaping process which is capable of yielding surface characteristics similar to those of a sanded fin, such, for example, as by embossing or removing small areas of fin material to create a stippled effect.

It will be immediately recognized that the chordal distance over the planar areas of which the disclosed treatment is to be applied (the distance A-B in FIG. 5) will vary widely depending upon the dimensions and configuration of the hydrofoil and upon the operational speed of the projectile. In all cases, however, it is the trailing edge of the missile fin which is to be modified in the manner set forth.

The process herein described is readily carried out by a minimum of equipment and at a relatively low cost in terms "of both materials and manpower. A substantially complete elimination of the Von Karman vortex street in the manner taught does not measurably increase the cavitation effect which, when present to any appreciable degree, is itself a source of hydrodynamic noise.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. The method of reducing the hydrodynamic noise level of an underwater torpedo having at least one hydrofoil, said method including the step of roughening the surface area of such hydrofoil adjacent the trailing edge thereof so as to disturb the fluid flow over such surface when said torpedo is in motion, the said surface area of the hydrofoil being roughened by applying a granular substance thereto as an outer coating.

2. The method of reducing the hydrodynamic noise level of a missile designed to pass through a liquid medium during operation, said missile being stablized by a plurality of longitudinally-disposed fins arranged circumferentialy in spaced-apart relation, certain of which fins form an angle of 45 to the vertical when said missile is in operation and which 45 fins each create an oscillatory turbulence in the wake thereof during passage of said torpedo through said liquid medium, such turbulence bringing about a vibration of the 45 fins when the natural resonant frequency of the latter approaches the turbulence frequency, said method including the step of toughening a portion of the surface of each 45 fin adjacent the trailing edge thereof to modify the fluid flow over such roughened fin surface and hence alter the frequency of the resulting turbulence.

3. The method of claim 2 in which the chordal distance over the planar'areas of which each 45 fin is to be roughened is determined in accordance with the following formula 5 6 Where References Cited by the Examiner f=the turbulence frequency d =the elfective thickness of the 45 fin aft portion UNITED STATES PATENTS v=the velocity of the fluid 1,7 ,4 9 Crook 244-75 The method of fabn'cafing'a P f qesigned g 5 51333132 1653; iiiiiit 'jjjjjjj:III-2533353 use on an underwater torpedo Whlch cornpnses coating 7 3,076,725 2/63 Boggs an aft portlon of the hydrofo1l surface u ith a fluid ad- 3,096,739 7/63 Smith hesive, applying .to said adhesive a comnnnuted material a in such a manner that the particles thereof are substan- FOREIGN PATENTS tialy evenly distributed over the said hydrofoil surface 10 7 490,501 8/38 Great Britain. portion, and then alowing the fluid adhesive to harden 580,806 9/46 Great Britain.

soas to'retain the particles of comminuted material in the respective locations Where they were originally BENJAMLN BORCHELT Pnmary Exammel" applied. 15 SAMUEL. FEINBERG, Examiner.

Claims (1)

1. THE METHOD OF REDUCING THE HYDRODYNAMIC NOISE LEVEL OF AN UNDERWATER TORPEDO HAVING AT LEAST ONE HYDROFOIL, SAID METHOD INCLUDING THE STEP OF ROUGHENING THE SURFACE AREA OF SUCH HYDROFOIL ADJACENT THE TRAILING EDGE THEREOF SO AS TO DISTURB THE FLUID FLOW OVER SUCH SURFACE WHEN SAID TORPEDO IS IN MOTION, THE SAID SURFACE AREA OF THE HYDROFOIL BEING ROUGHENED BY APPLYING A GRANULAR SUBSTANCE THERETO AS AN OUTER COATING.

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