Retractable strake and method

Abstract

A retractable strake is disclosed that includes an assembly of flexible finger elements or flexible faces. The assembly, when coupled to a structure, deflect at wind speeds greater than approximately twenty mph or wind pressures greater than approximately one psf. Accordingly, the strakes' retractable or flexing feature does not contribute to the wind load when the wind load is overly severe, while providing vortex shedding benefits within a wind speed profile range.

Description

FIELD OF THE FIELD OF THE INVENTION

[0002] The present invention relates generally to methods and devices for reducing vortex shedding. More particularly, the present invention is directed to a retractable strake for reducing the susceptibility of an object to vortex shedding.

BACKGROUND OF THE INVENTION

[0003] It is well known that antenna structures or other tall cantilevered cylindrical structures are subject to vibration caused vortex shedding from wind forces. Vortex shedding refers to the phenomenon that occurs when wind forces exert an unbalanced crosswind pressure on opposite sides of the object. Vortex shedding occurs as the air moving over the surface of a cylinder separates from its hind quarters at about the four to five and seven to eight o'clock positions forming a regular train of downwind swirling eddies or vortices. The separation of the vortices for a given size cylinder is periodic over a range of windspeeds. The pressure exerted by the air is reduced as it separates from the cylinder. Thus the separating vortices cause unbalanced pressure fluctuations on the sides of the cylinder. If the frequency of the pressure fluctuations is near or at the natural frequency of the structure, large amplitude, potentially damaging cross wind vibrations can occur.

[0004] For example, an antenna structure is typically surrounded by a radome. A radome is a hollow cylindrical mast, typically made from fiberglass, that is placed around an antenna structure to protect it from elements, such as snow and ice, that could affect the performance of the antenna. When a radome enclosed antenna structure is erected and subjected to wind, the wind flows around the circumference of the radome.

[0005] As shown in FIG. 1, when wind 10 flows around the radome it separates on the downwind surface. As the air separates from the cylinder a vortex is created. This typically occurs on one side at a time. As the newly created vortex moves away downwind another vortex is forming as the air separates from the opposite side of the cylinder. Each time this occurs the side to side pressure on the cylinder is unbalanced. FIG. 1 shows a possible pattern of vortices created. The frequency of the shedding vortices is dependent on the kinematic viscosity of the fluid (in this case air), the wind speed, and the geometry of the object. The frequency of vortex shedding can be either random or periodic. For a fixed geometry (ie radome or smoke stack) the frequency is most strongly dependent upon windspeed.

[0006] Antenna structures are designed to withstand established maximum expected wind speeds as the local and national standards dictate. The antenna structures are designed to withstand the expected maximum wind speeds, which are measured from a reference point location at or near ground level, occurring over a given time period of fifty years or so. Typically, the maximum design wind speeds are in excess of seventy mph. The wind pressure at the antenna is scaled up to account for the increase in wind speed that occurs as with the increased height of the structure.

[0007] Vortex shedding frequencies are either random or significantly higher than any of the potentially damaging modes of structural vibration at the expected maximum windspeed used for structural design. The greatest vortex shedding problem occurs at low wind speeds. The frequency of vortex shedding is periodic at low wind speeds. A vortex will shed off of one side and then the other at regular intervals, producing a periodic oscillating side to side force. This can be damaging if the frequency of vortex shedding is slightly above the first structural mode and the wind speed driving the structure is greater than ten mph. This will cause resonance, a condition where there is very little resistance to oscillatory motion. Large displacements can develop causing damage or failure. As shown in FIG. 1, the vortices 14, 16, 18 are spiraling circles of wind that tend to increase the pressure exerted on the radome 12.

[0008] When the pressure on one side of a structure differs from the pressure on the opposite side of the structure, at a point in time, the structure may move in a direction toward the side that is lower in pressure. As the wind traverses the structure, the pressure exerted on opposite sides of the structure may continue to fluctuate, and cause the structure to vibrate, i.e., sway in response to the alternating low pressure sides. For example, as shown in FIG. 1, vortex 14 will cause radome 12 to move downward, while vortex 16 will cause the radome 12 to move upward.

[0009] Several approaches have been used to eliminate or alleviate vortex shedding. One of the most common is to use Scruton Windings, which are helical rigid strakes added to the upper third of the structure to disrupt the formation of the vortices. The rigid strakes disrupt and diffuse the flow of wind around the radome, such that the development of periodic vortices, which may cause the antenna structure to resonate, is reduced. The negative side of using helical rigid strakes is a large amount of windload is added since the overall projected area has increased as well as the shape is changed from a lower drag full cylinder to more flattened cross-section.

[0010] Typically, radome enclosed antenna structures, such as a television broadcasting antennas, experience vortex shedding at wind speeds in the range of ten to twenty mph and/or at wind pressures at or below one pound per square foot (psf). Thus, strakes are mostly needed at wind speeds below approximately twenty mph and/or wind pressures below one psf.

[0011] However, since the addition of the strakes to a radome increases the cross-sectional area of the radome, the radome is susceptible to greater wind loads that could affect the stability of the antenna. Thus, components of the antenna structure, such as an antenna mast and a supporting tower structure, have to be built stronger to withstand the increased wind loads. As a result of the added strakes, the cost to manufacture the antenna structure, or any structure that endures wind forces, increases.

[0012] Accordingly, it would be desirable to provide a strake that may reduce the susceptibility of wind-bearing structures such as towers, chimneys, smoke stacks, masts, etc. to vortex shedding, while reducing the contribution of the strake to the wind load of the structure.

[0013] Further, it would be desirable to provide a strake that helps to prevent vortex shedding without significantly increasing the costs of associated structures, such as in the case of an antenna, the antenna masts and supporting tower structures.

SUMMARY OF THE INVENTION

[0014] In various embodiments of the present invention, system(s) for reducing vortex shedding on an object are provided. In one aspect of the present invention, the system comprises a strake having a substantially polygonal shape, and flexible attachment feet, wherein the strake is attached to the feet and the feet are attached to the object.

[0015] In another aspect of the present invention, the system comprises vortex shedding means for shedding vortices about the object, wherein the shedding means is flexibly retractable, and attachment means for attaching the vortex shedding means to the object, wherein at least one of the vortex shedding means or the attachment means flexs when at least one of wind speeds are greater than approximately twenty mph and wind pressures are greater than approximately one psf.

[0016] In another aspect of the present invention, a method is provided for manufacturing the present invention, comprising a method for manufacturing an apparatus for reducing vortex shedding on an object, comprising arranging a plurality of truncated two-side strake elements into an assembly of attachment elements, and coupling the assembly of attachment elements to the object, such that the assembly of strake elements deflects when at least one of wind speeds are greater than approximately twenty mph and wind pressures are greater than one psf.

[0017] There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.

[0018] In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.

[0019] As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 illustrates a radome subjected to vortex shedding.

[0021] FIG. 2 is a top view of a retractable strake in accordance with the present invention.

[0022] FIG. 3 is a front elevation view of a retractable strake in accordance with the present invention.

[0023] FIG. 4 is a front view and side views of another retractable strake in accordance with the present invention.

[0024] FIG. 5 is a series of perspective views of the retractable strake of FIG. 4 arranged about an antenna radome and a smoke stack, in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] Referring now to the figures, wherein like reference numerals indicate like elements, there is shown in FIG. 2, a retractable strake 20, 22 in accordance with the present invention, that may be utilized to reduce vortex shedding. For purposes of example, the present invention is described with respect to a radome 12. However it should be understood by one of ordinary skill in the art that a strake 20, 22 in accordance with the present invention may have other applications, for towers, chimneys, smoke stacks, masts, etc.

[0026] In an exemplary embodiment of the present invention, the strake 20, 22 is constructed from an assembly of finger elements 24, 26, 28. The individual finger elements 24, 26, 28 are bristle elements manufactured from a non-metallic material, for example, a plastic, a nylon material, or a polyethylene material. In another exemplary embodiment of the present invention, the finger elements 24, 26, 28 are formed from strips of a plastic material, for example polyethylene.

[0027] It should be understood by one of ordinary skill in the art that a strake 20, 22, when utilized in connection with an antenna system, is made from a non-metallic material to prevent interference with the transmission of signals from the antenna. However, a strake 20, 22 of the present invention, when utilized for other applications, such as preventing the occurrence of vortex shedding on, for example, metal chimney stacks, may be manufactured alternately from a metallic material.

[0028] Shown in FIG. 2, the assembly of finger elements 24, 26, 28 are arranged according to a predetermined pattern. The pattern is designed such that the maximum height of the assembly of finger elements 24, 26, 28 is approximately ten percent of the overall diameter of the radome 12. In an exemplary embodiment of the present invention, the diameter of the radome 12 is forty inches and the maximum height of the assembly of finger elements is approximately four inches.

[0029] In an exemplary embodiment of the present invention, at least one side of the assembly of finger elements 24, 26, 28 is curved, such that the strake 20, 22 can be curvedly positioned about the radome 12.

[0030] Shown in FIG. 3, strakes 20, 22, in accordance with present invention, are positioned on an exterior surface of a radome 12. In an exemplary embodiment of the present invention, the strakes 20, 22 are positioned about the exterior surface of the radome, such that they form a helical or nearly helical pattern about the exterior surface of the radome.

[0031] By positioning the strakes 20, 22 in a helical-like type of pattern about the radome 22, instead of straight out from the radome 12, the strakes 20, 22 cover more surface area of the radome 12, and are able to diffuse the wind flow, and prevent the development of vortices, such as vortices 14, 16, 18 shown in FIG. 1. Also, by configuring the strakes in a semi-helical manner, the strakes are able to disrupt the air flow from any direction.

[0032] During operation, a strake 20, 22, in accordance with the present invention, is retractable. For example, at wind speeds of approximately twenty mph or less and/or wind pressures of approximately one pound psf or less, when vortex shedding typically occurs, the strake 20, 22 is erect, stiff and/or stable. Accordingly, the strake 20, 22 creates the necessary turbulence to avoid the development of vortices that could affect the stability of, for example, a radome enclosed antenna structure.

[0033] However, the strake 20, 22 is designed such that, at wind speeds above approximately twenty mph and/or wind pressures greater than approximately one psf, when vortex shedding typically does not occur, the strake 20, 22 deflects in the direction of airflow, as the wind speeds and/or wind pressures increase. Thus, the cross-sectional area of the radome 12, with the added strake, decreases. Accordingly, the amount of wind load that the radome 12 is susceptible to also decreases. The deflection serves to retract the strake.

[0034] In an exemplary embodiment of the present invention, at wind speeds of approximately twenty miles per hour, and/or wind pressures of twelve and one-half psf, the assembly finger elements 24, 26, 28 of a strake 20, 22 completely deflect, and lay along the surface of the radome 12.

[0035] In an exemplary embodiment of the present invention, a strake 20,22 is coupled to the radome via an adhesive. In an exemplary embodiment of the present invention the radome 12 has openings/ports through which the finger elements 24, 26, 28 are inserted, and secured with adhesive, such as an epoxy. In a second exemplary embodiment of the present invention, the individual finger elements 24, 26, 28, of a strake 20, 22, are secured to the exterior surface of the radome 12 with an adhesive.

[0036] FIG. 4 is a front view and side views of an exemplary retractable strake 41. The retractable strake 41 is illustrated in the front view as being upright and having a generally rectangular shape. The retractable strake 41 is a preferably, but not necessarily, made from a UV resistant elastomer and is fabricated in relatively short sections. In this example, the retractable strake 44 is of approximately 5 inches in length or less. The retractable strake 41 employs a notch running along the edge adjacent to the surface of the structure 43 to help elevate the strake 41, to add flexibility to the strake 41, and also to provide an alternate flow path around the strake 41, and is attached to the surface via mounts 45. The mounts 45, shown here for illustrative purposes, as bolts or hex head screws secure the strake 41 and joined to the strake 41 through the feet 47, which are attached to the strake 41 at its perimeter. The feet 47 are made from a material that is pressure sensitive, specifically wind loads, to result in the strake 41 having a bent configuration as shown in the high speed illustration. The feet 47 serve as both the hinge point and spring tension to return the strake 41 to its original low vortex shedding position.

[0037] FIG. 5 is a series of perspective views of an exemplary embodiment of this invention arrayed about an antenna radome 50 and a smoke stack 60, respectively. The individual strakes 41 are placed around the antenna radome 50 and smoke stack 60 in an overlapping helical pattern to provide vortex shedding from any angle of wind impingement. The strakes 41 are positioned with a slight gap between adjacent strakes 41 to enable the adjacent sections to fold without interference. The gap in the exemplary embodiments is about 0.50 inches, however, the gap may be changed according to design preferences. Similar to the benefits afforded to the antenna radome 50, the smoke stack 60 can be protected from oscillatory vibrations caused by vortex shedding.

[0038] While FIG. 5 shows the exemplary retractable strakes according to this invention being placed about an antenna radome 50 and a smoke stack 60, it should be appreciated that the exemplary retractable strakes may be placed about other structures, such as sailing masts, towers, buildings, etc., to provide vortex shedding. Also, while the exemplary retractable strakes are shown in FIGS. 4 and 5 as having a predominantly rectangular shape, alternate shapes may be used as deemed appropriate. Additionally, alternate positioning of the exemplary feet on the strake may be facilitated based on the choice of materials used or the type of application. For example, instead of using two feet, one foot may be used either at a midpoint of the strake edge or at another desired location. Furthermore, it should be appreciated that while the exemplary embodiments illustrated the use of flexible feet to accomplish a retractable function of the invention, other mechanisms may be used without departing from the spirit and scope of this invention. For example, mechanical springs may be attached to the strake to enable the strake to retract under a given wind load. Alternatively, portions of a section of the face of the strake may be made of a material that allows the strake to flex when subject to a given wind load rather than the entire face of the strake.

[0039] Furthermore, while the surface of the strake in FIGS. 4-5 is shown as having a smooth face, alternative strake face designs may be used utilizing any one of holes, grooves, dimples, patterns, etc. that result in a distinct wind drag attribute to the strake. Thus, strakes aligned about a structure may have an assortment of wind effect profiles. For example, alternate strakes in the helical pattern may flex at different wind speeds, according to design preference. Additionally, it should be appreciated that a non-helical arrangement of strakes may be implemented, or a non-continuous arrangement, as desired.

[0040] In various exemplary embodiments of the present invention, a strake may be assembled on a non-metallic support structure and/or base that is molded into the structure of, or coupled to the wind-bearing object with non-metallic hardware. In various other exemplary embodiments of the present invention, the strake is assembled within a non-metallic frame structure that is coupled to the wind-bearing object with non-metallic hardware. It should be understood by one of ordinary skill in the art that there may be various other methods for coupling the strake to a wind-bearing object.

[0041] The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims

1. A system for reducing vortex shedding on an object, comprising: a strake having a substantially polygonal shape; and flexible attachment feet, wherein the strake is attached to the feet and the feet are attached to the object.

2. The system of claim 1, wherein the strake deflects at wind pressures greater than approximately one psf.

3. The system of claim 1, wherein the strake deflects at wind speeds greater than approximately twenty mph.

4. The system of claim 1, wherein the strake's inner edge is notched.

5. The system of claim 1, wherein the strake is attached to the object by a single foot.

6. The system of claim 1, wherein the strake is substantially rectangular.

7. The system of claim 1, further comprising: a plurality of strakes, the plurality of strakes being arranged about the object in a semi-helical form.

8. The system of claim 1, wherein the plurality of strakes are arranged about the object in a plurality of semi-helical forms.

9. The system of claim 1, wherein the strake is molded to the feet.

10. The system of claim 1, wherein the feet are molded into the object.

11. The system of claim 1, wherein the strake is less than five inches wide.

12. The system of claim 1, wherein a maximum height of the strake is approximately 10 percent of an overall diameter of the object.

13. The system of claim 1, wherein a surface of the strake is patterned.

14. The system of claim 1, wherein the object is a radome.

15. The system of claim 1, wherein the object is a chimney.

16. The system of claim 1, wherein the object is a mast.

17. The system of claim 7, wherein the strakes of the plurality of strakes are displaced from each other by approximately {fraction (1/2)} inches.

18. An apparatus for reducing vortex shedding on an object, comprising: vortex shedding means for shedding vortices about the object, wherein the shedding means is flexibly retractable; and attachment means for attaching the vortex shedding means to the object, wherein at least one of the vortex shedding means or the attachment means flexs when at least one of wind speeds are greater than approximately twenty mph and wind pressures are greater than approximately one psf.

19. The apparatus of claim 17, wherein the shedding means and the attachment means are positioned in a semi-helical arrangement about the surface of the object.

20. The apparatus of claim 18, wherein the attachment means is an adhesive.

21. The apparatus of claim; 18, wherein the shedding means is primarily a truncated flat surface.

22. The apparatus of claim 18, wherein the object is a radome.

23. A method for manufacturing an apparatus for reducing vortex shedding on an object, comprising: arranging a plurality of truncated two-side strake elements into an assembly of attachment elements; and coupling the assembly of attachment elements to the object, such that the assembly of strake elements deflects when at least one of wind speeds are greater than approximately twenty mph and wind pressures are greater than one psf.