VORTEX SHEDDING AND DRAG FORCE REDUCTION

Abstract: Smoothly curved protuberances (8, 26, 34) are added to an elongate body (2, 16, 18, 20, 22, 32) to modify the fluid flow to reduce the drag Force Fdrag and the forces induced by vortex shedding Fvortex. The protuberances can be arranged in diametrically opposed pairs with longitudinally adjacent pairs having different radial directions first to cope with fluid flow from a variety of different directions.

Description: VORTEX SHEDDING AND DRAG FORCE REDUCTION This invention relates to techniques for modifying fluid flow so as to reduce the effects of drag and vortex shedding. More particularly, this invention relates to such techniques that may be applied to elongate bodies. When an elongate body, such as a chimney, is positioned within an environment where it is subject to fluid flow, in the case of a chimney airflow, then a drag force is exerted on the elongate body and vortex shedding can occur inducing forces that can lead to undesirable vibration. The drag force of passing fluid flow often means that the elongate body has to be produced with a strengthened structure to resist such a drag force. The cost of strengthening the structure in this way can be significant. In the case of vortex shedding, the forces this exerts vary with time in a manner that can establish highly damaging undesirable vibrations within an elongate body. It may be that these vibrations will stimulate a resonance with potentially destructive consequences. It is known to fit fairings to structures in order to modify fluid flow around those structures to reduce drag. A problem with such fairings is that they are usually only able to cope with fluid flow from a single direction and if the fluid flow direction changes, then they may be ineffective, or in fact increase drag. The fairings may be made movable to accommodate different flow directions, but this disadvantageous^ increases their cost and complexity. It is also known to attach structures to elongate bodies in an attempt to reduce vortex shedding. An example of this is a helical strake that can be applied to the outside of a chimney. Whilst such a helical strake may reduce vortex shedding, it often has the effect of increasing drag with a disadvantageous need to increase the strength of the chimney. An alternative is the use of a perforated shroud over a chimney. Such perforated shrouds have been found to be too expensive to be practical. Discussions of vortex shedding may be found in E. Naudascher, D. Rockwell "FLOW-INDUCED VIBRATIONS an Engineering Guide", IAHR-AIRH, Hydraulic structures design manual, A. A. Balkema/Rotterdam/Brookfield/1994, 160-176 and M. M. Zdravkovich, "Review and Classification of Various Aerodynamic and Hydrodynamic Means for Suppressing Vortex Shedding," Journal of Wind Engineering and Industrial Aerodynamics, 7 (1981) 145-189. A description of a unidirectional fairing for use on a drilling riser to reduce vortex induced vibration is described in United States Patent US-A-6,048,136. Viewed from one aspect the present invention provides an elongate body having a plurality of longitudinally spaced apart smoothly curved protuberances extending therefrom, said protuberances being shaped and dimensioned to modify fluid flow around said elongate body in a manner that reduces forces upon said elongate body produced by drag and vortex shedding. The invention recognises and exploits the phenomenon whereby a smoothly curved protuberance (smooth at least in the sense of how it modifies the fluid flow) from an elongate body can be made to modify the fluid flow around that body in a manner that reduces the forces exerted on the body by drag and vortex shedding. The protuberance is advantageously simple and inexpensive to provide with or add to an elongate body. Whilst the invention could be used in situations where the fluid flow was unidirectional, in preferred embodiments of the invention said protuberances extend in a plurality of different radial directions from a longitudinal axis of said elongate body. This feature of the invention allows fixed protuberances that are inexpensive and simple to reduce drag and vortex shedding that can occur from fluid flow incident from any radial direction around the elongate body. This is strongly advantageous since, for example, a chimney or a drilling platform leg may be subject to fluid flow from any radial direction. It will be appreciated that the differences between the radial direction of adjacent protuberances may vary over a range of values. It has been found that a preferred range of values for the differences between radial directions is 30 degrees to 90 degrees inclusive. A particularly preferred arrangement that works well in many cases is when the difference in radial direction between adjacent protuberances is substantially 45 degrees. The protuberances could be applied to a single side of the elongate body. However, in preferred embodiments the protuberances are arranged in pairs at the same longitudinal position along the elongate body and with opposite radial directions. This has been found to be constructionally convenient and provide good omni-directional performance. The size of the protuberances can vary significantly depending upon the circumstances. Generally speaking, more dense fluids may require more pronounced protuberances than less dense fluids. It will also be appreciated that the protuberances should not be too large or they may result in an undesirably large increase in drag when the fluid flow is not favourably aligned with them. Compared with the maximum diameter D of the cross-section of the elongate body, a preferred range of protuberance sizes has been found to be one in which the protuberances extend from an outer surface of the elongate body by a distance within the range 0.1D to 0.75D. A more highly preferred range is 0.25D to 0.5D. The longitudinal spacing of the protuberances can also vary. Placing the protuberances too close together will increase cost and weight whilst it may also reduce the effectiveness of the protuberances in modifying the fluid flow in the desired manner. Similarly, placing the protuberances too far apart will make them ineffective. In preferred embodiments of the invention the longitudinal spacing of the protuberances is such that said radial directions of said protuberances vary along said longitudinal axis in a repeating pattern with a repeat distance within the range 3D to 9D inclusive. It will be appreciated that the smoothly curved protuberances could have a wide variety of cross-sectional shapes. The protuberances should be smoothly curved and blend well into the shape of the rest of the elongate body so as to reduce drag. However, within this constraint, the shape may vary widely. A preferred shape that has been found to produce good results is when the cross-sectional shape of the protuberances is at least a portion of an ellipse. When the protuberances are paired together, they may be arranged in a fashion in which the back-to-back protuberances have a combined cross-sectional shape that is a full ellipse. The elongate body to which the protuberances are attached could similarly have a range of cross-sectional shapes. However, a circular cross-sectional shape is common in bodies that are subject to the drag and vortex shedding forces which the invention seeks to reduce and this shape has been found to benefit well from the technique of the present invention. The elongate body around which the fluid flow is modified by the technique of the present invention could be part of a wide variety of different structures. Examples of structures that may particularly benefit from the technique of the invention are an offshore riser, a support member of an offshore platform, a pipe, an underwater cable, chimney and a support tower for a wind turbine. It will be appreciated that the fluid which gives rise to the drag and vortex shedding may be either a liquid or a gas. The protuberances could be integrally formed with the elongate body with which they are associated. However, in preferred embodiments of the invention the protuberances may take the form of fairings (e.g. an element added to modify fluid flow) that are attached to an elongate body. The engineering of many elongate bodies is in many cases already fixed and the form of the invention as add-on fairings is particularly convenient and simple together with allowing the possibility for retrofitting. Viewed from another aspect the present invention provides a method of reducing fluid flow induced forces upon an elongate body produced by drag and vortex shedding, said method comprising the step of providing a plurality of fluid flow modifying longitudinally spaced apart smoothly curved protuberances extending from said elongate body. Viewed from a further aspect the present invention provides a kit for modifying fluid flow around an elongate body, said kit comprising a plurality of smoothly curved fairings for fixing to said elongate body and a plurality of fairing fasteners for fixing said fairings to said elongate body to form a plurality of longitudinally spaced apart smoothly curved protuberances extending therefrom, said protuberances being shaped and dimensioned to modify fluid flow around said elongate body in a manner that reduces forces upon said elongate body produced by drag and vortex shedding. Supplying the fairings and associated fasteners as a kit is a likely way in which the invention may be embodied in circumstances when it is desired to retro-fit existing structures. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically illustrates fluid flow past a circular cross-section body with associated drag and vortex shedding; Figure 2 illustrates a cylindrical elongate body having flow modifying protuberances attached thereto; Figure 3 illustrates cross-sectional views of a pair of flow modifying protuberances; Figure 4 illustrates a range of protuberances sizes applied to a cylindrical body; Figure 5 illustrates a kit form of the protuberances; and Figures 6, 7 and 8 illustrate possible uses of the invention. Figure 1 schematically illustrates a cylindrical body 2 positioned within a fluid flow 4. The fluid flow 4 gives rise to a drag force F ra acting in the same direction as the fluid flow 4. Vortices 6 are shed from alternating sides of the cylinder 2 and moved downstream within the fluid flow 4. As these vortices 6 are shed, they subject the cylinder 2 to a varying vortex shedding force F VOrt ex that is of a generally periodic nature. The vortex shedding force F vortex can vary in magnitude, direction and timing. The drag force F dra g can necessitate an undesirable need to increase the structural strength of the cylinder 2. The vortex shedding force F VOrtex can similarly require the structure of the cylinder to be strengthened as well as raising the possibility of inducing undesirable vibrations, or even resonance, within the cylinder 2. Figure 2 illustrates an elongate body in the form of a cylinder to which smoothly curved protuberances have been added. These protuberances are arranged in diametrically opposed pairs with the radial direction of the protuberances varying by substantially 45 degrees between adjacent pairs of protuberances. In a test the arrangement illustrated in Figure 2 produced a 24% drag reduction compared with the plain cylinder and also led to significantly less vortex-induced vibration. In the specific example illustrated in Figure 2, the protuberances have an elliptical cross-section and protrude by 0.5D from the surface of the cylinder where D is the diameter of the cylinder. The protuberances are spaced at an interval of 1.75D along the length of the cylinder in an arrangement where the orientation of the protuberances repeats at a distance of 7D. Figure 3 schematically illustrates cross-sectional views through a pair of protuberances as illustrated in Figure 2. The end view shows the elliptical form of the protuberances. In the illustrated example, the major axis of the ellipse is W in length and the minor axis of the ellipse is D in length corresponding to the diameter of the cylinder D on which the protuberance is mounted. The plan view shows the protuberances to have a plan cross-section that is part of a circle of diameter W. Figure 4 illustrates three example cylinders with attached protuberance pairs of differing sizes. In each case, adjacent protuberance pairs are rotated by 45 degrees with respect to one another. The lower example shows relatively less pronounced protuberances that might be suitable for use within a less dense fluid (e.g.air) whereas the top most example shows relatively pronounced protuberances that may be more suitable for use in a more dense fluid (e.g. water). Figure 5 illustrates two fairings 8 that may be fixed to a cylinder 2 to form the protuberances for drag and vortex shedding reduction. These fairings 8 may be retro- fitted to an existing cylinder 2. The fairings 8 have fasteners 10 by which they may be fixed together and to hold the fairings 8 in place upon the cylinder 2. The fasteners 10 could take a wide variety of forms, e.g. in one form the fastener could simply be an adhesive for sticking the fairings 8 to the cylinder 2. Many alternative mechanical fasteners such as straps, screws, bolts etc, could also be utilised. In the form of a kit, the invention could be embodied as the fairings 8 and the associated fasteners 10 to be applied to an existing elongate body, such as the cylinder 2. Figure 6 illustrates one example environment in which the present invention may be used. A sea current 12 impinges upon an offshore platform 14. The sea current 12 may come from any direction making uni-directional fairings ineffective. The support legs 16, tension legs 18 and risers 20 are all fitted with appropriately dimensioned protuberances having differing radial directions to cope with the different directions of the sea current 12. The effect of the technique reduces the structural stresses upon the support legs 16 and the tension legs 18. The reduction in the forces on the risers 20 may mean that they can be more closely packed without risk of them banging together, which in turn means that a smaller platform 14 may be practical. Figure 7 illustrates another use of the invention. ' In this example, a steel chimney 22 is subject to a wind 24 that can impinge from any direction. Protuberances 26 are attached to the chimney 22 and reduce the wind drag and vortex induced vibrations. Figure 8 illustrates a further example of the use of the present invention. In this case a wind turbine 28 is of the type in which the turbine blade 30 is downwind of the support tower 32 in the direction in which the turbine 30 will try to self-align. Wind flow disturbance produced by the support tower 32 reduces the efficiency of the turbine 30 in extracting energy from the wind flow. Accordingly, the protuberances 34 attached to the support tower 32 reduce the vortex shedding from the support tower 32 in a manner in which enables the turbine 30 to more efficiently extract energy from the wind flow. Furthermore, less wind drag is exerted on the support tower 32 which means that its construction can be less expensive.

Claims: CLAIMS 1. An elongate body having a plurality of longitudinally spaced apart smoothly curved protuberances extending therefrom, said protuberances being shaped and dimensioned to modify fluid flow around said elongate body in a manner that reduces forces upon said elongate body produced by drag and vortex shedding. ' 2. An elongate body as claimed in claim 1, wherein said protuberances extend in a plurality of different radial directions from a longitudinal axis of said elongate body. 3. An elongate body as claimed in claim 2, wherein longitudinally adjacent protuberances have radial directions differing by an angle within the range 30 to 90 degrees inclusive. 4. An elongate body as claimed in claim 3, wherein longitudinally adjacent protuberances have radial directions differing by an angle of substantially 45 degrees. 5. An elongate body as claimed in any one of the preceding claims, wherein at least two of said protuberances are arranged as a pair of protuberances having a common longitudinal position along said elongate body and extending in opposite radial directions from said elongate body. 6. An elongate body as claimed in claim 5, wherein all of said protuberances are arranged as pairs of protuberances. 7. An elongate body as claimed in any one of the preceding claims having a maximum cross-sectional diameter excluding said protuberances of D. 8. An elongate body as claimed in claim 7, wherein said protuberances extend from an outer surface of said elongate body excluding said protuberances by a distance within the range 0.1D to 0.75D inclusive. 9. An elongate body as claimed in claim 8, wherein said protuberances extend from an outer surface of said elongate body excluding said protuberances by a distance within the range 0.25D to 0.5D inclusive. 10. An elongate body as claimed in claims 3 and 7, wherein said radial directions of said protuberances vary along said longitudinal axis in a repeating pattern with a repeat distance within the range 3D to 9D inclusive. 11. An elongate body as claimed in any one of the preceding claims, wherein said protuberances have a cross-sectional shape substantially corresponding to at least a portion of an ellipse. 12. An elongate body as claimed in any one of the preceding claims, wherein said elongate body has a cross-sectional shape substantially corresponding to a circle. 13. An elongate body as claimed in any one of the preceding claims, wherein said elongate body is one of: an offshore riser; a support member of an offshore platform; a pipe an underwater cable; a chimney; and a support tower for a wind turbine. 14. An elongate body as claimed in any one of claims 1 to 12, wherein said fluid flow is liquid flow. 15. An elongate body as claimed in any one of claims 1 to 12, wherein said fluid flow is gas flow. 16. An elongate body as claimed in any one of the preceding claims, wherein said protuberances are formed as fairings fixed to said elongate body. 17. A method of reducing fluid flow induced forces upon an elongate body produced by drag and vortex shedding, said method comprising the step of providing a plurality of fluid flow modifying longitudinally spaced apart smoothly curved protuberances extending from said elongate body. 18. A kit for modifying fluid flow around an elongate body, said kit comprising a plurality of smoothly curved fairings for fixing to said elongate body and a plurality of fairing fasteners for fixing said fairings to said elongate body to form a plurality of longitudinally spaced apart smoothly curved protuberances extending therefrom, said protuberances being shaped and dimensioned to modify fluid flow around said elongate body in a manner that reduces forces upon said elongate body produced by drag and vortex shedding.

Inventors: BEARMAN PETER WILLIAM, OWEN JOHN, SZEWCZYK ALBIN ANTHONY, BEARMAN, PETER, WILLIAM, OWEN, JOHN, SZEWCZYK, ALBIN, ANTHONY
Applicants: BEARMAN PETER WILLIAM, IMPERIAL COLLEGE, OWEN JOHN, SZEWCZYK ALBIN ANTHONY, BEARMAN, PETER, WILLIAM, IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE, OWEN, JOHN, SZEWCZYK, ALBIN, ANTHONY
Application year: 2001
Main classification: F15D1
IPC codes: F15D1