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Helical strake

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A two-start helical strake on a chimney

A helical strake, also known as a Scruton strake, is a helix-shaped fin arranged along a structure to disrupt fluid flow. It serves to mitigate fatigue by disrupting the vortexes that induce vibration. Introduced by Christopher Scruton and D. E. J. Walshe in 1957, helical strakes have been widely adopted as an engineering solution for controlling the oscillations caused by airflow and water flow.

Description

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Helical strakes are fins that follow a helix down a structure;[1] they are commonly used on cylindrical bodies[2] such as chimneys and pipelines. Starting with screw heads, strakes continue along the structure and disrupt the vortexes that form as a result of fluid flow[3] by ensuring that flow is shed at different heights and no structured vortexes can be formed.[4] In this manner, strakes reduce vibrations and mitigating fatigue;[5] multiple strakes are often used, as this minimizes the possibility of alternate vortexes forming.[6] They may be used on their own, or in conjunction with other devices such as fairings.[3]

Strakes need not follow the entirety of the structure; vibration suppression is most evident in the areas exposed to higher flow velocities, while strakes in lower-velocity areas have less effect.[7] Consequently, many structures are only covered in helical strakes where the vibrations are greatest;[8] many are limited to the upper third of the structure.[6] In deepwater structures, for instance, strakes may only cover the portion exposed to surface currents, while areas exposed to the slower waters below are left tubular.[8]

Sections of helical strakes are generally assembled from shorter lengths.[9] Strakes thus commonly consist of a series of segments, with the number determined by the length of the structure covered. Metal strakes are cut from larger plates to a radius of curvature informed by the number of revolutions around the stack.[10] Conventionally, strakes have a pitch of 16 to 17.5D, though other pitches may be used.[11]

Helical strakes are less sensitive to flow direction than alternatives such as fairings.[2] However, they have several shortcomings. They significantly increase mean drag,[12] and as a result loading is increased.[6] Strakes are less effective under a certain mass-damping parameter. Strakes have reduced effectiveness when flow is turbulent. Strakes likewise are less effective when located downstream from each other;[12] the wake behind the first strake is unstable, and thus the efficacy of downstream strakes varies significantly. Staggered helical strakes are likewise less effective.[13] Structures perpendicular to a cylinder with helical strakes will also reduce their effectiveness.[14]

History

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Slender structures are prone to fatigue damage from vortex-induced vibration,[15] a result of external fluid flow creating alternating vortices behind them.[16] Consequently, extensive research has been undertaken to understand and suppress these vibrations, thereby mitigating damage to structures exposed to airflow and water flow. Numerous geometric forms have been proposed as a passive means of vibration mitigation,[15] including tripping wires and splitting plates.[17] Other approaches, such as electrical methods, rotary oscillations, and feedback control, have been used as active methods.[18]

Helical strakes are designed to disrupt vortex shedding.

In 1957, Christopher Scruton and D. E. J. Walshe of the National Physics Laboratory in the United Kingdom explored the use of helical strakes with rectangular cross-sections as a means of suppressing vortex-induced vibration by disrupting the vortex shedding process.[19] After the success of this experiment, which led to helical strakes sometimes being known as Scruton strakes,[1] subsequent researchers sought to improve strake performance and explore the design's effectiveness in water.[20] In engineering, strakes began to be widely adopted in the 1960s as an element of wind engineering; ocean-borne versions were introduced in the 1970s.[21] Ultimately, strakes became "one of the most successful and widely used means of suppressing eddy shedding."[22]

Research into strakes has investigated the effect of various elements on design efficacy, including not only the pitch, height, density, and shape of fins but also the number of start screw heads. Generally, higher strakes have been found to be more effective. Likewise, triple-start designs have generally been found to be effective in both air and water environments.[5] Several studies have investigated the potential for new geometries intended to reduce strakes' effect on drag. Proposed new designs have included serrated strakes,[23] as well as inverted helical strakes wherein flow is disrupted not by fins but by internally protruding grooves.[24]

References

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  1. ^ a b Feldmeier 2020, p. 136.
  2. ^ a b Xu 2022, p. 2110.
  3. ^ a b Bai & Bai 2005, p. 429.
  4. ^ Xu & Ma 2024, p. 185.
  5. ^ a b Xu et al. 2017, p. 439.
  6. ^ a b c Escoe 2011, p. 57.
  7. ^ Allen & Liapis 2014, p. 84.
  8. ^ a b Allen & Liapis 2014, p. 85.
  9. ^ Allen & Liapis 2014, p. 87.
  10. ^ Escoe 2011, pp. 58–59.
  11. ^ Bai & Bai 2005, p. 430.
  12. ^ a b Naudascher & Rockwell 2012, p. 165.
  13. ^ Xu & Ma 2024, p. 188.
  14. ^ Zdravkovich 1981, p. 154.
  15. ^ a b Xu et al. 2017, p. 438.
  16. ^ Assi & Crespi 2020, p. 287.
  17. ^ Xu & Ma 2024, p. 187.
  18. ^ Rashidi, Hayatdavoodi & Esfahani 2016, p. 57.
  19. ^ Xu et al. 2017, p. 438
  20. ^ Zdravkovich 1981, p. 152.
  21. ^ Xu 2022, pp. 2109–2110.
  22. ^ Zdravkovich 1997, p. 778.
  23. ^ Assi & Crespi 2020, p. 288.
  24. ^ Kilner et al. 2018.

Works cited

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