Elimination of vortex induced vibration in sample probes and thermowells using helical strakes; a report on the development, testing and proving of a helical design
Vortex induced vibration (VIV) is a phenomenon experienced by bluff bodies placed directly in the flow of fluids traveling at velocity in a prevalent direction. As the fluid passes and moves around the body in the flow, vortices form and are shed from the lateral surfaces of the body in an alternating pattern. The alternating formation and release of these vortices from the lateral surfaces generate an ordered wake downstream of the body that remains apparent for a distance several times that of the diameter of the flow-disturbing body, before dissipating and re-establishing the flow pattern of the media that was present prior to interaction with the body in the flow.
The possibility of VIV of cylindrical bluff bodies is a serious design constriction for many applications and hence is a subject of significant research, given the theoretical and practical importance – (Scruton, 1957), (Sarpkaya, 1979), (Zdravkovich, 1981), (Bearman, 1984), (Parkinson, 1989), (JSME, 1998) and (Williamson, 2004).
With strakes, the disruption to the wake is enhanced by having them arranged in a helical pattern rather than along the cylinder axis. For a vortex to develop behind a cylinder, the flow has to separate at a certain angle around the cylinder, but also the separation has to be sustained over a length of cylinder. By having a helical strake, the separation position is always varying so it not possible for a vortex to develop and so, any coherent vortex shedding structure is destroyed and with it any oscillatory forces.
It is immediately apparent that with the solid bar, (Fig. 8), there are noticeable peaks in the data and there are none for the straked probe. This is an immediate indication that there is no coherent vortex shedding behind the straked probe.
At higher velocities it can be seen that the differences between the straked probe and the bar are smaller than at lower velocities. This would tend to support the findings pointed out earlier in the theory that at lower velocities there is a more distinctive vortex structure behind a circular cylinder that the strakes will have a significant impact on whereas at higher velocities the wake is more turbulent and containing vorticity and the effect of the strakes will be less apparent, though it will still disrupt any shedding behaviour.
Considering the data obtained in these tests it is certain that vortex shedding from the straked probe will not occur at any velocities higher than those tested here and the same behavior is expected if the probe was to be tested.