Preventing Vortex Shedding with a Helical Strake
Have you ever seen something on TV and thought ‘how fascinating – but I’ll never get to use that’. That’s what I thought when I saw Richard Hammond explaining about the Helical Strake in his series ‘Richard Hammond’s Engineering Connections’. Although some of it is pulp science, the principles explained are sometimes genuinely useful. The Helical Strake is one such example. This is a device, often seen on factory chimneys and tall structures, designed to reduce turbulence and thus make the structure more stable in the face of ever-present winds. When a fluid (air) flows past a cylindrical object (chimney or pole) it causes a phenomenon known as vortex shedding. Little eddies, or vortices, form and detach on alternate sides of the cylinder, in a pattern known as the Von Kármán vortex street.
This vortex pattern, shown in the above animation, creates an area of low pressure on alternate sides of the structure, which tends to move toward the low pressure region. As the vortex alternates from side to side, so the structure vibrates back and forth. If this vibration approaches the resonant frequency of the structure, it could be in serious trouble! Here is an awesome real-life illustration of the Von Kármán vortex street done by the Dept. of Aeronautics at Imperial College, London, which uses a laser sheet to show the vortex shedding around a pole. The segment beginning at 1m 50s shows the phenomenon in 3D.
Von Kármán vortex street, Dept. of Aeronautics, Imperial College, London
So what use is all this, you may ask. Good question. I have a Davis Vantage Pro2 weather station that is mounted on an aluminium aerial pole. Until recently this was fixed to the chimney stack of my house and I noticed that in even light wind conditions, the pole would vibrate vigorously. This caused me concern that the mechanical fixings may weaken and fail and that the accuracy of the readings obtained may be affected. Hammond’s TV programme demonstrated the principle of a helical strake using a very similar pole, a wind tunnel and a length of nylon cord wound around the pole. The results were impressive and I resolved to try out a helical strake on my weather station installation when the chance arose; a recent house move has provided the opportunity.
The Davis Vantage Pro weather station consists of an integrated instrument station containing various sensors and a solar powered wireless transmitter. The anemometer and wind vane is typically mounted atop a pole and the other sensors lower down, the two being connected with a length of cable. Although not ideal for the job, the cable can be wound around the pole forming the helix required. When I re-mounted the station in its new location, I used this technique. My technique is not very scientific but subjectively, I think this has made a difference to the stability of the pole. There seems to be less vibration even in moderate to strong winds. Ideally, this could be improved by threading the cable through a sleeve, to thicken the strake so that it presents more surface area to the air flow. My technique has the advantage of being simple and free. This technique may be applicable in many situations where something is mounted on a pole, such as TV aerials, flag poles and masts, even tall chimneys. I wonder whether the spiral shape of the chimneys on this Staffordshire manor were accident or design?
