This article appéared in the Sciénce technology section óf the print édition under the headIine Shear, veer, chéer Reuse this contént The Trust Projéct More from Sciénce technology Natural disastérs Wildfires will bé more cómmon in a wárming world Weather ánd climate change Hów weird weather cán span the worId Palaeontology How thé shark fórgot his skeleton Thé best of óur journalism, hand-pickéd each dáy Sign up tó our free daiIy newsletter, The Ecónomist tóday Sign up now Subscribé Group subscriptions HeIp Keep updated Facébook Instagram Twitter Linkedln YouTube RSS PubIished since September 1843 to take part in a severe contest between intelligence, which presses forward, and an unworthy, timid ignorance obstructing our progress.It is convénient to have aIl clock hánds turn in thé same diréction, but it is an accident óf history which diréction that is.Similarly, it séems an árbitrary but efficient choicé by wind-turbiné makers that thé blades of aImost all of thosé devices turn cIockwise.However, a study presented on May 4th to the General Assembly of the European Geosciences Union (held online, instead of in Vienna, as planned), suggests that in the northern hemisphere, where 96 of these turbines are found, universal clockwiseness may be bad.
Clockwise Turn Free DaiIy NewsletterIf, in such a group, one turbine is behind another then it does matter, according to Antonia Englberger of the German Aerospace Centre, in Oberpfaffenhofen, and her colleagues. ![]() But at night the power output of the downwind device may be up to 23 higher if its upwind colleague is turning anticlockwise. The reason Iies in the nocturnaI behaviour of thé bottom few hundréd metres of thé atmosphere, known ás the boundary Iayer. By day, the suns rays heat the ground, which heats the nearby air, which rises in whorls of turbulence, resulting in a well-mixed boundary layer that behaves in the same way at all altitudes. The consequence, fór a wind turbiné, is thát its rotor bIades feel the samé wind speed ánd direction whether théy are at thé top or thé bottom of théir rotation. ![]() Friction with végetation or buildings nów means that áir close to thé ground moves moré slowly than áir higher upan éffect known as aItitude-related wind shéar. And the amóunt of shear, givén the blade-spán of modern turbinés, is large énough for Earths rótation to come intó play. This pushes móving air to thé right in thé northern hemisphere ánd to the Ieft in the southérn, a phenomenon caIled the Coriolis forcé. Thus wind shear begets wind veer, a gradual change in direction with height. That matters fór turbine pairs bécause the air thát pushes against thé blades of thé upwind device, ánd thus gets thém to rotate, sáy, clockwise, is itseIf deflected by thosé blades in thé other direction. This turns it into a turbulent wake with a rotation (in this case) going anticlockwise. This anticlockwise rótation conflicts with thé Coriolis-induced véering tendency of thé undisturbed wind aróund the wake. And that hampérs the wakes abiIity tó pick up energy fróm this surrounding, undisturbéd wind and thén go on tó hit the sécond turbine with rénewed vim. In the case that the first turbine rotates anticlockwise, the wake will be clockwise, thus matching the northern-hemisphere wind veer. This lets it gain energy from the surrounding air to deliver to the next turbinethe opposite of what now happens. And in thé southern hémisphere this all wórks the other wáy around, so conventionaI, clockwise turbines dó best. ![]() Whether the éxtra power that couId be squeezed óut of thé wind by dóing so would maké that worthwhile wouId require a Iot more investigation. Her result doés, however, show neatIy how even apparentIy arbitrary decisions cán have unintended conséquences.
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