Abstract
After almost two years of testing its 1 MW-rated ducted twin vertical axis tidal turbine (2-VATT), HydroQuest wants to validate its design tools to predict the full-scale behaviour of its future turbines. To do so, a 1/20th scale model of the demonstrator was designed and tested in the Ifremer wave and current flume tank. Whether the flow comes from one side or the other of the turbine, at flood or ebb tide, the two counter-rotative rotors relative direction of rotation is reversed and the base bulk upstream differs. The experimental database is analysed to study the effect of the two reverse flow directions on the ducted 2-VATT behaviour. The results show that average drag and power coefficient are similar in the two configurations. However, the optimal tip speed ratio is 7 % lower and the power fluctuations are 1.5 times higher in the ebb tide configuration. The wakes interaction of the two VATT columns is also significantly different between the two cases as the wakes merge shortly downstream at flood tide while they remain distinct much further at ebb.
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