Near wake characteristics of a model horizontal axis marine current turbine under steady and unsteady inflow conditions

Near wake measurements on a 0.8 m diameter two-bladed horizontal axis tidal turbine model were obtained with two Nortek Vectrino+ Acoustic Doppler Velocimeters (ADV) sampling simultaneously at 200 Hz. The test matrix covered four radial positions from r/D=0.3 to 0.5 and five axial positions from d/D=0.9 to 0.95. All measurements were performed at the nominal tip speed ratio (TSR) of 7.4. Two different cases were investigated. The first case had steady inflow conditions, i.e. constant carriage speed and the second case had a constant carriage speed and incoming regular waves with a period of 1.6 seconds and 0.09 m wave height. The distribution of mean velocities for the steady inflow case exhibit significant spatial variability in the wake region. Mean streamwise velocity show a decrease in the axial direction for all radial locations with magnitudes ranging from 0.55 at r/D=0.49 to 0.35 at r/D=0.3. Vertical and lateral mean velocities are small but consistent with counterclockwise fluid angular momentum for a clockwise rotor rotation. The Reynolds shear stresses consistently show elevated levels for measurements near the rotor tip (r/D=0.49) and are significantly reduced by x/D=0.6 downstream. This suggests low turbulence levels in the wake which is consistent with very low inflow turbulence verified by the upstream ADV. For the case with waves, evidence of enhanced turbulence intensities and shear stresses within spatial coverage of the experiment suggest increased localized turbulence production in the blade tip region over the entire near wake region.