A Wind Tunnel Study on Wind Turbine Hub Height Effect

The rapid increase in wind turbine rotor diameters has been accompanied by a systematic reduction in hub-height-to-rotor-diameter ratios ( h/D ), particularly for large offshore and floating turbines. Motivated by a survey of more than 100 commercial and conceptual designs, this study examines the influence of hub height on wind turbine wake development and the validity of Gaussian wake similarity assumptions. High-resolution stereoscopic particle image velocimetry measurements were conducted in a neutral atmospheric boundary layer wind tunnel for three hub-height ratios ( h/D = 0.55, 0.70, and 1.00). The results demonstrate that proximity to the ground fundamentally alters wake recovery and expansion. Lower hub heights exhibit sharper vertical velocity deficit gradients, reduced lower entrainment, and suppressed vertical wake growth due to ground-induced friction and confinement. Despite higher ambient turbulence intensity at lower elevations, ground proximity acts as a dominant constraint on vertical wake expansion below hub height. Wake similarity analysis further shows that increasing hub height delays the onset of a fully developed Gaussian regime, reflecting the height dependence of atmospheric turbulence and near-wake length. A complementary theoretical analysis reveals that conventional Gaussian wake models neglect ground effect when deriving momentum conservation. Based on these findings, refinements to analytical wake models are proposed.