Statistical characteristics of natural frequency and damping ratio of operating wind turbines: Experimental investigation through wind tunnel tests

Monopile-supported wind turbines (MWTs) are continuously subjected to wind loads during operation. These loads are transmitted through the tower to the foundation, where the supporting behavior of the pile-soil system exhibits pronounced nonlinearity. As the load level varies, the stiffness and damping of the pile-soil system fluctuate accordingly, leading to noticeable variations in the natural frequency and damping ratio of the wind turbine structure. Such fluctuations introduce considerable uncertainty into the dynamic performance of MWTs, potentially affecting their operational safety, serviceability, and structural reliability. To accurately reveal the influence of pile-soil interaction on the fluctuation characteristics of MWT dynamic behavior, this study derives joint scaling laws considering wind-structure-soil interaction (WSSI) and develops scaled MWT models accordingly. A series of wind tunnel tests are conducted under operating conditions with different monopile slenderness ratios to capture the variations in natural frequency and damping ratio. The statistical characteristics of these parameters are systematically analyzed, and a concise probabilistic distribution model is proposed to describe their distribution patterns, with particular emphasis on examining the influence of monopile slenderness ratio on their probabilistic behavior. This study provides new insights into the fluctuation behavior of wind turbine dynamic performance and offers a scientific basis for the reliability assessment and dynamic design of monopile-supported wind turbines.