Assessment method of sound radiated by cyclically operating Wells turbines

The oscillating water column (OWC) is a system for sea wave energy conversion. This paper deals with the axial Wells turbine. Main objectives of this study are the experimental determination of aerodynamic and aero-acoustic characteristics of a laboratory scale turbine and their subsequent usage for full scale turbine performance prediction.For that a model Wells turbine was built and tested on a laboratory test rig at the University of Siegen. Assuming that the steady-state characteristics hold in unsteady operation, mean quantities such as a mean efficiency and an equivalent sound power level are defined that allow the quantification of the overall performance during cyclic operation. The steady-state quantities are used to assess various control strategies and to predict the aero-acoustic performance of the full scale LIMPET turbine on Islay, Scotland.In general, scaled-up acoustic laboratory test rig data correlates well with measurements from the full scale LIMPET turbine. A theoretical assessment of different control strategies under regular wave conditions demonstrates the potential of a variable speed control for reduction of equivalent sound power. Turbine operation which admits stall yields a typical signature in the equivalent sound power spectrum: a stalled Wells turbine emits low frequency noise at a level far above all levels encountered in the unstalled flow regime and hence significantly contributes to the equivalent sound power.