A lumped parameter model to explain the cause of the hysteresis in OWC-Wells turbine systems for wave energy conversion

A Wells turbine is an axial-flow turbine consisting of a rotor usually with symmetric (uncambered) blades staggered at a 90 degree angle relative to the machine's axis. This turbine is used within oscillating water column systems: during its normal operation, the blades experience a continuous change in incidence angle, that according to many authors is at the origin of a hysteresis in its force coefficients. Aerodynamic hysteresis in rapidly moving airfoils is a well-known phenomenon, but happens only at non-dimensional frequencies significantly larger than the ones encountered in Wells turbines. This work presents a re-examination of the two phenomena, that shows the unlikeliness of the presence of any aerodynamic hysteresis in Wells turbines. A simple yet effective lumped parameter analysis is used to prove how the real cause of the hysteresis is to be found in a different phenomenon. Results are compared to experiments and CFD analyses for the same problem, with an excellent agreement.