DIRECT NUMERICAL SIMULATION OF NONLINEAR SECONDARY INSTABILITIES ON THE PRESSURE SIDE OF A SAVONIUS STYLE WIND TURBINE
Résumé
Savonius-style wind turbines are a class of vertical axis wind turbine usually used for off-grid applications. It appears to be promising for energy conversion because of its better self-starting capability and flexible design promises. The blades are characterized by relatively large surface, which are thin circular shape to produce large drag, which is used for power generation. Typically, the suction side of the advancing blade is submitted to strong adverse pressure gradient, causing a well known vortex shedding process, which is responsible for the wake flow. This topic has been the subject of many researches in the past decades, as it obviously depends on tip speed ratio (TSR) and directly influences the turbine efficiency. The flow on the pressure side of the blade is generally considered as fully attached and is characterized by high pressure, low velocity level that produces most of the drag used in the energy conversion. However, because of the gap between the two blades, the flow is accelerated on the pressure side of the returning blade and a thicker boundary layer develops at this side. Because of the concave curvature of the blade and the small scale of the turbine, centrifugal instabilities may occurs, depending on the flow regime that can cause natural transition on the blade. Moreover, these vortices induce different mechanisms of ejections and sweeps, causing thereby strong transverse variations of the drag coefficient, which results in the formation of hot spots near solid walls. This can leads to a rapid degradation of mechanical structures and materials fatigue. In this paper, Direct Numerical Simulations (DNS) are carried out in order to capture the flow instabilities and transition to turbulence occurring on the pressure of a conventional design Savonius wind turbine blade. Simulations are conducted with the open source code NEK5000, solving the incompressible Navier-Stokes equations with a high order, spectral element method. Because of the relatively high Reynolds numbers considered (Re-xi = 90,000), the computational domain of the Savonius blade is reduced to the pressure side, whereas no turbine rotation is considered, which avoid the large scale vortex shedding that occurs on the suction side. The results suggest that Gortler vortices can occurs and cause the flow to transit to turbulence, which modify the pressure distribution and the drag force significantly.