Assessment of Reynolds Averaged Navier-Stokes method for modeling the start-up regime of a Darrieus rotor

RAMIREZ, JOSÉ MIGUEL; SARAVIA, CÉSAR MARTÍN

PHYSICS OF FLUIDS

AMER INST PHYSICS

Año: 2021

1070-6631

Abstract In this paper, we have investigated the suitability of Reynolds Averaged Navier-Stokes (RANS) method for predicting the dynamic response in the start-up process of small scale Darrieus vertical axis wind turbines (VAWT). The aerodynamic forces predictions are strongly affected by dynamic stall during the start-up regime. By utilizing OpenFOAM, we have executed numerical simulations to make an assessment of the regions of the rotor revolution where the prediction of the aerodynamic coefficients is expected to give unreliable results. We have evaluated qualitative aspects of the prediction of the self-starting of the VAWT for tip speed ratio (TSR) from 0 to 3. For modeling a more realistic condition, we have implemented resistive forces acting on the rotor. These resistive forces are the mechanical friction and the generator magnetic forces. These forces were implemented numerically through a viscous damping model and therefore, a new OpenFOAM library was developed. For implementing this new library, the Object-Oriented Programming methodology and C++ language were adopted. From the results, we have revealed that for a TSR equal to 0.4 (start-up regime), the aerodynamics forces are overestimated for a portion of 82% of the complete rotor revolution. For a TSR equal to 2.70 (stationary condition), this portion is decreased to 52%. This means that the aerodynamic forces and also the power output are overestimated for a large percentage in a complete rotor revolution. Results allow us to conjecture that RANS modeling may not seem to be an adequate approach to accurately assess the aerodynamic response of VAWTs.