Abstract
Laminar to turbulent transition in vertical-axis wind turbines has a dramatic effect on overall rotor performance, especially in fast rotating machines, where reliable prediction of the total drag coefficient for high values of tip speed ratio is one of the most critical topic in CFD simulations.
This paper presents a 2D numerical investigation of the capability of the γ–θ transition model to predict the laminar to turbulent transition and consequent friction drag over a NACA 0012 airfoil for a Reynolds number of 360,000, which is typical of vertical-axis wind turbine blades during operation. The analized range of angles of attack varies from 0 deg to 10 deg. The commercial CFD solver ANSYS FLUENT® is used.
In particular, the sensitivity to grid resolution is investigated for four different architectures: a completely unstructured mesh, a hybrid structured-triangular one and two distinct hybrid structured-triangular meshes where the wake region behind the airfoil is discretized using a fully structured grid. The effect of freestream turbulence intensity on the transition onset is also analyzed.
Finally, CFD results are compared to experimental data, although affected by some uncertainty, and to the predictions of an interactive program for the design and analysis of subsonic isolated airfoils (XFOIL), showing a very good agreement provided that the value of freestream turbulence intensity is known.
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