Interaction between Instabilities in Vertical Axis Turbine Blades

Abstract

This work investigates five different instability mechanisms which may occur in straight-bladed H-rotor vertical axis turbines. Specifically, it considers the static instabilities of divergence and centrifugal buckling as well as the dynamic instabilities of flutter, main resonance, and parametric resonance. This is done by applying Theodorsen’s model for unsteady aerodynamics with traditional finite element methods. The resulting model is comprised of bend-twist-coupled elements which inherently account for fluid and centrifugal effects. A validation study is conducted to assess the accuracy and limitations of the model using experimental data from the existing literature. Eigenvalue analysis is used on time-averaged equations to predict divergence, flutter, and centrifugal buckling, as well as identify regions where main and parametric resonance could be excited. Time history analysis is then used to confirm these predictions and show main and parametric resonance explicitly. Results show good agreement in the prediction of centrifugal buckling and main and parametric resonances excited with respect to twisting. Neither type of resonance is observed with respect to bending. Flutter is predicted to occur via eigenvalue analysis but is not observed via time history analysis due to the limited applicability of Theodorsen's model in the time-domain. A discrepancy with the divergence boundary is also observed due to an unexpected region of "parametric stabilization" which is found to extend into regions predominated by static instability.