A coupled hydro-structural design optimization for hydrokinetic turbines

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Abstract
An optimization methodology for a stall regulated, fixed pitch, horizontal axis hydrokinetic turbine is presented using a combination of a coupled hydro-structural analysis and Genetic Algorithm (GA) based optimization method. Design and analysis is presented for two different designs: a constant chord, zero twist blade, and a variable chord, twisted blade. A hybrid approach is presented combining Blade Element Momentum (BEM), GA, Computational Fluid Dynamics (CFD), and Finite Element Analysis (FEA) techniques. The preliminary analysis is performed using BEM method to find the hydrodynamic performance and flap-wise bending stresses in turbine blades. The BEM analysis used for the current study incorporates effect of wake rotation, hub loss and tip loss factors, and effect of Reynolds number on hydrodynamic data. A multi-objective optimization is then performed to maximize performance and structural strength of turbine. The results of optimization for a constant chord, zero twist blade design are validated with detailed three dimensional CFD and finite element analysis. The fluid domain is coupled with the structural domain through one way coupling and fluid-structure interaction analysis is carried out to find the effect of blade geometry and operating conditions on the stresses developed in the blades. The hydrodynamic performance of a constant chord turbine was found to be limited by the high stresses developed in turbine blades. Hence, in an effort to reduce the stresses in turbine blades, a variable chord, twisted blade design was developed; and a multi-objective optimization is presented for the variable chord twisted blade turbine for hydro-structural performance improvement. The final-optimized variable chord, twisted blade design was found to improve the power coefficient by 17% and resulted in lower overall stresses.

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