Aerodynamic shape optimization of bluff body for galloping-based wind energy harvester by the class function/shape function transformation technique

This paper presents an aerodynamic modification scheme by using the class function/shape function transformation (CST) method to determine the optimal shape of the bluff body and hence improve the performance of the associated galloping-based wind energy harvester (WEH). The CST method, accompanying with the Latin Hypercube sampling method, is used to generate 50 representative samples of galloping bluff bodies with varying sectional geometries, by using only four parameters to describe each sectional geometry. Wind tunnel tests are conducted to measure the output power of each WEH and the transverse wind force acting on the associated bluff body. A Kriging surrogate model is constructed based on the results from wind tunnel test, to determine the optimal shape in terms of the output power. Results show that the optimal shape has the tapered leeward side and concave windward side, which can double the output power compared to a square shape. The underlying mechanism for the high performance is explained by analyzing the measured transverse force in the wind tunnel and the vorticity contours of the flow field generated by CFD analysis. On the one hand, the tapered leeward side avoids flow attachment on the two lateral sides of the shape, providing considerable transverse force that sustains the unstable steady-state galloping. On the other hand, the concave windward side promotes the development of large circulation bubbles, causing significant asymmetry of wind flow and hence remarkable galloping. The findings of the research can guide the design for the bluff body of the galloping-based WEHs, and the CST method can be used as a powerful tool for aerodynamic shape optimization.