Design and Optimization of a Multi-Element Hydrofoil for a Horizontal-Axis Hydrokinetic Turbine

Jonathan Aguilar,Ainhoa Rubio-Clemente,Laura Velasquez,Edwin Chica

doi:10.3390/en12244679

Jonathan Aguilar, Ainhoa Rubio-Clemente + Show 2 more

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https://doi.org/10.3390/en12244679

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Abstract

Hydrokinetic turbines are devices that harness the power from moving water of rivers, canals, and artificial currents without the construction of a dam. The design optimization of the rotor is the most important stage to maximize the power production. The rotor is designed to convert the kinetic energy of the water current into mechanical rotation energy, which is subsequently converted into electrical energy by an electric generator. The rotor blades are critical components that have a large impact on the performance of the turbine. These elements are designed from traditional hydrodynamic profiles (hydrofoils), to directly interact with the water current. Operational effectiveness of the hydrokinetic turbines depends on their performance, which is measured by using the ratio between the lift coefficient (CL) and the drag coefficient (CD) of the selected hydrofoil. High lift forces at low flow rates are required in the design of the blades; therefore, the use of multi-element hydrofoils is commonly regarded as an adequate solution to achieve this goal. In this study, 2D CFD simulations and multi-objective optimization methodology based on surrogate modelling were conducted to design an appropriate multi-element hydrofoil to be used in a horizontal-axis hydrokinetic turbine. The Eppler 420 hydrofoil was utilized for the design of the multi-element hydrofoil composed of a main element and a flap. The multi-element design selected as the optimal one had a gap of 2.825% of the chord length (C1), an overlap of 8.52 %C1, a flap deflection angle (δ) of 19.765°, a flap chord length (C2) of 42.471 %C1, and an angle of attack (α) of –4°.

Highlights

For applications requiring high lift forces at large angles of attack and low flow velocities, the traditional hydrofoils tend to produce separation of the flow near the trailing edge, causing a decrease in the hydrofoil performance
The results concerning the initial sampling, the design suggested by the surrogate model, the starting design (Eppler 420 hydrofoil), and the selected multi-element design based on the CL /CD ratio are depicted
The designs supplied by the surrogate model contributed to the Pareto front with new designs that fill the gaps in the Pareto front of the initial sampling plan and move it forward

Summary

Introduction

For applications requiring high lift forces at large angles of attack and low flow velocities, the traditional hydrofoils tend to produce separation of the flow near the trailing edge, causing a decrease in the hydrofoil performance. Multi-element hydrofoils constitute a proper alternative since the lift force is increased, leading to a rise in the camber when operating at a high angle of attack and a delay of the flow separation near the trailing edge. This delay in the flow separation on the deflected flap element is achieved by introducing a slot ahead of the flap for boundary layer control [1]. In the automotive industry, cars competing in Formula One races use multi-element foils to increase the down-force produced by the rear wing [3]

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Conclusion