Darrieus vertical-axis water turbines: deformation and force measurements on bioinspired highly flexible blade profiles

Abstract

The characteristics of the fluid-structure interaction on the flexible blades of a horizontal-axis water turbine are studied; this bioinspired technology features mechanical simplicity, performance and lifetime improvement, and low fish impact risk, all characteristics of a truly sustainable renewable energy exploitation. A surface-tracking method synchronized with force measurements was applied on a surrogate model of single-bladed, vertical-axis water turbine in a water channel. This allows for the characterization of the structural deformations and their link to the hydrodynamic forces, over a large range of turbine designs and operating points. It is shown that the phase angles of the maxima in blade deformation coincide with those of the load maxima on a rigid blade in identical flow conditions. The influence of the turbine's tip/speed ratio on the blade deformation and blade load is investigated. With the chosen blade design, hydrofoil deformation is found to be maximum in the operating points where the performance improvement is maximized (k o = 0.3). With lower values of reduced frequency (corresponding to lower rotation speed), fluid-induced forces dominate the fluid-structure interaction. Conversely, at higher frequencies, structural inertia dominates the interaction, and blade deformation is again reduced. Results suggest that the optimal blade rigidity may depend on the operating point; nevertheless the potential of the flexible-blade design is demonstrated through clear linking of fluid-induced forces and blade deformation in the complex flow conditions of the Darrieus water turbine.

Cross-Flow Tidal Turbines with Highly Flexible Blades—Experimental Flow Field Investigations at Strong Fluid–Structure Interactions

“…A chordwise flapping effect can be observed in the upwards motion from −12°to −6°, which is in accordance to the structural deformation measurements in a previous study [25,32]. This profile flapping is of stronger amplitude and can be linked to the shedding of the leading-edge vortex after convection of the profile, leading to significant deformations of the structure.…”

“…The deformations of the highly flexible rotor blades, which result from the flow adaptation, amount up to 20% of the chord length and provide passive control of the flow. Potential efficiency increases of 20% along with structural load reduction of 25% could be achieved, as shown experimentally by Hoerner et al 2020 [25] on a reduced turbine model consisting of a pitching hydrofoil.…”