大友 衆示

With an increasing demand for small energy generation in urban areas, small-scale Savonius wind turbines are growing their share rapidly. In such an environment, Savonius turbines are exposed to low mean velocity with highly turbulent flows made by complex geographies. Here, we report the flow-induced rotation of a Savonius turbine in a highly turbulent flow (18% turbulence intensity). The high turbulence is realized by using the far-field of an open-jet. Compared to low turbulence inflow (1% turbulence intensity), the turbine rotates 4% faster in high turbulence since the torque/power increases with turbulence intensity. The wake measurement by hot-wire anemometry and particle image velocimetry reveals the suppression of vortex shedding in high turbulence. In addition, a newly developed semi-empirical low-order model, which can include the effect of turbulence intensity and integral length scale, also confirms high turbulence intensity contributes to the rotation of the turbine. These results will boost more installation of small Savonius turbines in urban areas in the future.

Tidal currents are renewable and predictable energy sources that could prove fundamental to the transition to a sustainable use of renewable energy resources. Over a tidal period, changes in the flow speed in a tidal channel require that the blade pitch is adjusted to maximise power extraction. This is currently achieved with active pitch actuation, which however increases the capital and maintenance cost of the turbine. Furthermore, because of turbulence in the tidal stream, turbine yaw, wave-induced currents, etc., tidal turbine blades experience high-frequency velocity fluctuations that result in power and thrust unsteadiness, both of which are transmitted to the generator, the tower, and the active pitching mechanism, shortening the operating life due to fatigue loading. A passive morphing blade concept capable of reducing the load fluctuations without affecting the mean loads has recently been formulated and demonstrated with low-order simulations (https://doi.org/10.1016/j.renene.2021.10.085) and measurements (https://doi.org/10.1016/j.renene.2023.01.051). The system allows both passive pitch adjustment to changes in the mean flow speed over the tidal period, and the mitigation of high-frequency fluctuations. In this paper, we present the recent progress on the development of morphing blade technology, including with numerical simulations and experimental tests on a 1.2-m diameter turbine. Two different design concepts have been tested in the FloWave facility at the University of Edinburgh and in the recirculating channel at the Institute for Marine Engineering of the Italian National Research Council, respectively. Experimental results show that the amplitude of power variations over a wide range of flow speeds is substantially decreased, while thrust variations with changes in freestream speed are essentially suppressed. The detrimental effect of yawed inflow is, in addition, almost entirely cancelled. The fluctuations in the root-bending moment, thrust and torque are consistently reduced over a broad range of tip-speed ratios. We also show that such a system, if improperly designed, could result in a negative starting torque, and we show the steps necessary to avoid this issue. Furthermore, we present a theoretical and numerical framework that allows the design of passive pitch blades that can cancel either thrust or power fluctuations in specific flow conditions, as well as mitigating both types of fluctuations over a wide range of conditions. Specifically, we show that for any quasi-steady change in the relative flow speed and direction, there is a pitching axis that allows a chosen force component to be kept constant. High-frequency force fluctuations can also be substantially mitigated, and the extent of the mitigation depends on the inertia and friction in the system. Overall this paper demonstrates experimentally the effectiveness of morphing blades for tidal turbines and presents a theoretical and numerical framework for the future development of this technology.

The ability to accurately predict the forces on an aerofoil in real-time when large flow variations occur is important for a wide range of applications such as, for example, for improving the manoeuvrability and control of small aerial and underwater vehicles. Closed-form analytical formulations are only available for small flow fluctuations, which limits their applicability to gentle manoeuvres. Here we investigate large-amplitude, asymmetric pitching motions of a NACA 0018 aerofoil at a Reynolds number of 3.2 × 10 4 using time-resolved force and velocity field measurements. We adapt the linear theory of Theodorsen and unsteady thin-aerofoil theory to accurately predict the lift on the aerofoil even when the flow is massively separated and the kinematics is non-sinusoidal. The accuracy of the models is remarkably good, including when large leading-edge vortices are present, but decreases when the leading and trailing edge vortices have a strong interaction. In such scenarios, however, discrepancies between the theoretically predicted and the measured lift are shown to be due to vortex lift that is calculated using the impulse theory. Based on these results, we propose a new limiting criterion for Theodorsen’s theory for a pitching aerofoil: when a coherent trailing-edge vortex is formed and it advects at a significantly slower streamwise velocity than the freestream velocity. This result is important because it extends significantly the conditions where the forces can be confidently predicted with Theodorsen’s formulation, and paves the way to the development of low-order models for high-amplitude manoeuvres characterised by massive separation. Graphic abstract: [Figure not available: see fulltext.].

© 2019 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. An unsteady lifting-line theory for time-domain problems with arbitrary kinematics is presented. This is formulated by matching a vortex particle based 2D inner model with a 3D vortex lattice wake. This and a small-amplitude frequency-domain unsteady lifting-line theory are then verified against experiment and computational fluid dynamics for the case of a flat rectangular plate oscillating in heave at aspect ratios 3 and 6. Both lifting-line theories were found to generally be in good agreement with experimental and CFD results, providing reasonable solutions even in cases dominated by LEV shedding. For larger amplitude problems, the new non-linear geometry lifting-line theory predicted larger amplitudes and higher average lift coefficient than the small-amplitude lifting-line theory.