Three-dimensional computational fluid dynamics (3D-CFD) was performed to simulate the flow field around an aluminum circular-blade butterfly wind turbine, which is a vertical-axis-type turbine with four circular blades and a diameter of 2.06 m. Under the assumption of a loss factor of 0.8 due to a generator and an AC–DC converter, the CFD results agreed with the experimental results. Although tip vortices were observed at the top and bottom portions of the blades, the vorticity intensity was weaker than that of the straight-blade rotor case. In addition, the cross-sectional shape of the tip vortices seemed to be elliptical for circular blades rather than circular as for straight blades. As the tip speed ratio was less than 2, vortices arising from dynamic stall at maximum-radius portions of blades were observed at the downwind half-cycle as well as the upwind half-cycle. A feature of the vortices shed from a circular blade at the downwind half-cycle was the looped shape.

To reduce costs involved in manufacturing small wind turbines, an aluminum circular-blade butterfly wind turbine (ACBBWT) has been developed, in which four blades of the turbine were extruded and bent to shape then attached directly to a rotating flange. The ACBBWT is a vertical axis wind turbine (VAWT) and the rotor diameter of the prototype is 2.06 m. Experiments to obtain the output performance were conducted outdoors using an axial blower; however, the data obtained were rather scattered due to the effects of natural wind. Therefore, performance curves in the high wind speed range are predicted by fitting theoretical curves based on the Blade Element Momentum (BEM) theory, in which modification of virtual incidence due to flow curvature effects is included. Three-dimensional computational fluid dynamics (CFD) analysis of a circular-blade wind turbine model (dia. 2 m) with a shape almost identical to that of the experimental rotor is performed. The results assuming an energy-conversion efficiency of 0.8 agree well with the experimental results at 7 m/s. CFD analysis shows that tip vortices are shed from the top and bottom parts of a circular blade, as with straight-blade VAWTs. However, vorticity in the circular-blade case is lower than that in the straight-blade case, and the cross-section of each tip vortex shed from circular blades appears to be in the shape of a deformed ellipse. In cases of small tip speed ratios, vortex shedding caused by the dynamic stall phenomena is observed around the equator plane in both the downstream and upstream regions, and the vortex shed in the downstream region by a circular blade forms a looped shape. Since distributions of surface pressure and skin friction obtained by 3D-CFD have a similar pattern in both the upstream and downstream regions, which is related to vortex shedding, it is considered that the vortex in the upstream region is likely to also have a looped shape.

Beach changes in Kugami area of the Iwami coast in Tottori Prefecture were investigated through the comparison of bottom sounding data and field observation. This coast is a pocket beach of 2.2km length. On the west end of the beach, a breakwater of the Higashi fishing port was extended, resulting in the formation of wave calm zone behind the breakwater. Due to the wave sheltering effect of the breakwater, westward longshore sand transport was induced over the natural headland with a point depth of 3.5m in the middle of the pocket beach. Erosion and accretion were quantitatively analyzed.

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