From the viewpoint of installing small wind turbines (SWTs) on rooftops, this study investigated the effects of wind direction and horizontal aspect ratio (HAR = width/length) of a high-rise cuboid building on wind conditions above the roof by conducting large eddy simulations (LESs). The LES results confirmed that as HAR decreases (i.e., as the building width decreases), the variation in wind velocity over the roof tends to decrease. This tendency is more prominent as the angle between the wind direction and the normal vector of the building’s leeward face with longer roof edge increases. Moreover, at windward corners of the roof, wind conditions are generally favorable at relatively low heights. In contrast, at the midpoint of the roof's windward edge, wind conditions are generally not favorable at relatively low heights. At leeward representative locations of the roof, the bottoms of the height range of favorable wind conditions are typically higher than those at the windward representative locations, but the favorable wind conditions are much better at the leeward representative locations. When there is no prevailing wind direction, the center of the roof is more favorable for installing SWTs than the corners or the edge midpoints of the roof.

We have investigated the effects of the rotating blades of an upwind-type three-blade horizontal-axis wind turbine (HAWT) on the basic characteristics of aerodynamic forces acting on its tower by conducting improved delayed detached-eddy simulations (DESs). Three tip-speed ratios were considered for the operating conditions of the HAWT: λ = 3 (low), λ = 6 (optimum), and λ = 10 (high). The diversion of the flow approaching the tower by the rotating blades and the low-pressure region that formed downwind of the blades significantly affected the aerodynamic forces acting on the tower. For example, the azimuth angle around the tower at which the pressure reached a maximum at each height shifted significantly in the direction of the movement of the blade passing the tower because of the diversion of the flow by the blades. Fluctuations in the lift force of the tower were significantly larger than those in its drag force because of the low-pressure region downwind of the blades.

To properly conduct a micro-siting of an orthopter-type vertical axis wind turbine (O-VAWT) in the built environment, this study investigated the effects of horizontal shear flow on the power performance characteristics of an O-VAWT by performing wind tunnel experiments and computational fluid dynamics (CFD) simulations. A uniform flow and two types of shear flow (advancing side faster shear flow (ASF-SF) and retreating side faster shear flow (RSF-SF)) were employed as the approaching flow to the O-VAWT. The ASF-SF had a higher velocity on the advancing side of the rotor. The RSF-SF had a higher velocity on the retreating side of the rotor. For each type of shear flow, three shear strengths (Γ = 0.28, 0.40 and 0.51) were set. In the ASF-SF cases, the power coefficients (C_p) were significantly higher than the uniform flow case at all tip speed ratios (λ) and increased with Γ. In the RSF-SF cases, C_P increased with Γ. However, when Γ = 0.28, the C_P was lower than the uniform flow case at all λ. When Γ = 0.51, the C_P was higher than the uniform flow case except at low λ; however, it was lower than the ASF-SF case with Γ = 0.28. The causes of the features of C_P were discussed through the analysis of the variation of blade torque coefficient, its rotor-revolution component and its blade-rotation component with azimuthal angle by using the CFD results for flow fields (i.e., horizontal velocity vectors, pressure and vorticity). These results indicate that a location where ASF-SFs with high Γ values dominantly occur is ideal for installing the O-VAWT.

The effects of splitter plate as a passive turbulence control on the transverse vibration characteristics of a cantilevered rectangular prism with the different finite length (aspect ratio L/H) and depth ratios (side ratio D/H of 0.2 and 0.5) was investigated in a water tunnel. A splitter plate was installed behind the prism with a small gap and sufficient length to prevent vortex shedding intrusion. It enhances the response amplitude ascended linearly with respect to non-dimensional velocity. In the case of the prisms with critical points, interferences of amplitude signal spectra for the period of displacement vanished, yielding a uniform waveform on the response amplitude. Inserting a splitter plate improves the increment rate of the dynamic response of the test models. A brief numerical calculation was performed to find the quantities of flow features around a plain and rectangular prism with a detached splitter plate. The alteration of flow wake characteristics is found by diminishing unsymmetrical vortices shedding and reduce flow entrainment at the trailing edge. The essential findings of the prism with a detached splitter plate are the base pressure improvement, reducing drag force, and vanishing sharp peaks of wake FFT spectra.

Three-dimensional flow features of the slender rectangular prism with cross-section height (H) to streamwise depth (D) ratio or side ratio (D/H = 0.5) were investigated numerically using the Large-eddy simulations (LES) turbulence model with Reynolds number Re = 22000. Four different aspect ratios (L/H = 2.5 – 10) were employed in this research to study the effect of the spanwise variation of the prism model on the flow pattern around the prism. Moreover, the instability-induced motion of the prism was modeled to predict the alteration of flow characteristics of stationary to vibrating states of the test model. The global quantities such as drag force, pressure coefficient, and Strouhal frequency characteristics are presented, which suggests that the structure end tip effect plays an essential role in the dependency of flow features variation. The velocity vector variations at streamwise and spanwise positions are also demonstrated. The prism model with a small aspect ratio (L/H = 2.5) exhibited Karman vortex suppression at the prism's vicinity, and the streamwise vortices region shrank. The flow features of the vibrating prism show different behaviors from the stationary prism model

To develop the power generation system using the magnetostrictive material and the vibration in the flowing water, we focus on transverse-galloping vibration for a rectangular prism with a critical depth section of less than 0.6. Free-vibration tests of two cantilevered rectangular prisms with side ratios of 0.5 and 0.2 in a side-by-side arrangement were performed in a water tunnel to investigate the effects of the gap between two prisms on the flow-induced transverse-galloping vibration and the performance of vibration power generator using an iron-gallium alloy that is a kind of magnetostrictive material. Rectangular prisms with a side ratio of 0.2 vibrate only the characteristic frequency of a prism, and the gap ratio between two prisms has little influence on the response characteristics of vibration. The response characteristics of vibration and the frequency of the rectangular prisms with a side ratio of 0.5 are dependent on gap ratio. The maximum electric power using the rectangular prisms with a side ratio of 0.2 arranged with gap ratio of 0.5 was 10.8 mW at a reduced velocity V_r=2.6.