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A wind tunnel experiment was performed to investigate the suitable conditions in which a localized turbulent region can easily be generated in a flat-plate boundary layer. An artificial disturbance of zero-mass flux was introduced upstream using a combination of a short-duration jet and suctions to prepare a potentially unstable environment. The disturbed region by itself decayed downstream. Another jet was then ejected downstream at several different timings and two different spanwise locations relative to the passage of the locally disturbed region to promote transition to turbulence. Although the jet was too weak to trigger the turbulence transition by itself, an isolated turbulent region, the so-called turbulent spot, was generated when ejected against the disturbed region. The optimum conditions were found when the jet was ejected between the high- and low-speed areas of the convecting unstable region.

The streamwise interface of an isolated turbulent region in a laminar flat plate boundary layer is investigated by performing a wind tunnel experiment. In the experiment, bimorph-type piezoceramic actuators are used to generate a trapezoidal-shaped turbulent region that has a wide interface in the spanwise direction. With the use of the piezoceramic actuators, the interference of the spanwise interface with the streamwise interface of the turbulent region is eliminated. Therefore, the streamwise interface can be considered as an independent interface. The experimental results show that the rms value of the velocity fluctuation and the turbulent fluctuation rapidly changes at the leading edge of the turbulent region. On the other hand, the rms value of the velocity fluctuation remains high after the turbulent fluctuation disappears at the trailing edge of the turbulent region. It is also shown that the traveling speed of the trailing edge of the turbulent region is faster than the ensemble-averaged streamwise velocity at the trailing edge of the turbulent region near the wall (η≤1.3). In addition, the velocity profile at the trailing edge of the turbulent region indicates the existence of the acceleration of the turbulent fluid near the wall. From the above results, it is found that the inverse transition occurs at the trailing edge of the turbulent region near the wall due to the acceleration of the turbulent fluid.

This paper describes the analysis of flows around a straight wing vertical axis wind turbine. Hotwire measurements in two-dimensional wind tunnel and numerical simulations are conducted. The rotor has two blades, whose airfoil is NACA0012 and the chord length is 0.1 of the rotor diameter. Reynolds number based on the uniform velocity and rotor diameter is 8.0 × 104. The tip speed ratio is 2.0. Incompressible Navier-Stokes equation is used for numerical simulation. A rotating coordinate system, which rotates at the same speed of the rotor, is employed. Both the experimental and numerical results are compared. As a result, the velocity fields show good agreements at each rotation angle, and the aerodynamic torque varies according to the flow condition, especially the flow separation.

The interface characteristics in a millimeter-scale channel are investigated in order to find a technique to speed up fluid mixing. The channel is fabricated on an acrylic-resin plate, where a cavity is attached in the downstream region of a T-shaped conduit. The mixing effect is evaluated using a blue dye and a colorless liquid, which are alternately injected into the channel by two syringe pumps. The important factors for highly efficient mixing under the combination of alternate inflow and cavity are investigated. Two-dimensional and three-dimensional numerical simulations are also performed, and the results are compared those of the experiment. It is confirmed that the fractal dimension and interface length inside a cavity can be used as indicators to predict the mixing level in the downstream region, although they are not perfect. The importance of focusing on both the stretching rate of the interface and the concentration gradient where the stretching of the interface occurs is presented.

This paper describes the effect of a thin plate as a static controlling device to suppress cavity noise when it is inserted vertically into a cavity. Experiments using small and large wind tunnels are carried out to invistigate the size effect of the cavity model. From the experimental results, it is shown that there are two optimum plate locations for the noise reduction. One is near the upstream edge and the other is just in front of the downstream edge. By comparing the experiments at small and large wind tunnels, it is found that in the large wind tunnel, noise reduction is not so effective as in the small one.

A passive control of aerodynamic noise generated by a laminar separating flow over a cavity is carried out in a wind tunnel test and by numerical simulation, using a thin plate inserted into the cavity as a passive flow-controlling device. In the experiment, a noise suppression effect up to 14 dB is achieved by placing a plate in a proper location. It is shown that a plate of smaller vertical size is more effective. In the numerical simulation, it is shown that the location where the shear layer rolls up and the path of vortices are both very sensitive to plate position.

Breakdown process of the Klebanoff mode and origin of turbulence are experimentally investigated in detail. An acoustic excitation using a 2D roughness attached on the wall are used for T S wave generation. Piezo-ceramic actuators are used to introduce the Klebanoff mode fluctuation. It is shown that the turbulence region starts from two locations, one is beneath the head of a A vortex and the other is the high-shear layer accompanying by the A vortex.

Interaction between a vortex ring and shear layer vortices was simulated by the three-dimensional vortex blob method. The purpose of this research is clarifying interaction between a vortex ring and shear layer vortices, and is also the basic research of active control of the shear layer by introducing the self-propelling vortex ring from outside of the shear layer. Interaction of the vortex ring is effective in enhancing the local growth of the shear layer. Numerical simulation was performed in order to clarify the circulation ratio between the vortex ring and the shear layer vortex tube which promote the interaction between a vortex ring and shear layer vortices most. In the case of circulation ratio ΓR/ΓF of 1.0, shear layer vortices and the vortex ring interact strongly. Large vortices were generated by amalgamation between the shear layer vortices and the vortex ring. The vorticity thickness attains a maximum when circulation ratio ΓR/ΓF is 1.0. However, enhancement of the growth of the shear layer saturated when the ratio exceeds about 1.5. This is because the vortex ring which has large circulation passes through the shear layer.