Dielectric barrier discharge (DBD) plasma actuators were used for the active control of flow separation on a curved wall simulated suction surface of a gas turbine blade at three different mainstream velocities, U_MS = 2.2 m/s, 4.1 m/s, and 6.3 m/s. Owing to the change in mainstream velocity, the Reynolds number was varied as Re = 1.7 × 10⁴, 3.1 × 10⁴, and 4.7 × 10⁴, respectively. Particle image velocimetry system was used to obtain two-dimensional velocity field measurements. The amplitude of input voltage for the plasma actuator was changed from ±2.0 kV to ±4.0 kV. At the lower mainstream velocity, U_MS = 2.2 m/s (Re = 1.7 × 10⁴), the separated flow induced on a curved wall was consider-ably reduced by the flow control using the DBD plasma actuator. Moreover, the effect of flow control by the plasma actuator was gradually reduced at the higher mainstream velocities, U_MS = 4.1 m/s and 6.3 m/s (Re = 3.1 × 10⁴ and 4.7 × 10⁴, respectively). The flow control effect was improved by changing the position of the plasma actuator. When the plasma actuator was positioned immediately before the separation point, it exhibited better flow control effects than when positioned immediately behind the separation point.

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 flow characteristics of sub- and supersonic under-expanded jets issued from circular nozzles have been studied well; however, this does not apply to sub- and supersonic jets from an orifice nozzle. In this study, the flow characteristics of sub- and supersonic jets issued from an orifice or notched special orifice nozzle are examined experimentally based on the results of a visualized flow pattern by the Schlieren method and the measurements of velocity distribution by a thin supersonic Pitot tube. That is, the effects of the nozzle shape on the flow characteristics such as the decay of the jet centerline velocity, velocity distribution at the cross section, increment of the jet- width and -flow rate to the downstream are examined experimentally. Results indicate that the jet issued from the notched orifice nozzle with a rectangular notch demonstrates excellent entrainment performance. In addition, to improve the performance of a cylindrical ejector, the application of the results is examined.

The flow characteristics in a centrifugal bladeless mixer were investigated by using particle image velocimetry (PIV) and large eddy simulation (LES). The numerical result is compared with experimental one in terms of the phase-averaged velocities in radial and tangential directions. The result was analyzed by comparing with that of a conventional flat-blade mixer. A spirally structured vortex tube pair was observed around the centrifugal bladeless rotor. In a vertical cross-section of the spirally structured vortex tube pair, a zigzag street of counter-rotating vortices like a reverse von Kármán vortex street was formed. It was found that the rotating jet flow was enhanced by the reverse von Kármán vortex street of the spirally structured vortex tube pair. Furthermore, the sustained rotating jet flow constructed a couple of large toroidal structured vortex tube near the tank wall. The large toroidal structured vortex caused a wider circulation in the stirred tank, which was advantage to stir the fluids over the whole field.

This study proposes a shape optimization approach for the cross-sectional shape of the inner pipe of a counterflow concentric tube heat exchanger. The cross-sectional shape of the inner pipe is expressed by an algebraic expression with a small number of parameters, and their heat transfer performance is evaluated by a commercial Computational Fluid Dynamics (CFD) solver. The optimization is conducted by the Non-Dominated Sorting Genetic Algorithm II (NSGA-II) assisted by the Kriging surrogate model, and the NSGA-II finds the optimal cross-sectional shape with many protrusions around the perimeter of the inner channel to improve the heat transfer performance. In this study, heat transfer performance is evaluated from the temperature drop at the outlet of the high-temperature fluid. The present optimization finds the optimal channel with many protrusions, which achieves lower outlet temperature than a circular channel even with the same heat transfer surface area. This result indicates that the number of protrusions plays important roles which contribute not only to increase heat transfer area but also to improve heat transfer performance.

Unsteady pressure distributions around a simplified sedan automobile model were investigated by conducting dynamic wind-tunnel testing using the newly developed forced oscillating apparatus, HEXA-X3, which can produce 6-degrees-of-freedom motion. The effects of heaving and pitching oscillation were investigated as the model simulated a vehicle running on a flat road at approximately 40 m/s and 1 Hz oscillation. The effects of the ground plate on unsteady pressure distributions over the model surfaces were measured while simulating heaving and pitching motion at Strouhal-number conditions similar to those for actual vehicles. The influence of the tubing on the frequency response of the pressure sensor was evaluated to be negligible by conducting a calibration experiment first. In the static case, the overall pressure distribution was consistent with that for a typical sedan, and the influence of the local relative flow velocity changes due to the contraction effect was observed in the underside of the model. In the forced oscillation tests, the effect of heaving and pitching motions on the flow around the underside was investigated. Effects of oscillation parameters, specifically amplitude and frequency, were investigated using the gain and phase-lag normalized by data from the steady model. Results of the test indicate that there is a characteristic distribution in pressure fluctuation, and the phenomena that become dominant in the flow around the underside vary according to location. The dynamic heaving motion was shown to change the pressure distribution, possibly due to changes in the effective angle of attack in addition to the static effect. The pitching test showed that a dynamic camber effect works in addition to those effects.

Wall shear stress fluctuations are measured using an electrochemical method in pipe flow. The relationship between the mass transfer and the momentum transfer, studied by Hanratty and Campbell (1983) and the Chilton–Colburn analogy (Chilton and Colburn, 1934), are compared to each other with respect to the mean wall shear stress and the wall shear stress fluctuation. Chilton–Colburn analogy has been applied to measure the mean wall shear stress in the fully developed concentration boundary layer. There are few studies concerning the feasibility of the Chilton–Colburn analogy for instantaneous wall shear stress measurements. In the present study, with the new designed overall electrode, the feasibility of Chilton–Colburn analogy for predicting the wall shear stress fluctuations is studied for the first time. The friction factor is used to compare the mean wall shear stresses. It is found that both methods can accurately predict the mean wall shear stress. The wall shear stress fluctuations are compared via the probability density function and frequency spectrum. When the probability density fluctuation is normalized by the standard deviation and the spectrum is normalized by the typical time scale, these two methods show the similar statistical properties. It is confirmed that with the new designed overall electrode, Chilton–Colburn analogy is available for predicting the wall shear stress fluctuations.