Norimasa Miyagi

Statistical analysis on wall shear stress of turbulent boundary layer in a channel flow using micro-shear stress imager

Abstract Measurements of wall shear stress of turbulent boundary layers in the channel flow were carried out using a micro-electro-mechanical-system (MEMS)-based micro-shear stress imaging chip. The study was carried out in a turbulent channel flow facility. One array of 25 micro-shear stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous span-wise distribution of the surface shear stress. The characteristics of high shear stress area (streaks) were described with statistics. Based on the measurement, the physical quantities associated with the high shear stress streaks, such as their length, width with the high shear stress level, were obtained. To further explore the relationship between the shear stress slope and the peak shear stress, the probability density function (PDF) of the ratio of peak shear stress to shear stress slope at different Reynolds number Re is examined. As for the distribution of PDF, it was found that the distribution concentrated towards a certain value in each Re . This result is extremely important because it points to the possibility of predicting the peak shear stress level based on the shear stress distribution at the leading edge of the streaks.

Jet Diffusion Control Using Plasma Actuators

The present study investigated the jet diffusion mechanism using a coaxial dielectric barrier discharge plasma actuator. A coaxial plasma actuator was installed at a nozzle exit to promote mixing, and a hotwire sensor was used to measure the jet velocity fluctuations downstream of the nozzle exit. It was found that the jet velocity near the nozzle exit exhibited three different fluctuation modes depending on the duty cycle of the plasma actuation. Modifications to the flow structure and the corresponding mechanism under different actuation conditions are described in this paper.

Diffusion Control of Jet by Acoustically Driven Secondary Film Flow

Active control of the diffusion of a circular jet was attempted by application of a secondary film flow around the jet. Sinusoidal acoustic excitation of the film flow was carried out for VR values 0.5, where VR is the ratio of the film flow velocity to the main jet velocity. For VR = 0.5, the diffusion of the jet was suppressed compared to that of a single jet but it was somewhat enhanced by the acoustic excitation. We conclude that secondary film flow can control diffusion and that acoustic excitation can enhance the diffusion of jet flows. In this paper, it was re-configuration to confirm the condition of the diffusion suppression of the jet. It has analyzed the jet structure by visualization experiment.

Study of Active Jet Control by Acoustically driven Secondary Film Flow

Active control of jet is applied at control of burner flame or air-conditioning. In this study, secondary film flow is built on the outside of axisymmetric jet. And it is driven by the acoustic excitation for active jet control Furthermore, the velocity ratios of mainstream and film flow and acoustic Strouhal number were changed. The jet structure was visualized by LLS in axial and cross section for research of change in each jet ejecting condition. As a result, it was confirmed that the velocity ratio of film flow influenced to outbreak and growth of vortex ring corresponding with shear stress with surrounding air, and the acoustic excitation influenced to outbreak period of vortex ring in film flow. And it was presumed that there were both modes which rolled up to the mainstream or surrounding air by frequency of an acoustic wave to give. For controlled jet, it was confirmed a mutual interference process of vortex ring and the blade which were base of the vortex structure of jet.