Shunsuke Yamada

Flow Behavior behind a Circular Cylinder by DBD Plasma Actuators in Low Reynolds Number

The aim of this study is to investigate on frequency shift of vortex shedding from a circular cylinder by introducing jet with dielectric barrier discharge (DBD) plasma. The electrode of DBD plasma was mounted on the circular cylinder. A constant AC wave with voltage of 2 kV and frequency of 10 kHz is applied to the electrode. The forward and backward jets to the main flow direction were induced by electrode arrangements. The timeaveraged velocity profiles and the vortex shedding frequency in the wake behind the circular cylinder were measured by I-type hot-wire anemometer at a Reynolds number of 1.0×10 3 . We discussed on the wake using the forward jets and backward jets by Hot-Wire anemometer and flow visualization. The vortex shedding frequencies are changed by the forward jet and backward jet of the plasma actuators. From the flow visualization, the frequency shift of vortex shedding is closely related to a length scale of the vortex induced by the forward and backward jet.

Spatio-Temporal Characteristics of Heat Transfer in Separated and Reattaching Flows

Spatio-temporal characteristic of heat transfer accompanied by the flow separation and reattachment was investigated using a high-speed infrared thermograph that recorded the temperature fluctuation on a heated thin-foil. In this study, the heat transfers behind a backward-facing step, behind a forward-facing step, and on a blunt plate were investigated. In all configurations, the heat transfer in the flow reattaching region had spot-like characteristics, which spread with time and overlapped with others to form a complex feature in terms of spatio-temporal characteristics of the heat transfer. The mean Nusselt number distribution behind the reattaching region was approximately proportional to 2/3 power of Reynolds number, as indicated in the previous researches. The time-space distribution of the heat transfer had a typical spanwise wavelength and fluctuating frequency in the reattaching region, which was likely to be related to the dimension of the separation region (characteristic length H or the separation length xR ). This suggests that the origin of these periodicities is not the instability upstream of the flow separation, but due to some instability, which is accompanied by the flow separation and reattachment.Copyright

A Study of Periodic Flow Behavior over Backward Facing Step in Low Reynolds Number

A study on the low Reynolds number flow such as the flow in or around the downsized device is strongly required along with the development of the micro manufacturing technology. It is important to clarify the flow behavior of the separating shear layer in small channel or around small device. The low Reynolds number flow over a backward facing step is selected as an example of the recirculation vortex in the present study, because the mixing promotion is expected from an oscillatory motion of the vortex in the separating and reattaching shear layer by the step edge. The Reynolds number based on step height is set at 130 to 3700 in the micro sensing wind tunnel. The behavior of the near wall flow is measured by Micro Flow Sensor (MFS), and the flow field is visualized with a high-speed video camera. The results show that the frequencies of 36 Hz and 24Hz appear in the flow on the lower and upper wall downstream of the step. To identify the periodic motion near the wall, the dominant frequency of the separating shear layer is analyzed by the linear instability theory.

Development of Closed-loop Control System of Reattachment Flow in Low Reynolds Number - Validity of Sensor Position -

There is the separation and reattachment process generated in or around the fluid machinery, as a significant factor to affect the energy efficiency of the fluid machinery in the low Reynolds number. It is important to control the separating and reattaching flow in a backward facing step flow in low Reynolds number. In the present study, a synthetic jet is employed as actuator because the acceptability of the frequency in the separating shear layer was confirmed in the previous study. The orifice of the synthetic jet is placed discretely in the spanwise direction. The effect of the orifice pitch on the spanwise uniformity of the reattachment process is investigated. The normalized pitch by the orifice diameter ranges from 10 to 40. In the orifice pitch range of 10 to 30, it is confirmed that the flow of the reattachment point on the lower wall has the two-dimensional structure in spite of the threedimensional disturbance by the synthetic jets.

Control of Backward Facing Step Flow in Low Reynolds Number by Synthetic Jets -Flow Structure in Common-phase and Counter-phase Injection-

In this study, a synthetic jet array which can be actuated and injected individually at the different phases is employed. Influence of the phase difference on reattachment process and flow structure in the low Reynolds number flow over a backward facing step is discussed. Flow visualization and measurement of near-wall flow using the micro flow sensor are conducted. In counter-phase injection, the excitation of the separating shear layer and the transverse vortex structure are different from those in common-phase injection. It is also found from the wall shear stress measurement that the transverse vortex structure is affected by the strength of the vortex which is induced by each synthetic jet.

Wake Structure Behind Circular Cylinder by Plasma Actuators

The separation flow causes the decrease of the driving efficiency of the fluid machines. It is important to control the separation on the bluff body, boundary layer and so on. The purpose of the present study is to control the separation on a circular cylinder and investigate the wake structure using the induced by dielectric barrier discharge (DBD) plasma. The electrode of the DBD plasma was mounted. In previous study, it is reported that the three dimensional wake structures are effective for the drag reduction. We investigate the three dimensional structure of the wake due to three dimensional jets of plasma actuators. The plasma actuators have pulse driving frequency of 0, 0.22, 1.0 and 2.0. A voltage of 4 kVpp and a frequency of 10 kHz are applied to the electrode. The velocity profiles behind the circular cylinder were measured by X type hot-wire anemometer at a Reynolds number of 1.0 × 103 . The wake structure changes because the roll up of vortex is suppressed using pulse driving of the plasma actuators. The effect of plasma actuators is evaluated based on the half width and Reynolds stress in the wake.Copyright

Streamwise and Spanwise Flow Structures Over Backward Facing Step in Low Reynolds Number

A study on the low Reynolds number flow such as the flow in or around the micro device is strongly required along with the development of the micro manufacturing technology. The low Reynolds number flow over a backward facing step is selected as one of the representative examples of the vortex dominant flows in the present study, because the mixing promotion is expected by an oscillatory motion of the vortex in the separating and reattaching shear layer over the step. It is important to clarify the flow fields in small channel or around the small device by flow visualization, since minimum disturbance in the measurement is achieved due to non-intrusive method. The objective of the present study is to clarify the flow behavior in the cross section in the spanwise, transverse and streamwise direction by the flow visualization using a high speed video camera. The Reynolds number based on the step height and the bulk velocity is set at 380 to 960. The visualization results in the cross section in the spanwise direction show that the separating shear layer from the step edge introduces a series of the primary vortices which have a rotation axis around the spanwise direction, and the main stream has a regularly whipping, wavy motion caused by the vortices moving toward the downstream direction along the upper and lower walls. The observation in the cross section in the transverse direction indicates that a scale of the vortex length in the streamwise direction is almost constant, but the primary vortex shows a periodic change in the spanwise direction, as the Reynolds number increases.Copyright

Measurement of Ionized Particle Distribution in Axisymmetric Jet

The aim of this study is to measure distribution of ionized particles in an axisymmetric jet. Recently, the effects of ionized particles, such as disinfection of bacteria and neutralizations of static electricity, are attracting much attention from many industry circles. Micro- and nano-size particles with electric charge are called “ionized particles” in this study. The ionized particles were usually carried to the objects in the air, which should be disinfected or neutralized, by a fluid force like an axisymmetric jet generated by a blower and a purge nozzle. High transportation of ionized particles improves the efficiency of the industrial products which use ionized particles. However, it is considered that behavior of ionized particle in axisymmetric jet is easily influenced by not only fluid force, but flow turbulence, Brownian motion and coulomb’s force. In this study, we measured ionized particle distribution in an axisymmetric jet by an ion count system which was a measuring device to count the number of ionized particles. The axisymmetric jet has an advantage of the high mass transportation and the diffusion capability by its flow characteristics. We conducted wind tunnel experiments to clarify the relationship between the axisymmetric jet and ionized particles. The experiment was performed in the flow velocity measurement by hot wire anemometer and the density measurement by an ion count system which was able to measure the number of ionized particle in the air. The results suggested that the behavior of ions in the jet was influenced by fluid force more than by the coulomb’s force. The aim of this study is to measure the distribution of ionized particles introduced into the axisymmetric jet.Copyright

Effects of Sway Motion and Flow-Induced Vibration on Vortex Structure Behind Flexible Rectangular Plate

Vortex structure behind a flexible rectangular plate with sway motion and flow-induced vibration was experimentally investigated by wind tunnel experiment by using Particle Image Velocimetry (PIV). The flexible rectangular plate, which was made of a polyurethane block, was cantilevered on a flat plate. On the opposite end, top free end showed a sway motion in the downstream direction. Increasing sway angle, the top free end involved the flow-induced in-line vibration which has a small amplitude. This is a typical example of fluid-structure interaction problem. However more experimental research for the effects of the sway motion and the flow-induced vibration on vortex structure behind the rectangular plate is required. In this paper, we focus attention on the phase-averaged vortex structure when the amplitude of vibration is the largest and smallest case. PIV measurement was conducted to clarify the phase-averaged and the instantaneous vortex structure behind the swaying plate. We discussed the effect of sway motion and flow-induced vibration on vortex structure.Copyright