Shinji Honami

Optimal injection condition of a single pulsed vortex generator jet to promote the cross-stream mixing

Abstract The flow field of a pulsed vortex generator jet which is injected periodically through a single hole into a boundary layer is described. Measurements of phase-averaged velocity and wall shear stress are made by a hot-wire anemometer and a high response micro-wall flow sensor, respectively. Optimal mixing conditions, based on three parameters, (i) velocity ratio, (ii) duty ratio and (iii) pulse injection frequency, are discussed. The longitudinal vortex structure characteristic of steady injection is obtained at the stable stage of the evolution of the pulsed jet. A peak of the wall shear stress is also observed at the final stage. These mechanisms in the pulsed vortex generator jet enhance the cross-stream mixing to inhibit the boundary layer separation.

Behaviors of the Laterally Injected Jet in Film Cooling: Measurements of Surface Temperature and Velocity/Temperature Field Within the Jet

This paper presents the behaviors of the injected jet on the flat surface in lateral injection of the film-cooling. Simultaneous velocity and temperature measurements were made by the double-wire probe. The test surface was also covered with an encapsulated temperature-sensitive liquid crystal. The image processing system based on the temperature and hue of the liquid crystal calibration provides the surface temperature distributions. The tests were conducted at three kinds of mass flux ratio of 0.5, 0.85, and 1.2. The laterally injected jet has an asymmetric structure with a large scale of vortex motion in one side caused by the interaction with the primary stream. Asymmetry is promoted with mass flux ratio increased, resulting in low film-cooling effectiveness.Copyright

Noncontact Bubble Manipulation in Microchannel by Using Photothermal Marangoni Effect

A noncontact bubble manipulation by an optically induced local surface tension gradient is described in this paper. In microfluidic devices, the effects of interfacial phenomena become dominant with decreasing of the length scale. An appropriate control of the local gradient of the interfacial properties can provide a powerful method for bubble handling in the microfluidic system. Here, the photothermal technique is used to induce the gradient in surface tension around a microbubble. The thermocapillary force around the bubble is induced and controlled by local laser focusing. The bubble in a microchannel filled with silicone oil with varied viscosity was examined to verify experimentally the optical manipulation method of bubbles based on the photothermal Marangoni effect. As a result, three applications of bubble manipulation by using photothermal Marangoni effect are obtained: detachment from channel wall, trapping in liquid phase, and transportation in the channel. Additionally, minimum optical power for the manipulation was evaluated. The effects of bubble size, liquid viscosity, and irradiated optical power on the manipulation characteristics can be summarized by a dimensionless number that is a ratio of the thermocapillary force to viscous drag force for the bubble.

Vortex Behavior of Vertical and Inclined Synthetic Jets in Cross Flow at Low Reynolds Number

An experimental investigation to obtain the three-dimensional flow structure of the synthetic jet in low Reynolds number cross flow has been conducted for the purpose of examining the effective method of vortex generation for flow control. A synthetic jet is a promising technique to promote the mixing and to control the flow separation because it introduces flow perturbation without a net mass injection and the device can be easily downsized. Jet orifice diameter is 0.5 mm, and Reynolds number based on channel height is 650. The phaseaveraged three-dimensional flow structures of synthetic jet with the vertical and the inclined injection patterns were measured by scanning stereoscopic PIV. Then, it becomes clear that both injection patterns generate the hairpin vortex. The inclined injection pattern cannot generate the higher vorticity compared with the vertical one, but it is suitable for the control of flow separation because the hairpin vortex exists near wall region.

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.

Dynamic Characteristics in a Control System of Backward Facing Step Flow by Vortex Generator Jets

*† The present study is conducted in an attempt to construct a closed loop control system of the reattachment process for a backward facing step flow by using a row of vortex generator jets. Two-dimensional reattachment is found to occur when the spanwise pitch of the injected jets is 10d to 20d, where d is the jet diameter, because measurement of the forward flow fraction indicates that the reattachment line is approximately parallel to the spanwise direction. It is necessary to examine the dynamic characteristics of the injection jet on the reattachment process in the design of the control system. The coherence and the frequency transfer characteristics between the injection jet and the near-wall flow in the reattachment region are examined in an open-loop system. The experimental result shows that the coherence of the jet velocity with the surface static pressure is higher than that with the surface friction velocity. The surface static pressure has better frequency transfer gain characteristics near the reattachment point. A step response of the near-wall flow is also conducted in order to investigate the transient characteristics. Damped oscillation with respect to the surface static pressure is found to occur immediately after a step change in the jet velocity.

Effect of Cross-Shaped Circular Jet Array on Impingement Heat Transfer

The purpose of this study is to clarify heat transfer characteristics for the high cooling performance with multiple jet impingement. In the present study, the influence of the interaction among adjacent impinging jets on heat transfer of target surface is experimentally investigated. The study is focused on the effect of jet injection shape on the heat transfer. 3×3 square array of cross-shaped circular jet is tested. Injection distances L are 2 and 4 jet hole diameters, and jet-to-jet spacing S are 4, 6 and 8 jet hole diameters. Experiments are conducted for a constant Reynolds number Re = 4,680 based on the jet hole diameter. Steady state thermochromic liquid crystal technique is employed to measure local and area averaged Nusselt numbers. The flow field is visualized by smoke-wire and oil flow techniques. It is found that the cross-shaped circular jet array improves heat transfer at the intermediate area enclosed by four impinging jets compared to that of circular jet array at the narrow injection distance. In the case of cross-shaped circular jet array, the wall jet produces a stronger turbulence than that of circular jet, which makes the heat transfer push up toward the apex of square detachment line at injection distance L/D = 2 and jet-to-jet spacing S/D = 6 and 8.Copyright

Modeling of Boundary Condition for Turbulent Boundary Layer Bleed

In this paper, a new model of boundary condition for bleed with porous holes is proposed. This model is based on the experimental evidence that the local bleed rate is primarily a function of the local flow conditions. We propose a model to simulate flow fields of a shock wave/turbulent boundary layer interaction with bleed at freestream Mach number of 2.5 with 6.0 and 8.0 deg shock generator angles. The computational results are compared with experimental data which contain wall static pressure and bleed flow rate distributions. It is shown that the agreement is improved by using our method for the bleed boundary condition.

Three-dimensional flow characterization of a square array of multiple circular impinging jets using stereoscopic PIV and heat transfer relation

AbstractImpinging jets are one of the most industrially essential methods of cooling, for example, the cooling of gas turbine blades and electronic devices, drying or annealing of glasses. Usually, jets are configured for a specific purpose, but the flow tends to be very complicated as each jet interacts with the others, especially near the impingement surface. In the present study, complicated three-dimensional flow from a square array of circular impinging jets was revealed experimentally to investigate the effect on heat transfer characteristics. The flow fiel d was measured by scanning stereoscopic particle image velocimetry to confirm the detailed spatial features of the multiple circular impinging jets as nozzle-to-surface distance and jet-to-jet spacing were changed. Adjacent jets generated vortex rings and roll-up toward the nozzle plate, vortex rings and roll-up sizes changing depending on the experimental parameters. Differences in vorticity and dispersions of velocity from the jets were also observed. The temperature field of the impingement surface was measured using a thermosensitive liquid crystal technique. The spatial distribution of heat transfer coefficient was related to the flow field near the impingement surface.Graphical abstract

Recent Advances in Technology for Flow Control Based on Development of Actuator Induced Aerodynamics

The strategy on the selection of the actuator in the flow control system is strongly required, since the separating shear layer associated with the vortex structure has receptivity of the different kinds of disturbances. Many kinds of the passive control devices for flow separation have been employed in the engineering application since 1950. Many studies on an actuator started in 1980’s after the passive one. The advanced actuators such as the synthetic jet, plasma actuator jet and active dimple were suggested in late 1990’s or early 2000’s. The aerodynamics around the device is also discussed, since the remarkable progress in the micro sensing system like the stereo-scopic Particle Image Velocimetry has been made. The recent progress in both actuator device and instrumentation is attributed to the development of MEMS and Laser Optics based technologies. Some examples of the flow control by the recent actuators such as synthetic jet and plasma actuator are discussed. Topics also include the actuator induced flow behavior and vortex interaction. An appropriate selection of the actuator device is a key in control for flow separation.