Motoaki Kimura

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.

Measurements of wall shear stress of a turbulent boundary layer using a micro-shear-stress imaging chip

Measurements of wall shear-stress streaks of a turbulent boundary layer in the channel flow were carried out using a MEMS-based micro-shear-stress imaging chip, which contains about 100 sensors. The chip is designed and fabricated by surface micromachining technology. One arrray of 25 micro-shear-stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous spanwise distribution of the surface shear stress. The statistics of high shear-stress streaks were established. Based on the measurement, the physical quantities associated with the high shear-stress streaks, such as their length, width and peak shear-stress level, were obtained. We found out that a high correlation exists between the peak shear-stress level and front-end shear-stress slope of a high shear-stress streak. This important property is currently being applied to the deisgn of a real-time flow control logic.

MEMS for Aerodynamic Control

Multiple arrays of micromachined micro shearstress sensors have been designed and fabricated to spatially and temporally resolve the small streamwise streaks in the near-wall region of a turbulent boundary layer. By using these sensors, the instantaneous surface shear-stress distribution of a turbulent boundary layer at high Reynolds numbers has been measured. Based on the measurement, the physical quantities associated with the high shear-stress streaks, such as their length, width and peak shear-stress level, are obtained. We found that a high correlation exists between the peak shear stress level and the front-end shear-stress gradient of a high shear-stress streak. This important property is currently being applied to the design of a real-time flow control logic. The control logic will be used to initiate a micro actuator, which interacts with the detected streak for the purpose of reducing local shear stress. The interaction between an actuator with near wall vortices was first investigated in a laminar boundary layer. The results show that the level of reduction is related to the product of the actuators peak amplitude and oscillating frequency. This result suggests the possibility of achieving shear-stress reduction by using high-frequency and low-amplitude actuators.

Visualization of Shear Stress With Micro Imaging Chip and Discrete Wavelet Transform

The two-dimensional low-speed structure of a turbulent boundary layer has been clearly visualized by a combination of a shear stress sensor using micro electro mechanical systems and the discrete wavelet transform. The application of two-dimensional discrete wavelet transforms to the visualization of wall shear stress data obtained using the micro shear stress imaging chip is described. The experiment was carried out under various Reynolds number conditions

Separation of Condensation Jet Image Using Filter Effect of Wavelets Transform

The present paper describes the application of discrete wavelet transform to the analysis of condensation jets in order to clarify the fluid and heat transfer phenomenon. The condensation jets in the nozzle vicinity are experimentally visualized via the laser light sheet method to obtain condensation particle density images of the jets. The image of the condensation particle density in the jet is decomposed to mean and fluctuation images via wavelet multiresolution. The dominant temperature boundary and the mean component outside the boundary were obtained from wavelet separation images. The boundary was compared to the experimentally obtained temperature distribution.

Stabilizing Effect and Diffusion Control on Diffusion Flames by Magnetic Field

Diffusion control and stabilizing effect on diffusion flame of propane gas were found in a magnetic field. Magnetic field strength was changed at 0.11.38xl05A/m at the center of the field. Experiment of diffusion control was carried out with a nozzle of 8-mm diam. Experiments of stabilizing effect were carried out with a nozzle of 2-mm diam. These nozzles were set at the center position of the uniform magnetic field and propane gas was run through with these nozzles at a constant flow rate. Property of propane gas and flame are diamagnetism, on the other hand air and oxygen gas are paramagnetism. Particularly volumetric susceptibility of oxygen gas is very large value. The magnetic pressure on flames was attributed to paramagnetic (air) and diamagnetic (flame) properties. As magnetic field was applied, the flame changed from an oscillating flame to a stable flame with a low frequency and small oscillating amplitude. The stabilizing effect was examined by temperature and luminosity measurement by observation with the schlieren method and high-speed photographs. The width of the outer swelled combustion gas region became narrower and keeps a constant width when the magnetic field was applied. In the condition that magnetic field was not applied, when oxygen gas was injected coaxially in the outer nozzle as secondary flow of gas, the length of the diffusion flame changed extremely short. Under this condition when magnetic field was applied, the flame length increased with increasing in magnetic field. This means that the diffusivity of oxygen gas can be controlled by magnetic field. In this study, flow control of fuel and stabilizing effect to oscillating flame are proved by using magnetic field. These effects were expressed by pressure acting on the flame by magnetic force.

Induction Heating Super Steam by Solar Thermal Using a Point Condensing Type Heat Collector

As the issue of global warming increases in severity, the introduction of renewable energy has become a global challenge. Technique of using sunlight especially has existed from old days and has been closely related in our life. Solar light proves to condense rays of light with Fresnel lens that the highest of 1,400 degrees Celsius is got. Now, we tried to get a high temperature superheated steam from water experimentally. Heat exchanger in a flask filled with olive oil heated by sunlight to condense rays of light with Fresnel lens. Heating medium used olive oil..The discharge of heat exchanger is changed depending on the water flow to determine water flow experimentally when saturated steam becomes140 degrees Celsius. 1. はじめに 我が国においては,脆弱なエネルギー供給構造は依然 と解決されていない.また,東日本大震災の影響で原子 力発電への信頼が大きく揺らいだため,再生可能エネル ギー等の活用がこれ迄以上に求められる事になった .そ こで,我々は資源が豊富で,環境への負荷が小さい太陽 光をエネルギー源として用いる事に注目した.本研究で は,フレネルレンズを用いた点集光装置で内部に水を流 した熱交換器を加熱する事で過熱蒸気を発生させる装置 の試作をし,5.0MPa,100 °C以上の高圧高温の水,或い は過熱蒸気の生成法を確立し,その評価,考察を行う. また,熱交換器の熱媒体として安価で入手が容易で安全 である食用油を用いた.今回は,オリーブオイルの熱伝 達性の影響と油浴の有効性の評価も併せて考察する. 2. 理論 2.1 流量 熱交換器における熱平衡より,それ以外の部分が完全 断熱であるとすると,流出水温To[K]となる時の水の流量 mf[kg/s]は,

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.

Jet Control Using the Coaxial Type DBD-PA by Burst Modulation

In this study, a jet control using the coaxial type DBD plasma actuator (=DBD-PA). The coaxial type DBD-PA is an axisymmetric nozzle, and the jet is ejected from this nozzle. DBD-PA is driven by burst modulation control and the induced flow is intermittently generated. The coaxial type DBD-PA controls the jet by controlling vortex generation in the jet. We control the jet at Reynolds number Re = 10,000. As a result, the induced flow of the coaxial type DBD-PA synchronizes vortex generation of the jet in a specific burst modulation control frequency range. This phenomenon is the phenomenon of lock-in. Driving the coaxial type DBD-PA with burst modulation, it is possible to generate an axisymmetric vortex within frequency of burst modulation control in which the phenomenon of lock-in occurs. Consequently, the jet is controlled by the coaxial type DBD-PA within frequency of burst modulation control in which the phenomenon of lock-in occurs.

Influence of Nozzle Exit Velocity Distribution on Flame Stability Using a Coaxial DBD Plasma Actuator

In this study, a coaxial dielectric barrier discharge plasma actuator (DBD-PA) is applied to control the flame shape, the control to maintain excellent flame stability is attempted. DBD-PA consists of an exposed electrode, an insulated electrode, and a dielectric layer. In a convergent round nozzle having an exit inner diameter of d = 6 mm, the electrode set was arranged coaxially with the nozzle. DBD-PA is applied to the control of the flame stability on condition equivalence ratio: ϕ = 0.85, 1.00. Premixed gas of air and propane was used for fuel. The applied voltage is 5–8 kV and OFF, and frequency is 8 kHz constant. Flame stability limit is taken with a high speed camera. The center of inner flame assumes a crown shape. It is thought that this is due to a change in velocity distribution in the free boundary layer by the effect of the DBD-PA induced flow.