Kazuhiro Onishi

Simultaneous measurement of all three velocity components and pressure in a plane jet

Simultaneous measurement of all three velocity components and static pressure in a plane turbulent jet is performed by developing a new probe for the measurement. The combined probe consists of two X-type hot-wire sensors and a static pressure tube placed at the center of hot-wires. The static pressure tube is miniaturized with a micro electromechanical system fabrication technique to improve spatial resolution. The results of the calibration test show that the measurement accuracy of the pressure fluctuation with the pressure tube is 9%. Further, we are able to compensate for the cross-flow error of the pressure tube by instantaneously measuring all the velocity components with two X-type hot-wire sensors. The profile of the production term and diffusion term in the turbulent energy transport equation, directly estimated with the measured data, also shows slight improvement compared to our previous studies. This is due to the improvement in the spatial resolution of the combined probe and the improvement in the measurement accuracy of both the velocity-kinematic energy correlation and that of the velocity–pressure correlation by measuring all velocity components. Further, it is also found that the pressure diffusion of the turbulent energy in the plane jet is mainly caused due to the fluctuation whose frequency is from 20 to 40 Hz, which corresponds to the flapping frequency in the present experiment condition.

Improvement of Constant Temperature Anemometer and Measurement of Energy Spectra in a Plane Turbulent Jet

A constant temperature anemometer (CTA) is a useful instrument for measuring the velocity fluctuations in turbulent flow. However, in our calibration test, the actual frequency response of a typical CTA was no more than 5 kHz under normal laboratory conditions: for example, the diameter of the hot wire is 5 μm and the free stream velocity is 20 m/s. Therefore, in some cases, a typical CTA is not enough to measure accurately turbulent velocity fluctuations for fine scale structures. In this paper, we present a rearranged CTA circuit to obtain a faster frequency response so that in turn fine-scale structures can be more accurately investigated.A typical CTA circuit consists of a Wheatstone bridge and a feed back circuit. To improve the frequency response, the ratio of the electrical resistance of the Wheatstone bridge is set to 1 and two operational amplifiers with a gain-band width product of 100 MHz and a slew rate of 20 V/μs are used in the feedback circuit.An experiment to estimate the frequency response of the rearranged CTA circuit is performed with a free stream velocity of 20 m/s and using hot wires of diameter 5 μm and 3 μm. Experimental results show that the roll-off frequency of the rearranged CTA circuit is improved from 5 kHz to 20 kHz for the 5 μm hot wire and from 6 kHz to 40 kHz for the 3 μm hot wire.Velocity measurements are made using the rearranged CTA circuit in a plane turbulent jet where the value of the Taylor microscale λ is 3.2 mm and the Taylor-scale Reynolds number Reλ is 440. Measurements shows that the power spectrum obeys the reliable numerical profile derived by a LDIA (Lagrangian Direct-Interaction Approximation) theory until more than 0.20 of the non-dimensional wave number κ1η, which is a wider range in comparison with the results obtained when using a typical CTA circuit. Here, κ1 is the axial wave number and η is the Kolmogorov microscale.Further, velocity measurements are performed taken using the rearranged CTA circuit with a square jet where the value of λ is 6.3 mm and Reλ is 1,720. Measurements shows that the power spectrum obeys the numerical profile by the LDIA theory in the range 0.04 < κ1η < 0.20, which is a much wider range than the results obtained when using a typical CTA circuit (0.04 < κ1η < 0.08).These results indicate that the rearranged CTA circuit can be used to investigate fine-scale structures in turbulent flows more accurately.Copyright

A Technique for the Measurement of Wall Shear Stress Based on Micro Fabricated Hot-Film Sensor

The objective of this study is to establish a technique for accurately measuring the wall shear stress in turbulent flows using a micro-fabricated hot-film sensor.Previously, we developed a hot-film sensor with a flexible polyimide-film substrate. This sensor can be attached to curved walls and be used in various situations. Furthermore, the sensor has a 20-μm-wide, heated thin metal film. However, the temporal resolution of this hot-film sensor is not very high owing to its substrate’s high heat capacity. Consequently, its performance is inadequate for measuring the wall shear stress “fluctuations” in turbulent flows.Therefore, we have developed another type of hot-film sensor in which the substrate is replaced with silicon, and a cavity has been introduced under the hot-film for reducing heat loss from the sensor and achieving high temporal resolution. Furthermore, for improving the sensor’s spatial resolution, the width of the hot-film is decreased to 10 μm. The structure of the hot-film’s pattern and the flow-detection mechanism are similar to those of the previous sensor.Experimental results show that new hot-film sensor works as expected and has better temporal resolution than the previous hot-film sensor. As future work, we will measure the wall shear stress for a turbulent wall-jet and discuss the relationship between a large-scale coherent vortex structure and wall shear stress based on data obtained using the new hot-film sensor.Copyright

209 Study on simultaneous measurement of velocity and temperature and mixing enhancement in an axisymmetric jet

1. 緒論 噴流は混合・拡散・化学反応などの工業的な場面で多用さ れており,省エネルギー化・地球温暖化防止の観点から効率 的な利用が求められている.このため,噴流の制御に関する 研究が数多く行われ,制御の効果や制御時の噴流場に関して 考察が進められている.しかし,噴流中における混合・拡散・ 反応の効率の評価において重要な知見を与える,速度とスカ ラ量(物質の濃度や流体の温度など)の結合統計量については, その測定の難しさから未だに計測事例が少なく,より効率的 な制御手法の構築や制御効果の高精度な評価に向け,同時計 測による結合統計量の取得が求められている. そこで本研究では,速度測定用の熱線流速計と温度測定用 の冷線温度計を製作し,それらを適切に構成・配置して軸対 称加熱噴流中において速度と温度の同時測定を行い,結合統 計量などを考察した.また,加熱噴流の吹出口に設置した稼 働式ボルテックスジェネレータ(以下,VGと表記)を用いて加 熱噴流と周囲流体との温度の混合拡散促進を試み,その効果 を速度と温度の同時測定結果から考察した.

The Development of Extremely-Compact Static Pressure Probe for the Simultaneous Measurement of Pressure and Velocity in the Turbulent Flows

A new static pressure probe was developed to improve the space resolution and the measurement accuracy of the combined probe for the simultaneous measurement of the static pressure and the velocity in turbulent flows. The external diameter of the static pressure tube is 0.3 mm and its internal diameter is 0.2 mm. There are 8 static pressure holes on the wall of the static pressure tube and their diameters are 0.1 mm. The MEMS microphone is used as the pressure sensor and embedded inside the flare of the static pressure tube. The diameter of the MEMS microphone is 2.54 mm and has the wide range flat frequency response. The measurement results by the new static pressure probe in the two-dimensional turbulent jet show that the measurement accuracy of the static pressure probe is sufficient and the seven-thirds power law is clearly observed in the power spectra of the fluctuating pressure measured at the position of a half width of the mean velocity distribution in the cross-streamwise direction apart from the jet center line. In addition, the yaw angle characteristics of this new pressure probe shows that the measurement accuracy of the static pressure has less dependency on the yaw angle of the probe to the flow direction than the one of the previous static pressure tube (its external diameter is 0.5 mm). From these results, it is found that the new static pressure probe is effective for the measurement of static pressure in turbulent flows and useful to improve the space resolution and the measurement accuracy of the combined probe for the simultaneous measurement of the velocity and the static pressure. By using this static pressure tube, the space resolution of the combined probe is reduced approximately 40%. Further, by combing two X-type hot-wire probes with the new pressure probe, the simultaneous measurement of three velocity components and static pressure is realized.© 2011 ASME