Masato TAGAWA's research while affiliated with Nagoya Institute of Technology and other places

In the present study, we have developed a two-dimensional velocity vector sensor using two identical hot-wires placed close together and parallel to each other. To clarify the principle of operation of the sensor, we have used the OpenFOAM software and performed numerical analysis of the velocity field around the sensor consisting of the two hot-wires. As a result, it is shown that the thermal boundary layers developing from the close vicinity of each hot-wire can induce the thermal interference between the hot-wire measurements. Thus, a wall-normal velocity component existing in wall turbulent shear flows will affect the output of each hot-wire of the sensor. In the present study, measurement accuracy of the two component velocity fluctuations obtained by the proposed sensor is appraised experimentally using a canonical flat-plate turbulent boundary layer. Then, it was verified that the present velocity sensor can reproduce not only the DNS results but also the experimental ones measured by the standard I- and/or X-type hot-wire probes. A notable advantage of the proposed hot-wire sensor should be that it can be brought closer to the wall than the X-type one.

A novel hot-wire technique consisting of two parallel hot-wires is proposed. The gap between the two parallel wires is filled with an adhesive, which can be expected to improve the sensitivity to the flow angle at high velocity regions. In the present study, measurement accuracy of the two component velocity fluctuations is appraised experimentally in a flat-plate turbulent boundary layer. As a result, it was verified that the present probe can reproduce not only the DNS database but also the measurements obtained by the standard I- and X-type hot-wire probes. In addition, the proposed probe can be brought closer to the wall than the X-type one. However, it is observed that the turbulent intensity measurements in the streamwise direction are slightly reduced, and the power spectrum of the streamwise velocity fluctuations is attenuated accordingly in a high-frequency range. Thus, fluid-dynamical and thermal effects of the adhesive need to be investigated in more detail in the future.

In this study, in order to observe and investigate characteristics of heat transfer phenomena of round impinging turbulent jet in a finite circular vessel, DNS of turbulent heat transfer phenomena is conducted. Also, LES is carried out to evaluate predictions of LES in such heat transfer phenomena as compared with DNS result. As the result of evaluation, predictions of LES almost give good agreements with DNS results, but the slight differences in thermal field can be found near the stagnation point. On the other hand, to reveal an effect of impingement distance giving influence to heat transfer phenomena, turbulent statistics, structures and heat transfer mechanism in round impinging turbulent jet are investigated in detail, in which the effect for turbulent heat transfer on the impinging wall can be clearly found. However, similar local Nusselt numbers at the stagnation point are obtained in both the cases of H/D = 2 and H/D = 4. Concerning the reason for obtaining similar values, it is found that the distributions of temperature and the wall-normal turbulent heat flux near the impinging wall are related, in which the wall-normal turbulent heat flux remarkably affects in the case of H/D = 4. As for the efficiency of heat transfer, the wall friction coefficients in all cases are larger than the Nusselt numbers around the point taking the maximum value of friction coefficient. Thus, it is found that the effective heat transfer rate is mainly obtained around stagnation point.

In a turbulent thermal boundary layer, to accurately measure the temperature fluctuation near the wall, a thin-wire temperature sensor with high spatial resolution is indispensable for capturing small-scale temperature fluctuations in the immediate vicinity of the wall. On the other hand, in order to minimize the deterioration in the dynamic response of the thin-wire sensor, its length-to-diameter ratio need generally exceed 1000. To meet these conflicting requirements, the wire diameter of the sensor must be extremely thin. In this study, as a novel approach, we proposed a new temperature probe consisting a V-shaped thin tungsten wire, and verified its measurement accuracy by using a standard cold-wire probe as a reference. As a result, the turbulence characteristics measured in a fully-developed thermal boundary layer—r.m.s. values, power spectra, instantaneous signal traces of temperature fluctuations, etc.—show that the V-shaped thin-wire temperature sensor can improve the spatial resolution of the sensor while enabling us to take full advantage of the response compensation technique developed for the standard cold-wire probe.

We have developed a thermofluid scanner that enables us to visualize the spatial distributions of velocity and temperature of fluid flows. This novel measuring device can scan the flow field to be measured with a rod-shaped probe equipped with hot- and cold-wires to measure velocity and temperature fields simultaneously. By combining the time-series data from the velocity and temperature sensors and the image measurement data of the probe trajectories obtained by a high-speed camera, the spatial velocity and temperature distributions can be visualized by making the best use of digital signal processing. In this study, we have verified the measurement accuracy by applying the thermofluid scanner to the measurement of a circular jet of heated air. By using a mutual compensation scheme newly developed for the constant-temperature hot-wire anemometer and the response delay of the cold-wire (resistance) thermometer, it was shown that quantitative visualization measurement can be easily realized even in a non-isothermal field with turbulent fluctuations.

To measure concentration fluctuation accurately by a fast-response flame ionization detector (FFID), the frequency response function should be known, and measured signals need to be compensated appropriately. In the present study, we numerically analyzed the convection and diffusion equations of concentration developing in a laminar pipe flow. Then, we have obtained the response functions of the cross-sectional mean concentration at several downstream locations by varying the frequency of concentration fluctuation at the inlet of the pipe. It is found that the theoretical and numerical results are in good agreement. Based on the response function obtained, we may compensate the attenuated concentration signals rationally.

The objectives of this study are to observe and investigate characteristics of near-wall turbulent heat transfer phenomena caused by a round impinging jet by means of direct numerical simulation (DNS). In order to accurately investigate such phenomena, DNS is very useful tool as far as we know. The DNS of near-wall turbulent heat transfer phenomena caused by round impinging jet with isothermal impingement wall is conducted under conditions of various impingement distances to investigate effect of impingement distance. In a round impinging jet, since a bulk flow rate decreases toward streamwise direction on the impingement wall due to increase in the cross-sectional area, heat transfer is also suppressed in the down-stream region on it. Thus, because a remarkable enhancement of heat transfer occurs around a stagnation point, it is important to know the effect of impingement distance for the near-wall heat transfer phenomena. As results of DNS, both detailed characteristics and statistics of near-wall turbulent heat transfer phenomena caused by a round impinging jet are clearly obtained.

For accurate measurement of fluid temperature fluctuations by a fine-wire temperature sensor (coldwire), it is necessary to keep the sensor length about 1000 times larger than its diameter to avoid the deterioration in temporal response. However, a longer sensor unfortunately reduces its spatial resolution. In the present study, we have tested the performance of fine cold-wires of several different sensor lengths, and examined the effects of the temporal and spatial resolutions on the rms values, power spectra and instantaneous signal traces of temperature fluctuations in a canonical turbulent thermal boundary layer. It is found that applying the response compensation to the outputs of cold-wires with a relatively small length-to-diameter ratio enables us to avoid the deterioration in spatial resolution and to obtain sufficiently accurate measurement results.

The object of this study is to investigate and observe characteristics of turbulenet heat transfer phenomena in an entrance region of wall-heated pipe flow by means of direct numerical simulation (DNS). In order to reproduce the entrance region of both velocity and thermal turbulent boundary layers in a pipe, an obstacle which induces a turbulence is mounted on the wall near the inlet of pipe, and the wall is set under isoflux thermal condition. Such conditions in DNS is successfully carried out to reproduce the entrance region of pipe. Thus, characterisc dtributions of turbulent statistical quantities in entrance of a pipe are clearly obtained by the DNS. In the downstream region, the velocity and thermal fields simultaneously develop, and almost development turbulent flow with heat transfer in a pipe can be observed. Therefore, DNS reveals characteristics and structures of entrance region of pipe.