高垣 直尚

気候変動の予測において大気・海洋間運動量フラックスを算出する際に用いられる抵抗係数の正確なモデル化が重要である．そこで海洋シミュレーション装置である風波水槽が有用であるが，水槽全長を超える吹走距離での海洋環境の再現が不可能である．既往研究11)では水槽出口部の波を水槽入口部で生成する波ループ法を考案することで解決した．しかし完全に吹走距離を延長するには，水槽出口部の気流も水槽入口部で生成する必要がある．本研究では，水槽出口部の風速鉛直分布を可動翼を用いて水槽入口部で生成することで水槽全長(6.5m)を超える吹走距離12m地点における気流場の再現に成功した．さらに波・気流ループを組み合わせた気流・波ハイブリッドループ法の検討した結果，ループによって波が発達する傾向が示され，吹走距離延長の可能性が示された．

<p>Micro-tube heat exchangers are often used for heat exchange between fluid and gas. This heat exchanger is equipped with fine circular tubes to perform heat exchange efficiently and the miniaturization of tube diameter and the increase of installation density are promoted with the recent development of processing technology. However, the miniaturization of the circular tube leads to an increase in the wall thickness, it is necessary to calculate the temperature considering not only the heat transfer in the radial direction of the pipe but also the heat transfer in the axial direction of the pipe. Therefore, in this analysis, numerical analysis of tubes with wall thicknesses was conducted, and the flow characteristics of the fluid and the heat transfer characteristics inside the tube were investigated.</p>

<p>Heat exchangers are installed in the cycle of car air conditioners to improve the fuel efficiency of automobiles. This higher performance of the internal heat exchanger leads to further improvement of fuel consumption and reduction of the influence of global warming. As the way to do it, research and development of laying dimples inside equipment is attracting attention for the purpose of increasing the heat transfer area and disturbing the fluid with low pressure loss to improve the heat exchange efficiency. However, there are still many unclear matters such as optimum arrangement and optimum shape of dimples, and the Reynolds number of the actual machine is on the order of hundreds of thousands, so it is necessary to clarify the influence of dimples on the turbulent flow field. Therefore, in this research, numerical analysis was performed by using in-house code in the channel where multiple dimples were laid to clarify the effect of laying dimples and the relationship between turbulence statistics and heat transfer characteristics. As a result, it was found that the Nusselt number at the bottom and the Reynolds stress in the vicinity become larger near the dimple trailing edge. In addition, it was found that the magnitude of the local Nusselt number between dimples in the center of the flow field and the peak value of Reynolds stress near the bottom decrease as going downstream.</p>

<p>Boiling heat transfer using immiscible liquid mixtures is an innovative cooling technique for high-heat-density electronic devices. Immiscible liquid mixtures which are composed of more-volatile liquid with higher density and less-volatile liquid with lower density such as combination of FC-72 and Water are discussed here. In the case of pool boiling using immiscible liquid mixtures, more-volatile liquid on the heating surface is started boiling firstly, and then the more-volatile liquid reaches the critical heat flux as the heat flux increases. Subsequently, the less-volatile liquid is replaced with the more-volatile liquid and moves onto the heating surface, and this liquid is started boiling. This phenomenon is called a Boiling Refrigerant Transition, and the characteristics of heat transfer and flow behavior during Boiling Refrigerant Transition are not clear. Therefore, here the experiments under the conditions of various heights of more-volatile liquid layer and various mixture composed ratio by using two combinations of immiscible mixtures are carried out. The experimental results show that characteristics of heat transfer including Boiling Refrigerant Transition are only depending on the height of more-volatile liquid layer. And the relation between the observation of boiling behavior on the heating surface and characteristics of heat transfer is discussed.</p>

<p>The drag coefficient is very important parameter that is used to calculate the air-sea momentum flux. Although the many estimation models of the drag coefficient have been proposed and discussed (conditions with extreme wind, wind wave, swell, etc.), the agreement has not reached yet. Comparison and evaluation of the model's characteristics have been performed. However, assessment related climate change has not been done. For a global phenomenon associated with climate change due to change in wind speed, we focused on the El Nino and La Nina phenomenon. In this study, using the sea surface wind data, we investigated the influence of the air-sea momentum flux estimation for the year when the phenomenon occurred and did not occur. CCMP (Cross-Calibrated Multi-Platform) of NASA was used the sea surface wind data. The period is from January to December of El Nino phenomenon, La Nina phenomenon, and normal period. Studies of Charnock (1955) and Takagaki et al. (2012) (consider the extreme wind speed range) were used for the drag coefficient model. The annual mean global air-sea momentum flux showed that the maximum and minimum air-sea momentum flux value corresponded the normal period and the El Nino phenomenon, respectively. And the difference was as small as 7.4% and 7.0% in both models. In every month, it showed the maximum and minimum is the normal period and the El Nino phenomenon in both models, and the difference was as big as approximately 13.9% and 13.8%. We also investigated the difference of the air-sea momentum flux for each phenomena in 10 degree latitude bands and the proposed seven sea areas. As a results, the difference between the maximum and the minimum value corresponded to the La Nina and El Nino phenomenon showed approximately 18% from the north latitude 50° to the north latitude 60° in the high wind speed region. The wind speed in the North Atlntic showed that sea wind speed is a very large value for the El Nino and La Nina phenomenon. Therefore, in this region, the value of air-sea momentum flux corresponding to the El Nino and La Nina phenomenon was larger than that corresponding to the other phenomena.</p>

Drag coefficient is an important parameter when estimating the air-sea momentum flux correctly. The drag coefficient, however, hasn't been accurately established due to variations in the data from field observation. Thus, a number of drag coefficient models have been formulated. Since these models define an effective low wind speed range (e.g., 6 m/s), it is important to correctly estimate the air-sea momentum flux in such an effective low wind speed range. Nevertheless, with regard to such models, the air-sea momentum flux is commonly extrapolated out of the effective low wind speed range that is defined for each model. Therefore, such an estimated drag coefficient is not always correct, and the difference in the drag coefficient is reflected by the particular model that is used. In this study, we investigated the effect of the various drag coefficient models concerning the air-sea momentum for the low wind speed range in two processes: (1) calculating the drag coefficient in the effective low wind speed range, and (2) extrapolating the drag coefficient out of the range. Six drag coefficient models were used (Charnock, 1955; Smith, 1980; Large and Pond, 1981; Yelland and Taylor, 1996; Large and Yeager, 2004; Takagaki et al., 2012). We found the largest difference between the maximum and the minimum annual mean global air-sea momentum flux on the estimated data in the effective low wind speed range at 98.5% while 19.1% was observed on the extrapolated data. When taking into consideration both the 10-degree latitude and the proposed seven sea areas, we also found that significant impact on the air-sea momentum flux was apparent when the occurrence frequency of low wind speed was high. These results show that the parametrization of the drag coefficient is imperative for the low wind speed range.

Air-sea CO₂ gas transfer velocity is used to estimate the air-sea CO₂ gas flux and is generally expressed as a function of wind speed. There has been considerable research on the air-sea CO₂ gas transfer velocity including wind speeds due to the fact that the difference in wind speeds impacts the air-sea CO₂ gas flux. On the other hand, CO₂ solubility to estimate the air-sea CO₂ gas flux and the Schmidt number to measure the air-sea CO₂ gas transfer velocity are expressed as a function of sea surface temperature. Given this, different data sets of global sea surface temperature have been proposed. Therefore, it is imperative to evaluate the effect of the air-sea CO₂ gas flux caused by the difference in sea surface temperature. In this study, we estimated and then investigated the global air-sea CO₂ gas flux using wind and wave data sets by ECMWF, as well as seven kinds of sea surface temperature data sets (ERA40, JRA-25, JRA-55, NCEP R1, NCEP R2, ERA-interim-high, and ERA-interim-low). Our findings show that the largest difference of the data sets in annual global air-sea CO₂ gas flux was 13.2%, and the largest difference by 10-degree latitude was 0.11 (PgC/year) at 60–70 degrees south latitude. To conclude, these results clearly demonstrate that the difference in sea surface temperature has a significant effect on the air-sea CO₂ gas flux.

The air-sea CO₂ gas transfer velocity is generally expressed as a function only of the wind speed U₁₀. However, there exists considerable disagreement among the observed values. The disagreement is especially large in the context of the different sea surface conditions (wind wave growth and swell etc.). Consequently, many models of the air-sea CO₂ gas transfer velocity are proposed by field and laboratory experiments. In this study, we evaluate the estimated global air-sea CO₂ gas flux using the different some air-sea CO₂ gas transfer velocity models (field experiment model, laboratory experiment model and hybrid model considering wave breaking). The 6-hourly wind speed and mean period of wind and wave data sets by ECMWF were used. The maximum difference of annual global air-sea CO₂ gas flux was 0.76 PgC/year. The annual global air-sea CO₂ gas flux of each laboratory experiment models were the smallest value, and each hybrid models were near value to each field experiment models. The difference of each model in low latitude is large, same as the difference in middle latitude. This shows that the difference of the result of each model in low latitude is significant for the estimation of air-sea CO₂ gas flux.

Drag coefficient is an important parameter in order to correctly estimate the air-sea momentum flux. However, the parametrization of the drag coefficient hasn't been established due to the variation in the field data. Instead, a number of drag coefficient model formulae have been proposed, even though almost all these models haven't discussed the extreme wind speed range. With regards to such models, it is unclear how the drag coefficient changes in the extreme wind speed range as the wind speed increased. In this study, we investigated the effect of the drag coefficient models concerning the air-sea momentum flux in the extreme wind range on a global scale, comparing the difference in the drag coefficient models between Charnock (1955) and Takagaki et al. (2012). Interestingly, the former model didn't discuss the extreme wind speed range while the latter one considered it. We have found that the difference of the models in the annual global air-sea momentum flux was small because the occurrence frequency of strong wind was approximately 1% with a wind speed of 20 m/s or more. However, we also discovered that the difference of the models was shown in the middle latitude where the annual mean air-sea momentum flux was large. In addition, the estimated data showed that the difference of the models in the drag coefficient was large in the extreme wind speed range and that the largest difference became 23% with a wind speed of 35 m/s or more. These results clearly show that the difference of the two models concerning the drag coefficient has a significant impact on the estimation of a regional air-sea momentum flux in an extreme wind speed range such as that seen in a tropical cyclone environment.