One of the authors has made clear the bubble behaviors around the bubble collapsing period as well as the patterns of bubble collapse near solid wall through instantaneous measurement of an impulsive force and bubble appearance under a really flowing condition using a high speed video system composed of an impulsive force sensor and a high speed video camera. In this report, a simultaneous observation of bubble collapsing behavior and cavitation erosion pits is tried in separated vortex flow behind a circular cylinder to experimentally prove the ability of damage for the bubble collapsing patterns reported in the previous papers. As a result, the simultaneous observation of the bubble collapsing behavior and the erosion pits are successfully conducted with the present measurement system so that the damage ability is confirmed, at least, for a collapsing pattern of the axial collapse type, and the details of its collapsing behavior are also shown at the same time.

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Much attention has been focused on vortex cavitation bubbles because of their highly erosive power. In this study, vortex cavitation flow developing in the wake behind a circular cylinder with a typical separated vortex is experimentally examined to solve the process on collapse and impulse of cavitation bubble in the context of the cavitation erosion. From the comparison with the erosion pit distribution obtained in erosion tests due to vortex cavitation, it is experimentally proved that the collapsing process is classified into four patterns, using high-speed-video observations of vortex cavitation bubbles. As a result it is shown that the collapsing process has two different sequential stages so that a vortex cavity of large spatial scale does not collapse all at once.

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任意形状物体まわりの2次元有限空どう流れに関する新らしい非線形理論を示す.空どう後流をその排除効果に着目し, 排除厚さに準拠した排除面により置き換えモデル化し, (2次元流れにおいて) 2方向の運動量保存の条件を用いて合理的に解析する方法を提示してある.さらに, 曲線状物体の形状をフーリエ展開を用いて表わし, 流れの基礎式をフーリエ係数を含めた非線形方程式群に帰着させ解く方法を示してある.計算例として, 円弧形のスーパキャビテーション翼形を採り上げその特性を定量的に示すと共に実験値とも比較してある.

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It is known that erosion of fluid machinery can be caused by collapes of a cavitation bubble under high speed flow conditions. To solve this cavitation erosion problem, we performed some experiments on the cavitation process from a subcavitation to a supercavitation stage with a measurement system combining a high-speed video camera and an impulsive pressure sensor. This study focuses in particular on a vortex cavitation bubble in the near wake of a triangular body at the partially cavitating stage which is well known as a highly erosive pattern. Erosion tests were conducted regarding the mechanism of highly impulsive force generation, and bubbl collapsing behaviors were observed. The results show that three characteristic patterns of bubble collapse and erosion occur within the near-wake region.

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Temperature and humidity environments inside an evaporative-fog air cooling greenhouse with natural ventilation were measured on a clear day in summer. The fog was sprayed intermittently (spraying time: 1 min at 3 or 4-min intervals) from nozzles installed 2 m above the floor inside the greenhouse. The side, end, and roof ventilators of the greenhouse remained open to increase air circulation and fog evaporation during cooling. The results of the experiment are summarized below. (1) The inside air temperature reduced to approximately the inside wet bulb temperature within 1 min after the start of fog spraying. (2) The vertical air temperature differences below the nozzles (plant growing zone) were within 1 °C, and the horizontal air temperature differences at 1.5 m above the floor in the greenhouse cooled with fog spray system were within 2.5°C. (3) The reduction of leaf temperature of the tomato plant by fog spray was smaller than that of air temperature. The difference between the leaf temperature and the air temperature inside the greenhouse decreased as the saturation deficit of the air increased. (4) From these measurements, when fog fills the greenhouse atmosphere, the air is rapidly cooled at a rate of 10°C per min, a small sensor, such as a small diameter thermocouple in which heat capacity is small, is recommended to prevent errors in measuring air temperature.

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The spray atomization has been believed to be enhanced by various mechanisms such as internal flow disturbance caused by existence of cavitation bubbles and shock wave generated by bubble collapse. This report focuses on the shock wave generated when the bubble collapses. As the first stage, the quantity of dissolved air was changed, which played an important role for the nucleus of cavitation bubbles. Cavitation images were obtained by a high speed video camera and the pressure distribution in the throat of the orifice were measured by a manometer. Moreover, the impact acceleration of the bubble collapses was measured with an acceleration sensor. A comparison between the influence of dissolved air quantity and the degree of the shock wave was carried out. As a result, the impact acceleration becomes larger by decreasing the dissolved air quantity, whereas there is no changed in pressure distribution in the throat of the orifice.

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Cavitation inception and desinent behaviors in a cylindrical orifice under transient flow condition are observed using a high-speed video camera. Pressurized sample water with some conditions of air content is accelerated or decelerated by opening or closing of solenoid valve. Pressures upstream and downstream of test section, and the flow rate are measured simultaneously with high-speed video observation. The test orifice is made of transparent acrylic resin. The orifice has nozzle diameter of 1.6mm and throat length in 6mm without inlet roundness. As a result from observation, the time delay of cavitation inception becomes shorter with the increase in upstream pressure in the accelerated flow and longer with the decrease in air content. Growth rate of cavitation also becomes higher with the increase in upstream pressure. For the decelerating flow, cavitation desinence is delayed with the increase in upstream pressure.

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従来、キャビテーション壊食を防止する有効な手段とされていたキャビテーション発生域への通気の効果をベンチュリを通る流れを例にとり詳細に検討した。得たる結果によれば、キャビテーションに対する通気の効果は通気前のキャビテーションのタイプ、通気位置、通気量の違いによっておおいに異なる。特定の条件の下では、キャビテーションが誘発あるいは助長されることもある。

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Experimental investigations on the unsteady cavitating flow around an elliptic cylinder of axis ratio 3 : 1 were carried out for several angles of attack and a wide range of cavitation numbers. The flow characteristics were studied by measuring the fluid force and the pressure fluctuation. Under non cavitating conditions, a dramatic change of flow state was observed in the critical Reynolds number regime. This change of flow state has a great influence on the unsteady characteristics of cavitating flow.

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