The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. Axial velocity was measured by a hot-wire anemometer. The turbulence induced by the jet within the boundary layer developed into turbulent patches which grew in the axial, circumferential and radial directions downstream. Turbulent fluctuations within the patch were maximum a little inside from the leading edge in the axial direction and a little inside from the peripheral interface in the circumferential direction. Non-turbulent fluid was entrained into the patch through the leading and trailing edges. This was the main reason for the axial growth of the patch downstream. If the entrainment is suppressed, the axial growth is inhibited. The axial distance between the leading edge and maximum fluctuation position varied little downstream. On the other hand, the distance between the maximum fluctuation position and the trailing edge increased downstream due to the increase in the entrainment of non-turbulent fluid across the trailing edge. とは異なることを明らかにした上で，噴流流量や噴流噴出周波数が乱流塊の長さや内部の変動速度に及ぼす影響 を調べた (7) ．また乱流塊形状，境界層との位置関係及び成長の様相を調べ，HP 流れの乱流パフ，スラグや平板境 界層乱流斑点と比較した (8) ．しかし乱流塊内部の乱れの様相や乱流塊の成長の機構までは明らかにされていなか った． そこで本論文は集合平均した速度からの変動すなわち不規則変動速度を求め，孤立乱流塊内部の乱れの様相を 調べた．また乱流塊の成長機構を調べるために，前縁や後縁界面に相対的な流線や，両界面において乱流または 非乱流で条件を付けた条件付平均速度を求めて，両界面において非乱流流体が乱流塊内に取り込まれ乱流化する 様相を調べた．これらによって，HP 流れの乱流パフやスラグとの差異，また平板境界層乱流斑点との差異を明 らかにする． お も な 記 号 a, D ：円管半径 30mm と直径 60mm実験装置は前報 (7)(8) に示したものと同じで，直径 D = 60 mm，全長約 6.2 m (104D)のアクリル製円管であり，そ の下流の送風機によって空気が吸い込まれる．円管は長さ約 1m のもの６本をなめらかに接続した．円管の軸方 向速度の軸対称性は良好である． 読者の便宜のために図 1 に流れ場および座標系を再掲する． 原点から x j = 107mm 下流において管壁に垂直にあけた直径 2mm の一つの孔から，局所一点撹乱として空気噴流を管半径方向に周期 的に噴出した． 本研究において噴流を周期的に噴出させるために，エアポンプから放出された空気を電磁弁に導き，フォトイ ンタラプタから周期的に発生する矩形電圧を電磁弁の制御信号に用いた．円管内への噴流噴出時間 t j 1 = 0.1s に対 周期的吹き出しによる円管助走部境界層の乱流遷移

The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. Axial velocity was measured by a hot-wire anemometer. The turbulence induced by the jet within the boundary layer developed into turbulent patches which grew in the axial, circumferential and radial directions downstream. Turbulent fluctuations within the patch were maximum a little inside the leading edge in the axial direction and slightly inside the circumferential interface in the circumferential direction. Non-turbulent fluid was entrained into the patch through the leading and trailing edges. This was the main reason for the axial growth of the patch downstream. If the entrainment is suppressed, the axial growth is inhibited. The axial distance between the leading edge and maximum fluctuation position varied little downstream. On the other hand, the distance between the maximum fluctuation position and the trailing edge increased downstream due to the increase in the entrainment of non-turbulent fluid across the trailing edge. Keywords

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