Necessary Condition of External Disturbances for the Generation of Puffs in a Pipe.

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A turbulent blob called a puff is generated in a transitional pipe flow. Introducing disturbances of a given intensity, we determined the limits of the disturbance intensity for the generation of the puffs. We also tried to understand the mechanism of the generation of the puffs. It was found that upper and lower limits of the intensity exist for the generation. When the disturbance intensity is stronger above the upper limits, fluctuations decrease near the wall and there is not enough turbulent energy necessary for the generation of puffs. On the other hand, when the disturbance intensity is weaker below the lower limit, the turbulent energy is insufficient even immediately after the introduction of the disturbance. In both cases therefore, no puffs are generated.
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