The fluctuations of turbulence intensity in a pipe flow around the criticalReynolds number is difficult to study but important because they are related toturbulent-laminar transitions. We here propose a rare-event sampling method tostudy such fluctuations in order to measure the time-scale of the transitionefficiently. The method is composed of two parts: (i) the measurement oftypical fluctuations (the bulk part of an accumulative probability function)and (ii) the measurement of rare fluctuations (the tail part of the probabilityfunction) by employing dynamics where a feedback control of the Reynolds numberis implemented. We apply this method to a chaotic model of turbulent puffsproposed by Barkley and confirm that the time-scale of turbulence decayincreases super-exponentially even for high Reynolds numbers up to Re = 2500,where getting enough statistics by brute-force calculations is difficult. Themethod uses a simple procedure of changing Reynolds number that can be appliedeven to experiments.
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