Pipe-flow relaminarization by temporal acceleration

An experimental and computational study was conducted in temporally acceler ated pipe flows under initially turbulent conditions. Relaminarization was identified when the imposed pressure gradient was of the same order as that required for relaminarization under steady conditions. Unlike the steady c ase, integral development of the mean flow does not characterize relaminari zation. Rather, mean-flow effects were confined to the near-wall region, wi th significant departures from fully turbulent flow only evident at Y(+)les s than or equal to 50. Streamwise turbulence fluctuations in the near-wall region initially decreased and then increased, and in the pipe core region the turbulence fluctuations were effectively frozen. (C) 1999 American Inst itute of Physics. [S1070- 6631(99)02311-9].