The transition to turbulence in boundary flow of superfluid 4He is investigated using a vibrating wire. An object in a superfluid can easily generate the turbulence by oscillation even at very low temperatures, because remanent vortices are attached to it from the beginning. However, these vortices affect the transition to turbulence, occasionally causing intermittent switchings between laminar and turbulent flows. Recently, we have successfully devised a condition of vortex-free superfluid, in which a vibrating wire cannot generate turbulence. This condition prevents an oscillator from the switchings, enabling us to study the transition from turbulent to laminar flows. Using a vortex-free vibrating wire and a generator of vortex rings, we find that a turbulent state has a lifetime at driving forces near the transition, while it seems to be permanent at high driving forces. Lifetime data scatter widely, suggesting that the transition from turbulent to laminar flows is a probability event.
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