Precursor of transition to turbulence in an oscillatory flow

The measurements of velocity on the turbulence occurring in a purely oscillatory pipe flow were performed by hot-wire anemometer for the Reynolds number Re (defined using the amplitude of cross-sectionally averaged velocity) of 30 000 and for the value of the Stokes parameter λ = a/(ν/ω) ^1/2 (a = internal radius of tube, ν = kinematic viscosity, ω = angular frequency) of 8. The transition to turbulence occurring in an oscillatory flow is evaluated in terms of turbulent intensity, skewness and flatness factors. The peak of skewness and flatness factors appears in front of the peak of turbulent intensity. Particularly, the rise of the flatness factor preceding the transition to turbulence signifies the appearance of an intermittent nature of turbulence and is characterized as a precursor of the transition to turbulence.