Experiments on transition to turbulence in an oscillatory pipe flow

Experiments on transition to turbulence in a purely oscillatory pipe flow were performed for values of the Reynolds number R δ , defined using the Stokes-layer thickness δ = (2ν/ω) ½ and the cross-sectional mean velocity amplitude Û, from 19 to 1530 (or for values of the Reynolds number R e , defined using the pipe diameter d and Û , from 105 to 5830) and for values of the Stokes parameter λ = ½ d (ω/ 2ν ) ½ ( ν = kinematic viscosity and ω = angular frequency) from 1·35 to 6·19. Three types of turbulent flow regime have been detected: weakly turbulent flow, conditionally turbulent flow and fully turbulent flow. Demarcation of the flow regimes is possible on R λ , λ or R e , λ diagrams. The critical Reynolds number of the first transition decreases as the Stokes parameter increases. In the conditionally turbulent flow, turbulence is generated suddenly in the decelerating phase and the profile of the velocity distribution changes drastically. In the accelerating phase, the flow recovers to laminar. This type of partially turbulent flow persists even at Reynolds numbers as high as R e = 5830 if the value of the Stokes parameter is high.