| Abstrakt: |
Oscillatory boundary layer (OBL) flows over a smooth surface are studied using laser Doppler velocimetry in a large experimental oscillatory flow tunnel. The experiments cover a range of Reynolds numbers in the transitional regime ($ {\textit {Re}}_{\delta }=254 - 1315$). Motivated by inconsistencies in the literature, the focus is to shed light regarding the phase shift ${\rm \Delta} \phi$ between the bed shear stress and the free stream velocity maxima. Details of the mean flow structure and turbulence characteristics in transitional OBL flows indicate the emergence of a logarithmic profile, which for $ {\textit {Re}}_{\delta }=763$ appears at the middle of the deceleration and as the $ {\textit {Re}}_{\delta }$ increases, it appears for a longer part of the period and for a larger region of the boundary layer. Turbulence statistics profiles approach those of equilibrium, unidirectional boundary layer flows with similar $ {\textit {Re}}_{\theta }$ , defined using the local free stream velocity and momentum thickness $\theta$. Analysis of the ensemble-average bed shear stress variation reveals that for $ {\textit {Re}}_{\delta } a single peak, associated with the laminar regime, occurs during the acceleration phase. For $ {\textit {Re}}_{\delta }=552$ a second peak, associated with the transition to turbulence, appears towards the middle of the deceleration phase. This turbulence peak becomes larger than the 'laminar' one for $ {\textit {Re}}_{\delta } \sim 763$ and lags with respect to the free stream velocity maximum. For $ {\textit {Re}}_{\delta }>1036$ the laminar peak disappears under the effect of the turbulence peak. The presence of the phase lag is discussed using data from this study and the literature, and a revised ${\rm \Delta} \phi$ diagram is introduced for the whole range of flows, from laminar to fully turbulent. [ABSTRACT FROM AUTHOR] |
