Flow field properties local to near-wall shear layers in a low Reynolds number turbulent boundary layer

Abstract

Combined hot-wire anemometry and flow visualizations are used to explore the properties of near-wall shear layers in a low Reynolds number turbulent boundary layer (Rθ=U∞θ/ν≅1200). The experiments employed two four-element probes positioned in the buffer layer. Data from these sensors were acquired synchronously with high speed, dual view, 35 mm movies of vaporized mineral oil fog illuminated via two orthogonal laser sheets. Near-wall shear layers were identified as satisfying the visual criterion of a narrow fissure of passive marker moving upward through the buffer layer in concert with a strong deceleration in the axial velocity trace. The present results reveal that the local flow fields satisfying these criteria regularly contain adjacent and spatially compact regions of opposing-sign spanwise vorticity ωz in the streamwise/wall-normal plane. A number of features were identified relative to whether ωz at the upper probe was positive or negative. In this regard, the configuration with positive ωz located above a region of negative ωz correlated with a more rapid wall-normal transport of the passive marker. Conditionally averaged measurements indicate that the flows local to near-wall shear layers have an attenuated opposing effect relative to the long time vωz contribution to the Reynolds stress gradient. Overall, these observations point to the change-of-scale mechanism rather than gradient transport in establishing the large Reynolds stress gradient in the buffer layer. Both spatial configurations of ωz were revealed to have association with an advective acceleration upstream of the detected shear layer. The scale of this accelerated region was larger for the condition of negative ωz located above a region of positive ωz. The detection of near-wall shear layers had a strong visual correlation with clusters of spanwise vortices, often occurring in counter-rotating pairs.