Spatial Structure of Negative ∂ũ/∂y in a Low Rθ Turbulent Boundary Layer

Abstract

Multiple line Molecular Tagging Velocimetry (MTV) measurements are used to explore the flow fields associated with wall region negative ∂ũ/∂y in a low Rθ turbulent boundary layer. Velocity and velocity gradient statistical profiles establish the present flow as a canonical flat plate boundary layer. Spatial correlations of ∂u/∂y in the (x, y) plane, and the probability of observing negative ∂ũ+ /∂y+ exceeding a negative threshold of −0.25 reveal the highly significant presence of negative ∂ũ/∂y in the region 25 ≤ y+ ≤ 35. Examination of the instantaneous flow fields underlying the conditional pdf indicates that there are two primary contributing motions: (i) streamwise shear layers and (ii) compact regions containing ±∂ũ/∂y. The importance of these motions to wall region turbulence production is discussed.