3D flow motions in the viscous sublayer

TL;DR

This study uses advanced holography to capture and analyze 3D flow patterns in the viscous sublayer of turbulent channel flow, revealing complex unsteady motions and their impact on drag reduction.

Contribution

It introduces a novel holographic measurement technique to observe 3D sublayer flows and uncovers detailed flow structures and statistics not captured by traditional DNS.

Findings

Identification of unsteady, diverse flow patterns in the sublayer

Discrepancies between measured and DNS wall shear stress PDFs

Strong intermittency and small-scale motions linked to high accelerations

Abstract

We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayer of a smooth-wall turbulent channel flow at Re{\tau}=400. The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale (on the order of viscous wall units) spanwise meandering motions. The probability density functions (PDFs) of wall shear stresses show a clear discrepancy in high stress range with those from direct numerical simulation (DNS), which is attributed to the unresolved streamwise and spanwise motions by DNS. Moreover, the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenous isotropic turbulence, indicating strong intermittency in the sublayer. We find a significant fraction of high accelerations is associated with the small-scale meandering motions. Our study helps explain the effect of sublayer-scale roughness on reducing drag and flow separation reported in the literature.