We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns i...We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(onthe order of viscous wall units)spanwise meandering motions.The probability density functions(PDFs)ofwall shear stresses show a clear discrepancy in high stress range with those from direct numerical simu-lation(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 homogenousisotropic turbulence,indicating strong intermittency in the sublayer.We find a significant fraction of highaccelerations is associated with the small-scale meandering motions.Our study helps explain the effectof sublayer-scale roughness on reducing drag and flow separation reported in the literature.展开更多
文摘We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(onthe order of viscous wall units)spanwise meandering motions.The probability density functions(PDFs)ofwall shear stresses show a clear discrepancy in high stress range with those from direct numerical simu-lation(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 homogenousisotropic turbulence,indicating strong intermittency in the sublayer.We find a significant fraction of highaccelerations is associated with the small-scale meandering motions.Our study helps explain the effectof sublayer-scale roughness on reducing drag and flow separation reported in the literature.
