Three dimensional flow motions in the viscous sublayer

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.

Snow-powered research on utility-scale wind turbine flows

This paper provides a review of the general experimental methodology of snow-powered flow visualization and super-large-scale particle image velocimetry(SLPIV),the corresponding field deployments and major scientific findings from our work on a 2.5 MW utility-scale wind turbine at the Eolos field station.The field measurements were conducted to investigate the incoming flow in the induction zone and the near-wake flows from different perspectives.It has been shown that these snow-powered measurements can provide sufficient spatiotemporal resolution and fields of view to characterize both qualitatively and quantitatively the incoming flow,all the major coherent structures generated by the turbine(e.g.,blade,nacelle and tower vortices,etc.)as well as the development and interaction of these structures in the near wake.Our work has further revealed several interesting behaviors of near-wake flows(e.g.,wake contraction,dynamic wake modulation,meandering and deflection of the nacelle wake,etc.),and their connections with constantly-changing inflows and turbine operation,which are uniquely associated with utility-scale turbines.These findings have demonstrated that the near wake flows,though highly complex,can be predicted with substantial statistical confidence using supervisory control and data acquisition(SCADA)and structural response information readily available from current utility-scale turbines.Such knowledge can be potentially incorporated into wake development models and turbine controllers for wind farm optimization in the future.