Signature of varicose wave packets in the viscous sublayer

Waviness of the wall-shear stress (WSS) field of a turbulent boundary layer is investigated experimentally. Arrays of flexible micropillars are used as sensors for WSS in the turbulent boundary layer of a flat plate in a zero pressure gradient flow channel. The pillars and flow conditions are chosen such that accurate dynamic measurements of the two-dimensional (2D) WSS vectors at the sensor locations are obtained for the mean and the fluctuating part simultaneously. Time-resolved measurements reveal for the first time the orientation of the 2D WSS vectors and their spatiotemporal correlation. The results show trains or packets of varicose wavelike disturbances of the near wall flow with strong anticorrelation of the spanwise WSS component, which appear periodically and can be related to the signature of coherent packets of vortex loops. In addition these structures may represent the near-wall state of secondary varicose instability of sublayer streaks. (C) 2008 American Institute of Physics.