Statistical evidence of hairpin vortex packets in wall turbulence

The structure of velocity in the outer region of turbulent channel flow (y( +) greater than or similar to 100) is examined statistically to determine t he average flow field associated with spanwise vortical motions. Particle i mage velocimetry measurements of the streamwise and wall-normal velocity co mponents are correlated with a vortex marker (swirling strength) in the str eamwise-wall-normal plane, and linear stochastic estimation is used to esti mate the conditional average of the two-dimensional velocity field associat ed with a swirling motion. The mean structure consists of a series of swirl ing motions located along a line inclined at 12 degrees -13 degrees from th e wall, The pattern is consistent with the observations of outer-layer wall turbulence in which groups of hairpin vortices occur aligned in the stream wise direction. While the observational evidence for the aforementioned mod el was based upon both experimental and computational visualization of inst antaneous structures, the present results show that, on average, the instan taneous structures occur with sufficient frequency, strength, and order to leave an imprint on the statistics of the flow as well. Results at Re-tau = 547 and 1734 are presented.