Statistical properties of vortical structures with spanwise vorticity in zero pressure gradient turbulent boundary layers

Turbulent boundary layers at large Reynolds numbers up to Re =17 500 are obtained in a very large recirculating water tunnel and are analyzed through PIV measurements in streamwise-wall- normal planes. Vortical structures in the outer region are tracked using a wavelet based technique. The adopted method, which can be applied also to time series obtained from pointwise measurements, provides the separation of coherent structures from the background flow and the measure of their length scale and of other relevant quantities. The range of Re under test is extended by analyzing also a PIV database obtained in a different laboratory flow public domain data provided by Adrian and co-workers at lower Re 1015 and 7700. In this way, statistical properties of the spanwise coherent structures are parametrically studied in terms of Re. The analysis of conditional vortices demonstrates the capability of the identification method to correctly track the spanwise vortical structures, to determine their length scales and, at low Re, to evidence the formation of vortex packets. The probability distribution functions of relevant quantities such as the length scale, the local energy, and the vorticity of the educed vortices are shown to collapse well when normalized with respect to low order statistical quantities mean and variance. Analytical approximations useful for extrapolating the observed statistical properties at a large Reynolds number are also presented and discussed.