Towards capturing large scale coherent structures in boundary layers using particle image velocimetry

Here we describe the implementation of a large-scale multicamera particle image velocity (PIV) measurement to resolve the large range of spatial scales prevalent in moderate to high Reynolds number turbulent boundary layers. The objective of these experiments is to capture instantaneous snapshots of recently discovered large coherent structures which are known to inhabit the log and outer regions of boundary layers, while still maintaining sufficient fidelity to resolve a large proportion of the smaller spatial scales that coexist. Measurements are conducted across friction Reynolds number range from Reτ = 2000 to 8000 in a planar PIV configuration on a streamwise/wallnormal plane with a streamwise field of view in excess of 14 boundary layer thicknesses. These datasets are supplemented by planar PIV measurements on a streamwise/spanwise plane which spans a similar streamwise extent. Flow statistics are compared to recent numerical simulation databases at moderate Re, and to well resolved single point hot-wire anemometry measurements at higher Re to validate and assess the quality of the PIV velocity fields. The results reveal good agreement and also indicate that a simple box filter approach can be used to estimate spatial attenuation for PIV measurements. Comparisons between the pre-multiplied energy spectra of the streamwise velocity fluctuations from the PIV and hotwire databases also show good agreement over the field of view when Taylor’s frozen turbulence hypothesis is assumed.