DOI: 10.1615/TSFP10
Large coherence of spanwise velocity in turbulent boundary layers
SINOPSIS
This study investigates the spatial signature of spanwise velocity coherence in turbulent boundary layers. A series of unique large field-of-view multi-camera particle image velocimetry (PIV) experiments are conducted in a planar arrangement at a friction Reynolds number of Reτ ~ 2600. The experiments are configured to capture streamwise/spanwise slices of a turbulent boundary layer in both the logarithmic and wake regions. Consistent with numerical observation of Sillero et al. (2014), we observe a pronounced oblique pattern of spanwise velocity coherence with an orientation dependant on the sign of the spanwise velocity. This behaviour is sufficiently strong to be distinguished in the instantaneous flow fields and mostly obvious in the outer most part of the layer. Additionally, we observe that this oblique arrangement dictates the alignment of the intermittent turbulent bulges, which leads to a pronounced impact on the geometry of the turbulent/non-turbulent interface. Finally, we test whether a boundary layer constructed using a model based on the attached eddy hypothesis can produce similar behaviour for the coherence of spanwise velocity. Comparisons between the model and experimental velocity fields reveal some degree of similarity with both exhibiting coherent regions of spanwise velocity that are arranged diagonally.