The large-scale structure of turbulent pipe and channel flows

In recent years there has been renewed interest in the large-scale behaviour of wall-turbulence. This is largely due to the insights of Adrian and co-workers (see Adrian, {\it Phys. Fluids}, {\bf 041301}, 2007) and more recently Hutchins \& Marusic ({\it J. Fluid Mech.}, {\bf 579}, 2007). Much of the existing work has centered around turbulent boundary layer measurements, leaving scope for further investigation into pipe and channel flows. Following the work of Hutchins \& Marusic (HM07), the authors sought to study the large-scale structure of duct flows using multi-probe hot-wire arrays. The arrays were custom made to minimise blockage and consisted of up to 15 hot-wires. The results show that `superstructures', identified by HM07 for boundary layers, exist qualitatively similarly in pipes and channels. These coherent structures have lengths of up to 20 pipe radii or channel half-heights. Furthermore, it was found that the spanwise width of the structures was similar in pipes and channels, but much larger than that in a boundary layer. This work forms part of an ongoing investigation into the structural similarities and differences between pipes, channels and boundary layers.