Azimuthal correlation in the outer layer of turbulent pipe flow

Two point hot-wire measurements of streamwise velocity were performed in the log-layer and wake region of turbulent pipe flow for Reynolds numbers ranging from 8$\times $10$^{4}$ to 8$\times $10$^{6}$ at four wall-normal positions with azimuthal probe separation. The azimuthal correlations were found to be consistent with the presence of very large scale coherent regions of low-wavenumber, low-momentum fluid observed in previous studies of wall-bounded flows. Within the log-layer the azimuthal scale determined from the correlations was found to be similar to that observed for channel flows. As wall-normal position increased outside the log-layer, there was a decrease in azimuthal scale relative to that of channel flow. Using cross-spectral analysis, high-wavenumber motion was found to grow azimuthally with wall-normal distance at a faster rate than the low-wavenumber motions.