Very large-scale motion in the outer layer

Very large-scale motions in the form of long regions of streamwise velocity fluctuation are observed in the outer layer of fully developed turbulent p ipe flow over a range of Reynolds numbers. The premultiplied, one-dimension al spectrum of the streamwise velocity measured by hot-film anemometry has a bimodal distribution whose components are associated with large-scale mot ion and a range of smaller scales corresponding to the main turbulent motio n. The characteristic wavelength of the large-scale mode increases through the logarithmic layer, and reaches a maximum value that is approximately 12 -14 times the pipe radius, one order of magnitude longer than the largest r eported integral length scale, and more than four to five times longer than the length of a turbulent bulge. The wavelength decreases to approximately two pipe radii at the pipe centerline. It is conjectured that the very lar ge-scale motions result from the coherent alignment of large-scale motions in the form of turbulent bulges or packets of hairpin vortices. (C) 1999 Am erican Institute of Physics. [S1070-6631(99)01002-8].