Very-large-scale fluctuations in turbulent channel flow at low Reynolds number

It is known that there exist very large features in turbulent channel flow at high Reynolds number as well as in pipe flow and turbulent boundary layer. In addition, a low frequency peak was observed in spectra of the streamwise velocity fluctuation in transitional and low-Reynolds-number but turbulent channel flows. In this study, the large-scale fluctuation observed at the low Reynolds number has been experimentally explored with increasing Reynolds number by means of a hot wire anemometry. There are two peaks in the streamwise velocity spectra for transitional flow for 1800 ≤ Re ≤ 2600, where Re is the Reynolds number based on the bulk velocity and the channel width. The high-frequency peak corresponds to the turbulent vortices that have the same order of magnitude as the channel width, while the low-frequency peak is due to passages of the turbulent patches whose streamwise scale is greater than ten channel widths. It is surprising that the low frequency peak remains even up to Re=3000 at which the flow is fully turbulent. Furthermore, a spectral plateau around the frequency corresponding to 25 channel widths is confirmed up to Re=4000, indicating that there exist very-large-scale fluctuations in turbulent channel flow even at low Reynolds number.