Quasi-steady description of modulation effects in wall turbulence
A theoretical description of the phenomenon of modulation of near-wall turbulence by large scale structures is investigated. The description given is simple in that the effect of large-scale structures is limited to a quasi-steady response of the near-wall turbulence to slow large-scale fluctuations of the skin friction. The most natural and compact form of expressing this mechanism is given by the usual Reynolds-number-independent representation of the total skin friction and velocity, scaled in wall variables, where the mean quantities are replaced by large-scale low-pass-filtered fluctuating components. The theory is rewritten in terms of fuctuations via a universal mean velocity and random mean square fluctuation velocity profiles of the small-scales and then linearised assuming that the large-scale fluctuations are small as compared to the mean components. This allows us to express the superposition and modulation coefficients of the empirical predictive models of the skin friction and streamwise fluctuating velocity given respectively by Marusic et al. (13th Eur. Turb. Conf., 2011) and Mathis et al. (J. Fluid Mech. 2011, vol. 681, pp. 537-566). It is found that the theoretical quantities agree well with experimentally determined coefficients.