Some aspects of linear and non linear scale interactions in wall turbulence

Simultaneous hotwire measurements across the vertical direction, obtained in the atmospheric surface layer in near-neutral conditions at ReT - 5uT jv ~ 10s, are used to study interactions among the turbulent scales, from the very large scale motions (VLSM of order of 6 - 10<S) lo the dissipative scales. In this contribution we want to focus on the linear and non linear character of such interactions. Two distinct ways are pursued, the first based on wavelet analysis, the second one based on the Mutual Information Content (MIC). The overall picture confirms that large and very large scale motions arc actively participating in near wall turbulence activity. By wavelet analysis we can quantify the temporal evolution of the energetic contribution of different large and small (strictly time) scale motions, and thus study if VLSM locally influence smaller time scale processes. Such interaction can be investigated also probabilistically as the mutual influence between two different quantities, namely a large scale velocity time series and a (varying) velocity differences time series Au{t,z) = u(t + T) - u(t), representative (for small time lag T) of the small scales of turbulence. MIC analysis allows a distinction to be made between the linear and non linear type of interaction, suggesting perhaps how much and where small scales are i) simply converted by or ii) "entangled" with large scales motions.