  The ability of linear stochastic response analysis to estimate coherent
motions is investigated in turbulent channel flow at friction Reynolds number
Re$_\tau$ = 1007. The analysis is performed for spatial scales characteristic
of buffer-layer and large-scale motions by separating the contributions of
different temporal frequencies. Good agreement between the measured
spatio-temporal power spectral densities and those estimated by means of the
resolvent is found when the effect of turbulent Reynolds stresses, modelled
with an eddy-viscosity associated to the turbulent mean flow, is included in
the resolvent operator. The agreement is further improved when the flat forcing
power spectrum (white noise) is replaced with a power spectrum matching the
measures. Such a good agreement is not observed when the eddy-viscosity terms
are not included in the resolvent operator. In this case, the estimation based
on the resolvent is unable to select the right peak frequency and wall-normal
location of buffer-layer motions. Similar results are found when comparing
truncated expansions of measured streamwise velocity power spectral densities
based on a spectral proper orthogonal decomposition to those obtained with
optimal resolvent modes.
