  This paper employs an input-output based approach to analyze convective
velocities and the transport of fluctuations in turbulent channel flows. The
convective velocity for a fluctuating quantity associated with
streamwise-spanwise wavelength pairs at each wall-normal location is obtained
through the maximization of the power spectral density associated with the
linearized Navier-Stokes equations with a turbulent mean profile and
delta-correlated Gaussian forcing. We first demonstrate that the mean
convective velocities computed in this manner agree well with those reported
previously in the literature. We then exploit the analytical framework to probe
the underlying mechanisms contributing to the local convective velocity at
different wall-normal locations by isolating the contributions of each
spanwise-streamwise wavelength pair (flow scale). The resulting analysis
suggests that the behavior of the convective velocity in the near-wall region
is influenced by large scale structures further away from the wall. These
structures resemble Townsend's attached eddies in the cross-plane, yet show
incomplete similarity in the streamwise direction. We then investigate the role
of each linear term in the momentum equation to isolate the contribution of the
pressure, mean shear, and viscous effects to the deviation of the convective
velocity from the mean at each flow scale. Our analysis highlights the role of
the viscous effects, particularly in regards to large channel spanning
structures whose influence extends to the near-wall region. The results of this
work suggest the promise of an input-output approach for analyzing convective
velocity across a range of flow scales using only the mean velocity profile.
