We describe the first DNS-based measurement of the complete mean response of
a turbulent channel flow to small external disturbances. Space-time impulsive
perturbations are applied at one channel wall, and the linear response
describes their mean effect on the flow field as a function of spatial and
temporal separations. The turbulent response is shown to differ from the
response a laminar flow with the turbulent mean velocity profile as base flow.Comment: Accepted for publication in Physics of Fluid
