  A technique for measuring the mean impulse response function of stationary
homogeneous isotropic turbulence is proposed. Such measurement is carried out
here on the basis of Direct Numerical Simulation (DNS). A zero-mean white-noise
volume forcing is used to probe the turbulent flow, and the response function
is obtained by accumulating the space-time correlation between the white
forcing and the velocity field.
  This technique to measure the turbulent response in a DNS numerical
experiment is a new research tool in that field of spectral closures where the
linear response concept is invoked either by resorting to renormalized
perturbations theories or by introducing the well-known Fluctuation-Dissipation
Relation (FDR).
  Though the results obtained in the present work are limited to relatively low
values of the Reynolds number, a preliminary analysis is possible. Both the
characteristic form and the time scaling properties of the response function
are investigated in the universal subrange of dissipative wavenumbers; a
comparison with the response approximation given by the FDR is proposed through
the independent DNS measurement of the correlation function. Very good
agreement is found between the measured response and Kraichnan's description of
random energy-range advection effects.
