We present a numerical investigation of the linear response of a turbulent flow to a perturbation at large scale. In the incompressible case, we show that part of the small scale action corresponds to the standardturbulent viscosity, but that there are also other terms which are important for the system dynamics and which do not reduce to a simple turbulent viscous effect. In the compressible case, we show that the small scales of the turbulent flow create aturbulent pressureforce which resists compression. Anadiabatic indexis estimated for this pressure and is found to decrease with time scale, from a stronglyadiabaticbehavior down to asubisothermalbehavior. ©1998 American Institute of Physics.