NUMERICAL STUDY OF THE EFFECTIVE VISCOSITY AND PRESSURE IN PERTURBED TURBULENT FLOWS

We present a numerical investigation of the linear response of a turbu lent flow to a perturbation at large scale. In the incompressible case , we show that part of the small scale action corresponds to the stand ard turbulent 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 a. turbulent pressure force which resists compression. An adiabatic index is estimated for this pr essure and is found to decrease with time scale, from a strongly adiab atic behavior down to a subisothermal behavior. (C) 1998 American Inst itute of Physics. [S1070-6631(98)02711-1].