The question raised in this paper concerns the meaning attributed to a direct simulation of a turbulent flow corresponding to a non-zero mean solution at large time constant. Indeed, both the mean field and the fluctuations field are affected by the viscosity of the fluid which transfers dissipation energy in the form of heat. The use of the equation of motion on the instantaneous velocity to obtain a turbulent solution inevitably leads to a relaminarization of the flow. This observation goes against the observations where a turbulent flow with a non-zero solution remains turbulent indefinitely. The consequence is that the viscous dissipation must be incorporated for the mean field but not for the fluctuations. The total energy of the initially introduced fluctuations must be maintained in the very long term provided that the equation of motion is conservative. This property is intrinsically ensured in discrete mechanics which conserves a priori the total acceleration but also other quantities like the angular acceleration. A fluctuation equation is thus derived without any assumption or simplification; it includes in particular terms of advection of the fluctuations by the mean flow. The resolution of this equation in time allows to find simultaneously the mean field and the field of fluctuations.