ENERGY INJECTION IN CLOSED TURBULENT FLOWS - STIRRING THROUGH BOUNDARY-LAYERS VERSUS INERTIAL STIRRING

The mean rates of energy injection and energy dissipation in steady re gimes of turbulence are measured in two types of flow confined in clos ed cells. The first flow is generated by counterrotating stirrers and the second is a Couette-Taylor flow. In these two experiments the soli d surfaces that set the fluid into motion are at first smooth, so that everywhere the velocity of the stirrers is locally parallel to its su rface. In all such cases the mean rate of energy dissipation does not satisfy the scaling expected from Kolmogorov theory. When blades perpe ndicular to the motion are added to the stirring surfaces the Kolmogor ov scaling is observed in all the large range of Reynolds numbers ( 10 (3) < Re<10(6)) investigated. However, with either smooth or rough sti rring the measurements of the pressure fluctuations exhibit no Reynold s number dependence. This demonstrates that, though the smooth stirrer s are less efficient in setting the fluid into motion, their efficienc y is independent of the Reynolds number so that the Kolmogorov scaling characterizes, in all cases, the dissipation in the bulk of the fluid . The difference in the global behaviors corresponds to a different ba lance between the role of the different regions of the flow. With smoo th stirrers the dissipation in the bulk is weaker than the Reynolds-nu mber-dependent dissipation in the boundary layers. With rough (or iner tial) stirrers the dissipation in the bulk dominates, hence the Kolmog orovian global behavior.