Experimental measurement of the scale-by-scale momentum transport budget in a turbulent shear flow

We report measurements linking velocity fluctuations with the turbulent drag in a turbulent closed flow, namely the von Karman flow. Making use of the angular momentum balance equation in integral form, we obtain a simple expression for the torque applied by the forcing mechanism, which we check against quantitative laser Doppler velocimetry measurements. We then decompose the angular momentum flux into contributions coming from the different spectral components of the flow. We provide evidence of the fact that the turbulent drag is dominantly generated by coherent structures at the largest scales of the flow. (C) 2004 American Institute of Physics.