Drag fluctuations of a disk in a turbulent jet: Effect of turbulent scales averaging

The drag fluctuations of a disk placed on the axis of a turbulent incompressible jet are studied at Re=70000. Statistics and spectra have been obtained for different disk sizes. A significant spatial averaging effect is observed in the symmetrization of probability distribution function and in the low-pass filtering of spectra. These effects are associated with a redistribution of the high-frequencies energy to the low frequencies. It is shown that this redistribution is done in such a way that the rms value of the drag fluctuations increases linearly with the disk surface. These results concerning the drag fluctuations are compared and found to be consistent with fluctuations of a global kinetic energy extracted from the turbulent field in front of the disk.