USE OF ELECTROCHEMICAL SENSORS FOR THE DETERMINATION OF WALL TURBULENCE CHARACTERISTICS IN ANNULAR SWIRLING DECAYING FLOWS

The wall shear stress and turbulence features in swirling decaying flo w were analyzed near the inner cylinder of a cylindrical annulus fitte d with a tangential inlet. The experimental method was based on the de termination of the limiting diffusional current on electrochemical sen sors mounted flush to the surface of the inner cylinder. The mean valu e of the diffusional current leads to the mean wall velocity gradient, and the power spectral density of the current fluctuations is linked to that of the velocity gradient fluctuations through a transfer funct ion. The analysis of the mean wall velocity gradient allowed the three -dimensionality of the swirl flow and the swirl decay along the flow p ath to be studied. Just downstream of the tangential inlet, a recircul ation zone is set up near the inner cylinder, followed by a nonmonoton ic decay of the swirl motion. These two zones define the three-dimensi onal flow region. The wall turbulence was characterized by both the tu rbulence intensity and the cutoff frequency, defined by the transition between the low-frequency plateau and the inertial subrange of the po wer spectral density. The turbulence intensity, which is quasi-indepen dent of the Reynolds number, decreases with axial distance from the in let section and has a more complex evolution with the circumferential position with respect to the axis of the tangential inlet. (C) Elsevie r Science Inc., 1997.