Frequency of sublayer bursting in a curved bend

Abstract

The characteristics of sublayer bursting in turbulent pipe flow through a 180° curved bend at a pipe Reynolds number of 50000 were investigated. In particular, the effects of bend curvature on the bursting frequency were studied in detail. A flush-mounted shear stress gauge was used to measure the wall shear stress and the VITA technique was applied to isolate the bursting events in the wall shear signals. The results show that the bursting frequency starts to drop rather dramatically at the inlet region of the inner bend (closest to the centre of curvature of the bend). Contrary to expectation, the bursting frequency remains fairly constant along the outer bend, but increases sharply as the flow comes out of the bend. Possible explanations for this behaviour are proposed based on the important external effects present in a pipe bend; that is, those due to centrifugal force, streamline curvature and the superimposed secondary flow. Even though wall pressure measurements show that the flow recovers to a fully developed straight pipe condition at a short distance downstream of the bend exit, the circumferential wall shear stress distribution, the spectral content of the wall shear signal and the associated bursting frequency suggest that the near-wall flow takes a much longer distance to return to an unperturbed straight-pipe condition.