The growth of swirl in curved circular pipes

Steady secondary currents in the entry region of curved circular pipes with different curvature ratios (R/a=4.66 and 16) have been delineated at moderate Dean numbers by pulsed-probe anemometry. When swirl is quantified using circulation loops, flow development can be scaled by the length parameter (aR)1/2. The initial growth of axial swirl seems consistent with a model of vorticity transport based on average streamline curvature, but does not progress monotonically to an asymptotic value. Instead, an abrupt relaxation occurs approximately 2(aR)1/2 downstream from the inlet, suggesting that boundary-layer current intensifies well before the axial velocity profile can be reordered by convection.