Secondary Currents in Curved Flows

Bottom friction in curved flows induces weak secondary flows in a vertical plane orientated at right angles to the direction of the depth averaged velocity. In a curved channel this results in weak flows towards the outside of a bend at the surface and weak inward flows near the bottom. When combined with the strong along channel primary flow, the net result is a helical motion as water moves around the bend. Though weak the cross-channel secondary flows are important because they move sediment towards the inside of the bend, significantly enhancing horizontal dispersion and creating upwelling. These secondary flows occur in channels within eddies and in the curved flow around headlands. This project used Acoustic Doppler Current Profiler measurements from a moving vessel to quantify the intensity of secondary flow in the Otago Harbour Channel. The plot on the lower right shows the averaged measured secondary flow velocity profile, with surface flows outward and near bottom flows inward.The measurements showed that the secondary flow was 50% stronger than predicted by a theoretical model.

Future work will look at how secondary flow varies across a channel and PhD student Pete Russell is making detailed measurements of secondary flow around Cape Saunders, a headland on Otago Peninsula.

Abstract

High Resolution Observations of the Intensity of Secondary Circulation along a Curved Tidal Channel - Ross Vennell and Chris Old, VOL. 112, Journal Geophysical Research doi:10.1029/2006JC003764, November  (2007).

High horizontal resolution moving vessel ADCP observations of the spatial pattern of cross-stream velocities in a curved tidal channel show radially outward surface velocities up to 10cm/s which are maximum mid-channel, consistent with helical secondary flow in a vertical plane normal to the depth averaged velocity. The 30m cross- and 150m along-channel resolution observations are from a 2700m long section of a 350m wide horizontally and vertically well mixed tidal channel with a radius of curvature 1- 5 km. The along-channel resolution allows the intensity of the curvature induced secondary flow to be estimated from the linear correlation between the observed cross-channel component of vertical shear and the shear estimated from the streamwise velocity and its varying curvature using an existing analytic model. The two shears are highly correlated and the regression line slope demonstrates that the observed curvature induced secondary flow is 30% more intense than that predicted by the model for a typical bottom drag coefficient. The secondary flow is 50% more intense than that predicted using the drag coefficient which best fits the streamwise velocity profile.Numerical solutions demonstrate that the intensity of the secondary flow is sensitive to small changes in the shape of the eddy viscosity profile, hence intensity may be sensitive to the way turbulence is modeled. Lagged correlation of the observations showed that the secondary flow adapts to changes in curvature and primary flow over a 300m length scale, or 20 water depths, consistent with the existing model and laboratory studies.