Coriolis generated secondary currents and their effects on turbulent channel flow

Sammanfattning: The influence of secondary currents on various hydrodynamic processes in channel flow is discussed. Different mechanisms that generate secondary currents are presented. The effect of the earth's rotation on channel flow is analysed and it is shown that in wide channels and for moderate Rossby numbers the relative magnitude of the horizontal secondary currents is approximately 4 times the inverse of the Rossby number. The effect of the channel width on the magnitude of the secondary currents is shown to be small for normal aspect ratios. The Coriolis generated secondary currents are shown to develop in 30-60 channel depths downstream from zero cross-stream velocities. The effect of secondary currents on friction losses is shown to be small for secondary velocities of the order of I % of the downstream velocity. However, secondary velocities of this magnitude are shown to have significant effect on the lateral mixing of a passive tracer in a channel. The distance needed for complete lateral mixing is clearly reduced with secondary currents. This effect is shown to increase when the source is located off the centre line of the channel. It is further shown that a diffusion model for the lateral mixing is not appropriate when the secondary currents are important.