Integral turbulent scales in unsteady rapidly varied open channel flows

Abstract

In an open channel, a rise in free surface elevation is called a positive surge and may occur in man-made channels as well as natural estuaries. Herein new unsteady velocity profiling measurements were performed in positive surges using a relatively large laboratory facility. An ensemble-averaged technique was applied by repeating 25 times each controlled flow condition. The velocity characteristics, Reynolds stress properties and integral turbulent scales were deduced to characterise the unsteady turbulence induced by the surge propagation. The profiling data indicated that the longitudinal velocity profile within the inner region of the turbulent boundary layer followed the log law before, during and after the surge passage. The integral turbulent time and length scale results indicated that the propagation of a positive surge generated an unsteady and anisotropic turbulence. Dimensionless turbulent time and length scale data yielded Lz,z/Lz,x∼1.1–7.3 and Tz,z/Tz,x∼1.1–14.4 for all the flow conditions, suggesting vortical structures with longer dimensions in the vertical direction compared to those in the horizontal directions.