An investigation on the outer bank cell of secondary flow in channel bends

Beside the curvature-induced secondary flow in curved channels, often a counter-rotation secondary flow cell is observed near the outer bank, called outer-bank cell. In the current study, experiments were conducted for three low subcritical Froude numbers typical for natural streams that complement the Froude number range investigated in previous studies. It was found that for increasing Froude numbers, the main cell of secondary flow expanded and strengthened. Simultaneously, the outer-bank cell reduced in size and retreated towards the boundary. A term-by-term analysis of the energy production and the vorticity equations was performed to substantiate this observation. In the outer-bank zone, for increasing Froude numbers, a reduction of the energy flux from turbulence to the mean flow was discernible; which confirms that the turbulence-induced processes affect the outer-bank cell. The analysis of the vorticity equation showed the balance between generative and degenerative terms in the streamwise vorticity equation.