Flow Separation in Sharp-Bend-Flow

Abstract

In sharp-bend-flow the hydrodynamic phenomena are different than in straight channel flow. Flow separation and hdelical flow are the most important sharp-bend-flow phenomena and they have a major morphological effect. Literature states that the transverse water surface tilt is an important factor in determining the internal processes of flow separation and helical flow and that flow depth and roughness influence the transverse tilt. Literature also states that the effects of flow separation are flow recirculation, contraction of the main flow, a shear layer, shear layer-induced vorticity, interaction between hydraulic phenomena and separation-sediment interaction. The exact causes and internal processes of flow separation remain unknown. Experimental laboratory data-sets of surface flow structures were analyzed and several maps were made to try to fill the knowledge gap. The results showed that an increase in flow depth and/or Froude number both lead to an increase in transverse water level tilt, thus promoting helical flow and flow separation by creating an adverse pressure gradients. The flow separation creates a sharp transition in flow velocity that starts halfway the inner bend and diffuses downstream while creating vorticity. Flow recirculation was not significant in the constant width bend, but outer bank widening was likely the cause of considerable flow recirculation. The larger the flow depth the smaller the extent of the recirculation zone. The shear layer “flaps” with a period of approximately 13 seconds over 6% of the flow width. An increase in flow depth resulted in a downstream movement of the shear layer and an increase in the Froude number caused the downstream end of shear layer to bend towards the inner bend.