Bistability of turbulent flow in open-channel expansion: Characterization and suppression

Abstract

Three-dimensional turbulent bistable flow (TBF) in an open-channel expansion is predicted using large eddy simulation. The free surface of TBF is tracked using the volume of fluid method, coupled with the level-set method. This paper aims to reveal the ensemble-average flow characteristics and explore effective ways to control bistability. For a given condition of flow approaching an expansion, either of two stable flow states can possibly occur, depending on the flow history. The predicted pressure field agrees well with experimental data. The velocity field is decomposed into deformation regions and eddy-rotation regions using the Okubo–Weiss parameter. Turbulent eddies initiated by shear instability dominate those associated with sidewall-friction force; this condition is responsible for the occurrence of bistability. Fitting a simple hump at a flat-bottom expansion is an effective way to suppress bistability. The presence of the hump shrinks eddy cores and breaks the interaction between eddies triggered by instabilities and eddies induced by friction forces; the result is an increase in flow uniformity and control of turbulence, flow separation, and vortex behavior.