A transient numerical investigation is employed in this study to evaluate the influence of channel aspect ratio varying between 2.0 and 12.0 on the secondary currents and other flow characteristics in an open-channel turbulent flow at mildly supercritical Froude numbers. The transient three-dimensional Navier–Stokes equations are numerically solved using a finite volume approach with detached-eddy simulation as the turbulence model. The commonly used rigid-lid approximation to model the free surface is found to be unsatisfactory. A flat wave model linked with the volume of fluid method is used to simulate the free surface at the water–air interface to bring forth the realistic flow structures in the region below the free surface and the side walls. The size of the structures is dependent on the water column height rather than the channel aspect ratio. It is shown that the streamwise velocity profile across the channel has a strong dependence on the channel aspect ratio. This profile has two recognizable points of inflection for aspect ratios between 3.0 and 6.0, which move toward the sidewalls as the channel aspect ratio increases. A region of inviscid-like flow is seen about the channel central plane above a specific vertical location for a small channel aspect ratio only. The distribution of the contour patterns of the ratio of mean vertical and transverse secondary currents is similar for a wide range, and it does not depend on the channel aspect ratio. The transverse profiles of the Reynolds stresses are impacted by the channel aspect ratio and the vertical location from the channel bed. More waves form at the water–air interface in the narrower channel compared to the wider one, which indicates that the free-surface deformation is dependent on the channel aspect ratio. It is highly recommended that to study the fluid structure interaction problems in open channels, it is best to use a channel aspect ratio of 12 or greater.
This study investigates numerically the effect of the aspect ratio (AR) on the velocity field characteristics of the turbulent flow of a straight open-channel flow. Five aspect ratio cases (AR = channel width/flow depth) are investigated ranging from a narrow case of AR = 1 to a wide case of AR = 9. The transient three-dimensional Navier–Stokes equations were numerically solved using a finite-volume approach with an improved–delayed detached-eddy simulation turbulence model. The objective of this study is to enhance our understanding of the effect of AR on the formation of secondary currents in a channel flow. The results revealed the formation of a pair of counter-rotating recirculation zones near the bottom corners of the channel, whose axes are aligned with the main flow direction. Each pair consists of a mean recirculation zone that resides near the bed (bed recirculation zone), and another one that resides near the sidewall (side recirculation zone). The AR appears to significantly influence the size and strength of the recirculation zones. With decreasing the AR, the momentum transport by the streamwise-vertical Reynolds shear stress in the horizontal direction appears to be inhibited at higher vertical locations, the momentum transport by streamwise-transverse Reynolds shear stress is considerably enhanced, and the momentum transport by vertical-transverse Reynolds shear stress is slightly enhanced.
The gap flow effect in a wake is investigated to develop an improved picture of the formation of fluid structures via a numerical simulation of flow past a bluff body with two different clearances from the bed. These two cases are compared with the no-gap case which is considered as a reference case. The transient three-dimensional Navier-Stokes equations are numerically solved using a finite volume approach with the detached eddy simulation as the turbulence model. The effect of the free surface is included in the model by using the volume of fluid method. The fluid structures that are generated in the wake are identified using the λ2-criterion. It is found that the gap flow influences the formation of various types of fluid structures in the wake region. The horseshoe vortex appears to be attenuated as the gap size increases. The turbulent structures at the core of the wake appear to be disorganized and have the ability to extend further in the transverse direction in the absence of the horseshoe vortex. A new structure is identified in the wake flow when the gap size exceeds a threshold value. This structure acts to enhance the positive wall-normal velocity and accelerate the restoration of the free surface to its original position at shorter distances downstream of the bluff body. The lateral entrainment to the wake region is also enhanced as the gap is introduced in the wake flow.
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