| Open channel flow in a bend is a common form in water conservancy projects.Due to the combined effects of gravity and centrifugal force,its own structures redistributes along both horizontal and vertical directions.The study on the flow characteristics of curved flow is one of the most important topics in hydraulics.This subject has both theoretical significance and engineering practical value.Therefore,this article takes open channel flow in a bend as the research object,and study the effect of geometrical parameters onn the flow structures by a combined method of experiment and numerical simulation.The main contents are as follows.1.Flow in a U-shaped bend with three different width-depth ratios(1.5,2,3)is finely measured by Acoustic Doppler Velocimetry(ADV).The experiments show that,the greater the width-depth ratio is,the more the position of the hydrodynamic axis begins to shift toowards the bank.This can explains that the mainstream line with small value of velocity flows along the convex bank,and it begins to shift to the concave bank when flowing out of the curve.However,mainstream line with large value of velocity is earlier to the concave bank.With the increase of width-depth ratio,the position where the lateral flow begins to fluctuate along the concave bank and that secondary circulation appears near the surface of concave bank are close to the upstream,subsequently.In these three conditions,the shearing force in the convex bank is larger than that in the concave bank,and it becomes larger from the entrance section to the curved section of the bend.In addition,the maximum shear force appears in the first half of the curve.It can be inferred that the erosion of sediment in the curve is more serious in this position.2.In this paper,the large-eddy simulation method based on high and density grid is used to simulate the flow in the bend channel.The simulation results show that the velocity distribution along transverse section is in accordance with the distribution of the main line in bend;In addition the phenomenon of water separation occurs near the convex bank of the bend direction.The circulation structure in the cross transverse section is complex and diverse,especially in the vicinity of the bend top section where the main circulation is nested with multiple small secondary loops and these secondary loops continue to develop,interacting with the main circulation.Several minor secondary circulations are nested in the main circulation,and these secondary circulations develop continuously and interact with the main circulation.In the surface of the water body,the flow rate is small,but the vortex is larger,and the distribution is even.In the lower part of the water body,the vortices near the center line of the bend are small and there are various vortices in the convex bank and the narrow area of concave bank of the bend top section.3.We conduct a three-dimensional numerical simulation of water and fixed bed flow within rectangular corners,the curvature of which are 30°,60°,90°,120°,150°,180°,210°respectively.Here,the RNG k-ε model is selected,and the wall function and VOF method is used to deal with the boundary and free water surface.The results show that when increasing the curve bending,the superelevation of the water surface increases,but the surface slope decreases.If the curve is more bending,the mainstream become more concentrated,and the mainstream of export cross-section of curve move more to the convex bank.The circulation strength also increases and the circulation center tends to move upward and to the convex bank with increasing the bending.4.The radius-to-width ratio is one of the geometric parameters of open channel and has an important effect on the flow structures.By use of a three-dimensional numerical simulation method(RNG k-ε model),the flow structures in 90° open channel bend corresponding to four radius-to-width ratios are investigated at the case of small width-to-depth ratio.The results show that the radius-to-width ratio is strongly associated with the surface pattern,flow field,turbulent kinetic energy and circulation strength.If the radius-to-width ratio is smaller,the gap of velocities of concave bank and convex bank will be bigger,and the distribution of velocity on cross section will be more uneven.Besides,the mainstream will be more concentrated,and the transverse velocity and the circulation strength will be higher.When radius-to-width ratio is more than 1.5,the traditional theoretical formula of ultra-height is still applicable.The water surface changes mildly,and the turbulent kinetic energy of flow out of the channel decreases gradually along the straight in the downstream.When radius-to-width ratio is less than 1.5,the traditional theoretical formula of ultra-height is not applicable.The flow is separated along with a large concave pit in the convex bank,and the turbulent kinetic energy of flow increases continuously in wide range of the straight in the downstream. |
