| Floating vegetation is often found in natural rivers,diversion channels,lakes and reservoirs and is widely used in river management.The presence of floating vegetation affects the structure of water flow,which in turn can affect sediment transport,riverbed deformation and the hydro-ecological water environment.However,research on the mechanical properties of the water under the influence of floating vegetation is uncommon and immature.In this thesis,a combination of flume physical experiments and numerical simulations is used to compare and analyze the flow characteristics of floating vegetation groups at different depths of entry,placement,deployment density,diameter and patch spacing.In the flume physics experiments,measurements were carried out using ultrasonic water level meters and Particle Image Velocimetry(PIV)instruments to obtain data on water level and flow velocity at typical sections,and to analyze the water level,longitudinal flow velocity,turbulent kinetic energy and Reynolds stress distribution.In the numerical simulation,SPACECLAIM software is used to establish a mathematical model in proportion to the physical experiment flume,and the hexahedral unstructured grid is divided and solved by FLUENT,and processed visually in TECPLOT and CFD-POST software.At the same time,the accuracy of the numerical simulation is verified by comparing with the physical experiment results.Finally,the variation of velocity and turbulent kinetic energy under different cases are analyzed.The main research results and conclusions are as follows:(1)The floating vegetation obviously obstructed the water level in the upper reaches of the vegetation area,resulting in a certain water slope.The water level and water surface slope along the way at the upstream of the vegetation area grow with the increasement of the length of the vegetation in the water.The influence of full-section vegetation layout on the water level and water surface slope is greater than that of half-section vegetation layout.The higher the vegetation density,the higher the water level and water surface slope.(2)With the increase of vegetation entry depth,the position of maximum velocity and maximum fluctuation move down.When floating vegetation is arranged in half section,the mean value of vertical velocity is higher than that in full section,and turbulent kinetic energy is maximum at the junction of vegetation area and non-vegetated area.With the increase of vegetation density,the fluctuation range of vertical velocity becomes larger,the turbulent kinetic energy becomes smaller,and the flow needs to go through a longer distance to recover to the average velocity of the section and maintain stability.(3)Along the water depth direction,the maximum value of Reynolds stress appears in the area where the vegetation roots transition to the free water below.Along the flow direction,the variation amplitude of Reynolds stress reaches the maximum value in the downstream of the vegetation group.The existence of floating vegetation groups is beneficial to the stability of river bed morphology and can reduce the scour deformation of river bed.(4)As the flow increases,the flow velocity increases in all sections and the variation in flow velocity along the way increases,the turbulent kinetic energy decreases;as the diameter of the vegetation increases,the longitudinal flow velocity is more evenly distributed along the lateral direction,the greater the resistance effect downstream of the vegetated area,the larger the peak turbulent kinetic energy behind the vegetation;the spacing between vegetation patches is smaller is more conducive to flow stability. |
