Research On The Fluid Flow Structures And Dynamics Around The Flat Plate Cascades

Fluid flow around flat plate cascades is a classical and fundamental topic of fluid mechanics,which can be applied widely in many different engineering design fields,such as hydraulic turbine(pump)blade,bridge piers,high-rise building group,offshore structure,intelligent transportation et al.This research takes the flat plate cascades as research object.The Large Eddy Simulation(LES)numerical method and Laser Doppler Anemometry(LDA)velocity measurement system are used to study flow structures and dynamic around flat plate cascades at various Reynolds numbers under different arrangements.It is funded by Natural Science Foundation of China(Vortex dynamics and its excitation mechanism of turbulent flow in pump,51576090).The main work and obtained results are as following.Firstly,based on the flow around blades in pump,a set of flat plates with circular leading edge are designed,and they are arranged as research models of two tandem flat plate cascades and four-plate cascades.A small closed water tunnel test platform is designed and built.The matching design is made for the diffusion section,the rectifying section,the shrinkage section and the test section in the water tunnel.In the end,a water tunnel system is constructed to meet the requirements of the experiment.Secondly,in order to capture the evolution processes of vortex structures at different scales accurately,a LES numerical method is explored,which is suitable for calculating the fluid flow around cascades.A case of single cylinder flow(Re=3900)and several cases of flat plate cascades flow are calculated,from which the St number,time-average velocity,vortex shedding frequency and vortex structures are compared with the experimental results from papers,LDA and PIV.The applicability and accuracy of the numerical method are verified.Thirdly,Based on the determinate LES calculation method,cases of three-dimensional turbulent flow around two tandem plate cascades are investigated over a wide range of Reynolds numbers from 0.5×10~4 to 10×10~4.The velocity filed in the gap and wake area,forces on the plate and vortex shedding frequency are analyzed carefully.The evolution processes of vortex structures around plate cascades are described under different flow conditions.Besides,the relationship between vortex structures and forces on the plate is constructed,and the change law of the St-Re is summarized.The study shows that if based on the distribution of time-averaged stream-wise velocity along the wake centerline,the gap and wake area can be divided into four regions:(?)reverse flow region,(?)rapid growth region,(?)fall back region,(?)gentle growth region.If according to the stream-wise velocity gradient along the z-axis,the wake region can be divided into:(?)vortex street interaction region and,(?)mainstream region.The wake of the upstream plate has a great influence on the lift force acting on the downstream plate,but has little impact on the drag force.The vortex shedding from trailing edge is the main factor affecting the forces on the plate.However,with the decrease of the Reynolds number,the influence of mature vortices separating from the plate surfaces becomes stronger.In the range of 0.5×10~4?Re?10×10~4,the Strouhal number depends strongly on the Reynolds number,while it stays unchanged for 4.8×10~4?Re?10×10~4,and the Strouhal number reaches a minimum at Re=1.0×10~4.Fourthly,cases of three dimensional turbulent flow around four-plate cascades under L_d=2.5D and 5D spacing are calculated.The influence of the spacing on the flow around the plate cascades is analyzed,and the differences of the flow structures in the gap area are revealed by the three-dimensional cloud map.It is found that the von Karman vortex can not be formed in the gap zone when the distance of the gap is only2.5D.The rise and drag coefficients of the plate are restrained obviously,and the vortex shedding frequency of the downstream plate decreases.However,when the distance of the gap is 5D,the von Karman vortex in the gap area is captured obviously.The fluctuation amplitude of lift coefficient of the downstream plate rises significantly,while the drag coefficient is lower than the upstream plate cascades.The vortex shedding frequency is the same for four plates.Furthermore,the LDA velocity measurement system is built.The velocity fields of two tandem plate cascades under different gap distance from 0 to 5D are measured accurately on the water tunnel test platform.The distribution law of time-averaged stream-wise velocity in the gap and wake area and turbulent kinematic energy are obtained.The law of instantaneous velocity distribution of a single monitoring point is analyzed and the vortex shedding frequency is extracted.Finally,the change regulation of the St,L_d and Re is obtained.The measurement results show that,when the gap distance is small,the vortices can not shed directly from the upstream plate trailing edge into the gap area,which leads to the time-averaged stream-wise velocity along the center-line increases first then decreases.And the turbulent kinetic energy in the gap zone is lower than in the wake area.When the spacing increases to the periodic vortices can be formed in the gap zone,the frequency of vortex shedding is both the same in the gap and wake area.The flow velocity also decreases first then increases.The turbulent kinetic energy rises significantly in the gap area.Besides,the vortex shedding frequency in the tandem plate cascades is always less than that of single plate,which means that the arrangement of tandem has an inhibitory effect on vortex shedding.With the increase of the distance,the distribution trend of the St number decreases first then increases.At the gap distance is 3D,the inhibitory effect reaches the maximum at various Re numbers.