Research On Fluid Flow And Force Characteristics Around D-Shaped Flat Plate Cascades

Flow around the flat plate is a classical and fundamental topic of fluid mechanics,which contains complex flow structures,such as boundary layer separation and wake.The flat plate is excited by fluid to induce structural vibration and flow noise.This phenomenon has been widely found in many engineering domains,such as hydraulic turbine,bridge,ocean engineering,transportation engineering et al.The study of flow around the flat plate can be provided fundamental theoretical support for engineering application.Compared with the ordinary flat plate,the D-shaped flat plate is modified the leading edge,which is changed the flow separation point,the vortex shedding pattern and the mechanical characteristics.The study of flow around the D-shaped flat plate can be provided reference for flow control,lower noise blade design and reliable data for numerical calculation and verification.It is funded by Natural Science Foundation of China(Vortex dynamics and its excitation mechanism of turbulent flow in pump,No:51576090).The main work and obtained research results are as follows:1.A D-shaped flat plate is taken as the research object in this paper.The Laser Doppler Anemometry(LDA)and Particle Image Velocimetry(PIV)techniques are used to study flow structures around a single flat plate and flat plate cascades.Based on the Proper Orthogonal Decomposition(POD),the wake flow around the flat plate is decomposed and the dominant modes with higher energy are extracted from huge amounts of data in the experiments,revealing the influence of the Reyonlds number,chord-to-thickness ratio(L/D),streamwise gap ratio(T/D)and transveral gap ratio(H/D)on wake flow around the flat plate.2.To study the force characteristics of flow around the D-shaped flat plate.Large Eddy Simulation(LES)is used to complete the exact numerical calaulation,which is can be accurately captured the unsteady flow structures.A case of a single flate plate model and a pair of staggered double flat plate models are used to verify the applicability and accuracy of the numerical method by comparing the claulation results with LDA and PIV experiments.3.An experimental of flow around a single flat plate with Re=3×10~3~4×10~4 and L/D=1~8 is studied.The time-averaged velocity,turbulent intensity,velocity frequency spectra characteristics and POD modes in the wake of flat plate are analyzed,and the change law of St-Re and St-L/D is summarized.The experimental results show that the change of Strouhal number with chord-to-thickness ratio is showed“zonal distribution”between 0.23~0.26.When the L/D?2,the mean velocity field distribution of each condition is highly coincident,the vortex shedding frequency and the flow pattern are tended to be consisitent at the same Reyonlds number.The variation of Srouhal number with Reyonlds number can be divided into three regions:descending region,transition region and dynamic stability region.With the increasing of Reyonlds number,the Strouhal number is tended to be 0.245 in the dynamic stability region.Compared with the ordinary flat plate,the wake of flow around the D-shaped flat plate is more sensitive to the change Reyonlds number.4.Based on the research results of flow around a single flat plate,a pair of flat plates(L/D=3.75)is selected as the research object.The experimental of flow around staggered double flat plates is studied under the Re=3×10~3~4×10~4,T/D=0~6,H/D=0~4.The experimental resuts show that the T/D=3 can be used as the critical gap ratio of the flow direction of the tandem double flat plates,where the Strohal number has a minimum value.When the value is lesser than critical value,the Strouhal number is almost constant.On the contrary,the Strouhal number is increased and tended to the Strouhal number of flow around a single flat plate.The H/D=2 can be used as the critical transveral gap ratio of the staggered double flat plates.The vortex shedding is only formed at the trailing edge of the downstream flat plate under H/D=1,which the corresponding larger-scale Karman vortex street and the vortex shedding of flow around a single flat plate are essentially belong to the same vortices.When the H/D=2,both the upstream and downstream flat plates have vortex shedding formation,but the difference of the vortex shedding frequency between the two flat plates is a maximum value.With the increasing of the transveral gap ratio,the difference is gradually reduced,and the flow structures are tended to be consistent.5.The LES method is used to numerically calculate the D-shaped single flat plate and staggered double flat plates.Due to the symmetry of the single flat plate flow field is destroyed in the staggered double flat plate models,the mean lift coefficient of the upstream and downstream flat plates is not to zero.Compared with a single flat plate model,the lift and drag coefficients of the upstream and downstream flat plates are changed.The mean lift coefficient of the upstream flat plate is significantly changed,which it is changed from zero to negative value.The drag coefficient change is smaller,which fluctuates around a certain positive value with smaller amplitude.