Research On Wake Structure And Force Characteristics Of A Flat Plate With Attached Splitters

Flow around a flat plate is a classical and fundamental topic of fluid mechanics,which is widely used in engineering applications,such as hydraulic machinery,ocean engineering,automobile transportation,bridge construction,aerospace engineering and other fields.The flow around a flat plate contains complex unsteady flow structures such as boundary layer separation,vortex shedding and wake.Vibration is excited by the flow around the plate,which affects the stability of plate structure.Flow wake and its control have become an important basic research content in the engineering field.Compared with the ordinary plate,a flat plat with attached splitters can effectively change the form of wake vortex shedding and the force characteristics of the flat plate,reduce the excitation force on the surface of flat plate,and realize the control of the wake around the flow of the plate structure as well as the reduction of drag and noise.The numerical method and experiment measurement are used to study wake structures and force characteristics around a flat plat with attached splitters at various Reynolds numbers under different chord-thickness-ratio.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 results are as follows.(1)A closed cycle tunnel test platform is designed and built as well as the Laser Doppler Velocimeter(Laser Doppler Velocimeter,LDV)and Particle Image velocimetry(Particle Image Velocimeter,PIV)optical test system.A test scheme of flow around a flat plat with attached splitters is determined.The proper orthogonal decomposition(POD)method is adopted to analyze the data and identify the main coherent structures in the flow field around the flat plate.(2)To study the three-dimensional vortex evolution and force characteristics,the Large Eddy Simulation(LES)method is used for the numerical calculation based on the comparison of the advantages and disadvantages of various numerical calculation methods.A case of a flate plate model is used to verify the applicability and accuracy of the LES method by comparing the claulation results with the literature and optical measurement results.(3)Experiment research of flow around a flat plat with attached splitters at L/D=0~3,Reynolds number 0.5×10~4?1×10~4 and 3×10~4 is studied by using Laser Doppler Velocimeter(LDV).The distribution of average velocity and turbulence intensity are obtained by analyzing the single point velocity data.The results show that the wake has the shortest length of the backflow region when L/D=0,and the length of the backflow region gradually increases with the increase of L/D,while the flow velocity and turbulence intensity distribution gradually decrease.There are unique values of characteristic frequencies in the velocity spectrum when L/D?2.When L/D>2,there is no characteristic frequency in the velocity spectrum as no periodic vortex shedding can be captured in the wake of the plate.(4)A Particle Image Velocimeter(PIV)experiment research of flow around a flat plat with attached splitters is carried out at L/D=0~3,Reynolds number 0.5×10~4?1×10~4 and 3×10~4.The Snapshot POD method is used to analyze and identify the vortex structures in the wake under different L/D.The study shows that the wake structure changes significantly with the increase of L/D.The POD mode shows that the karman vortex street is the main coherent structure in the flow field when L/D?2.0,but the vortex center gradually moves downstream with the increase of L/D.When L/D>2,the Karman vortex street disappears from the wake of flat plate,and the separation shear layers on both sides of the plate gradually approach and attach to the surface of the attached splitter.(5)Based on the LES method,cases of three dimensional turbulent flow around a flat plat with different attached splitters are carried out at Re=3×10~4.The wake filed,lift-drag characteristic and three dimensional vortex structures are analyzed carefully.Besides,based on three-dimensional vortex structure evolution law of time and space,the relationship between vortex structures and forces on the flat plate is constructed.The research shows that the averaged drag coefficient C_D is consistently decreasd while the RMS lift coefficient C_L'increasd firstly and then decreasd with the increase of L/D.Moreover,when L/D?2.5,C_L'is smaller than the origin plate(L/D=0).It is founded that a best inhibition effect on the wake of flat plate wake is achieved when the chord to thickness ratio of the attached splitters is L/D?2.5.