| The juncture structure formed by the combination of bluff body and flat plate exists widely in practical fields,such as traffic,architecture,and aerospace engineering.Junction flows occur when the juncture suffering from incoming flow.In turbulent flow conditions,junction flow is a three-dimensional unsteady flow,containing rich physical phenomenon with complex vortexes and non-periodic evolution.In different fields,the unsteadiness of junction flow gives rise to some favorable or unfavorable factors in the engineering application.It is of great significance to fully understand this kind of flow phenomenon to develop its advantage and reduce the harm via different flow control techniques.An in-house LES code(MPLES)has been employed to simulate flow around square cylinder and to reveal physical images of junction flow under the turbulent inlet boundary layer condition.The aspect ratio is 8 and the Mach number and Reynolds number based on cylinder width were 0.5 and 12,000 respectively.The inflow turbulent boundary layer was generated via Rescaling-Reintroducing(R-R)technique.The square cylinder was mounted in where momentum thickness Reynolds numbers equals 1,000.Several results could be discovered from mean flow.First,the pressure gradients around square cylinder produce a three dimensional separation in incoming boundary layer,as a consequence of a complex horseshoe vortex system,including main horseshoe vortex,secondary vortex and corner vortex.Second,it is different from traditional topology that the most upstream singular point is attached half-node in symmetry plane.Pressure coefficient corresponding to the first main horseshoe vortex(PV1)and the first corner vortex(CV1)on the end wall is local minimum.On the contrary,the shear stress near these two position is larger.Third,thanks to the square cylinder,the boundary layer presents strong three-dimensionality,even boundary layer thickness reduce near both sides.Last,the turbulent kinetic energy in junction region focus on PV1 and free shear layer.Distribution of Reynolds stress supports further conclusion that mainly two-part oscillation contribute the unsteady of PV1.The one is the oscillation of vortex head in streamwise and wall normal direction,the other one is the oscillation of vortex leg in wall normal and transverse direction.Instantaneous flows fields further verify that the horseshoe vortex system originate from the recombination of upstream boundary layer.Its oscillation is apparently non-periodic,but the law of motion should be analyzed from multiple angles.Spectrum analysis and animation demonstration suggest that there are significant associations between PV1 and free shear layer,here aimed as the transverse-direction fluctuation on vortex head of PV1 and swing on vortex leg of PV1 refer to periodicity of separated shear layer.Probability density distribution(p.d.f)of some space position on symmetry close to square cylinder appear bimodal,which indicated that zero-flow mode and back-flow mode exist in junction flow.Further analysis of the flow details shows that there is a complex interaction between the vortices.The evolution of the horseshoe vortex system follows the rule of quasi periodic motion of back-flow,zero-flow and back-flow.What's more,the proper orthogonal decomposition(POD)is carried to the part of junction region for intrinsic unsteady.Major modes and the main frequency of the flow are extracted from the complex flow field,thus the understanding of the flow is deepened. |
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