Study On The Spectral Properties And Structural Features Of Flow Past A Disk

The turbulent flow behind a bluff body has received abundant attention. This is not only because of its fundamental role in helping understand dynamical processes of the boundary layer separation, vortex shedding and energy transport, but also due to its practical applications. As a simple and typical bluff body, a circular disk has the axisymmetric geometry. However, the flow past which is much complex, with distinct three dimensional property therein. In this context, the wake of a circular disk at different Reynolds numbers is studied in detail both numerically and experimentally. The results by different means are compared and confirmed with each other, aiming to have an insightful understanding of the flow. Meanwhile, the flow past a porous disk is also investigated because of its wide applications in practical engineerings. The differences between the flow past a solid and porous disk are also preliminarily discussed, and our specific work includes the following aspects:(1) The characteristics of flow past a disk are analyzed theoretically, including the shape of wake, the characteristic parameters, the lift and drag coefficients, the frequency characteristics and the structure of wake. Moreover, the aerodynamics of flow past a porous disk are also discussed;(2) Flow visualizations are conducted on the flow past a disk with staining method at low Reynolds numbers, the evolution of the laminar wake of circular disk as Reynolds numbers is analyzed;(3) Based on the results of large eddy simulation and hot wire anemometry, the frequency spectrum characteristic of the wake is analyzed with the wavelet transform method;(4) The flow past a solid and porous disk at high Reynolds number is measured by the particle image velocimetry method, and the mean flow field, together with coherent turbulent structures are compared and analyzed;Based on the above study, the following conclusions can be drawn:(1) The flow visualizations indicate:when Re<135, the wake has a steady and axisymmetric toroidal vortex; when135≤Re<158, the wake becomes plane symmetric; when158≤Re<175, the wake develops to be hairpin vortices. The above findings are coincident with results of large eddy simulation;(2) The spectral analysis shows that the dimensionless natural vortex shedding frequency of the wake is approximate to Stv≈0.11～0.14, while the high frequency corresponding to Kelvin-Helmholtz instability is about StKH≈1.2～1.6. Besides, two relatively lower frequencies of StL1≈0.02and StL2≈0.03also exist. It is found that StL1≈0.02is related to the rotation of the azimuthal location of vortex shedding, while StL2≈0.03is related to the pumping motion of the recirculation bubble;(3) The results of particle image velocimetry reveal a new flow phenomena which has never been reported previously. There are two recirculation regions behind the porous disk. The small one is separated by small jets through the disk into several species, while the bigger one is located at0.9<x/D<1.75downstream. The comparison between the wake of a solid disk and that of a porous disk suggests that the porous structures decrease the velocity deficit and the rate of velocity recovery. The cross-stream distributions of Reynolds normal stresses exhibit symmetry while Reynolds shear stress shows asymmetry at x/D=1,3,5respectively.