Bluff Body Wakes Dynamics And Instabilities PDF Download

Fluid-structure interaction is encountered in most of the engineering and industrial flow applications. The interaction can lead to generation of undesirable forces acting on structures and causing fatigue or damage. One approach to understanding the fundamental wake flow dynamics and stability is to conduct simulations using forced oscillation of a cylinder. Two aspects of such a flow are considered. The first is the dependence of vortex shedding pattern symmetries on the forced oscillation amplitude, forcing frequency ratios and the direction of the oscillation relative to flow direction. The forced oscillation, depending on the amplitude and frequency of the oscillation, causes formation of various patterns which have specific symmetries. Numerical simulations and stability analysis are employed to determine the unstable wake flow modes and their bifurcations. The forced inline oscillation of a circular cylinder is studied to predict the wake modes over a prescribed range of amplitudes and frequencies. The two-dimensional numerical computations of the Navier-Stokes equations are performed using forced periodic boundary condition method in the range of forcing-to-shedding frequency ratios / [1 2] e s f f ; covering the harmonic and superharmonic excitation regimes with amplitude ratio in the range A/ DÎ[0-0.5]. Symmetric and asymmetric lock-on modes are observed for three different oscillation amplitudes and frequency ratios. The asymmetric 2S mode when / 1 e s f f = for A/D=[0.35-0.5], P+S mode at / 1.5 e s f f = for A/D=[0.175,0.5] and S mode at / 2 e s f f = for A/D=[0.175,0.5] are confirmed. Due to the nonlinearity of the Navier-Stokes équations and the complexity of the infinite dimensional flow dynamics, the primary modes are calculated by the proper orthogonal decomposition (POD) tool. A Galerkin procedure is the used to project the Navier-Stokes equations onto a low-dimensional space spanned by the first two POD modes. This method reduces the problem size from an infinite-dimensional space to a finite number of modes (degree-offreedom)representing the wake dynamics. The vortex shedding dominant modes are invariant Under their symmetry groups. The equivariant bifurcation theory is employed to develop the low order model using the symmetry properties of the primary modes. Thus, equivariant bifurcation and normal form theories are combined with numerical computations of the Navier-Stokes equations to precisely describe the spatio-temporal properties of the wake flow modes and their bifurcations. A linear analysis of the analytical low order model near the bifurcation point is performed to predict the bifurcation sequences observed in simulations.