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Transition of flows past a pair of side-by-side circular
cylinders are investigated by numerical simulations and the bifurcation
analysis of the numerical results. Various flow patterns behind the
cylinder-pair have been identified by the gap ratio (G) and Reynolds number (Re).
This study focus on transition of in-phase and anti-phase vortex shedding
synchronized forms. A nested Cartesian-grid formulation, in combination with an
effective immersed boundary method and a two-step fractional-step procedure,
has been adopted to simulate the flows. Numerical results reveal that the
in-phase and anti-phase vortex shedding flows at Re = 100 can
co-exist at 2.08 ≤G≤ 2.58. Hysteresis loop with increasing/decreasing G at constant Reynolds number Re = 100 is reported.
Multimoment hydrodynamics equations are applied to investigate the phenomena of appearance and development of instability in problem on a flow around a solid sphere at rest. The simplest solution to the multimoment hydrodynamics equations coincides with the Stokes solution to the classic hydrodynamics equations in the limit of small Reynolds number values, . Solution to the multimoment hydrodynamics equations reproduces recirculating zone in the wake behind the sphere having the form of an axisymmetric toroidal vortex ring. The solution remains stable while the entropy production in the system exceeds the entropy outflow through the surface confining the system. The passage of the first critical value is accompanied by the solution stability loss. The solution, when loses its stability, reproduces periodic pulsations of the periphery of the recirculating zone in the wake behind the sphere. The and solutions to the multimoment hydrodynamics equations interpret a vortex shedding. After the second critical value is reached, the solution at the periphery of the recirculating zone and in the far wake is replaced by the solution. In accordance with the solution, the periphery of the recirculating zone periodically detached from the core and moves downstream in the form of a vortex ring. After the attainment of the third critical value , the solution at the periphery of the recirculating zone and in the far wake is replaced by the solution. In accordance with the solution, vortex rings penetrate into each other and form the continuous vortex sheet in the wake behind the sphere. The replacement of one unstable flow regime by another unstable regime is governed the tendency of the system to discover the fastest path to depart from the state of statistical equilibrium. Having lost the stability, the system does not reach a new stable position. Such a scenario differs from the ideas of classic hydrodynamics, which interprets the development of instability in terms of bifurcations from one stable state to another stable state. Solutions to the multimoment hydrodynamics equations indicate the direction of instability development, which qualitatively reproduces the experimental data in a wide range of Re values. The problems encountered by classic hydrodynamics when interpreting the observed instability development process are solved on the way toward an increase in the number of principle hydrodynamic values.
Slitty bluff body is widely used as a
high-performance flame holder in power industry. To understand the flame
stability mechanism, the evolution of the near wake over a slitty bluff body in
cold state was numerically investigated using the renormalization group (RNG)
k-ε model at Reynolds number of 470,000.
The coherent structure of the near wake was identified by the vortex shedding
simulation. To explain the vortex shedding, a mechanism that single vortex of
large size suddenly immerses two shear layers was proposed. To quantitatively
compare the near wakes at different gap ratio, a vortex shedding character
dimension was first proposed. This character dimension has positive correlation
with flame stability. Particle-image velocimetry (PIV) measurements in a close
wind tunnel were also carried out to confirm the observation from the numerical
study. The evidence shows that the numerical results are of good agreement with
the cold-state experiments.