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The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = U ref D/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of f m = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time.

This paper presents a methodology for simulation of two-phase flows with surface tension in the framework of unstructured meshes, which combines volume-of-fluid with level-set methods. While the volume-of-fluid transport relies on a robust and accurate polyhedral library for interface advection, surface tension force is calculated by using a level-set function reconstructed by means of a geometrical procedure. Moreover the solution of the fluid flow equations is performed through the fractional step method, using a finite-volume discretization on a collocated grid arrangement. The numerical method is validated against two-and three-dimensional test cases well established in the literature. Conservation properties of this method are shown to be excellent, while geometrical accuracy remains satisfactory even for

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The flow past a circular cylinder at critical and supercritical Reynolds number combines flow separation, turbulence transition, reattachment of the flow and further turbulent separation of the boundary layer. The transition to turbulence causes the delaying of the separation point and, an important reduction of the drag force on the cylinder surface. In the present work, large-eddy simulations of the flow past a circular cylinder at Reynolds numbers in the range 2.5 × 10 5 -8.5 × 10 5 are performed. In this range, major changes in the pressure distribution occur, the pressure minimum gets more negative as its location moves towards the cylinder rear, whereas the base pressure increases. These changes are shown to take place first on one side of the cylinder and then on the other side as the drag completes its drop up to a minimum value of ∼0.23, registered at Re = 6.5 × 10 5 in this work. After that, the flow enters in the supercritical regime, with little changes in the wake configuration. Furthermore, these changes in the wake topology as the Reynolds number increases are also shown to be related to the increase in the vortex shedding frequency.

20Keywords: 21Bubble dynamics 22Level-set method 23Multiphase flow 24

The analysis of the resulting spectrum showed the footprint of Kelvin-Helmholtz instabilities in the whole range. It is found that the ratio of these instabilities frequency to the primary vortex shedding frequency matches quite well the scaling proposed by Prasad and Williamson (f KH /f vs ∝ Re 0.67 ).