Large eddy simulation of the subcritical flow over a v grooved circular cylinder

In this paper, a comparative numerical study of the subcritical flow over a smooth cylinder and a cylinder with V grooves (Re = 140,000) is presented. The implemented technique was the Large Eddy Simulation (LES), which according to Kolmogorov's theory, resolves directly the most energetic largest eddies and models the smallest and considered universal high frequency ones. The Navier-Stokes (N-S) equations were solved using the commercial software ANSYS FLUENT V.12.1, which applied the finite volume method (FVM) to discretize these equations in their unsteady and incompressible forms. The grid densities were 2.6 million cells and 13.5 million cells for the smooth and V grooved cylinder, respectively. Both meshes were composed of structured hexahedral cells and close to the wall of the cylinders, additional refinements were employed in order to obtain y^+<5 values. All cases were simulated during at least 15 vortex shedding cycles with the aim of obtaining significant statistical data. Results showed that for both cases (smooth and V grooved cylinder flow), the numerical code was capable of reproducing the most important physical quantities of the subcritical regime. Velocity distribution and turbulence intensity in the flow direction suffered a slight attenuation along the wake, as a consequence of grooves perturbation, which also caused an increase in the pressure coefficient in the front part of the cylinder, and energy recovery in the rear of the cylinder. Furthermore, in the valleys of the grooves, the firsts flow instabilities related to separation points suffered a shift upstream at θ = 60°, while in the crests, the flow transitional zones started at θ = 97°. The Power Spectral Density signal showed that grooves presence do not affect the oscillating frequency of the most energetic eddies. Although grooves caused an alignment of the flow structures especially near the cylinder wall, due to the inhibition of the momentum exchange in the spanwise coordinate, the dominant structures resembled to be as the smooth cylinder ones, at least for this flow regime.