Unsteady separated wake behind an impulsively started cylinder in slightly viscous fluid

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Abstract
The initial unsteady two-dimensional flow around an impulsively started circular cylinder is investigated using the random-vortex numerical method. To understand the mechanisms of the primary and secondary vortex formation, we investigate the relationship between the creation, diffusion and convection of vorticity to the genesis and evolution of the complex and ‘unstable’ flow structures of the recirculating zone behind the cylinder. Our simulation reveals detailed geometric features of the wake which are in agreement with experimental observations and with other numerical calculations. Numerical calculation at Reynolds numbers R = 3000, 9500 and 104 show that the numerical method is appropriately sensitive to changes of the Reynolds number. Numerical functionals such as the length of the wake, velocity on the wake axis and the angle of separation for our calculations are in satisfactory agreement with known experimental and numerical results. This numerical method gives results comparable to those of a previously published method but does so using much less memory and computer time.