The Wake Structure behind Porous Obstructions in a Steady Current

This paper presents results from two-dimensional direct numerical simulations and laboratory experiments that have been undertaken to examine the wake structure of a circular array of cylinders modelling a porous obstruction. Four wake regimes are observed in both the DNS and experiments, including: (i) vortex street wake (VS); (ii) ‘steady + shedding’ wake (SS); (iii) Kelvin-Helmholtz instability wake (KH); and (iv) coupled individual wake (CI). A key distinguishing characteristic between the different regimes is the decreasing degree of interaction between the shear layers on either side of the array from regime VS to CI. Linear stability theory applied to the array shear layers provides a reasonable prediction of the KH instability frequency in the regime KH. The normalised frequency of velocity oscillation in the vortex street of the wake (fU∞/D, where D is the array diameter and U∞ is the ambient velocity) always lies around the typical Strouhal number of 0.2 of an isolated solid cylinder, regardless of the formation mechanism of the vortex street.