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To control the vortex structures and separation region of a low-aspect-ratio cylinder wake, a passive flow control method that is to drill a horizontal hole from the front side surface to the rear side surface was proposed for a short circular cylinder having an aspect ratio H/D  = 1 with height H and diameter D of 70 mm. The PIV measurements were performed at Reynolds number of 8570 in a circulation water tunnel in order to compare the flow characteristics between the controlling and no-controlling wakes. Furthermore, to study the position effect of the horizontal hole, three kinds of the hole positions having different height h from wall were tested. It was found that the jet flow from a horizontal hole leads to suppress the vortex formation and to reduce the rear separation region. The vorticity, Reynolds shear stresses and TKE were evidently suppressed by the jet flow of the horizontal hole in the rear recirculation zone, resulting in the reduction of the drag acted on the cylinder. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone were broken down into several small-scale vortices by the jet. As increasing the height of hole, the downwash flow is evidently suppressed and the upwash flow is increased. Spanwise vortex shedding is altered from symmetric to asymmetric arrangement. [ABSTRACT FROM AUTHOR]

Abstract: The effect of initial conditions on a self-preserving plane wake has been investigated for two wake generators, i.e. a circular cylinder and a screen of 50% solidity. Measurements were made at (x is the streamwise distance downstream of the cylinder and θ is the momentum thickness of the wake)=580 for the circular cylinder and 830 for the screen using 8 X-wires in the plane of mean shear. A vector wavelet multi-resolution technique is applied for analysis of the hot-wire data. This technique decomposes turbulent structures into a number of components based on their central frequencies, which are linked with the turbulence scales. Sectional streamlines, vorticity contours and probability density functions of the fluctuating velocities at the same central frequency, i.e. the comparable scales of turbulent structures, are examined and compared between the two generators. Discernible differences are observed in the turbulent structures of large-down to intermediate-scales. The differences are further quantified in terms of contributions from the turbulent structures of different scales to the Reynolds stresses and vorticity variance. The vorticity behavior of the screen wake is different from that of the circular cylinder. The difference exhibits an extraordinary similarity to that in the near wake. [Copyright &y& Elsevier]