Elucidating the mechanism of surface perforation on the delay of boundary layer separation for flow over a cylinder

Surface perforation is an effective method of passive flow control. It has been demonstrated that perforated surface on a bluff body such as a cylinder with perforation holes connected to a common internal hollow space significantly reduces the unsteady aerodynamic loading known to cause drag and lift fluctuations, structural vibrations and acoustic noise (Sudalaimuthu and Liu, 2019). It is hypothesized that perforation coupled with the internal connection of the upstream and downstream flow fields helps reduce the magnitude of the adverse pressure gradient, thus effectively delaying and suppressing flow separation so as to alleviate the effect of the unsteady loading. This paper aims to elucidate this pressure gradient mechanism on boundary layer separation delay through Particle Image Velocimetry measurements of the velocity field around a perforated cylinder. The mean pressure gradient field is evaluated through the balance of the Reynolds-Averaged Navier-Stokes equation with each velocity-related term resolved with converged measurement data. The pressure field is further reconstructed with the Rotating Parallel Ray Omni-Directional method. The experiment is performed at a Reynolds number of 1.0 x 10⁵ based on a tunnel speed of 10 m/s and a cylinder diameter 0.152 meters.