Passive Flow Control over a Three-Dimensional Turret with a Flat Aperture

Airborne optical systems are susceptible to large losses in performance due to aero-optic distortion. For the case of laser-based systems, as the laser beam propagates through re- gions of high turbulence, the transmitted energy and focus of the beam are reduced. A study of the flow physics around a surface mounted turret with a flat aperture is described. The baseline flow is characterized by steady pressure measurements over the centerline of the model, unsteady pressure measurements on the aperture, and oil flow visualization along the surface for the range of Reynolds numbers from Re_H = 2.27 × 10⁵ − 5.10 × 10⁵ (Ma = 0.09-0.26). The evaluation of the static pressure measurements provides quanti- tative information on the separation location along the centerline of the turret. Oil flow visualization provides information on the three-dimensional flow topology over the turret. Spectra from unsteady pressure measurements impart information on unsteady fluctuation trends along the aperture. Several passive flow control configurations using pins are stud- ied. The evaluation of flow control performance is performed by analyzing both the surface streamlines from oil flow visualization and measurements of unsteady pressure fluctuations along the flat window. Results show that pins that penetrated though the boundary layer and were spaced less than four boundary layer thicknesses apart provided a reduction in the rms pressure on the aperture, a slight delay in separation, and an elongation of the recirculation region.