Flow field investigation behind a trapezoidal rib and the effect of the synthetic jet

Abstract

The motive of the present work is to investigate the combined effect of synthetic jet and chamfer angle (15°, 18° and 21°) of a trapezoidal rib on flow phenomena. However, detailed study is restricted to 21° chamfer angle, which is the optimum angle from the heat transfer perspective. The rib has an obstruction of 5% of the tunnel for an aspect ratio of 30. Emphasis has been given to identifying the optimal location of the synthetic jet at which the recirculation zone behind the rib turbulator becomes a minimum. Experiments are performed on a low-velocity subsonic wind tunnel by particle image velocimetry (PIV) and hotwire anemometry (HWA). Flow field has been characterized in terms of time-averaged velocity magnitude, vorticity, streamlines, turbulence quantities such as turbulence intensity, Reynolds stress and root mean square (rms) velocities as well as instantaneous flow field. The data are presented at Reynolds number 32,000, based on the hydraulic diameter of the test section. Along with the large primary recirculation, a secondary recirculation bubble is visible at the leeward corner predominantly for the angles 18° and 21°. The secondary bubble is considered to be responsible for maximum heat transfer from the near rib surface. The synthetic jet is placed at different upstream and downstream locations to see the effect on flow parameters. With the synthetic jet, the primary recirculation bubble moves closer to the rib, which ultimately reduces the recirculation length. A greater mixing is observed in the near wake of the obstacle, and the secondary recirculation zone near the surface diminishes. Out of six different synthetic jet locations studied, the maximum reduction of the recirculation bubble is observed when the synthetic jet is placed 0.5D upstream and 1.5D downstream from the rib front face. A comparison has been made between with and without synthetic jet for these flow parameters.