Experimental Investigation of Flow Separation Control Using an Array of Synthetic Jets

An experimental study of flow control using an array of three synthetic jets; has been undertaken in a separated laminar How over an inclined flat plate in a water channel. Particle image velocimetry was employed to obtain the information about the extent of flow separation delay at different synthetic jet operating conditions. A laser-induced florescence flow visualization technique was used to reveal the characteristics of the vortical structures produced by the synthetic jets, which result in a delay of separation. It was observed that the flow separation delay is typically associated with the presence of two or three streaks of high streamwise velocity in the separated flow. Based on the results from the present experiment, a parameter map indicating the flow patterns observed at different synthetic jet operating conditions is produced. In addition, a contour map showing the separation control effectiveness at the corresponding conditions is also obtained. It was found that the two-streak How pattern is mostly produced at ajet-to-freestream velocity ratio between 0.3 and 0.5, whereas the three-streak pattern occurs at a velocity ratio between 0.5 and 1.5. The laser-induced florescence images confirm that the two-streak flow pattern is caused by the hairpin type vortical structures produced by synthetic jets, whereas the three-streak pattern is caused by the tilted vortex ring type structures. For the range of actuator operating conditions tested in this study, operating the synthetic jets at a dimensionless stroke length of around 2, a velocity ratio of around 0.5, and a Strouhal number of around 1.6 would deliver the best flow control effect with the least energy consumption. Under these conditions, hairpinlike vortical structures are observed with a streamwise spacing of 44% of the local boundary-layer thickness at the orifices of the synthetic jets. In this experiment, the spacing between the jets and the distance between the jet array and the baseline separation line are fixed. As the level of interaction between neighboring synthetic jets and their flow control effectiveness are expected to alter upon changes in these two parameters, the generality of this finding remains to be established in future work.