Flow physics and sensitivity study of synthetic jets-based flow control of 3-D configurations

Abstract

Bridging the gap between sub-scale, fundamental studies of synthetic jet actuators on aerodynamic models and full-scale applications on actual vehicles requires a thorough understanding of how the synthetic jets interact with three-dimensional cross flows in conditions representative of flight, where separation and spanwise flow are present. This work targeted this in-between region with a detailed experimental study of the effects of an array of finite-span synthetic jets on a large, sub-scale, and fundamental model, with a symmetric cross-section and no taper, designed specifically for this research. The model’s interchangeability allowed sweep and control surface deflection angles to vary, and its generic geometry was applicable to any aerodynamic surface. Experiments included surface pressure measurements, tuft flow visualizations, hotwire measurements, and particle image velocimetry.

The findings of this research can be used to inform future testing of synthetic jets at larger model scales and in additional three-dimensional configurations. The increased understanding of the flow physics, and corresponding performance, of synthetic-jet actuators in three-dimensional configurations that was obtained through this research is a further step toward the implementation of active flow control on production aircraft.

A parametric study of the sensitivity of the lift coefficient and the behavior of the pressure distribution showed that the jets increased performance through separation reduction and circulation enhancement, as expected from literature. The sectional lift coefficient was increased by 26.3% and 15.9% in unswept and swept cases, respectively. Flow field investigations revealed that vortex rings were formed by the synthetic jets and accelerated the flow. The rings developed undulations and exhibited the effects of axis switching, such that the inboard and outboard edges were propelled ahead of the center portion of the ring and the upstream and downstream edges pinched together to resemble a double ring. The relative increased velocity of the flow through different ring segments caused the extent of separation reduction. In the unswept case, both inboard and outboard edges accelerated flow downstream and towards the center of the ring. In the swept case, the inboard edge was annihilated first due to spanwise cross flow, causing the accelerated flow through the outboard edge to be unmatched. The inboard-component of the flow through the outboard ring edge opposed the spanwise cross flow and reduced the effect of actuation. In general, the stronger the vortical structures were, the greater the separation reduction and performance enhancement were. With sweep, the largest separation reduction was achieved when the structures least opposed the spanwise flow. The insight into the flow physics of the vortical structures as separation severity, spanwise flow, and spanwise spacing were varied was the crux of this research.

This work was directly motivated by previous research on sub-scale, vertical tail models with high sweep angles (i.e., larger than 40°), which found that under certain conditions, aerodynamic force was increased when spanwise spacing between active jets increased. The present work investigated unswept and mildly swept (20°) configurations, and did not clearly observe this trend due to the smaller magnitude spanwise velocity component. In general, the sectional lift coefficient increased with decreased spacing and increased number of active jets. The results did indicate that an optimum spacing may exist in the tested conditions when the total number of active jets was constant.