Flow control of wall-bounded vortex structures

Abstract

Wall-bounded vortex structures exist over lifting bodies, bluff bodies, and even occur naturally through instabilities or turbulence. In application, wall-bounded vortex structures can reduce lift, increase drag, and also decrease stability. These undesirable effects can be mitigated with flow control devices such as jets. We considered both steady and synthetic jets as our flow control devices. Jets have been widely used for flow control applications, due to their ability to enhance mixing and mitigate separation, but it is unclear the role jet steadiness plays in flow control effectiveness. We explored the foundational differences between steady and synthetic jets issued into a laminar boundary layer crossflow. The jets had variable pitch and skew angles, changing the vortex structures and downstream impact. The coherent streamwise vortices produced by synthetic jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns. These differences are rooted in how vorticity is generated in the orifice, either through a Stokes layer (unsteady) or Blasius boundary layer (steady). ☐ When considering flow control metrics, we find that the synthetic jet produced greater added momentum in the boundary layer and added vorticity when compared to a momentum-matched steady jet. We found that orifice orientation specifically has a dramatic impact on the vortex production/organization and downstream flow field, where the aspect ratio and blowing ratio merely changed the strength and size of the flow structures. By analyzing the added momentum within the boundary layer and added enstrophy (a conduit for mixing), we discussed separation control effectiveness implications. It was shown that certain jet geometries and orientations may be best for separation control through added boundary layer momentum and large-scale mixing, depending on the flow scenario. ☐ We leveraged these studies to explore both vortex destruction and vortex generator mimicry. We generated longitudinal streamwise vortex structures using tab-style rectangular vortex generators. Downstream, we used synthetic jets to destroy the incoming vortex structure. We explored a parameter space of rectangular jet orifice pitch and skew angles to inhale coherent vortex structure and exhale weaker vortex structure. A vortex generator interacting with a synthetic jet shows that the synthetic jet can completely diminish the coherent vorticity of pre-existing vortex structure. ☐ Finally, to mimic a vortex generator, the vortex structures produced by rectangular tab-style vortex generators and synthetic jets were compared. Kutta-Joukowski theorem was coupled with thin airfoil theory to estimate the change in circulation with: length, angle, and freestream velocity for the vortex generators. The linear relationship between those three parameters and the circulation was borne out in the circulation plots. An estimate for the synthetic jet vorticity production was found using a Stokes boundary layer assumption, and also matched well with the experimental results. Synthetic jets can be tuned to replace rectangular tab-style vortex generators provided their size closely matches the relevant vortex generator. Overall, we were able to explore different wall-bounded vortex generating flow control devices such as steady and synthetic jets, and vortex generators. We were able to leverage this information to aid in destroying wall-bounded vortex structures using synthetic jets.