Collaborative Research: Reaping the Whirlwind: Re-energizing Boundary Layers by Targeted Manipulation of Coherent Structures

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In a turbulent flow, for example near the surface of an airplane wing or in the atmosphere as it flows over the Earth’s surface, there is a range of eddies of different sizes. The largest of these coherent eddies, called Large Scale Motions (LSMs), carry a significant portion of the turbulent kinetic energy and are largely responsible for mixing high-speed flow far away from the surface with the low-speed flow right near the surface. This mixing may be enhanced by selectively displacing the LSMs toward the surface, and this is very desirable in some applications. For example, bringing high-speed flow to the surface can keep a wing from stalling and losing its lift. The goal of this project is to prove via simulations and in matched experiments an approach to detect and manipulate LSMs to re-energize the near-surface flow. The success of the present approach is also expected to motivate the emergence of technologies such as controlling acoustic noise or heat transfer for underwater, ground-vehicles, or air-vehicles or combustion in turbomachinery. This project thus has the potential of initiating a new direction in turbulent flow control. The goal of this research is to develop an experimentally-validated system for active detection and manipulation of LSMs for turbulent boundary layer re-energization. Advances in the fundamental understanding of LSMs, actuator innovations, and advanced control theory make this goal achievable. The first step towards the goal is to demonstrate energizing a boundary layer by manipulating controlled, well-characterized, synthetically generated structures in well-coordinated boundary layer experiments and direct numerical simulations (DNS). The preliminary DNS successfully demonstrated optimization of control jet actuation using two different control methods, which deflected a prototypical LSM towards the surface. Preliminary work has created a suitable actuator based on a synthetic jet and demonstrated it in the lab. The specific tasks proposed in this collaborative 3-year project are: (1) develop and implement control algorithms to selectively move synthetically generated LSMs in matched experiments and DNS, (2) quantify the re-energization achieved by moving different kinds of LSMs to guide target prioritization, and (3) test and optimize the re-energization of a turbulent boundary layer with an eye toward separation control and wind power applications. The results of this project will be made publicly available through online platforms and enhanced classroom activity. The project will also help recruit graduate students from underrepresented groups and for several undergraduate research projects.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.