Uncovering the self-sustaining cycle in the outer region of turbulent boundary layers

Chaotic motions of fluid near surfaces (turbulent boundary layers) cause up to 70% of drag on ships and 50% of drag on planes. Certain fluid motions are repeatedly observed in turbulent boundary layers and contribute significantly to drag on surfaces. Mechanisms that sustain turbulent motions in the inner region of the boundary layer were previously identified and have inspired control strategies for drag reduction and simple models for prediction.

This research will seek to identify mechanisms for coherent motions in the outer region, which could then be better modelled to more accurately predict vehicle drag and could lead to drag reduction strategies. Graduate, undergraduate, and high school students will be trained and mentored in the laboratory through the research initiative and graduate students will be educated in a research-based class at the University of Illinois at Urbana-Champaign.

Additionally, math tutor and introductory engineering educational programs will be implemented at the Danville Correctional Facility, a medium security men’s state prison in Illinois.

The goal of the project is to evaluate a hypothesized cycle that generates and sustains coherent motions in the outer region of turbulent boundary layers. The project has three tasks: two that focus on mechanisms through which different coherent motions can interact and one that focuses on the characterization of the coherent motions involved. The interaction of a small von Karman vortex street with a turbulent boundary layer will be studied using time-resolved particle image velocimetry, yielding insights into the interaction of large- and small-scale coherent motions.

The amplification of small-scale coherent motions will be investigated by performing a resolvent analysis of the Navier-Stokes equation with a quasi-base flow, which includes both the mean and a single large-scale coherent motion. The small-scale coherent motions will be characterized using a new sampling approach for turbulent boundary layer data that leverages the current understanding of the interaction of coherent motions in turbulent boundary layers.

Together, these three tasks will identify the coherent motions and the mechanisms that sustain them in turbulent boundary layers, enabling prediction and control of drag on transportation vehicles.

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.