External Force Control of Transitional and Turbulent Flows

Authors

• Cesar Alberto Leos, University of Nebraska-LincolnFollow

First Advisor

Jae Sung Park

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Adam Larios, George Gogos, Nitesh Nama

Department

Mechanical Engineering

Date of this Version

4-27-2026

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy

Major: Mechanical Engineering

Under the supervision of Professor Jae Sung Park

Lincoln, Nebraska, May 2026

Comments

Copyright 2026, Cesar Alberto Leos. Used by permission

Abstract

Turbulence in wall-bounded flows governs momentum transport, drag, and energy efficiency in a wide range of engineering systems, yet its response to external forcing remains incompletely understood across transitional and high Reynolds number regimes. This dissertation investigates an active flow control strategy based on spanwise external forcing and demonstrates how it modifies the dynamics of transitional and turbulent channel flows with the objective of identifying the mechanisms through which forcing alters transition, flow physics, and turbulent drag reduction.

Firstly, direct numerical simulations are performed to examine the subcritical transition under external forcing. The onset and sustenance of turbulence are shown to depend strongly on the interaction between the forcing and coherent structures of perturbation fields. This dependence is clarified through a deterministic analysis based on the so-called exact coherent states, which reveals that forcing alters transition pathways differently for core-dominated and near-wall critical-layer perturbation fields. A predictive framework to provide quantitative estimates of the timing and intensity of the transition is established using sustained turbulent flows. These findings demonstrate that transition is governed not only by the magnitude of perturbation but also by the persistence and structural alignment of turbulent motions under forcing, providing a mechanistic basis for anticipating controlled transition dynamics. Secondly, in fully developed turbulence, the external forcing reorganizes flow physics through synchronization, producing intermittent symmetric states associated with reduced wall shear stress and increased bulk velocity. However, increasing drag reduction is accompanied by a decrease in the occurrence of these states, revealing a nontrivial tradeoff between drag reduction and flow organization. In addition, the influence of forcing on intermittency is further characterized through extreme events, whose frequency and magnitude increase substantially under external forcing control, indicating a redistribution of turbulent activity toward rare but intense fluctuations.

Finally, at moderate Reynolds numbers up to Re_τ=1000, drag reduction via external forcing decreases and follows a power-law dependence on the Reynolds number. Analysis using the Fukagata-Iwamoto-Kasagi (FIK) identity demonstrates that drag reduction via external forcing is dominated by the outer-region contribution up to Re_τ=800, whereas inner-region effects become increasingly significant at Re_τ=1000. These results highlight the multiscale nature of controlled turbulence and establish a unified framework in which external forcing modifies flow behavior through its interaction with structures across scales.

Recommended Citation

Leos, Cesar Alberto, "External Force Control of Transitional and Turbulent Flows" (2026). Dissertations and Doctoral Documents, University of Nebraska-Lincoln, 2023–. 469.
https://digitalcommons.unl.edu/dissunl/469