Session: Research Posters

Paper Number: 172821

Transition Control in Subcritical Channel Flow via External Forces 

The onset of turbulence in wall-bounded flows has significant implications for drag, heat transfer, and flow control in engineering systems. In a channel flow, the transition is subcritical, triggered by finite-amplitude disturbances despite the linear stability of the laminar base state. This makes the transitional regime particularly sensitive to external influences and a promising target for active control. While strategies such as wall-normal blowing and suction, spanwise wall oscillations, and streamwise traveling waves have been widely studied in fully turbulent flows, their ability to delay or promote transition remains less well understood. Among these, an external forcing has received limited attention, despite its potential to interact directly with streak amplification and secondary instabilities in the near-wall region.

To investigate this, direct numerical simulations (DNS) are conducted under constant mass flux across a range of friction Reynolds numbers 85 to 180, spanning the transitional regime. The flow is initialized with a laminar base state superimposed with a small three-dimensional perturbation, allowing for transition to turbulence. Control is introduced via an external spanwise force applied exclusively, defined by four parameters: amplitude, frequency, spanwise wavenumber, and penetration depth. In this study, the wavenumber and penetration depth are fixed, respectively, while amplitude and frequency are varied systematically. This open-loop, non-interactive configuration enables precise investigation of how externally imposed spanwise motion influences transition dynamics in the absence of feedback mechanisms.

The response of the flow to spanwise forcing is strongly dependent on both frequency and amplitude. At low frequencies, the flow transitions earlier than in the uncontrolled case, driven by enhanced formation and breakdown of streamwise streaks. In contrast, higher frequencies can delay transition by weakening streak amplification and suppressing secondary instabilities. This shift in transition timing reflects the intricate interaction between forcing induced perturbations and the self-sustaining mechanisms of near-wall turbulence. Flow visualizations reveal the presence of streamwise-elongated roller vortices near the wall, generated by the spanwise forcing. These rollers interact with the streaks, altering their coherence and stability. However, as the friction Reynolds number increases up to 180, the ability of these vortices to delay transition declines, and early breakdown becomes prevalent even under forcing. These changes in transition onset cause corresponding changes in skin-friction drag. When transition is delayed, drag is reduced due to the persistence of low-shear laminar or intermittent states. In contrast, early transition increases drag as turbulence develops sooner and enhances momentum transfer near the wall.

These results offer new insight into the physics of controlled transition in shear flows, especially in regimes where sensitivity to external disturbances is highest. Unlike wall-based techniques that primarily act on the viscous sublayer, volumetric spanwise forcing engages directly with dynamics in the buffer layer, where streaks and vortices drive the transition. The ability to tune forcing parameters provides a means to influence different stages of the breakdown process and explore limits of control authority across Reynolds numbers. Understanding the conditions under which spanwise forcing is effective or ineffective can guide the development of more reliable control strategies for internal and external aerodynamic applications. These observations can also be interpreted from a dynamical systems perspective, where the flow evolves through a phase space populated by exact coherent structures and their unstable manifolds. By modifying the control input, spanwise forcing perturbs the flow’s trajectory toward and away from these invariant solutions, influencing the likelihood of transition. At lower Reynolds numbers, the flow can remain near the laminar attractor, while higher Reynolds numbers introduce greater instability and reduce control effectiveness. This perspective highlights how spanwise forcing not only alters transition timing but also reshapes the underlying phase space dynamics that govern flow evolution.

Presenting Author: Cesar Leos University of Nebraska-Lincoln

Presenting Author Biography: Cesar Alberto Leos Jr was born on June 5, 1992, in Monterrey, Nuevo León, Mexico, and raised in McAllen, Texas. He began his undergraduate studies at The University of Texas Rio Grande Valley (UTRGV) in Fall 2014, earning a Bachelor of Science in Mechanical Engineering in Fall 2017. As an undergraduate, Mr. Leos served for two years as a research assistant in the Aerodynamics and Propulsion Systems Laboratory, where he contributed to projects in pulsed jet propulsion. He also held a teaching assistant position in mechanical engineering courses, supporting instruction in core subjects. He continued his graduate education at UTRGV and received a Master of Science in Mechanical Engineering in August 2020. In Fall 2021, Mr. Leos began his doctoral studies in mechanical engineering, where his research focuses on turbulent drag reduction and flow control using direct numerical simulation (DNS). He advanced to PhD candidacy in Spring 2024. His academic interests include computational fluid dynamics, turbulence modeling, hypersonic flow and active flow control, with an emphasis on high Reynolds number flows and aerospace applications. He is committed to advancing the fundamental understanding of wall-bounded turbulence and its manipulation through externally applied forces.

Authors:

Cesar Leos University of Nebraska-Lincoln
Jae Sung Park University of Nebraska-Lincoln