Jae Sung Park
Date of this Version
Rogge, A. (2020). Toward Understanding Underlying Mechanisms of Drag Reduction in Turbulent Flow Control. Unpublished Master’s Thesis, University of Nebraska-Lincoln.
The underlying mechanisms of three different flow-control strategies on drag reduction in a turbulent channel flow are investigated by direct numerical simulations. These strategies include the addition of a small concentration of long-chain polymers into a fluid, the incorporation of slip surfaces, and the application of an external body force. While it has been believed that such methods lead to a skin-friction reduction by controlling near-wall flow structures, the underlying mechanisms at play are still not as clear. In this study, a temporal analysis is employed to elucidate underlying drag-reduction mechanisms among these methods. The analysis is based on the lifetime of turbulent phases represented by the active and hibernating phases of a minimal turbulent channel flow. At a similar amount of drag reduction, the polymer and slip methods show a similar mechanism, while the body force method is different. The polymers and slip surfaces cause hibernating phases to happen more frequently, while the duration of active phases is decreased. However, the body forces cause hibernating phases to happen less frequently but prolong its duration to achieve a comparable amount of drag reduction. A possible mechanism behind the body force method is associated with its unique roller-like vortical structures formed near the wall. These structures appear to prevent interactions between inner and outer regions by which individual hibernating phases are prolonged. It should motivate adaptive flow-control strategies to fully exploit the distinct underlying mechanisms for optimal and robust control of turbulent drag.
Advisor: Jae Sung Park