Supplement: Relaminarization and Turbulence Suppression in Rotating Flows

Turbulent swirling and rotating flows involve intricate flow physics that are not well-understood, including turbulence suppression and relaminarization - one of the biggest mysteries in turbulence research. The axially rotating pipe is an exemplary prototypical model problem that exhibits these complex turbulent flow physics. For this flow, the rotation of the pipe causes a region of turbulence suppression and relaminarization which is particularly sensitive to the rotation rate and Reynolds number. While the laminarturbulent transition process has been studied in great detail for decades, there has never been a DNS study that covers the entire reverse (turbulent to laminar) transition process. Experimental studies are rare and do not comprehensively cover relevant rotation rates. Prior research studies for dierent classes of flow indicate, however, that hydrodynamic stability theory can be utilized to extract important information about the relaminarization process. The computational resources are utilized to conduct a high-fidelity numerical simulation in support of carefully designed experiments (funded by NSF) to obtain detailed insight about the ongoing flow physics of turbulence suppression and relaminarization at high Reynolds number. These one-of-a-kind direct numerical simulation will be used to characterize the turbulent flow, and temporally and spatially highly resolved experimental data is available for comparison. Here, local and global stability theory will be utilized to analyze the physical mechanisms that drive the relaminarization process. The results will be integrated through examination of turbulence budgets as well as applying several data reduction techniques, including higher-order spectral analysis, and modal decomposition. The key objective of this research is to obtain high-quality simulation data (in conjuction with ongoing experiments) that can be used to study the nature of the highly complicated flow physics of turbulent rotating flows. While numerous research studies have targeted the laminar to turbulence transition process, very few studies were concerned with investigating the reverse process - particularly within the framework of stability analysis. As such, the intricate nature and relevant physical mechanisms of this process are not well understood and the proposed research oers an opportunity to understand the physics of this process. A profound understanding of the relevant mechanisms can be employed to devise passive or active flow control strategies for turbulent flows. Another significant contribution of the proposed research is to improve current computational prediction capabilities of swirling and rotating flows. Swirling flows are an important class of flows, not only because of the complex flow physics, but also because of their relevance to many industrial applications, such as combustion, heat exchangers, cyclone separation, mixing, etc. To design the next generation of ever more ecient cars, aircraft, or energy systems it is important to understand turbulent swirling and rotating flows in order to predict and manipulate them to our benefit. Thus, enhanced understanding of the physical mechanisms for swirling and rotating flows and improved prediction capabilities for these types of flows are highly beneficial for key US industries where swirling and rotating flows appear, e.g., oil and gas, biomedical, energy harvesting, aerospace, etc. Access to an up-to-date extensive collection of DNS will also allow the development of new RANS/Hybrid RANSLES models by other research groups. This research is currently funded by the National Science Foundation and is being performed in close collaboration between experimentalists and computational fluid dynamicists of the same institution, fostering a full integration of these two research groups. The combined eort between researchers from the University of Kentucky and the University of Oxford within this research project will be the starting point for a close international collaboration between these two institutions, enabling student exchanges and exposure to research from across the globe.