Laminarization and Turbulence Suppression in Rotating Flows

Turbulent swirling and rotating flows are common to many engineering applications - from the flow inside pipes and ducts to the wing-tip vortices generated by airplanes. These types of flows involve intricate flow physics that are not well-understood, including turbulence suppression and relaminarization - one of the biggest mysteries in turbulence research. Swirling flows are not only an important class of flows because of the complex flow physics but they are relevant to many industrial applications, such as combustion, heat exchangers, cyclone separation, mixing, etc. To design the next generation of ever more efficient cars, aircraft or energy systems it is crucial to both understand these type of flows, and most importantly to predict and manipulate them to our benefit. An excellent prototypical model problem that exhibits complex flow physics due to swirl and rotation is an axially rotating pipe 1,2. For this flow, the rotation of the pipe (following an initial non-rotating section) causes a complex and not well-understood region of turbulence suppression which is particularly sensitive to the rotation rate. After this region of turbulence suppression (roughly 50 - 100D after the rotation begins), the flow recovers to a state of isotropy, but even the amount of turbulence suppression in this final state is again sensitive to the rotation rate and Reynolds number. For even higher rotation rates, the flow can be fully relaminarized. There is, however, no direct numerical simulation (DNS) data available at these high rotation rates and not for realistic Reynolds numbers 3,4,5,6,7 (up to Re = 7, 400 in Feiz et al. 8). Further, there has never been a DNS study that covers the entire reverse (turbulent to laminar) transition process. The available experimental data is relatively old 2, and/or does not comprehensively study a range of spin rates and crucially the long pipe (200D), which demonstrates the complex transition between turbulence suppression and recovery. In the proposed research, high-quality experiments in conjunction with high-fidelity numerical simula- tions are used to obtain a detailed insight about the ongoing flow physics of turbulent suppression and relaminarization at realistic Reynolds numbers and rotation rates. State-of-art experimental measurement techniques are used to characterize the unsteady flow and the temporally and spatially highly resolved ex- perimental data will be used to validate the direct numerical simulations. The turbulent flow is analyzed by computing turbulence budgets as well as applying several data reduction techniques, including higher-order spectral analysis, and modal decomposition.