Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G01: Nonlinear Dynamics: Turbulence & Turbulent Transition 
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Chair: Daniel Borrero, Willamette University Room: Georgia World Congress Center B201 
Monday, November 19, 2018 10:35AM  10:48AM 
G01.00001: The characterization of the interaction between two convection rolls from two cubic containers Dandan Ji, Eric Brown We present characterization of the largescale circulations (LSCs) of turbulent Rayleigh Bénard convection in two cubic cells. The two cells are connected through a halfopen shared wall. We found two states when changing the tilting angle: counter rotating when the tilting angle is small, corotating when the tilting angle is big. We looked at how the states and the orientations of the LSCs change along the tilting angle, we found a plot of the preferred orientation as a function of tilting angle has hysteresis loop. We are testing whether an extended stochastic model based on Brown and Ahlers (Phys. Fluids, 2008) can describe the observed behaviors.

Monday, November 19, 2018 10:48AM  11:01AM 
G01.00002: Generation and decay of turbulence in an abruptly stopped TaylorCouette flow Harminder Singh, Arnaud Prigent, Innocent Mutabazi This study presents an innovative approach towards the generation and decay of turbulence in the TaylorCouette system. The cylinders were brought to an abrupt stoppage that generated turbulence in the system, which was initially in the laminar flow region. Two different experimental approaches, namely visualizations and stereoPIV measurements, were used to better understand the presented phenomenon for three different configurations at multiple rotation rates but always starting in the laminar flow region: only external cylinder, corotation and counterrotation. For only external cylinder rotation, the lowest threshold limit of outer Reynolds number was found to be Re_{o}=693, which could be lowered with co or counterrotation to a minimal value of Re_{o}=433 . Axial wavenumber increased with increasing rotation rate, and decay time to achieve complete laminar state was found to be around 40 seconds irrespective of the initial rotation rate. On the other hand, if the initial state before abrupt stoppage was turbulent the turbulence decayed in the matter of few seconds. In contrast to the study of Verschoof et al. (2017), selfsimilar decay of turbulence was not observed. 
Monday, November 19, 2018 11:01AM  11:14AM 
G01.00003: Energy flux enhancement, intermittency and turbulence via Fourier triad phase dynamics in the 1D Burgers equation Miguel D Bustamante, Brendan P Murray We present a theoretical and numerical study of Fourier space triad phase dynamics in 1D stochastically forced Burgers equation at Reynolds number Re ≈ 2.7×10^{4}. We show that Fourier triad phases over the inertial range display a collective behaviour characterised by intermittent periods of synchronisation and alignment, reminiscent of Kuramoto model (1984) and directly related to shock collisions in physical space. These periods of synchronisation favour efficient energy fluxes across the inertial range towards small scales, resulting in strong bursts of dissipation and enhanced coherence of Fourier energy spectrum. The fast time scale of the onset of synchronisation relegates energy dynamics to a passive role: this is further examined using a reduced system where Fourier amplitudes are fixed in time  a phaseonly model. In it, we find intermittent triad phase dynamics without amplitude evolution and recover many features of the full Burgers system. Finally, for both full Burgers and phaseonly systems the physical space velocity statistics reveal that triad phase alignment is directly related to the nonGaussian statistics typically associated with structurefunction intermittency in turbulent systems. Published in JFM 850, 624645 (2018). 
Monday, November 19, 2018 11:14AM  11:27AM 
G01.00004: Sparse and randomized sampling methods for scalable turbulent flow networks Zhe Bai, N. Benjamin Erichson, Muralikrishnan Gopalakrishnan Meena, Kunihiko Taira, Steven L Brunton This work demonstrates the effective use of scalable algorithms in randomized and sketched linear algebra to perform networkbased analysis of complex fluid flows. Network theoretic approaches can help reveal the connectivity structure among a set of fluid elements and analyze their collective dynamics. These approaches have recently been generalized to analyze highdimensional turbulent flows, for which network computations can become prohibitively expensive. In this work, we propose efficient methods to approximate leading network quantities, such as the leading eigendecomposition of the adjacency matrix, using sparse and randomized techniques from linear algebra. First, we explore importance sampling to identify key locations to sample in the turbulent vorticity field that are most correlated with network quantities of interest. Importance sampling is then combined with the Nystr\"om method to approximate the leading eigendecomposition, resulting in significant computational savings. The effectiveness of the proposed technique is demonstrated on two and threedimensional isotropic turbulence. 
Monday, November 19, 2018 11:27AM  11:40AM 
G01.00005: The turbulent flow in a slug Rory T Cerbus, Jun Sakakibara, Gustavo Gioia, Pinaki Chakraborty In seminal experiments reported in the 1970s, Wygnanski and coworkers made detailed measurements of transitional pipe flow, an intermittently laminar and fluctuating flow. Their findings led to the sharp distinction between ``puffs”, a fluctuating domain of fixed length, and ``slugs”, an evergrowing chaotic region. Puffs showed apparent differences with fullydeveloped turbulent flows, for which there is no laminarturbulent intermittency. When Wygnanski compared slugs and fullydeveloped turbulence, however, he concluded that ``The structure of the flow in the interior of a slug is identical to that in a fully developed turbulent pipe flow.” This conclusion has become an unquestioned cornerstone in our understanding of transitional pipe flow. We have repeated Wygnanski’s measurements using modern experimental techniques and computer simulations and find that his conclusion comes with important caveats, which are missing in the original work, and that his evidence requires a thorough reevaluation. Our results reveal a richer picture of equivalence between slugs and turbulence that also unveils the crucial role of the Reynolds number. 
Monday, November 19, 2018 11:40AM  11:53AM 
G01.00006: Understanding finite lifetimes of Newtonian turbulence James Hitchen, Alexander N Morozov Recently, our understanding of the transition to turbulence has significantly changed due to the discovery of exact solutions of the NavierStokes equations and the introduction of the selfsustaining process in parallel shear flows. This theory has been very successful in describing the main features of weakly turbulent states, including the metastable nature of turbulence close to the transition and the superexponential dependence of its lifetime on the Reynolds number. The main strength of this approach is that it allows for a semianalytical description of the turbulent dynamics in the form of a rather lowdimensional model. The exact form of such models is typically guided by one’s intuition and DNS. In this talk we present a systematic way of deriving lowdimensional models for plane Couette flow that requires no previous intuition of the system in question or its dynamics. We find that the model exhibits a subcritical transition to turbulent dynamics, contains stable periodic orbits, exact coherent structures and finite turbulent lifetimes. We demonstrate that the superexponential nature of the lifetimes requires interactions between exact coherent structures of different symmetries and discuss the implications of this discovery for the transition. 
Monday, November 19, 2018 11:53AM  12:06PM 
G01.00007: Optimal forcing to destabilise turbulence in a pipe flow Elena Marensi, Ashley P Willis, Rich Kerswell Recent experiments (Kuehnen et al. 2018) have shown that flattening a turbulent streamwise velocity profile in pipe flow destabilises the turbulence so that the flow relaminarises. We show that a similar phenomenon occurs for laminar pipe flow profiles in the sense that the nonlinear stability of the profile increases as it becomes more flattened by a body force. Significant turbulent drag reduction is found even in cases where the amplitude of the forcing is not large enough to avoid transition being triggered. The artificial body force is designed to mimic a baffle used in the experiments of Kuehnen et al. (2018) and the nonlinear stability is measured by the size of the energy of typical disturbances (here taken to be localised random initial conditions) needed to trigger transition. A new fully nonlinear optimisation problem is then constructed, whereby the "minimal forcing", i.e. the forcing characterised by the lowest amplitude or minimum work done, is sought to just destabilise turbulence. The resulting optimal forcing is related to the experiments of Kuehnen et al. (2018) to provide insights and guidelines on how to exploit this promising direction of flow control in the most efficient way. 
Monday, November 19, 2018 12:06PM  12:19PM 
G01.00008: The Effects of Rotation on FiniteAmplitude Perturbation Thresholds for Transition in Subcritical TaylorCouette Flow Daniel BorreroEcheverry, Katherine M. LaChasse While famous for its transition to turbulence via a hierarchy of centrifugal instabilities of ever increasing complexity, TaylorCouette flow (i.e., the flow between independently rotating cylinders) can also bypass linear instabilities and become turbulent via a subcritical route. In this case, the laminar state is linearly stable but sufficiently large finiteamplitude perturbations can destabilize it and cause the system to jump directly to a state with a high degree of spatiotemporal complexity. Here, we present some preliminary results regarding the effects of rotation on the minimum perturbation strength required to trigger turbulence. The perturbations are introduced by small jet emanating from the inner cylinder wall. By differentially rotating the cylinders and probing the systems with jets of different strengths, we can separate the influence of rotation from that of pure shear on the stability of the flow, as proposed by Dubrulle et al. (Phys. Fluids 17, 095103 (2005)). Our preliminary results suggest that rotation can have a strong effect on the stability of TaylorCouette flow to finiteamplitude perturbations depending on experimental conditions. 
Monday, November 19, 2018 12:19PM  12:32PM 
G01.00009: A relaminarization mechanism by sustainable thin airfilms for turbulent drag reduction Cong Wang, Morteza Gharib Reduction of hydrodynamic frictional drag through introduction of air bubbles or films at the wall region has been tried by several groups in the past. However, this method requires energy input to pump air bubbles. Previously, we have demonstrated a novel technique that can stably maintain and acoustically modulate oscillating airfilms in fully turbulent boundary layer. This technique generates large drag reduction effect. We will also present a relaminarization mechanism in the near wall region due to the oscillation of airfilms. This mechanism can explain the observed large drag reduction. 
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