Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B46: Turbulence, Instabilities, Pattern Formation and Nonlinear Dynamics 
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Sponsoring Units: DFD GSNP Chair: Peter Monkewitz, Swiss Fed Inst Tech Room: LACC 506 
Monday, March 5, 2018 11:15AM  11:27AM 
B46.00001: The Statistical Properties of UltraStrong Turbulence Christian Küchler , Gregory Bewley , Eberhard Bodenschatz How do the twopoint statistical quantities of the velocity field depend on the turbulence level? Is turbulence universal? Answering these questions requires access to a sufficiently wide inertial range over which the turbulence can be considered universal. The Variable Density Turbulence Tunnel at the Max Planck Turbulence Facility can reach extremely high turbulence levels under experimentally resolvable conditions. The VDTT is a closedloop wind tunnel of 80 m^{3} volume filled with pressurized sulfurhexaflouride at 0.5 bar to 15 bar. This allows to change the turbulence levels over two orders of magnitude, while keeping the geometry of the flow the same. The turbulence is generated by an unique active grid consisting of 111 individually controllable flaps. By using nanoscale thermal anemometry probes developed at Princeton University we record ultra long time series of the turbulent velocity field at a single point 9 m downstream of the active grid. Using these techniques we compiled datasets at turbulence levels that heretoforth could not experimentally resolved. In the talk we report the evolution of twopoint statistics as a function of turbulence level and present insights on the inertial range scaling and intermittency effects of turbulent flows. 
Monday, March 5, 2018 11:27AM  11:39AM 
B46.00002: A Stable NonLaminar Invariant Solution in Pipe Turbulence Kimberly Short The features of the chaotic saddle in a short pipe at Re = 2200 and the ergodic trajectories that visit it are detailed. The domain contains a handful of unstable solutions of the incompressible NavierStokes equations (NSE), as well as a marginally stable solution, the first nonlaminar stable solution in pipe to be reported. As in longer pipes, the lifetimes of the turbulent trajectories in a short pipe are found to be exponentially distributed, an indicator of memorylessness, and a regular feature of chaotic saddles that give rise to chaos. In addition, a positive correlation between the lifetimes of the ergodic trajectories and the number of sufficiently close passages to the invariant solutions is observed: in general, the longestlived ergodic trajectories spend more time near invariant solutions. 
Monday, March 5, 2018 11:39AM  11:51AM 
B46.00003: Construction of Pitot Corrections
for the Zagarola & Smits Superpipe Data Peter Monkewitz The original turbulent pipe flow experiments in the Princeton “Superpipe” by Zagarola & Smits (PRL 1997) at unprecedented laboratory Reynolds numbers R^{+} have started an ongoing debate on the logarithmic law in the mean velocity profile U^{+}(y^{+}) and the question of Pitot probe corrections for shear, viscous effects and turbulence level. Considering that the Pitot probe diameter d^{+} exceeded 7000 wall units at the highest R^{+}, the various corrections had to be extended into uncharted territory. In this contribution, the inverse approach is adopted, where the result of the corrections is taken to be the model for U^{+} developed by Monkewitz in PRFluids 2, 2017. The latter has an inner part identical, up to higher order corrections, to the zeropressuregradient turbulent boundary layer profile and switches around y^{+} ≈ 500 to a pipespecific logarithmic overlap layer. The simplicity of the resulting global Pitot correction proportional to d^{+}(R^{+})^{}^{1/2}, with only two fitting parameters, indirectly supports this model. In addition, the higher order correction for the overlap layer, originally proposed by Afzal & Yajnik (J. Fluid Mech. 1973) and tied to the pressure gradient by Luchini (PRL 2017), is extended to the entire profile and shown to provide an excellent fit to the data. 
Monday, March 5, 2018 11:51AM  12:03PM 
B46.00004: Chiralsymmetry breaking and triad interactions in active turbulence Jonasz Slomka , Piotr Suwara , Jorn Dunkel Generalized NavierStokes (GNS) equations describing 3D active fluids with flowdependent spectral forcing possess numerical solutions corresponding to spontaneous generation of parityviolating Beltramitype chaotic flows that can sustain an upward energy transfer. To rationalize these findings, we study the triad truncation of two GNS models. Utilizing a previously unknown cubic invariant, we show that the asymptotic triad dynamics reduces to that of a forced rigid body coupled to a particle moving in a magnetic field. This analogy allows us to classify the triadic interactions by their asymptotic stability: unstable triads correspond to rigidbody forcing along the largest and smallest principal axes, whereas stable triads arise from forcing along the middle axis. This suggests that the unstable triads dominate the initial relaxation stage of the full GNS equations, which is characterised by helicity growth, whereas the stable triads determine the statistically stationary state. To support this hypothesis, we simulate a new active turbulence model, which develops an energy spectrum with Kolmogorovtype 5/3 scaling. Our results suggests that Beltramitype flows and an inverse energy cascade are generic features of 3D active turbulence models with flowdependent forcing. 
Monday, March 5, 2018 12:03PM  12:15PM 
B46.00005: Exact invariant solutions in the nearwall region of boundary layer turbulence Sajjad Azimi , Carlo Cossu , Tobias Schneider In boundary layer flows, smallscale streaky structures with a typical spacing of roughly one hundred wall units dominate the dynamics close the wall, where they control dissipation and wall drag. 
Monday, March 5, 2018 12:15PM  12:27PM 
B46.00006: Numerical Simulation of MultiMaterial Flows and Turbulent Mixing with High Order Eulerian Methods Baolin Tian , Yousheng Zhang , Zhiwei He He Compressible multimaterial flows and turbulent mixing can be found in many engineering and nature science fields, such as inertia confined fusion (ICF), astrophysics and so on. In this work, a high order multiphysics code, CFD^{2} (Code of Finite Difference for Compressible Flow Dynamics), was developed for the simulation of compressible multimaterial flows under extreme conditions. The code solves the equations of multidimensional hydrodynamics with high order accuracy in space and time, including a series of high order difference schemes, such as WENO, MP, GVC and WCNS schemes based on flux splitting techniques. The CFD^{2} code is implemented on nonuniform mesh and can simulate flow problems with billion mesh cells on thousands CPU cores with MPI parallelization. The code can simulate multiphysics flow problems, such as detonation, radiation, and so on. Some recent progress on high order schemes has been integrated into the code in order to prevent the nonphysical oscillations near material interfaces. A series of typical model problems have been simulated to validate the code. Moreover, turbulent mixing induced by RM and RT instability was simulated systematically to study the turbulent mixing mechanism. 
Monday, March 5, 2018 12:27PM  12:39PM 
B46.00007: Inertial Particle Dynamics and Coherent Structures in Geophysical Fluid Flows Alexa Aucoin , Eric Forgoston , Philip Yecko , Lora Billings Lagrangian Coherent Structures (LCS) provide a skeleton for the underlying structures in geophysical flows. We consider two types of geophysical flows with Coriolis force included and numerically investigate the dynamics of inertial particles. In particular, we use finitetime Lyapunov exponents (FTLE) to characterize the attracting and repelling LCS and show how inertial particles aggregate with respect to LCS for a range of Stokes numbers and density ratios. Using the MaxeyRiley equation we examine the inertial FTLE by tracking inertial particle trajectories. Lastly, we discuss the connection between these theoretical/numerical results and experimental results. 
Monday, March 5, 2018 12:39PM  12:51PM 
B46.00008: Emergence of Invariant Solutions underlying Oblique TurbulentLaminar Stripes in Plane Couette Flow Florian Reetz , Tobias Kreilos , Tobias Schneider

Monday, March 5, 2018 12:51PM  1:03PM 
B46.00009: On the spectrum of shallow water gravity waves generated by confined twodimensional turbulence Claudio Falcon , Edgar Knobloch We apply Lighthill's theory of aeroacoustic sound generation to shallow water gravity waves generated by spatially confined twodimensional turbulence. We show that the frequency spectrum of surface waves at large distances from the source of turbulence is, under suitable conditions, proportional to the spatiotemporal spectrum of the energymomentum tensor associated with the turbulent fields acting as the wave source and hence that it follows a powerlaw behavior. We compute the exponent for shallow water waves generated by isotropic twodimensional turbulence and show that the integrated power radiated scales as ω^{2/3} when the turbulent fluctuations arise from an inverse energy cascade and as ω^{2} when they arise from the enstrophy cascade. 
Monday, March 5, 2018 1:03PM  1:15PM 
B46.00010: Stability of an accelerated hydrodynamic discontinuity Daniil Ilyin , Yasuhide Fukumoto , William Goddard , Snezhana Abarzhi While looking from a far field at the accelerated interface separating ideal fluids of different densities, we identify, for the first time to our knowledge, a new type of hydrodynamic instability that develops when the acceleration magnitude exceeds a critical value. The flow dynamics conserves the fluxes of mass, momentum and energy at the interface, has potential velocity fields in the fluid bulk, and is shearfree at the interface. The growth rate and the flow fields' structure of this unstable dynamics depart substantially from those of other interfacial hydrodynamic instabilities, thus suggesting new opportunities for stabilization, diagnostics, and control of the interfacial dynamics. 
Monday, March 5, 2018 1:15PM  1:27PM 
B46.00011: Generalized Stability Analysis of Capillary Flow in Slender VGrooves Nicholas White , Sandra Troian Spontaneous capillary flow, an especially rapid process in slender open microchannels resembling Vgrooves, is of significant importance to many applications requiring passive robust flow control. Many types of biomedical devices for pointofcare use in developing countries are being designed around this principle. Important fundamental work by Romero and Yost (1996) and Weislogel(1996) elucidated the behavior of Newtonian films in slender Vgrooves driven to flow by the streamwise change in capillary pressure due to the change in radius of curvature of the circular arc describing the interface of wetting or nonwetting fluids. Selfsimilar solutions describing Washburn type dynamics were found but other solutions are possible. Here we extend the Romero and Yost model to include a variety of inlet and outlet boundary conditions and examine the transient growth and generalized stability of perturbations to steady state and selfsimilar flows. In total, the results support decades of experimental work which has found this method of flow control to be especially reliable, robust and selfhealing. 
Monday, March 5, 2018 1:27PM  1:39PM 
B46.00012: Flow Instability due to Pipeline Leakage Roes Budiman , Vishash Sharma Leakage in a pipeline is modelled by presence of one small hole on pipeline wall. The hole sets up a localized asymmetric pressure difference which alters the steady state velocity of the fluid flowing inside the pipeline. We use Reynolds's averaging technique and Prandtl's mixing length to study the turbulent flow perturbation to an initially laminar steady state flow. The resulting equations are twodimensional by assuming negligible velocity gradient along the pipeline length. We propose a potential function expression to account for the angular dependence from the asymmetry. Results from the analysis give the leakage flow rate as a function of leakage radius and pressure difference between the pipe inside and the ambient. 
Monday, March 5, 2018 1:39PM  1:51PM 
B46.00013: 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 also looked at the strength of LSCs at different states, and we found the discontinuity in the strength VS tilting angle measurement for the LSC that switched the orientation. We are testing whether an extended stochastic model based on Brown and Ahlers (Phys. Fluids, 2008) can describe the observed behaviors. 
Monday, March 5, 2018 1:51PM  2:03PM 
B46.00014: Untangling Superfluid Vortices Dustin Kleckner , Martin Scheeler , Hridesh Kedia , William Irvine , Louis Kauffman Previous work has shown that simple knotted vortices will untie in both viscous fluids and superfluids. Does the same behavior hold for complexly tangled vortices, irrespective or shape and topology? By simulating large numbers of vortex configurations in the GrossPitaevskii equation, I will show that the spontaneous unknotting of vortices is a universal feature of undriven fluids. I will also discuss the connection to conservation of helicity and topological features of the unknotting process. 
Monday, March 5, 2018 2:03PM  2:15PM 
B46.00015: Symbolic dynamics applied to a numerical simulation of a perturbed Hill's spherical vortex. Joshua Arenson , Spencer Smith , Kevin Mitchell 3D homotopic lobe dynamics (HLD) is a new symbolic method of describing topological dynamics for fully 3D systems. Here we apply this new method to a numerically computed perturbed Hill's spherical vortex flow. We consider the scattering of passive tracers that are drawn into and then ejected from the vortex. We focus on the numerical computation of fractal scattering functions—the time advected particles are trapped within the vortex as a function of two impact parameters. We compare the fractal selfsimilarity of these scattering functions to those predicted by 3D HLD. Our new method also produces a lower bound on the topological entropy of our system which approaches the true topological entropy as we add more numerical data. 
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