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 M38: Turbulence Theory II |
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Chair: Toshiyuki Gotoh, Nagoya Institute of Technology, Japan Room: Georgia World Congress Center Ballroom 1/2 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M38.00001: State estimation in transitional and turbulent cylindrical-Couette flow Mengze Wang, Tamer A. Zaki Transitional and turbulent cylindrical-Couette flows are very sensitive to initial conditions and the start-up process. As a result, in numerical simulations, it is difficult to prescribe appropriate initial conditions that achieve target flow states, and comparisons to experiments become challenging. This problem is addressed using an adjoint-variational state-estimation algorithm. By combining simulations with limited measurements, we predict the appropriate initial conditions that track the correct flow states. We first consider estimation of a superposition of saturated wavy vortices from wall measurements. The computed initial condition contains all the relevant modes, and accurately reproduces the true energy spectra, mean-flow distortion and flow structures within the time intervals where measurements are available. We subsequently investigate the more challenging turbulent case with coarse-grained velocity data. The velocity field is reconstructed on a much finer simulation grid. The estimation error is 70% lower than space-time interpolation, with a better prediction of large-scale structures and vorticity. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M38.00002: Fundamental limitations in initial-state estimation using surface measurements in wall turbulence Qi Wang, Tamer A. Zaki Estimating the initial state of turbulent channel flow from surface measurements is formulated as an adjoint variational minimization. The cost function is the difference between the true measurements and their prediction from the reconstructed state. The accuracy of the state reconstruction and the fundamental limitations in its prediction from wall measurement are examined in detail. The analysis relies on evaluation of the Hessian matrix of the cost function, in the vicinity of the true solution, and analysis of the Hessian eigen-spectra. The leading eigenmodes correspond to the highest sensitivity of measurements to the flow; these modes are concentrated in the near-wall region upstream of the sensor location. Their structure explains the fundamental difficulty of estimating the flow state from wall measurements. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M38.00003: Extracting the Spectrum by Spatial Filtering Mahmoud Sadek, Hussein Aluie The spectrum of a flow quantifies the energy content of spatial scales and can yield valuable information about the cascade ranges, dissipation, and turbulence intensity. It is often calculated using Fourier series, which are formally justified only in periodic domains, and are inherently global in space, incapable of characterizing the flow locally. We will show that the spectrum of a flow field can be extracted within any local region by straightforward filtering in physical space. This can be as simple as unweighted averaging of adjacent grid-cells. We derive the conditions under which such a "filtering spectrum" is meaningful, and discuss its close connections and important differences with the wavelet spectrum. We demonstrate the approach using synthetic fields, 2D turbulence from a Direct Numerical Simulation, and 3D turbulence from the JHU Database.
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Tuesday, November 20, 2018 8:39AM - 8:52AM |
M38.00004: Input-output based analysis of convective velocity in turbulent channels Chang Liu, Dennice F Gayme This work employs an input-output based approach to computing convective velocities in turbulent channel flows based on stochastically forced Navier-Stokes equations linearized about a turbulent mean velocity profile. We use this model to examine both the mean convective velocity for fluctuating quantities as a function of wall-normal location, and the convective velocity associated with individual streamwise--spanwise wavelength pairs at each wall-normal location. We focus on the streamwise velocity and demonstrate the model's ability to reproduce the shape of the convective velocity profile, particularly its tendency toward a constant value in the near-wall region. The model predicted convective velocities also shows the Reynolds number invariance observed in computations using DNS data. We then isolate the contributions of each scale as well as each linear term in the momentum equation to examine their contribution to the convective velocity and their role in the known deviation from Taylor's hypothesis in the near-wall region. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M38.00005: A self-sustaining process theory for uniform momentum zones and internal layers in wall turbulence Brandon Montemuro, Gregory Chini, Joseph Charles Klewicki, Chris White The instantaneous streamwise velocity in turbulent wall flows exhibits a staircase-like profile, with uniform momentum zones (UMZs) separated by internal layers of concentrated spanwise vorticity (termed `vortical fissures', or VFs) across which the streamwise flow speed jumps by a few multiples of the friction velocity. A fundamental challenge is to identify a mechanism that can account for observed properties of the UMZ/VF profiles while respecting the constraints imposed by the mean momentum balance. Specifically, measurements show that outboard of the near-wall peak in the Reynolds stress: (i) the characteristic VF thickness decreases as the friction Reynolds number Reτ increases; and (ii) the mean momentum equation is inertially dominated. Supported by a new large-Reτ asymptotic analysis of the Navier-Stokes equations, a multiple spatial scale self-sustaining process for UMZs and interlaced VFs is proposed that satisfies these two requirements. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M38.00006: Abstract Withdrawn
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Tuesday, November 20, 2018 9:18AM - 9:31AM |
M38.00007: Scale interactions and spectral energy transfer in turbulent channel flow Minjeong Cho, Yongyun Hwang, Haecheon Choi Spectral turbulent energy transport is studied in the form of triadic wave interaction by focussing on the logarithmic layer. At the integral length scales, the main kinetic energy balance is formed between turbulence production and nonlinear turbulence transport, the latter of which plays the key role in the energy cascade. Visualisation of triadic wave interactions further revealed that there exist two new types of scale interaction processes highly active in the near-all and the lower logarithmic regions. First type of the scales interactions is associated with the energy cascade of relatively small energy-containing motions located in the region close to the wall, and considerable part of their energy transfer mechanisms from the integral to the adjacent small length scale in the energy cascade is found to be provided by the interactions between larger energy-containing motions. Second type of the scale interactions is the existence of a non-negligible amount of energy transfer from small to large integral scales. This scale interaction is predominant only for the streamwise and spanwise velocity components, and it plays a central role in the formation of the wall-reaching inactive part of large energy-containing motions which locally scales in inner units. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M38.00008: Transitions between large-scale flow states in turbulent 3D Kolmogorov flow Cristian C Lalescu, Michael Wilczek For most turbulent flows in nature, the idealization of statistical homogeneity and isotropy only applies to the small scales. The large scales are typically non-universal with pronounced inhomogeneities resulting from walls, large-scale driving etc. This motivates the investigation of the large-scale structure, turbulent fluctuations as well as their interaction in a canonical flow. Here we present results on a generalized turbulent Kolmogorov flow in three-dimensional periodic domains with aspect ratios larger than one. The flow is forced on a single Fourier mode and is subject to large-scale friction. The flow develops large-scale vortex patterns similar to two-dimensional Kolmogorov flow, even in the presence of intense three-dimensional small-scale fluctuations. We characterize transitions between different large-scale flow states as the large-scale friction is modified, including a regime reminiscent of noise-suppressed hysteresis. In addition to the large-scale flow features, we address the question of small-scale isotropy in the presence of large-scale anisotropies in this flow. Our results help to clarify the role of fluctuations for transitions in fully developed turbulence. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M38.00009: The Okubo-Weiss Type Criteria in Two-Dimensional Hydrodynamic and Magnetohydrodynamic Flows Bhimsen Shivamoggi, Gert Jan Van Heijst, Leon Kamp The Okubo-Weiss type criteria for 2D quasi-geostrophic (QG) flows (via the potential divorticity framework) and 2D magnetohydrodynamic (MHD) flows are formulated, and are interpreted in terms of the intrinsic metric properties of the underlying appropriate vorticity manifold. The Okubo-Weiss parameter is shown to remain robust under the beta-plane approximation to the Coriolis parameter, while the Okubo-Weiss type parameter is shown to provide a useful diagnostic tool to parametrize the magnetic field topology in 2D MHD flows. The ability of the Okubo-Weiss type criteria to separate the flow-field into coherent elliptic structures and hyperbolic flow configurations is validated via numerical simulations of quasi-stationary vortices in QG flows and 2D MHD flows in the decaying turbulence regime (in the presence of viscosity). |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M38.00010: Interaction between cloud droplets and turbulence Toshiyuki Gotoh, Izumi Saito, Takeshi Watanabe, Tatsuya Yasuda We investigate the mechanism for the modification of turbulence spectra due to turbulence-cloud droplet interactions reported in the previous DNS study (Saito and Gotoh, 2018, New J. Phys, 20, 023001), where it was found that spectra of scalar variances (temperature and vapor mixing ratio) tend to be k-1/3 unlike the classical Obukhov-Corrsin spectrum k-5/3. In order to explore the physical mechanism, we explore the mechanism of the spectral modification by using the cloud microphysics simulator under the conditions simpler than for the previous study. It is found that the particle inertia plays an essential role in the spectra modification. The classical -5/3 spectrum for the scalar variance at low wavenumers changes to the shallower spectrum at moderate wavenumbers, which resembles the characteristics reported in previous observational and DNS studies. We examined the dependence of the transition in the spectra on various parameters, such as gravity, Stokes numbers, Reynolds numbers, and so on. |
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