Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A11: Turbulence: Wall-bounded Flows I |
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Chair: Jason Hearst, NTNU Room: 3B |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A11.00001: Direct Calculation of Eddy Viscosity of Turbulent Channel Flow Danah Park, Ali Mani This talk discusses a direct measurement of the eddy viscosity of turbulent channel flow using macroscopic forcing method (MFM), a statistical technique that utilizes DNS to determine turbulence closure operators. We introduce the eddy viscosity tensor, the leading-order form of the generalized eddy viscosity, and a methodology to compute this tensor for the turbulent flow using MFM. Our results provide the anisotropic eddy viscosity tensor as a function of distance from the wall. Additionally, for a subset of these tensorial components, we present the full eddy viscosity operator, which indicates a quantification of non-local effects beyond the leading-order eddy viscosity. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A11.00002: A minimal quasilinear approximation of turbulent channel flow Yongyun Hwang, Bruno Eckhardt Townsend's model of attached eddies for boundary layers is analysed within a quasi-linear approximation. The velocity field is decomposed into a mean profile and fluctuations. While the mean is obtained from the nonlinear equations, the fluctuations are modelled by replacing the nonlinear self-interaction terms with an eddy-viscosity-based turbulent diffusion and a stochastic forcing. The colour and amplitude of the stochastic forcing are then determined self-consistently by solving an optimisation problem which minimises the difference between the actual Reynolds shear stresses and the model. When applied to turbulent channel flow in a range of friction Reynolds number from $Re_\tau=500$ to $Re_\tau=20000$, the resulting turbulence intensity profile and energy spectra exhibit exactly the same qualitative behaviour as DNS data throughout the entire wall-normal location, thereby reproducing the early theoretical predictions of Townsend and Perry within a controlled approximation to the Navier-Stokes equation. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A11.00003: Minimal-span direct simulation of~transient, accelerating channel flows and application with wall riblets. Saurabh Pargal, Junlin Yuan, Giles Brereton Minimal-span simulation of wall~turbulence is an attractive approach for reducing simulation~cost~when~focusing~on~near-wall~phenomena~~associated with frictional drag modification due to surface riblets~or roughness. We~evaluate the capability~of~such~simulations~for~a periodic smooth-wall channel flow subjected to~rapid~acceleration from a friction Reynolds number of 180 to one of 420. Compared to a~full-span simulation, the single-point statistics, two-point correlations and~spectral analyses~indicate that the minimal span is sufficient to capture the~pseudo-laminarization phenomenon, since the stabilization effect of the increased~ensemble-averaged shear is~confined to~the near-wall region. As the increased shear is relaxed, the~retransition to a new equilibrium state, though starting from the wall, is slightly~delayed~compared to that in a full channel.~The wall is~then covered with saw-tooth riblets of a uniform height, which is 7.5~wall units at the start of, and 17.5 at the end of acceleration. Lower Reynolds~stresses (in wall units) and a~slower retransition are observed in the riblet flow, compared to a smooth-wall channel flow.~The results demonstrate both advantages and~limitations of minimal-span~simulations of non-equilibrium wall turbulence. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A11.00004: Orr-Sommerfeld and Squire modes in fully-turbulent channel flow Ryan McMullen, Kevin Rosenberg, Beverley McKeon The Orr-Sommerfeld (OS) and Squire (SQ) operators from classical stability literature have gained renewed interest in the context of linear analyses of turbulent shear flows. We demonstrate that the recently-proposed decomposition of the resolvent operator into two distinct families related to the OS and SQ operators (Rosenberg {\&} McKeon, 2019) results in accurate low-order representations of the second-order statistics in turbulent channel flow. It is shown that the ability to match all components well is due to the isolation of the v response in the OS modes. This enables competition between the OS and SQ vorticity, which is interpreted as a phase difference between the two sets of modes. Additionally, the relative Reynolds number scalings for the two families of resolvent weights are derived for the universal classes of resolvent modes presented by Moarref et al. (2013). Implications for modeling nonlinear interactions in wall-bounded turbulence are discussed. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A11.00005: Direct Numerical Simulations of transverse acoustic forcing in fully developed channel flow turbulence Andrea Gruber, Melissa Kozul, James R. Dawson Hydrogen-firing of gas turbines is a promising direction for clean and efficient carbon-free power generation. However, premixed combustion of undiluted hydrogen in low-emission combustion systems can result in unstable operating modes for gas turbines and their development must solve issues posed by flame flashback (off-design upstream propagation), and flame combustion dynamics (coupling of longitudinal or transversal acoustic waves with fluctuating heat release). The causal relationship between longitudinal acoustically-induced bulk flow fluctuations (due to combustion dynamics) and the occurrence flashback in gas turbine burners has been established for some time (Fritz et al., \textit{J Eng Gas Turbines Power }2004) while the key role of the near-wall `streaks' of the turbulent boundary layer in controlling the flashback process was only explained more recently (Gruber et al., \textit{J Fluid Mech} 2012). In this context, it is important to examine the effect of transversal acoustic waves, resulting from azimuthal modes resonating in annular combustors, on the characteristic streaky structure of near-wall turbulence. We present DNS results from an idealized channel flow configuration that employs periodic boundary conditions in the streamwise and spanwise directions, where sinusoidal pseudo-acoustic waves are imposed transversally to the bulk flow, mimicking a limit cycle. We report the response of the near-wall streaks of the velocity field to the frequency and amplitude of the imposed acoustic waves. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A11.00006: Lifetimes of the large-scale motions in channels up to $Re_\tau \approx 5000$ Miguel Encinar, Alberto Vela-Martin, Javier Jimenez The energy spectra of (wall-bounded) turbulent flows is unmistakeably one of the most important tools in turbulent research. Due to past limitations of computational resources, direct numerical simulations have traditionally focused on the computation of the spatial spectra, neglecting its temporal counterpart. We compute the spatio-temporal spectra of the large scales (larger than one fifth of the channel half-height) of channels up to $Re_\tau \approx 5000$. Using the spectra, it is possible to extend the definition of `correlation time' to provide a meaningful estimation of the lifetime for every spatial wavelength, and at every height. We find the lifetimes to increase with the distance to the wall and with the stream and spanwise wavelengths. There is almost no difference between in the lifetime of a given wavelength between velocity components. Nevertheless, the expected lifetime of each component is different due to the difference in their energy spectra, i.e. the streamwise component lives longer because has more energy in long wavelengths. We also found that the lifetimes scale across Reynolds numbers with the channel half-height and the bulk velocity, instead of the friction velocity. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A11.00007: Turbulent mean flow profile in 2D channel flow Vilda Markeviciute, Rich Kerswell Two-dimensional channel flow is well known to exhibit chaotic behaviour in which structures common to turbulent flows such as vorticity sweeps, large-scale intermittency, and quasi-periodic bursting are observed (Jimenez 1990). Recently, Falkovich and Vladimirova (2018) have found that the time-averaged turbulent mean flow at constant pressure gradient exhibits a remarkably simple wave-like structure which is known to exist for low Reynolds numbers (Re), including below the linear instability threshold (Re=5772). Trying to confirm this finding, here we revisit this flow imposing constant mass flux driving instead. At least in short channels, we find unexpectedly long-lived asymmetric states beyond a critical Reynolds number where the turbulence is confined to one of the channel walls. This poses an interesting question of how long one needs to wait to recover the expected symmetric mean flow or whether there really is bistability of turbulent states centred on one or other of the walls. [Preview Abstract] |
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