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 P15: Turbulent Boundary Layers II |
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Chair: Angeliki Laskari, Caltech Room: 310 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P15.00001: Reynolds stress spectra in pipes and boundary layers up to \textit{Re}$_\tau\approx 10000$ Spencer Zimmerman, Jimmy Philip, Joseph Klewicki Turbulent pipe flows and zero-pressure-gradient boundary layers (BLs) are frequently compared with the goal of elucidating the role of boundary conditions on turbulent wall flows. Since at least the early study by Schubauer (J. Appl. Phys., \textbf{25} (2), 1954, pp. 188--196), differences in the Reynolds stress (RS) profiles of the two flows have been described as resulting primarily from the turbulent/non-turbulent intermittency of the boundary layer. More recently, however, a number of researchers have shown differences in the streamwise velocity variance contributions per scale between the two flows even below the intermittent range. Despite these differences, Monty \textit{et al.} (J. Fluid Mech., \textbf{632}, 2009, pp. 431--442) showed that the streamwise variance profiles of the two flows match over most of the domain at matched friction Reynolds number. Extending this work, Zimmerman \textit{et al.} (J. Fluid Mech., \textbf{869}, 2019, pp. 182--213) showed experimental evidence that differences in the wall-normal and spanwise RS profiles can extend to within the log-layer. Here, we use the same experimental dataset to discuss features of the spectra common to both flows as well as the scales and wall-positions at which systematic differences are observed. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P15.00002: Spectral analysis of Reynolds shear stress in high Re turbulent channel flows Robert Moser, Myoungkyu Lee We perform spectral analysis of the terms in the transport equation Reynolds shear stress, $\langle u'v' \rangle$ in high $Re$ wall-bounded turbulent channel flows, using the analysis technique from Lee \& Moser (J. Fluid Mech., vol 860, 886-938). Specifically, a log-polar representation of two-dimensional spectra are used to study the interactions of turbulence at different length-scales and wall-normal distances. The analysis results show that $\langle u'v'\rangle$ production occurs primarily in the streamwise-elongated modes. Inter-scale transfer at fixed wall-normal distances transfers shear-stress to modes that are elongated in the spanwise direction, especially away from the wall. Wall-normal transport then moves this streamwise-elongated stress to the other side of the channel. This exchange of stress between the two sides of the channel is the primary balance of the production, since the dissipation is relatively weak. Wall-normal transport of streamwise-elongated modes is more complex, with $\langle u'v' \rangle$ exchange driven by the increasing spanwise scale of the dominant stress carrying modes. Finally, away from the wall, the characteristic length scales of production and wall-normal transport mechanisms grow linearly with wall-normal distance. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P15.00003: Identification of nonlinear interaction of the self-sustaining process in wall-bounded turbulence using resolvent analysis H. Jane Bae, Beverley McKeon The nonlinear interaction in the self-sustaining process of wall-bounded shear flows is investigated using resolvent analysis. Resolvent analysis (McKeon \& Sharma 2010) is used to identify the principal forcing mode which produces the maximum amplification in the minimal channel (Jim\'enez \& Moin 1991). The identified mode is then removed from the nonlinear term of the Navier-Stokes equations at each time step from a direct numerical simulation of a minimal channel. The results show that the removal of the principal forcing mode is able to laminarize the flow, while the removal of subsequent modes only marginally affects the flow. Using conditional averaging, the flow structures that are responsible for generating the principal forcing mode, and thus the nonlinear interaction to self-sustain turbulence, are identified to be spanwise rolls interacting with meandering streaks. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P15.00004: Two-Scale Interaction in Near-Wall Turbulence Patrick Doohan, Ashley P. Willis, Yongyun Hwang It has been shown that the dynamics of individual energy-containing eddies in the hierarchy of wall-bounded turbulence (Townsend,1980) are governed by the self-sustaining process (SSP) (Hwang \& Bengana,2016). However, multiscale flows also exhibit interaction between structures of different scales, notably the energy cascade, but the temporal dynamics of multiscale turbulence are not well understood. In this study, the temporal dynamics of a two-scale near-wall flow are investigated using a shear stress-driven model (Doohan et al.,2019). In addition to the SSP at each scale, the energy cascade and feeding from small- to large-scales are identified as the primary scale interaction processes. The energy cascade is most active during the streak-breakdown stage of the large-scale SSP and the timescale of the resulting small-scale dissipation matches that of the large-scale motion i.e. non-equilibrium cascade. The wall-normal cascade can also fuel small-scale production, driving the small-scale SSP. The feeding of large-scale structures is correlated with the streak-breakdown stage of the small-scale SSP and results in increased large-scale pressure transport and dissipation. In the presentation, the dynamics of the two-scale interaction system will be discussed in detail. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P15.00005: Linear mechanisms sustaining wall turbulence Adrian Lozano-Duran, Marios-A. Nikolaidis, Michael Karp, Navid C. Constantinou Turbulence is the primary example of a highly nonlinear phenomenon. Nevertheless, there is evidence that the energy-injection mechanisms sustaining wall turbulence can be ascribed to linear processes. The different scenarios stem from linear stability theory and comprise modal instabilities from mean-flow inflection points, transient growth from non-normal operators, and parametric instabilities from temporal mean-flow variations, among others. These mechanisms, each potentially capable of leading to the observed turbulence structure, are rooted in simplified theories. Whether the flow follows any or a combination of them remains unclear. In the present study, we devise a novel collection of numerical experiments in which the Navier-Stokes equations are sensibly modified to quantify the role of the different linear mechanisms. This is achieved by direct numerical simulation of turbulent channel flows with constrained energy extraction from the streamwise-averaged mean-flow. We demonstrate that (i) transient growth alone is not sufficient to sustain wall turbulence and (ii) the flow remains turbulent when the modal instabilities are suppressed. On the other hand, we show that the parametric instability due to the time-varying mean-flow is essential to maintain turbulence alive. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P15.00006: Statistical characterization of inter-component energy exchange in turbulent channel flows Yongseok Kwon, Javier Jimenez In turbulent channel flows, it is well understood that the fluctuating velocity extracts energy from the mean flow via the lift-up mechanism (as represented by the production term in the energy budget equations). In this process, the streamwise velocity perturbation receives energy from the mean flow by the linear advection of the streamwise momentum along the cross-shear direction. However, what is still unclear is the precise mechanism by which the strong cross-shear velocity is triggered. In this presentation, this process is investigated from the perspective of the mean turbulent kinetic energy exchange and the inter-component energy exchange processes are statistically characterized. In particular, the primary focus is put on the non-linear interactions leading to the inter-component energy exchange, to shed light on their roles in the sustenance of turbulence in channel flows. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P15.00007: Large field of view volumetric measurement of a turbulent boundary layer Felix Eich, Matthew Bross, Daniel Schanz, Matteo Novara, Andreas Schr{\"o}der, Christian J. K{\"a}hler In order to understand the complex dynamics and interaction processes between coherent flow motions within turbulent boundary layers, time reolved 3D data is necessary. Therefore, a unique time resolved 3D measurement was performed at $Re_\tau = 4200$ under zero and adverse pressure gradient conditions. Using 13 high speed PIV cameras, helium filled soap bubbles as tracer particles in combination with LED illumination, it was possible to measure a turbulent boundary layer flow over $2.9 \times 0.6 \times 0.25\, \mathrm{m^3}$ at a recording frequency of 1 kHz. The acquired data was analysed with Lagrangian particle tracking techniques to generate velocity fields. The turbulent superstructures and their dynamics could be directly resolved. The interaction between the superstructures gives insight into the exchange of mass and momentum between the coherent flow motions. Furthermore the interaction between the superstructures with the flow separation line is studied. The results show that the superstructures have a significant effect on the dynamics of the line of separation. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P15.00008: Streamwise development of targeted coherent structures in turbulent pipe flow Tyler Van Buren, Leo Hellstrom, Ivan Marusic, Alexander Smits Our aim is to perturb specific naturally occurring energetic modes in turbulent pipe flow ({\$}Re\textunderscore $\backslash $tau $=$ 3486{\$}) using sections of pipe with a periodic non-circular cross-section. We track the downstream development of these perturbations with stereoscopic particle image velocimetry. Cross-sections with an azimuthally varying radius (sinusoidal) delicately agitate the flow through excitation of the Reynolds stresses. These sections significantly alter the mean flow, and add energy in the targeted structures while simultaneously reducing the energy in the non-excited structures. Supported under ONR Grant N00014-15-1-2402, Program Manager/Director Thomas and the Australian Research Council. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P15.00009: Application of the attached eddy hypothesis for turbulence characterization in marine boundary layer flows Dachuan Feng, Vikrant Gupta, Minping Wan, Larry K.B. Li Tidal current turbines usually operate in moderate to high current marine boundary layer (MBL) flows. Whereas the mean flow speed largely determines the average power extraction, it is the higher-order turbulence statistics that determine the structural load on turbines (required for device design) and the wake length behind them (required for array design). We propose to use Townsend's attached eddy hypothesis to characterize the turbulence in MBL flows. To this end, we perform large-eddy simulations of high-Reynolds-number MBL flows with seabed roughness varying from the transitional to the fully rough regime. We find that, within the log-layer, the horizontally-averaged spanwise turbulence intensity follows a log-linear law while the wall-normal component remains nearly constant. These findings are consistent with the attached eddy model (Townsend 1976, The Structure of Turbulent Shear Flow, CUP), with the constants being dependent on the seabed roughness. The present work provides a reduced-order framework for studying the effect of boundary layer turbulence on turbines. [Preview Abstract] |
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