Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F29: Turbulence: Structures in Wall Bounded Flows and JetsShear layer Turbulence

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Chair: Javier Jimenez, Universidad Politecnica de Madrid Room: 205 
Monday, November 20, 2017 8:00AM  8:13AM 
F29.00001: Affects of spanwise heterogeneity and topographic height on Amplitude and Frequency modulation in channel flow turbulence. Ankit Awasthi, William Anderson We study the affects of spanwise heterogeneity on amplitude and frequency modulation of smallscale roughnesssublayer structures due to the passage of largescale structures in the logarithmic region. Recent studies on amplitude and frequency modulation (Mathis et al. 2009: J. Fluid Mech., 628) have prompted the development of a predictive model for nearwall dynamics. Such a model is of great interest to largeeddy simulation (LES), since nearwall processes are, by definition, never resolved. Here, we have used LES to model flows over a series of spanwiseheterogeneous topographies, where a domain with very long streamwise extent is used to ensure that verylargescale motions are (or, can be) resolved. We report that the secondary flows globally disrupt the turbulence from channel physics, wherein the ``outer peak'' is either shifted to different wavelengths or nonexistent. This spectral density redistribution is assured to alter amplitude and frequency modulation rates within the roughness sublayer, and we present correlations of the small and large scales to demonstrate precisely that (following the wavelet decomposition, as outlined by Baars et al., 2015: Exp. Fluids). Thus, spanwise heterogeneity should be regarded as a model parameter in any roughwallgeneralized prognostic wall models. [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F29.00002: Coherent instability in wallbounded turbulence M. J. Philipp Hack Hairpin vortices are commonly considered one of the major classes of coherent fluid motions in shear layers, even as their significance in the grand scheme of turbulence has remained an openly debated question. The statistical prevalence of the dynamic process that gives rise to the hairpins across different types of flows suggests an origin in a robust common mechanism triggered by conditions widespread in wallbounded shear layers. This study seeks to shed light on the physical process which drives the generation of hairpin vortices. It is primarily facilitated through an algorithm based on concepts developed in the field of computer vision which allows the topological identification and analysis of coherent flow processes across multiple scales. Application to direct numerical simulations of boundary layers enables the timeresolved sampling and exploration of the hairpin process in natural flow. The analysis yields rich statistical results which lead to a refined characterization of the hairpin process. Linear stability theory offers further insight into the flow physics and especially into the connection between the hairpin and exponential amplification mechanisms. The results also provide a sharpened understanding of the underlying causality of events. [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F29.00003: The intermittency of the inner layer of turbulent boundary layers. James Wallace, Xiaohua Wu, Parviz Moin, Adrian LozanoDuran, Jinhie Skarda, JeanPierre Hickey Uncovering the constitutive coherent structure inside the inner region ($y^+ < 100$) of the canonical turbulent boundary layer has remained a central research focus for decades. At last year’s DFD meeting we showed the ubiquity of spatially intermittent concentrations of vortices within the buffer layer and viscous sublayer with characteristics remarkably like those of transitionalturbulent spots. We call these concentrations of vortices with high swirling strength "turbulentturbulent" spots because they originate and grow within the developed turbulence region of the flow. The turbulentturbulent spots cause indentation, segmentation and termination of the passive viscous sublayer streaks, confirming and explaining, for the first time, the experimental visualization of viscous sublayer "pockets" of Falco (1980). The turbulentturbulent spots also coincide with local concentrations of high levels of Reynolds shear stress, enstrophy and temperature fluctuations. In this presentation we will quantify the spatial intermittency characteristics of these turbulentturbulent spots and compare them with those of the transitionalturbulent spots. See PNAS 114 (27) 2017 for complete details. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F29.00004: Studying the formation of nonlinear bursts in fully turbulent channel flows Miguel P. Encinar, Javier Jimenez Linear transient growth has been suggested as a possible explanation for the intermittent behaviour, or `bursting', in shear flows with a stable mean velocity profile. Analysing fully nonlinear DNS databases yields a similar Orr+liftup mechanism, but acting on spatially localised wave packets rather than on monochromatic infinite wavetrains. The Orr mechanism requires the presence of backwardsleaning wallnormal velocity perturbations as initial condition, but the linear theory fails to clarify how these perturbations are formed. We investigate the latter in a timeresolved waveletfiltered turbulent channel database, which allows us to assign an amplitude and an inclination angle to a flow region of selected size. This yields regions that match the dynamics of linear Orr for short times. We find that a short streamwise velocity ($u$) perturbation (i.e. a streak meander) consistently appears before the burst, but disappears before the burst reaches its maximum amplitude. Liftup then generates a longer streamwise velocity perturbation. The initial streamwise velocity is also found to be backwardsleaning, contrary to the averaged energycontaining scales, which are known to be tilted forward. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F29.00005: Localised burst reconstruction from spacetime PODs in a turbulent channel Adrian GarciaGutierrez, Javier Jimenez The traditional proper orthogonal decomposition of the turbulent velocity fluctuations in a channel is extended to time under the assumption that the attractor is statistically stationary and can be treated as periodic for longenough times. The objective is to extract space and timelocalised eddies that optimally represent the kinetic energy (and twoevent correlation) of the flow. Using timeresolved data of a smallbox simulation at $Re_\tau=1880$, minimal for $y/h\approx 0.25$, PODs are computed from the twopoint spectraldensity tensor $\Phi(k_x,k_z,y,y',\omega)$. They are Fourier components in $x$, $z$ and time, and depend on $y$ and on the temporal frequency $\omega$, or, equivalently, on the convection velocity $c=\omega/k_x$. Although the latter depends on $y$, a spatially and temporally localised 'burst' can be synthesised by adding a range of PODs with specific phases (Moin & Moser, JFM 1989). The results are localised bursts that are amplified and tilted, in a timeperiodic version of Orrlike behaviour. [Preview Abstract] 
Monday, November 20, 2017 9:05AM  9:18AM 
F29.00006: Attached flow structure and streamwise energy spectra in a turbulent boundary layer John Christos Vassilicos, Sricharan Srinath, JeanPhilippe Laval, Christophe Cuvier, Michel Stanislas, JeanMarc Foucaut On the basis of (i) Particle Image Velocimetry data of a Turbulent Boundary Layer with large field of view and good spatial resolution and (ii) a mathematical relation between the energy spectrum and specifically modeled flow structures, we show that the scalings of the streamwise energy spectrum in a wavenumber range directly affected by the wall are determined by wallattached eddies but are not given by the TownsendPerry attached eddy model. Instead, this spectrum's wavenumber exponent is 1p where p varies smoothly with distance to the wall from negative values in the buffer layer to positive values in the inertial layer. The exponent p characterises the turbulence levels inside wallattached streaky structures conditional on the length of these structures. [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F29.00007: Role of wallattached structures in the interface of the quiescent core region in turbulent pipe flow Jongmin Yang, Jinyul Hwang, Hyung Jin Sung The effects of low and highspeed structures on the interface of the quiescent core region are explored using direct numerical simulation data of turbulent pipe flow. The quiescent core region is a uniform momentum zone located at the center of the pipe flow, which contains the highest streamwise momentum with a low level of turbulence. The interface of the quiescent core region can be identified from the probability density function of the streamwise modal velocity. In the vicinity of the interface of the quiescent core region, the streamwise velocity changes abruptly. The abrupt jump in velocity causes an increase of the velocity gradient. The interface of the quiescent core region is similar to the laminar superlayer in turbulent/nonturbulent interface. The interface of the quiescent core region contains the low and highspeed structures. They can be classified into wallattached and detached structures depending on the distance between the structures and the wall. The influence of the detached structures accounted for most of the number of detected structures is negligible due to its small volume. Conversely, the wallattached structures adjacent to the interface have a huge influence on the statistical amount of the interface, such as entrainment characteristics. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F29.00008: Wallattached structures of streamwise velocity fluctuations in turbulent boundary layer Jinyul Hwang, Hyung Jin Sung The wallattached structures of streamwise velocity fluctuations ($u)$ are explored using direct numerical simulation data of turbulent boundary layer at $Re_{\tau } =1000$. We identify the structures of $u$, which are extended close to the wall. Their height ($l_{y})$ ranges from the nearwall region to the edge of turbulent boundary layer. They are geometrically selfsimilar in a sense that the length and width of the structures are proportional to the distance from the wall. The population density of the attached structures shows that the tall attached structures (290 \textless $l_{y}^{\mathrm{+}}$ \textless 550) follow the logarithmic probability distribution ($\sim l_{y}^{1} )$, a reminiscent of the distribution for hierarchy scales of the attached eddies addressed by Perry and coworkers. The streamwise turbulent intensity of these tall attached structures follows the logarithmic distribution with the distance from the wall. The wallattached structures of $u$ identified in the present work are a proper candidate for Townsend's attached eddy hypothesis and these structures exist in the low Reynolds number turbulent boundary layer. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F29.00009: Evolution of local structure along pathlines in a turbulent round jet Joseph Mathew Studies of the velocity gradient tensor (VGT) have revealed several features of the local structure of flows. In homogeneous, isotropic turbulence, conditional mean trajectories in the phase space of 2nd and 3rd invariants ($Q$ and $R$) of the VGT spiral clockwise to the origin (Ooi et al., 1998). Local topologies change from sheets to stretching vortices to compressing ones, repeatedly. In this study, the evolution of many fluid particles were computed alongside a temporal DNS of a round jet. Trajectories in $Q$$R$ space from successive positions along fluid pathlines were found to be qualitatively different from the conditional mean trajectories found earlier. Large departures occur in the 2nd and 1st quadrants of $Q$$R$ space. The implied local topology is focalthat of stretching and compressing vortices; dissipation was large, arising from changes to vortex stretching. On these selected curves large changes to $Q$ occur almost all the time. [Preview Abstract] 
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