Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L7: Turbulent Boundary Layers VI |
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Chair: Ronald Adrian, Arizona State University Room: 310 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L7.00001: Three-dimensional structure of momentum transfer in turbulent channels Adri\'an Lozano-Dur\'an, \'Oscar Flores, Javier Jim\'enez The three-dimensional structures of the intense tangential Reynolds stress in plane turbulent channels (Qs) are studied by quadrant analysis, with emphasis on the logarithmic and outer layers. Wall-detached Qs are isotropically oriented background stress fluctuations, common to most turbulent flows, and do not contribute to the mean stress. Most of the stress is carried by a self-similar family of larger attached Qs, increasingly complex away from the wall, with fractal dimensions $D\approx 2$. They are ``sponges of flakes,'' while vortex clusters are ``sponges of strings.'' Although their number decays away from the wall, the fraction of the stress that they carry is independent of their heights, and a substantial part resides in a few objects extending beyond the centreline, reminiscent of the VLSM of several authors. The predominant logarithmic-layer structures are side-by-side Q4-Q2 pairs of sweeps and ejections, with an associated cluster, with dimensions and stresses similar to Townsend's conjectured attached eddies. They align themselves streamwise, but not strongly enough to explain the very long structures in the channel centre. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L7.00002: Hairpin vortices in the transitional and developed turbulence of a flat-plate boundary layer George Ilhwan Park, James Wallace, Xiaohua Wu, Parviz Moin Using Vortex lines to reveal vortical structures in turbulent flows has been in disfavor for some time. They are field lines that can be drawn wherever the flow is rotational, regardless of whether a true vortex exists there or not. However, their virtues are that the vortices they can reveal do not depend on setting a detection threshold, unlike all the vortex identifiers based on the velocity gradient tensor or based on a low pressure criterion, and they can isolate individual vortices. Such individual hairpin vortices have been identified in the transition region near $Re_\theta = 500$ of a recent flat-plate boundary layer and in the developed turbulence near $Re_\theta = 1950$. The vortices in these two regions emerge out of sheets of unorganized vorticity in the viscous sublayer and have quite similar characteristics. An octant analysis based on the combinations of signs of the velocity and temperature fluctuations, $u$, $v$ and $\theta$, shows that the momentum and heat fluxes in both the transitional and developed regions are predominantly of the mean gradient type. The fluxes appear to be closely associated with wall layer vortices that transport momentum and heat toward and away from the wall. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L7.00003: The structure of the vorticity field in the near-wall region of turbulent channel flow at high-Reynolds number Koji Morishita, Takashi Ishihara, Yukio Kaneda The structure of the vorticity field in turbulent channel flow is studied by using direct numerical simulations of the incompressible Navier-Stokes equations with up to $2048\times 1536\times 2048$ grid points; the maximum friction Reynolds number is $Re_\tau=2560$. Instantaneous vortex-line plots show the presence of $\Omega$-shaped hairpin vortices in the near-wall region of turbulent channel flow. The $\Omega$-shaped hairpin vortices in the near-wall region are well displayed by a bundle of vortex lines starting from points along a line near $y^+=10$ parallel to the mean stream. They suggest that the hairpin vortices are formed by instabilities and roll-ups of sheets of spanwise vorticity in the buffer layer of turbulent channel flow. The three-dimensional structure of the low-speed region of the streamwise velocity near the wall ($y^+<100$) is discussed in view of these vortex lines starting from the buffer layer. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L7.00004: Flow Visualization of Artificially Generated Hairpin Vortices Daniel Sabatino, Mark Palframan To investigate the potential mechanisms for hairpin packet formation in fully turbulent boundary layers, a flow visualization study of artificially generated hairpin vortices in an otherwise laminar boundary layer is performed. The experiments are conducted in a recently constructed free surface water channel at Lafayette College. A new method to artificially generate individual hairpin vortices is employed which utilizes a flexible membrane which is inflated to create transient hemispherical protrusions on a flat plate, zero pressure gradient laminar boundary layer. By controlling the duration of time the membrane protrudes above the wall, a single vortex can be reliably generated. This technique avoids the need for fluid injection in order to ensure uniform particle seeding for subsequent PIV measurements. Multiple generation sites are placed at different streamwise locations to allow hairpins of different maturity to interact. The characteristics of single hairpin vortices will be compared to those described in the literature along with a qualitative analysis of the interaction of two hairpin vortices. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L7.00005: Dynamical Properties of Vortex Furrows in Transitioning Boundary Layers Peter Bernard A vortex filament simulation of the spatially growing transitional boundary layer reveals the presence of low speed streaks underlying furrow-like streamwise oriented folds in the surface vorticity layer (AIAA J. Vol. 48, 2010; Proc. ETC13, 2011). The putative hairpin vortices and packets widely observed in boundary layers are found to be an illusion created by assigning the status of structure to the visualized form of regions of rotational motion created by the vortex furrows. Thus, at best, hairpins roughly describe the shape taken by that part of the vorticity within the furrows that directly causes rotation while ignoring the ``invisible'' and considerable non-rotational part. The life history of the furrows is discussed here including a description of how they grow and the dynamics of the vorticity field within them. Long lived furrows represent ``factories'' within which initially spanwise vorticity progresses from arch to either one or two-lobed mushroom-like structures in a continuous stream. Furrows grow by this same process. At the heart of the furrow phenomenon is a self-reinforcing process by which streamwise vorticity begets more streamwise vorticity. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L7.00006: Evolution of Reynolds stresses in a turbulent boundary layer Roeland de Kat, Lian Gan, James Dawson, Bharathram Ganapathisubramani Understanding Reynolds shear-stress events in a turbulent boundary layer is of crucial importance for modelling and controlling turbulent wall-flows. In this study, we examine the evolution in time and space of these shear-stress events by performing time-resolved PIV measurements in a stream-wise wall-normal plane of a turbulent boundary layer at $Re_\tau\approx2500$. The conditions are similar to the experiment by Dennis \& Nickels (J. Fluid Mech. 2011, vol. 673), who performed measurements at $Re_\theta=4700$. Four high-speed cameras positioned next to each other, 4-5 m downstream of a glass rod trip, captured a region of flow spanning approximately $2\delta$ in stream-wise and $0.5\delta$ in wall-normal direction. This zoomed-in field-of-view enables high spatial, $l^+\approx20$, and temporal resolution, $\Delta t^+\approx 1$ which will allow us to describe the evolution of shear-stress events in time and space. In the talk, detailed analyses including instantaneous tracking of Reynolds shear-stress events, quadrant decomposition and spectra of the stream-wise, wall-normal and Reynolds shear-stress fluctuations will be presented. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L7.00007: The topology of the footprints of wall-turbulence Jason Monty, Min Chong, Ivan Marusic When studying the topology of turbulent flows, the three invariants of the velocity gradient tensor are often used. For incompressible flow the first invariant $P$ is zero and the topology of the flow structures can be investigated in terms of the second and third invariants, $Q$ and $R$ respectively. For example, isosurfaces of $Q$ above a certain threshold are often used in an attempt to identify vortical structures in the flow. In wall-turbulence, however, these invariants are zero on a no slip wall. Therefore, analysis tools relying on these invariants cannot be used to topologically study the footprint of turbulence on the wall. In this paper, it is proposed that the ``flow'' field on a wall can be described by a no slip Taylor-series expansion. This provides a new tensor relating skin friction to streamwise and spanwise coordinate. Like the velocity gradient tensor, it is possible to define invariants $\mathcal{P}$, $\mathcal{Q}$ and $\mathcal{R}$ of the so-called ``no slip'' tensor. It will also be shown that it may be possible to investigate the topology of the flow field on a no slip wall in terms of these invariants. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L7.00008: Space-Time Correlation of Large-Scale Structures in a Turbulent Boundary Layer Nicolas Buchmann, Callum Atkinson, Matthias Kuehn, Julio Soria Taylor's hypothesis is often used to project temporal data into the spatial domain and has been used in the past to show the presence of large-scale structures ($>10\delta$) in the log and lower wake region of the turbulent boundary layer (TBL). To investigate the spatial and temporal evolution of such large-scale structures, the present study employs time-resolved Particle Image Velocimetry (PIV) in several streamwise-spanwise planes in the log-layer of a TBL ($Re_\theta=2,000$). In order to capture the full extent of these structures, four high-speed, high-resolution PIV systems are combined to span a region of approximately $3\delta \times 12\delta$ and a continuous time sequences of $\approx 50\delta/U$. Such data sets are currently unavailable from previous experimental investigations and reveal the existence of long and very long ($>8\delta$) low- and high-speed structures. Two-point space-time correlations are employed to examine the temporal extent and meandering nature of these structures with respect to their size and spacing in the log-layer. Furthermore, the validity of Taylor's hypothesis is tested for such long projection distances. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L7.00009: High spatial resolution measurements of large-scale three-dimensional structures in a turbulent boundary layer Callum Atkinson, Nicolas Buchmann, Matthias Kuehn, Julio Soria Large-scale three-dimensional (3D) structures in a turbulent boundary layer at Re$_\theta$ = 2000 are examined via the streamwise extrapolation of time-resolved stereo particle image velocimetry (SPIV) measurements in a wall-normal spanwise plane using Taylor's hypothesis. Two overlapping SPIV systems are used to provide a field of view similar to that of direct numerical simulations (DNS) on the order of $50\delta \times 1.5\delta \times 3.0\delta$ in the streamwise, wall-normal and spanwise directions, respectively, with an interrogation window size of $40^+ \times 20^+ \times 60^+$ wall units. Velocity power spectra are compared with DNS to examine the effective resolution of these measurements and two-point correlations are performed to investigate the integral length scales associated with coherent velocity and vorticity fluctuations. Individual coherent structures are detected to provide statistics on the 3D size, spacing, and angular orientation of large-scale structures, as well as their contribution to the total turbulent kinetic energy and Reynolds shear stress. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L7.00010: Energy dissipating structures in turbulent boundary layers Marie Farge, Romain Nguyen van yen, Kai Schneider We present numerical experiments of a dipole crashing into a wall, a generic event in two-dimensional incompressible flows with solid boundaries. The Reynolds number $Re$ is varied from $985$ to $7880$, and no-slip boundary conditions are approximated by Navier boundary conditions with a slip length proportional to $Re^{-1}$. Energy dissipation is shown to first set up within a vorticity sheet of thickness proportional to $Re^{-1}$ in the neighborhood of the wall, and to continue as this sheet rolls up into a spiral and detaches from the wall. The energy dissipation rate integrated over these regions appears to converge towards $Rey$-independent values, indicating the existence of energy dissipating structures that persist in the vanishing viscosity limit. Details can be found in Nguyen van yen, Farge and Schneider, PRL, {\bf 106}, 184502 (2011). [Preview Abstract] |
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