Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session R20: Turbulent Boundary Layers XI: Nonequilibrium and Transition |
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Chair: Ugo Piomelli, Queen's University Room: 30A |
Tuesday, November 20, 2012 1:00PM - 1:13PM |
R20.00001: Osborne Reynolds' pipe flow: Direct computation from laminar through bypass transition to fully-developed turbulence Xiaohua Wu, Parviz Moin, Ronald Adrian, Jon Baltzer, Jean-Pierre Hickey The most fundamental internal flow, smooth pipe from a slightly perturbed laminar inlet state continuously through bypass transition to fully-developed turbulence, has been computed using DNS over an axial domain length of 250 pipe radii. In the fully-developed turbulent region, mean and second-order turbulent statistics including the rate of viscous dissipation show excellent agreement with those obtained from an additional simulation using the conventional streamwise periodic boundary condition over an axial domain length of 30 pipe radii. Friction factor follows analytical solution prior to breakdown, and agrees with Moody's correlation after the completion of transition. During transition it exhibits an overshoot. Breakdown of the laminar pipe flow is characterized by the formation of large Lambda-shaped vortices pointing upstream, followed by their subsequent generation of small hairpin packets inclined towards the downstream direction. [Preview Abstract] |
Tuesday, November 20, 2012 1:13PM - 1:26PM |
R20.00002: Effects of mean and fluctuating pressure gradients on turbulence in boundary layers Pranav Joshi, Xiaofeng Liu, Joseph Katz This study focuses on the effect of mean and fluctuating pressure gradients on boundary layer turbulence. The mean favorable pressure gradient (FPG) is imposed by a sink flow. The streamwise and wall-normal (x,y) components of the material acceleration (Du/Dt and Dv/Dt, respectively) are calculated from time resolved 2D PIV data, and integrated spatially to obtain the pressure distribution. The mean FPG prevents vortical structures from rising away from the wall, decreasing the Reynolds stresses in outer region. Large scale pressure fluctuation gradients involve three dimensional flow structures. In both, zero pressure gradient (ZPG) and FPG boundary layers, large scale fluctuating adverse pressure gradients ($\partial $p'/$\partial $x$>$0) are preferentially associated with sweeps, as fluid approaching the wall is decelerating. Consequently, the outward transport of small-scale turbulence is suppressed, and the near-wall enstrophy increases. Conversely, ejections, high wall-normal enstrophy flux, and viscous vorticity production occur mostly during $\partial $p'/$\partial $x$<$0 as the fluid accelerates by moving away from the wall. The near-wall enstrophy flux peaks due to the inherent near wall 3D structures when $\partial $p'/$\partial $x$<$0 and u'$>$0. [Preview Abstract] |
Tuesday, November 20, 2012 1:26PM - 1:39PM |
R20.00003: Investigation of the Spreading Mechanism of Turbulent Wedges and Spots Jeff Chu, David Goldstein We investigate the physics of turbulent wedge and turbulent spot spreading in a nominally zero pressure gradient boundary layer over a flat wall using incompressible spectral DNS and an immersed boundary method. Turbulent wedges are simulated over both physical and unphysical surfaces to identify the important factors leading to wedge spreading and turbulence regeneration. We examined both instantaneous as well as time averaged turbulent wedge flow. We find that there are low speed streaks that remain stationary in time near the outer edge of the wedge. It is plausible that turbulent wedge spreading stems from a succession of such streaks due to instabilities introduced by the streak immediately preceding it upstream. The spreading mechanisms of turbulent spots are also investigated. Turbulent spots are artificially triggered and allowed to develop over physical and unphysical surfaces. Attempts are made to view both spot spreading and turbulent wedge spreading in one coherent picture. [Preview Abstract] |
Tuesday, November 20, 2012 1:39PM - 1:52PM |
R20.00004: Development of Turbulent Spots in Bypass Transition Kevin Nolan, Tamer Zaki The transition region in a boundary layer experiences sporadic bursts of localized turbulent spots. These spots spread as they are convected, and merge to sustain the turbulent boundary layer downstream. In this work, turbulent spots are identified and tracked from their point of inception in Direct Numerical Simulations (DNS) of bypass transition. The spreading angle, spatial extent and volume are recorded for each turbulent spot. The variation of these parameters is investigated for different pressure gradients. While the spreading angle depends on pressure gradient, the volumetric growth rate is found to be insensitive. The instantaneous structure of the spots is also examined in isolated events and in the ensemble average. The early stage of spot growth comprises a large-scale structure in the form of a streamwise-oriented vortex pair. The ensemble-averaged statistics are computed and demonstrate the important contributions to the turbulent-kinetic-energy budget within the structure of the turbulent patches. [Preview Abstract] |
Tuesday, November 20, 2012 1:52PM - 2:05PM |
R20.00005: Measurement of entropy generation within bypass transitional flow Richard Skifton, Ralph Budwig, Donald McEligot, John Crepeau A flat plate made from quartz was submersed in the Idaho National Laboratory's Matched Index of Refraction (MIR) flow facility. PIV was utilized to capture spatial vectors maps at near wall locations with five to ten points within the viscous sublayer. Entropy generation was calculated directly from measured velocity fluctuation derivatives. Two flows were studied: a zero pressure gradient and an adverse pressure gradient ($\beta $ = -0.039). The free stream turbulence intensity to drive bypass transition ranged between 3{\%} (near trailing edge) and 8{\%} (near leading edge). The pointwise entropy generation rate will be utilized as a design parameter to systematically reduce losses. As a second observation, the pointwise entropy can be shown to predict the onset of transitional flow. This research was partially supported by the DOE EPSCOR program, grant DE-SC0004751 and by the Idaho National Laboratory. [Preview Abstract] |
Tuesday, November 20, 2012 2:05PM - 2:18PM |
R20.00006: Simulations of equilibrium accelerating turbulent boundary layers over rough walls Junlin Yuan, Ugo Piomelli Studies of favorable-pressure-gradient (FPG) turbulent boundary layer are important both for engineering and geophysical applications, and for the fundamental understanding of the inner-outer layer interactions in turbulent wall-bounded flows. The sink flow, a FPG flow that is statistically self-similar in the streamwise direction, removes the history-dependence of a developing flow and is a good test bed to perform quantitative comparison between roughness and FPG effects. We carry out direct and large-eddy simulations of rough-wall sink flows in the transitionally- and fully-rough regimes, with mild to medium acceleration. The results show that the Reynolds number and the friction coefficient are generally affected by FPG and the physical roughness height (blockage ratio), respectively. The mean flow and the roughness function weakly depend on the pressure gradient. Flow statistics show that center-line velocity and friction velocity absorb the effects of acceleration and roughness respectively; close to the wall, the roughness-FPG combined effects are mostly reflected in the total drag, with the wall-normal location of strong turbulence production noticeably affected by the roughness. Explanations are provided by an exam of the turbulence structures. [Preview Abstract] |
Tuesday, November 20, 2012 2:18PM - 2:31PM |
R20.00007: Non-stationary boundary layers and energy dissipation in incompressible flows Marie Farge, Romain Nguyen van yen, Kai Schneider We argue that d'Alembert paradox (1749) is still unresolved for very large Reynolds number flows. Prandtl (1904) assumed that there exists a viscous boundary layer attached to the wall and predicted that the drag force dissipates energy there at a rate proportional to $Re^{-1/2}$. Kato (1984) proved that, in the limit of infinite Reynolds number, the energy dissipation rate tends to zero if and only if the solution of the Navier-Stokes equation converges towards the solution of the Euler equations (with the same initial data) and then occurs in a very thin boundary layer of thickness proportional to $Re^{-1}$. By performing direct numerical simulations of a dipole crashing into a wall we show that Kato's scaling is more appropriate than Prandtl's scaling as soon as the boundary layer detaches from the wall. Details can be found in Nguyen van yen, Farge and Schneider, PRL 106, 184502 (2011). [Preview Abstract] |
Tuesday, November 20, 2012 2:31PM - 2:44PM |
R20.00008: Tow-tank investigation of the developing zero-pressure-gradient turbulent boundary layer Jung Hoon Lee, Yong Seok Kwon, Jason Monty, Nicholas Hutchins Experiments are conducted using image-based measurement techniques to analyse the development of a zero-pressure-gradient turbulent boundary layer from trip to a high Reynolds number state. The unique experimental facility consists of a 5m long plate towed through a 60 x 1.8 x 1.8 m tow tank at speeds of up to 3 m/s. Windows in the side of the tank permit high-speed image acquisition and particle image velocimetry as the plate passes by the static measurement system. The evolution of the boundary layer can then be analysed from inception to Reynolds numbers up to $Re_\tau = 6000$ (near the end of the plate). Here $Re_\tau = \delta U_\tau/\nu$ is the K\'arm\'an number where $\delta$ is boundary layer thickness, $U_\tau$ is wall-shear velocity, and $\nu$ is kinematic viscosity. The unique frame-of-reference for this experiment enables us to track coherent motions as they evolve in the developing boundary layer. An analysis of vortical motion associated with the spatially and temporally evolving boundary layer reveals the development of large-scale vortices that originate from the inner region and extend to the edge of the outer region. Furthermore, the lifetimes of such large-scale vortical events can be estimated. [Preview Abstract] |
Tuesday, November 20, 2012 2:44PM - 2:57PM |
R20.00009: Onset of turbulent mean dynamics in boundary layer flow Curtis Hamman, Taraneh Sayadi, Parviz Moin Statistical properties of turbulence in low Reynolds number boundary layers are compared. Certain properties are shown to approach an asymptotic state resembling higher Reynolds number flow much earlier during transition than previously thought. This incipient turbulence is less stochastic and more organized than developed turbulence farther downstream, but the mean dynamics and production mechanisms are remarkably similar. The onset of turbulence in our recent simulations is also similar to that observed in the bypass transition of Wu \& Moin where continuous freestream turbulence, rather than small-amplitude linear waves, triggers transition. For these inflow disturbances, self-sustaining turbulence occurs rapidly after laminar flow breakdown without requiring a significant development length nor significant randomization. Slight disagreements with FST-induced bypass transition are observed that correlate with the extra strain a turbulent freestream would impose upon the near-wall dynamics. Nevertheless, the turbulence statistics are similar shortly after the skin-friction overshoot independent of upstream receptivity. This early onset of deterministic turbulence provides support for reduced-order modeling of turbulent boundary layers based on non-linear stability mechanisms. [Preview Abstract] |
Tuesday, November 20, 2012 2:57PM - 3:10PM |
R20.00010: Low order oscillatory modeling of the inner layer of turbulent boundary layers Promode R. Bandyopadhyay, Aren M. Hellum The visualization of the viscous sublayer (VSL) by Einstein {\&} Li (1956) and others indicates an oscillatory character with varying periods of growth followed by Strouhal-like liquidation of spanwise vorticity into arrays of lifting hairpins. In streak PIV at 20 wall units due to Li, Adrian {\&} Hanratty (1996), we notice a preponderance of dislocations. Therefore, we assume the sublayer to be in a permanent state of near-bifurcation irrespective of Reynolds number. To the lowest order, we model this process by Stuart-Landau (SL) oscillator equation. It is assumed that within a VSL cell, the oscillator is diffusively coupled along the span, the surface-normal growth is also diffusive---slowing as it thickens---and the outer layer provides the disturbance vector. The sublayer growth is followed by breakdown, creating a new outer layer disturbance vector for the next cycle. The SL equation is modified accounting for the above processes. The initial value solution of spanwise vorticity shows the development of nonuniformity, numerous dislocations and meandering streak-like structures that persist over extraordinarily large number of oscillatory cycles. Variation of the oscillator time scale shows the effects of increasing Reynolds number. [Preview Abstract] |
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