Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session LE: Instability and Turbulence: Shear Layers |
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Chair: Carlos da Silva, IST/Technical University of Lisbon Room: Long Beach Convention Center 102C |
Monday, November 22, 2010 3:35PM - 3:48PM |
LE.00001: Linear stability analysis of planar reacting shear layers Yee Chee See, Matthias Ihme Non-premixed flames are controlled by the mixing of fuel and oxidizer. These flames are susceptible to instabilities, which arise from hydrodynamic and heat-release instability mechanisms. To characterize these instability modes and their effects on the flame evolution and mixing, a spatial linear instability analysis is performed. In this formulation, a flamelet-formulation is utilized, in order to account for effects of detailed reaction chemistry and variations in thermo-viscous properties. The resulting set of governing equations is solved by employing a matrix method with spectral discretization. By applying this analysis to a methane/air flame in a planar shear layer, effects of molecular transport and reaction chemistry on the flame stability are investigated, and model results are compared detailed computational simulations. [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LE.00002: Effect of carbon content on hypersonic shear layer instability Luca Massa Thermochemistry interacts with fluid-dynamic processes when kinetic and convective time-scales overlap. Non-linear parabolized Navier-Stokes equations are applied to the analysis of the instability and transition of carbon containing shear layers in hypersonic conditions. Linear parallel analysis shows an increase in growth rate of magnitude up to 20{\%}, for selected oxygen to carbon molar fractions. Localized maxima in linear growth rates are obtained for temperature close to the characteristic vibrational temperatures of carbon dioxide and for oxygen to carbon fraction of two, indicating potential destabilizing effect of carbon chemistry at high Eckert numbers. An increase in farfield temperature leads to an increase in growth rate, which is more marked at low carbon content. Non parallel effects are primarily related to streamwise relaxation for conditions away from equilibrium. The integrated kinetic energy from the parabolized analysis show a considerable change of kinetic energy growth with carbon content, identifying carbon dioxide as a destabilizing factor and causing energy increase of around 50{\%}. Energy transfers from kinetic to sensible and latent enthalpic modes are analyzed within the parabolized evolution. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LE.00003: Evolution of the mean dynamics of a shear-wake flow Marc Bamberger, Joseph Klewicki A shear-wake flow forms in the post-separation region downstream of a splitter plate dividing two boundary layer flows that have different freestream velocities. Thus, the upstream (in-flow) condition for the shear-wake is a wall-bounded flow, while the downstream (out-flow) state is a two-stream shear layer. Recent studies have revealed that the mean momentum equation of the turbulent boundary layer admits a four layer structure, and that in three of these layers the mean viscous force (gradient of the mean viscous stress) is of leading order. Conversely, the mean dynamics of the two stream shear layer are dominated by inertial terms (mean advection and Reynolds stress gradient). In this presentation we report on an experimental investigation of the evolution of the mean dynamics of a shear-wake flow. Single and x-array hotwire measurements are acquired in a relatively large scale shear layer facility at a velocity ratio of two. Special attention is given to better understanding the processes leading to the attenuation of the viscous force to lower order, and the evolution of a two-signed mean vorticity distribution to one that contains a single sign. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LE.00004: Is the growth rate of a turbulent mixing layer universal ? Saikishan Suryanarayanan, Roddam Narasimha The controversy on the universality of asymptotic spreading rates in turbulent shear flows remains unresolved even in the context of the widely studied mixing layer. This fundamental issue has deep implications on modeling. A strong case for the existence of a regime of universal spreading rate for a 2D temporal mixing layer simulated by a repeated array of point-vortices is presented. In the most extensive set of point-vortex simulations, involving up to 10000 vortices, ensemble averages over up to 108 realizations, it is established that the growth rate of momentum thickness varies by less than 1{\%} from a universal value of 0.0167 times the velocity differential in the linear overlap between the initial condition influenced `inner' region and the domain-size limited `outer' region for uniform random, Gaussian, bimodal and periodic initial conditions, with amplitudes spanning 8 orders. The conclusion remains unaffected on addition of diffusion through random walk or on desingularization of the point-vortices. A quantitative reproduction of experimental observations on forced mixing layers is presented. The possible reasons for observations of apparently non-universal spreading rates in experiments and simulations are discussed. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LE.00005: Similarity in 2-D spatially developing and long shear layers C. Carton de Wiart, G. Winckelmans, C. Bailly, P. Chatelain, F. Thirifay, A. Rhosko 2D shear layers are studied using a high accuracy vortex-in-cell (VIC) method. The case investigated is $U_2/U_1=0.38$, as in the Brown and Rhosko experiment. The inflow corresponds to a regularized vortex sheet with momentum thickness $\theta_0=\pi/4$ and $Re_0=54$. It then growths and smoothly undergoes transition, through TS waves and then K-H instabilities, to a ``turbulent shear layer'' developed at $x\approx 500$. Two computational domains are used: $L_1=2500$ and $L_2=3500$. Various outflow conditions are also used with $L_1$. We focus on self-similarity: profiles of $U/U_1$, $-\overline{u'v'}$, etc. as a function of $\eta=y/(x-x_0)$ (with $x_0$ virtual origin), and slopes $d\theta/dx$, etc. The results of the $L_1$ simulations agree well with each other; the region $x\in[1800, 2500]$ being affected by the outflow and thus dismissed. They also agree well with the results of the $L_2$ simulation, thus confirming the $L_1$ simulations validity. The region $x\in[2800, 3500]$ is dismissed in the $L_2$ simulation. A remarkable result is that we do not obtain one long region of self-similarity but, instead, multiple such regions: the region $x\in[900, 1200]$ with $d\theta/dx=0.0180$ and $-\overline{u'v'}_{\rm max}/(\Delta U)^2=0.0135$, then the region $x\in[1400, 1900]$ with $0.0146$ and $0.0115$, then the region $x\in[2100, 2600]$ with $0.0177$ and $0.0140$ (thus almost identical to the first region, potentially hinting at a recurring pattern). [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LE.00006: Identification of the Viscous Superlayer on the Low-Speed Side of a Single-Stream Shear Layer John Foss, Jason Peabody Image pairs (elevation/plan views) have been acquired of a smoke streakline originating in the irrotational region on the low-speed side of a high Re single-stream shear layer of Morris and Foss (2003). The viscous superlayer (VSL) is identified as the terminus of the streak; 1800 such images provide VSL position statistics. Hot-wire data acquired concurrently at the shear layer edge and interior are used to investigate the relationship between these velocity magnitudes and the large-scale motions. Distinctive features (plumes) along the streakline are tracked between images to provide discrete irrotational region velocity magnitudes and material trajectories. A non-diffusive marker, introduced in the separating (high speed) boundary layer and imaged at $x/\theta _o =352$, has revealed an unexpected bias in the streak-defined VSL locations. The interpretation of this bias clarifies the induced flow patterns in the entrainment region. The observations are consistent with a conception of the large-scale shear layer motions as ``billows'' of vortical fluid separated by re-entrant ``wedges'' of irrotational fluid, per Phillips (1972). Morris, S.C. and Foss, J.F. (2003). ``Turbulent Boundary Layer to Single Stream Shear Layer: The Transition Region.'' Journal of Fluid Mechanics. Vol. 494, pp. 187-221. Phillips, O. M. (1972). ``The Entrainment Interface.'' Journal of Fluid Mechanics. Vol. 51, pp. 97-118. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LE.00007: On the thickness of the turbulent/nonturbulent interface in shear layers Carlos da Silva, Rodrigo Taveira In free shear flows the flow field can be divided into two regions: the outer region where the flow is irrotational and the inner region where the flow is turbulent. The two regions are separated by a sharp interface: the turbulent/nonturbulent (T/NT) interface. The thickness of this interface has been observed to be between the order of the Kolmogorov or the Taylor micro-scale in several experimental and numerical works. We show that the thickness of the T/NT interface is equal to the radius of the large scale vortices (LVS) nearby this region. Direct numerical simulations (DNS) of planar jets at Reynolds numbers ranging from $Re_{\lambda} = 60 - 140$ using different initial conditions, and DNS of shear free irrotational/isotropic turbulence shows that the mean shear and the Reynolds number affect the T/NT interface thickness insofar as they define the radial dimension of the LVS near the T/NT interface, thus defining its thickness. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LE.00008: Effects of resolution on the fine scale features in the far field of a turbulent planar mixing layer Oliver Buxton, Sylvain Lardeau, Sylvain Laizet, Bharathram Ganapathisubramani The three-dimensional structure and behaviour of the rate of rotation and strain rate tensors is examined in the far field of a turbulent planar two dimensional mixing layer. The mixing layer is simulated using the incompact3d DNS code at Reynolds number based on inlet conditions of 1000. The study looks at the effect of spatial resolution on the length scales of strain dominated (dissipation) and rotation dominated (enstrophy) regions of the flow by filtering the data and interpolating it onto successively coarser grids. The length scales of these regions are observed by means of probability density functions and the topological evolution is characterised by the intermediate eigenvalue of the rate of strain tensor. Additionally, the structure and length scales of strain producing and enstrophy producing regions of the flow, and the effects of spatial resolution upon them, are investigated. The effects of spatial resolution upon the interaction between strain and rotation are observed by looking at the alignment angle between the vorticity vector and the eigenvectors of the strain rate tensor. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LE.00009: Dynamic mode decomposition of turbulent cavity flows for self-sustained oscillation Jin Lee, Hong Beom Park, Hyung Jin Sung Self-sustained oscillations in cavity flows are due to the unsteady separation of boundary layer at the leading edge. The dynamic mode decomposition is employed to analyze the unsteadiness in extracted modes without the explicit knowledge of evolution operator of the data. Two different data of the cavity flow with and without self-sustained oscillations have been analyzed possessing thin and thick incoming boundary layers. The ratios of the cavity depth to the momentum thickness (D/$\theta )$ are 40 and 4.5, and the cavity aspect ratio is L/D = 2. The dynamic modes extracted from the thick boundary layer show that both of the boundary layer structures and the internal disturbance generated due to the presence of cavity coexist with coincidence in frequency spectrum but with different wavenumber spectrum, whereas the structures of the thin boundary layer show complete coherence among them causing self-sustained oscillations. This result suggests that the hydrodynamic resonance causing self-sustained oscillations occurs when the upcoming boundary layer structures and cavity perturbations coincide not only of frequencies, but also of wavenumbers. The structures of cavity perturbations change with the cavity size and the upcoming momentum thickness. The effects of cavity dimensions and incoming momentum thickness are discussed for oscillations to be self-sustained. [Preview Abstract] |
Monday, November 22, 2010 5:32PM - 5:45PM |
LE.00010: Time resolved measurements of the pressure field generated by vortex-corner interactions in a cavity shear layer Xiaofeng Liu, Joseph Katz A 2D open cavity shear layer flow, especially its interaction with the trailing corner of the cavity, was investigated experimentally in a water tunnel at a Reynolds number of $3.8\times 10^4$. Time-resolved PIV with an image sampling rate of 4500 fps and a field of view of $25\times 25$mm was used to simultaneously measure the instantaneous velocity, material acceleration and pressure distribution. The pressure was obtained by spatially integrating the material acceleration (Liu and Katz, Exp Fluids 41:227-240). A large database of instantaneous realizations enables detailed visualization of the dynamic changes to shear layer vortices, such as deformation, breakup and trapping as they impinge and climb over the cavity trailing corner. These phenomena dominate the high pressure fluctuations near the corner, e.g. formation of a pressure minimum as the vortex is trapped on top of the corner. Ongoing statistical analysis examines the turbulence variables, focusing in particular on pressure-velocity and pressure-rate-of-strain correlations and their impact on the TKE balance. [Preview Abstract] |
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