Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AQ: Shear Layer Instabilities |
Hide Abstracts |
Chair: Donald Rockwell, Lehigh University Room: Hilton Chicago Stevens 2 |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AQ.00001: Oscillations of Shear Flow Past a Perforated Plate Bounded by a Cavity: Physical Mechanisms and Attenuation Emine Celik, Donald Rockwell Grazing flow of a turbulent boundary layer along a perforated plate, bounded by a closed cavity on its backside, can give rise to a long wavelength, self-excited instability. A cinema technique of high-image-density particle image velocimetry, which provides a space-time representation of the unsteadiness over an entire plane, is employed to characterize the coupling between distinctively different patterns of the flow structure along the back (low-speed) and front (high-speed) sides of the plate. Global cross-spectral analysis, attainable via the space-time imaging technique, leads to patterns of amplitude and phase of the predominant spectral component, along and across the plate. This approach, together with complementary types of image evaluation, delineates the physics of the oscillation, which includes: (i) downstream propagating disturbances along either side of the plate; and (ii) a coherent region of unsteadiness at its trailing-edge. Attenuation of the oscillation can be achieved by insertion of a ramp at the leading-edge, which generates a defect layer along the surface of the plate. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AQ.00002: Transitory Control of Large Scales in a Plane Shear Layer P. Gerardin, B. Vukasinovic, A. Glezer Large-scale motions are induced in a separating, single-stream shear layer by transitory modulation of high-frequency fluidic actuation where the frequency of the carrier waveform is nominally an order of magnitude higher than the natural frequencies of the baseline flow. While the induced vortical structures scale with the cross stream width of the shear layer, they do not result from \textit{direct} manipulation of natural instability modes and their evolution is fundamentally different than that of the naturally-evolved coherent structures because it is associated with the transient onset of the actuation and consequently the momentary disruption of the vorticity flux within the upstream boundary layer. It is shown that a sequence of large vortical structures can be induced in the shear layer over a broad range of frequencies \textit{independently} of the natural amplification of the baseline flow. Mixing can be enhanced by simultaneous control of large-scale entrainment and the small-scale motions that are effected by the high-frequency actuation. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AQ.00003: Influence of a parallel magnetic field on the instability of a free shear layer at low magnetic Reynolds number Anatoliy Vorobev, Oleg Zikanov We analyze the effect of a constant parallel magnetic field on the temporal instability and transition to turbulence in a free shear layer. The case of low magnetic Reynolds numbers is considered. It is known that the magnetic field changes the instability characteristics. It suppresses the growth of the perturbations and, at sufficiently large magnetic interaction parameter, can result in the most unstable perturbations accepting three-dimensional form (rolls at an oblique angle to the flow direction). It is clear that the secondary instability of the developed rolls changes as well. In our work we summarize the results concerning the primary linear instability of the erf-mixing layer. We also investigate the development of the instability and transition to turbulence using the DNS approach. Influence of the magnetic fields of the different strengths is considered. Part of the work was performed during the MHD Summer Program - 2005 at the Universite Libre de Bruxelles, Belgium. Support from the DOE Office of Basic Energy Science and NSF-MRI program is appreciated. [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AQ.00004: The growth of a localized vortex in a plane stagnation flow Jacob Cohen, Jimmy Philip The evolution of a finite amplitude 3D localized disturbance, having an initial dipole Gaussian vorticity distribution, embedded in an external, unbounded, irrotational plane stagnation flow (${\bf {U}}=(Ay,Ax,0)$) is investigated. Using the fluid impulse integral as a characteristic of such a disturbance, the viscous vorticity equation is integrated analytically. Accordingly, except for the specific case where the initial vortex is placed along $x=-y$, the associated fluid impulse decays and grows exponentially along the principal axes $x=y$ and $x=-y$, respectively. Numerical simulations, carried out for both linear and nonlinear disturbances at a Reynolds number of 40, confirm the above predictions. The simulations have been also compared with the solution of the linear viscous vorticity disturbance equation.\footnote{Leonard, A. 2000 \emph {Turbulence Structure and Vortex Dynamics} Cambridge University Press} While the solution predicts the vorticity distribution for the linear case, it fails to predict the essential characteristics of a nonlinear disturbance associated with its self induced movement. Finally, it is shown that the fluid impulse and the disturbance kinetic energy follow the same trend, i.e. when the fluid impulse increases with time so does the kinetic energy and vice-versa. The correlation between them suggests the use of the fluid impulse to predict the stability for a localized disturbance. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AQ.00005: Long Wave stability criteria for paralell miscible flow Marguerite D'olce, Jerome Martin, Nicole Rakotomalala, Dominique Salin, Yannis C. Yortsos We analyze the stability of miscible fluid paralell flow (x direction) with a given transverse viscosity (or velocity) profile, N(y) in 2D or N(r) for axisymmetric flow (pipe flow). From the Navier-Stokes and Convection-Diffision equations, we derive the eigenvalue problem. In the Long Wave limit which corresponds to Stokes flow, and for the diffusionless regime (Peclet numner infinite), we derive criteria for instability to occur depending on the shape of the velocity profile for shear and Poiseuille flows in 2D and for the axisymmetrical geometry. A series of generic transverse velocity profile examples are analyzed. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AQ.00006: Heat transfer enhancement by primary and secondary Gortler instabilities Ladan Momayez, Pascal Dupont, Hassan Peerhossaini Heat transfer along a concave surface is more efficient than on the classical flat plate since the boundary layer is subjected to different instability mechanisms: the centrifugal instability, referred to as the primary Gortler instability, which generates steady longitudinal vortices; and the shear instability, called the secondary Gortler instability, which causes unsteadiness and transition to turbulence. The systematic measurement of wall heat transfer and mean and turbulent velocities for different upstream perturbation conditions allowed the relative influence of each instability on the heat transfer. It is shown that low amplitude and large wavelength perturbations excite the primary instability, where as strong perturbation amplitudes and small wavelengths preferentially cause the secondary instability. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AQ.00007: On Holmboe's instability for smooth shear and density profiles Alexandros Alexakis The linear stability of a stratified shear flow for smooth density profiles is studied. This work focuses on the nature of the stability boundaries of flows in which both Kelvin-Helmholtz and Holmboe instabilities are present. For a fixed Richardson number the unstable modes are confined to finite bands between a smallest and a largest marginally unstable wavenumber. The results in this paper indicate that the stability boundary for small wavenumbers is comprised of neutral modes with phase velocity equal to the maximum/minimum wind velocity whereas the other stability boundary, for large wavenumbers, is comprised of singular neutral modes with phase velocity in the range of the velocity shear. We show how these stability boundaries can be evaluated without solving for the growth rate over the entire parameter space as was previously done. The results indicate further that there is a new instability domain that has not been previously noted in the literature. The unstable modes, in this new instability domain, appear for larger values of the Richardson number and are related to the higher harmonics of the internal gravity wave spectrum. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AQ.00008: An experimental investigation of the relaminarization of pipe flow Jorge Peixinho, Tom Mullin The appearance of turbulence in a pipe as the flow rate is increased is an unresolved problem, although new and interesting ideas continue to emerge. Theoretical investigations of the stability of the Hagen-Poiseuille indicate that the flow is linearly stable i.e. any infinitesimal perturbation introduced into the flow will decay. In practice, disordered flow can be self-sustained at Reynolds number above $\sim$ 2000. We present the results of experiments on the decay of turbulence below the threshold in a constant-mass-flux pipe. The study of this reverse transition (i.e. the change from turbulent to laminar flow) allows us to uncover consistent lifetime distributions for disordered motion. We also find evidence for coherent wave like structures which suggest connections with modern theoretical developments. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AQ.00009: Edge states in the transition to turbulence in pipe and other shear flows Bruno Eckhardt, Tobias Schneider, Joseph D. Skufca, James A. Yorke We study the boundary of the laminar region in pipe and other shear flows near the onset of turbulence. Approaching the boundary from the laminar side, the lifetime of perturbations increases, and it diverges when the boundary is reached. Once this critical amplitude is exceeded the trajectory swings up to the turbulent regime, but its lifetime varies sensitively with amplitude, consistent with the strange saddle picture of the turbulence proposed earlier. The edge trajectory is asymptotic to a single well defined state, independent of the type of perturbation. The edge then becomes the stable manifold of this structure. In the case of a model shear flow, the edge states are simple or period doubled or chaotic trajectories. The case of pipe flow shows less variability and the edge state seems to remain close to a state with simple vortex structure. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AQ.00010: Turbulent-laminar patterns in plane Couette flow Laurette Tuckerman, Dwight Barkley We study turbulent-laminar patterns in large-aspect-ratio plane Couette flow. These states consist of regular alternations of turbulent and laminar flow over large length scales. We simulate these patterns by extending the minimal-flow-unit methodology to computational domains with one long dimension that can be tilted at any prescribed angle to the streamwise direction. At a tilt of 24 degrees, we reproduce experimentally observed oblique patterns. As Re is decreased from 420, uniform turbulence is succeeded by intermittency at Re=410 and then by three well-defined bands at Re=390 which persist to Re=320 and are replaced by two bands at Re=310. Surprisingly, during this entire evolution, the temporally averaged total kinetic energy remains constant. Thus, the turbulence in the bands (which occupy only a portion of the domain) is more intense than the uniform turbulence, in such a way as to compensate for the laminar regions. In a geometry with a long streamwise and a short spanwise direction, turbulent patches repeatedly disappear abruptly and then re-nucleate gradually, for Reynolds numbers as low as 220. When the spanwise direction is long and the streamwise direction short, transition occurs abruptly from uniform turbulence to laminar Couette flow at Re=400. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700