Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session AL: Instability: General I |
Hide Abstracts |
Chair: Peter Vorobieff, University of New Mexico Room: Salt Palace Convention Center 250 F |
Sunday, November 18, 2007 8:30AM - 8:43AM |
AL.00001: Self-sustained Flow States in a Square Duct Hakan Wedin, Damien Biau, Alessandro Bottaro, Masato Nagata The transition from laminar to turbulent flow in a square duct is an intriguing problem of hydrodynamics. The laminar profile of the flow in a square duct is linearly stable and it is hence conjectured that transition to turbulence is caused by the emergence of finite amplitude solutions of the Navier-Stokes equations. Recent evidence suggests that these alternative solutions, in the form of traveling waves and with no connection to the laminar flow, provide the skeleton around which time-dependent trajectories in phase space can orbit, preventing relaminarization of the flow for long times. Here we present approximate nonlinear solutions or ``self-sustaining-states'' to the Navier-Stokes equations, obtained with an approach initiated by Waleffe. The nonlinear flow that emerges when using such states as initial conditions in direct numerical simulations is studied. Interestingly, the lifetime of such nonlinear solutions decreases with the increase of the stream-wise length of the computational domain, for values of Re near the edge of chaos. [Preview Abstract] |
Sunday, November 18, 2007 8:43AM - 8:56AM |
AL.00002: Triggering turbulence in pipe and other linearly stable flows Tobias M. Schneider, Bruno Eckhardt In pipe flow and some other shear flows the laminar profile is linearly stable and turbulence may only be achieved through finite amplitude perturbations. Regions of laminar and turbulent dynamics in the state space of the system are separated by the \emph{edge of chaos}. Using an iterated bracketing technique we can numerically trace the dynamics in this edge of chaos and determine the invariant relative attractors. We will show results for plane Couette flow, pipe flow in the full space and in a symmetry reduced subspace to illustrate the variety of relative invariant attractors that can occur. The significance of these states lies in their governing role for triggering turbulence as well as for relaminarization. [Preview Abstract] |
Sunday, November 18, 2007 8:56AM - 9:09AM |
AL.00003: Pipe flow dynamics on the critical threshold. Fernando Mellibovsky, Alvaro Meseguer Pipe flow undergoes transition to turbulence despite the linear stability of its basic laminar solution. Finite amplitude solutions in the form of travelling waves, coexisting with the basic flow, have been identified recently. While they have been proved to play a role in the turbulent dynamics, their involvement in the transition process seems to be simply ungrounded. Furthermore, some recent experimental results point at a transitory nature of turbulence, thus questioning the mere existence of a well defined critical threshold. The region of phase space dominated by turbulent dynamics would then be constituted by a surging amount of bifurcating complex solutions as the Reynolds Number is increased, acting as an attractor most of the time, but always retaining some probability that any trajectory finds its way back to laminarity. However transient may turbulence be, the notion of a threshold separating initial conditions that lead to transition from others that end up decaying still applies. It suffices to define the threshold as the point where the perturbation lifetime seems to diverge, possibly not to infinity if turbulence is a transient phenomenon, but still abruptly. Then, the threshold regains interest, and the question can be asked of how a solution wandering about criticality would look like. Starting from different initial conditions, and through accurate refinements, trajectories on the edge between turbulence and laminarity can then be analysed to elucidate which properties of a solution determine whether it belongs to the laminar or the turbulent basin of attraction. We analyse these trajectories to try and understand transition. [Preview Abstract] |
Sunday, November 18, 2007 9:09AM - 9:22AM |
AL.00004: Bifurcation phenomena in the flow through a sudden expansion in a circular pipe James Seddon, Tom Mullin, Mick Mantle, Andy Sederman We report the results of an experimental investigation of laminar and time-dependent flows through a sudden expansion in a circular pipe. The flow state was investigated using high resolution MRI imaging techniques which have allowed us to settle a long standing debate on the first instability that occurs. As \textit{Re} is increased, the flow passes through a steady symmetry breaking bifurcation such that the position of the recirculating eddy becomes asymmetric within the pipe. This in turn gives way to simple periodic motion via a Hopf bifurcation with further increase in \textit{Re}. [Preview Abstract] |
Sunday, November 18, 2007 9:22AM - 9:35AM |
AL.00005: Flow regimes inside an expanding channel Peter Vorobieff, Vakhtang Putkaradze The well-known Jeffery-Hamel similarity solutions describe
two-dimensional radial flow of viscous fluid inside an expanding
channel (wedge). At Reynolds numbers greater than a certain
critical value $R_C$, the theoretical solution becomes
non-unique, with the possibility of radial velocity profiles with
alternating inflow/outflow. While our earlier experimental
work\footnote{V.~Putkaradze, P.~Vorobieff, Phys. Rev. Lett.
\textbf{97}, 144502 (2006).} confirms the absolute stability of the
Jeffery-Hamel radial solution for $R |
Sunday, November 18, 2007 9:35AM - 9:48AM |
AL.00006: Stability of small divergent channel with wall slip A. Sameen, Kirti Sahu, Rama Govindarajan From a non-parallel linear analysis and a transient/ algebraic growth study, we investigate the instability of channel flow subject to two conditions frequently encountered at small scales, local wall divergences and wall slip. The two are known to have opposing effects: slip at the walls has been found recently to strongly stabilizes the linear mode, while wall divergence hugely destabilizes them. At large scales, linear instability is considered relatively unimportant in transition to turbulence, since the latter usually occurs at a much lower Reynolds number, either directly nonlinearly, or triggered initially by transient algebraic growth. We predict a bigger role for linear instability in small-scale flows. As the angle of divergence increases, the effect of slip progressively reverses, from being hugely stabilising to mildly destabilising. The transient growth of disturbances depends only on the Reynolds number and not on the wall slope. The mechanisms and scalings will be discussed at the meeting. [Preview Abstract] |
Sunday, November 18, 2007 9:48AM - 10:01AM |
AL.00007: The 3D flow structure of puffs in transitional pipe flow Jerry Westerweel, Casimir van Doorne, Dirk-Jan Kuik Time-resolved stereo-PIV measurements were used to study the 3D flow structure of a puff in a pipe. At the trailing edge of the puff, where the laminar flow undergoes a transition to turbulence, pairs of counter rotating streamwise vortices result in large mushroom-like structures as seen in the LIF flow visualizations. The stereo-PIV system is used to take time- resolved measurements over the entire cross-section of the pipe. When time is converted to a spatial coordinate (assuming Taylor's hypothesis of `frozen turbulence') we obtain the quasi- instantaneous flow structure of a turbulent puff. At the upstream end of the puff a quasi-periodic regeneration of streamwise vortices takes place. Initially the vortex structure resembles a travelling wave solution, but as the vortices propagate further into the turbulent region of the puff they continue to develop into strong hairpin vortices. The structure suggests a mechanism for the long persistence of these puffs. [Preview Abstract] |
Sunday, November 18, 2007 10:01AM - 10:14AM |
AL.00008: Recurrence of Travelling Waves in Transitional Pipe Flow Rich Kerswell, Owen Tutty Wall-bounded shear flows are of tremendous practical importance yet their transition to turbulence is still poorly understood. A new direction in rationalising this phenomenon revolves around identifying alternative solutions (beyond the laminar state) to the governing Navier-Stokes equations. Such solutions which take the form of travelling waves (TWs) have only recently been found in pipe flow (Faisst \& Eckhardt 2003, Wedin \& Kerswell 2004). Despite being unstable, experimental observations (Hof et al 2004) have nevertheless indicated that these solutions are transiently realised. To quantify this, we perform a series of numerical experiments in which the spatial signatures of these TWs are sought in transitional pipe flows (Kerswell \& Tutty 2007). [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