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 HT: General Stability II: Pipe Flow |
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Room: Hilton Chicago Stevens 5 |
Monday, November 21, 2005 1:20PM - 1:33PM |
HT.00001: Effect of Axially-Periodic Pipe Radius on the Linear Stability of Circular Poiseuille Flow D.L. Cotrell, G.B. McFadden, W.E. Alley, B.J. Alder We report linear stability results for flow driven by an axial pressure gradient in an axisymmetric pipe with axially-periodic radius. We assume that the base flow is steady, axisymmetric, axially-periodic with the forcing wavelength, and that there is no azimuthal velocity component. In these computations, we use finite-element methods for the base flow and stability problems, account for arbitrary disturbances of infinitesimal amplitude, and show that the critical disturbance has non-zero frequency and is of different axial wavenumber than the wall forcing. Results show that neutral curves can be strongly bi-modal, and that this bi-modal nature results in the critical axial wavenumber and wavespeed having two disjoint ranges of forcing amplitude where critical values vary only slightly (i.e., a low and high plateau). On the other hand, the critical Reynolds number is monotonically increasing as one decreases the forcing amplitude. [Preview Abstract] |
Monday, November 21, 2005 1:33PM - 1:46PM |
HT.00002: Nonlinear instabilities in a vertical pipe flow discharging from a cylindrical container Ramon Fernandez-Feria, Enrique Sanmiguel-Rojas We report results from three-dimensional numerical simulations of the incompressible flow in a vertical pipe of circular cross-section discharging from a cylindrical container. Natural Coriolis forces due to the Earth rotation triggers the instability of the axisymmetric flow, and nonlinear spiral waves with azimuthal wave number $|n|=3$ are formed above a critical Reynolds number based on the pipe flow rate ($Re_Q$). We characterize this critical Reynolds number as a function of the Coriolis parameter ($F$), that is proportional to the square of the radius of the container. As a difference with previous numerical works on nonlinear instabilities and transition in a pipe flow, here the nonlinear disturbances needed to trigger the instabilities are not artificially introduced inside the pipe flow, but naturally produced by Coriolis forces, the amplitude of these disturbances being characterized by a non-dimensional Coriolis parameter. We find that the pipe flow can be unstable for $Re_Q$ as low as $300$ for the largest value of $F$ considered. [Preview Abstract] |
Monday, November 21, 2005 1:46PM - 1:59PM |
HT.00003: The instability of decaying pipe flow Jim Denier, Nathaniel Jewell In unsteady pipe flow, the frictional aspects have long been calculated using steady-flow formulas, which are demonstrably incorrect. Previous work has limited application because it is based on empirical modelling, rather than on an understanding of the fundamentals of the fluid mechanics of boundary layers in unsteady flow. This talk will present some new results on the problem of the instability of the unsteady flow in a circular pipe that is suddenly blocked. [Preview Abstract] |
Monday, November 21, 2005 1:59PM - 2:12PM |
HT.00004: An experimental observation of a turbulence regeneration mechanism in pipe flow Bjorn Hof, Jerry Westerweel We report an experimental investigation into the structure of turbulent pipe flow. With the aid of a high speed stereoscopic PIV system we were able to confirm and extend our previous observation of recurring travelling wave transients in pipe flow. Within the turbulent flow we have identified a turbulence regeneration cycle underlying the observed wave transients. The cycle involves vortices of a streamwise orientation and a wavy instability of low speed streaks. This observation is in excellent agreement with predictions from theoretical models where it has been suggested that this cycle is a key mechanism in sustaining turbulence in shear flows. The periodicity of the cycle observed in the experiment is in close agreement with that observed in numerical calculations. [Preview Abstract] |
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