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 EI: Instability: Shear Layers II |
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Chair: Donald Rockwell, Lehigh University Room: Salt Palace Convention Center 250 C |
Sunday, November 18, 2007 4:10PM - 4:23PM |
EI.00001: Asymmetric, Mirror-Symmetric and Helical Traveling Waves in Pipe Flow Chris Pringle, Rich Kerswell New families of three-dimensional nonlinear travelling waves are discovered in pipe flow. In contrast to known waves (Faisst \& Eckhardt {\em Phys. Rev. Lett.} {\bf 91}, 224502 (2003), Wedin \& Kerswell, {\em J. Fluid Mech.} {\bf 508}, 333 (2004)), they possess no discrete rotational symmetry and exist at significantly lower Reynolds numbers ($Re$). First to appear is a mirror-symmetric travelling wave which is born in a saddle node bifurcation at $Re=773$ (the next lowest being the three-fold symmetric wave at $Re=1251$). As $Re$ increases, `asymmetric' modes arise through a symmetry-breaking bifurcation. These look to be a minimal coherent unit consisting of one slow streak sandwiched between two fast streaks located preferentially to one side of the pipe. These travelling waves have higher upper branches and lower lower branches than any previously found, with the asymmetric branch being closest of all to the laminar state. Helical and non-helical rotating waves are also found emphasizing the richness of phase space even at these very low Reynolds numbers. The helical asymmetric wave is the first know travelling wave to possess no symmetries of any kind. The gap in $Re$ from when the laminar state ceases to be a global attractor to turbulent transition is then even larger than previously thought. [Preview Abstract] |
Sunday, November 18, 2007 4:23PM - 4:36PM |
EI.00002: On the transition between distributed and isolated surface roughness and its effect on the stability of channel flow Jerzy M. Floryan The question of whether a system of roughness elements has to be viewed either as a distributed roughness or a set of individual, hydrodynamically independent roughness elements has been considered. The answer has been given in the context of definition of hydraulic smoothness proposed in [1] where a roughness system that cannot destabilize the flow is viewed as hydraulically inactive. Stability characteristics have been traced from the distributed to the isolated roughness limits. It has been shown that an increase of distance between roughness elements very quickly stabilizes disturbances in the form of streamwise vortices but coupling with disturbances in the form of traveling waves persists over long distances. The transition to the single roughness limit is achieved must faster in the case of roughness elements in the form of ``ridges'' rather then ``trenches''. \newline \newline [1] J.M. Floryan, Three-dimensional instabilities of laminar flow in a rough channel and the concept of hydraulically smooth wall, Eur. J. Mech. B/Fluids, 26 (2007) 305-329. [Preview Abstract] |
Sunday, November 18, 2007 4:36PM - 4:49PM |
EI.00003: Transient growth in channel flow past a compliant boundary M. Lisa Manning, Bassam Bamieh, Jean M. Carlson Researchers have shown that transient amplification of non-normal modes plays an important role in transition to turbulence in three-dimensional channel flows. Although there have been many theoretical investigations of two-dimensional flows past compliant boundaries, transient amplification in three-dimensional flows interacting with flexible walls is not well-understood. Borrowing from techniques in control theory, we present an ``input-output'' analysis of transient amplification in a 2D/3C model for the fluid-flexible wall system. We first analyze the most amplified modes for the spring-backed plate wall model alone as a function of frequency and spanwise wavenumber, and show that the most amplified modes correspond to the ``Class B surface waves'' first described by Benjamin and Landahl. We then show how surface waves in a spanwise-stretchable wall influence the streamwise vortices that are most amplified in rigid wall channel flow. [Preview Abstract] |
Sunday, November 18, 2007 4:49PM - 5:02PM |
EI.00004: Banded laminar-turbulent patterns: mean flows, symmetries and statistics Laurette Tuckerman, Dwight Barkley In large-aspect-ratio plane Couette flow, patterns of oblique bands, alternating between turbulent and laminar flow, are the intermediate regime between uniform turbulence and laminar Couette flow. The mean flows corresponding to these patterns, as well as the Reynolds stress force, are found to be represented almost perfectly by a single trigonometric function along the pattern wavevector, leading to a quantitative description in terms of six ODEs. The Fourier component corresponding to the pattern wavelength provides an order parameter for the transition, which can be described as a bifurcation in its probability distribution function. [Preview Abstract] |
Sunday, November 18, 2007 5:02PM - 5:15PM |
EI.00005: Attenuation of Self-Sustained Oscillations Due to Flow Past Slotted and Perforated Plates Philip Breneman, Donald Rockwell Turbulent inflow past a perforated or slotted plate, bounded on one side by a large cavity, can give rise to highly coherent, self-sustained oscillations. At low Mach number, and in absence of wall elasticity, these oscillations may be viewed as purely hydrodynamic. Attachments to the backside of the slotted or perforated plate have been critically evaluated as potential techniques of attenuation. High-image-density particle image velocimetry and unsteady pressure measurements lead to representations of the flow structure that are associated with maximum reduction of pressure amplitude. Time-averaged and phase-referenced images show that dramatic alterations of both the instability and the associated mean flow patterns can be attained. Moreover, global spectral analysis, based on simultaneous time records at thousands of grid points of the cinema imaging, provides insight into the spatial patterns of the attenuated pressure amplitude. For a given configuration, both the patterns of the instability along the plate and the patterns of pressure amplitude can be attenuated to a high degree. [Preview Abstract] |
Sunday, November 18, 2007 5:15PM - 5:28PM |
EI.00006: On vortex shedding in low Reynolds number flows over an airfoil Serhiy Yarusevych, Greg Kawall, Pierre Sullivan Development of coherent structures in a separated shear layer and wake of a NACA 0025 airfoil was studied experimentally. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. Flow characteristics were assessed via surface pressure measurements, hot-wire velocity measurements, and flow visualization. A laminar boundary layer separation occurred on the upper surface of the airfoil for all cases examined, resulting in the formation of a separated shear layer. Two types of coherent structures were identified and investigated in detail. Laminar-to-turbulent transition, which occurred in the separated shear layer, was associated with the formation of roll-up vortices. Shear layer roll-up occurred when naturally amplified disturbances reached sufficient amplitude. The resulting roll-up vortices were found to play an important role in the transition process. Wake vortices formed in the near-wake region and were shed alternatively on the upper and lower sides of the turbulent wake. It has been established that the fundamental frequency of the shear-layer disturbances exhibits a power law dependency on the Reynolds number, whereas the wake vortex shedding frequency displays a linear dependency on the Reynolds number. Based on the obtained results, a universal scaling for the wake vortex shedding frequency has been determined. [Preview Abstract] |
Sunday, November 18, 2007 5:28PM - 5:41PM |
EI.00007: PIV Experimental Investigation of a Single-Phase Turbulent Mixing Layer Fude Guo, Bin Chen, Liejin Guo, Zhiwei Wang, Ximing Zhang In this work single-phase turbulent mixing layer was investigated by PIV at three different velocity ratios. The results show that average vorticity along the streamwise central line decreases with the development of shear layer, and the decreasing speed is faster at the higher velocity ratio. Comparison of the average vorticity distribution on different cross section at similar Reynolds number indicates that the vorticity has a peak value at the high speed side and a vale value at the low speed side near the split edge. The interesting thing is that both of the peak and vale value decreased with the velocity ratio increasing for similar Reynolds numbers. Reynolds stress distribution along different cross-section of the mixing layer is increased with the increasement of the velocity ratio. [Preview Abstract] |
Sunday, November 18, 2007 5:41PM - 5:54PM |
EI.00008: PIV Experimental Investigation of a Bubbly Turbulent shear Layer Bin Chen, Fude Guo, Zhiwei Wang, Liejin Guo, Ximing Zhang In this work bubbly shear layer was investigated by PIV. The velocity ratio is 4:1 and the Reynolds number ranged from 22000 to 158400. PIV results show that almost all the bubbles are trapped by vortex structures. Compared with the single phase case, average vorticity changes little along the streamwise central line, but decreased with the increasement of Reynolds number. The average vorticity distribution along different cross-section of the shear layer is decreased at low Reynolds number but changes little at higher Reynolds number. The results indicate that Reynolds stress along different cross-section of the single phase case is increased with the development of shear layer but decreased with the increasement of Reynolds number, and this tendency is disordered with bubbles injection. [Preview Abstract] |
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