Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session BD: Transition to Turbulence |
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Chair: Edward White, Texas A&M University Room: 002B |
Sunday, November 23, 2008 10:30AM - 10:43AM |
BD.00001: Generation of Turbulence by Vibrating Forks in Superfluid $^4$He Ladislav Skrbek, Michaela Kralova, David Schmoranzer, William Vinen A study is reported of the drag on the prongs of a number of quartz forks vibrating in superfluid $^4$He, and particular attention is paid to the transitions from laminar to turbulent flow. Behavior in the normal phase is consistent with that for a classical fluid (PRE75 (2007) 025302). We focus on the observed behaviour of the drag coefficient, $C_D$, as a function of velocity. There is evidence that there is a sharp critical velocity at which turbulence starts to be generated in the superfluid component, and that this critical velocity may be preceded by the partial breakdown of laminar flow in the normal component. Behaviour in the superfluid phase is compared with that of other structures vibrating in superfluid $^4$He. At high velocities $C_D$ tends to that observed in a classical fluid, indicating that the two fluids, strongly coupled by mutual friction, are then behaving like a single classical viscous fluid. Observed values of both the critical velocity and the effective viscosity of the fully-coupled fluids are presented and discussed. It is suggested that the critical superfluid velocity is always closely similar to that expected classically for the coupled fluids, and a possible reason is discussed. [Preview Abstract] |
Sunday, November 23, 2008 10:43AM - 10:56AM |
BD.00002: Turbulent spot evolution in the ASBL J.H.M. Fransson Turbulent spots and their streamwise evolution play a major role in most transition scenarios. The appearance of turbulent spots in laminar boundary layers was first noted by Emmons,\footnote{Emmons, {\emph{J. Aero. Sci.}} {\bf{18}}, 490 (1951).} who proposed a spot probability appearance model, which recently\footnote{Fransson et al., {\emph{J. Fluid Mech.}} {\bf{527}}, 1 (2005).} has proven to work well for the free stream turbulence induced transition scenario. However, there are many fundamental questions still remaining unanswered, which are important in the striving after new transition prediction models. In this experimental study the effect of Reynolds number on turbulent spot evolution has been studied while keeping the boundary layer thickness constant. This type of study can only be performed in the asymptotic suction boundary layer where uniform suction through the wall is applied creating a boundary layer which does not develop in space. The velocity profile in the ASBL can readily be derived as, $ u(y) = U_\infty \{ 1 - \mathrm{e}^{yV_w/\nu} \}~, $ where $U_\infty$, $V_w$, and $\nu$ are the free stream velocity, the suction velocity ($<0$), and the kinematic viscosity, respectively. The Reynolds number based on the displacement thickness becomes $Re = -U_\infty / V_w$ allowing for $Re$-changes without necessarily changing the boundary layer thickness. [Preview Abstract] |
Sunday, November 23, 2008 10:56AM - 11:09AM |
BD.00003: DNS of Surface Textures to Control the Growth of Turbulent Spots James Strand, David Goldstein A spectral DNS code was used to study the growth and spreading of turbulent spots in a nominally laminar, zero-pressure gradient boundary layer. In addition to the flat-wall case, the interaction of these spots with riblets, fins, and spanwise-damping fins was examined. The flat plate, surface textures, and initial spot perturbation were simulated via an immersed boundary method, and a suction-wall allowed the available channel code to model a boundary layer. In all cases, self-similar arrowhead shaped spots formed. The spanwise-damping fins were very effective; the tallest damping fins were able to completely halt spot spreading. A decrease in spreading angle was also observed for several of the cases with real fins and with riblets. The best case of the real fins decreased the spreading angle 19 percent from the flat wall value, and the best case of the riblets decreased the spreading angle by 12 percent. [Preview Abstract] |
Sunday, November 23, 2008 11:09AM - 11:22AM |
BD.00004: Mathematical Singularity Behaviour of Turbulent Transition Hua-Shu Dou, Boo Cheong Khoo In our previous work, a criterion for turbulent transition has been proposed which is expressed by an energy gradient function which is the ratio of the transverse energy gradient and the streamwise energy loss of unit volumetric fluid in the base flow. Further, the threshold of the disturbance amplitude obtained is scaled with the Reynolds number by an exponent of -1, which is in agreement with the experimental results for pipe flow. In the present study, we show that turbulent transition can be excited via a singularity of the energy gradient function. This singularity of the energy gradient function corresponds to the case of infinite Re. When a laminar flow is stable (at low Re), the energy gradient function is located remotely from the mentioned singularity. It is found that the role of disturbance introduced to a laminar flow is to promote the energy gradient function to approach the singularity. When the Reynolds number is sufficiently large, a large disturbance may trigger the energy gradient function to enter the singularity. Once this function is trapped into this singularity, the fluid flow becomes increasingly ``chaotic'' tending towards the turbulent state. For the occurrence of the singularity, the amplitude of disturbance needs to reach a certain threshold for a given Reynolds number. These findings are in agreement with those in the literature. [Preview Abstract] |
Sunday, November 23, 2008 11:22AM - 11:35AM |
BD.00005: Escape from turbulence in shear flows Alberto de Lozar, Bjoern Hof, Dirk Jan Kuik, Jerry Westerweel The collapse of turbulence, observable in shear flows at low Reynolds numbers, raises the question if turbulence is generically of transient nature or becomes sustained at some critical point. Recent data have lead to conflicting views with the majority of studies supporting the model of turbulence turning into an attracting state. We have performed lifetime measurements of turbulence in pipe flow spanning eight orders of magnitude in time, drastically extending all previous investigations. We show that no critical point exists in this regime and that in contrast to the prevailing view the turbulent state remains transient. Our experiments also strengthen the picture that the turbulent dynamics quickly erase any memory of the initial perturbation. The behavior found here identifies turbulence in pipe flow as a type-II super-transient, which had been conjectured as a potential description of turbulence two decades ago. [Preview Abstract] |
Sunday, November 23, 2008 11:35AM - 11:48AM |
BD.00006: Is Turbulence a Transient? Daniel Borrero, Michael Schatz In shear flows, the transition to turbulence typically occurs through a subcritical bifurcation where a finite amplitude perturbation is required to take the system from the laminar state to a turbulent one. Experiments have shown that the lifetime of the turbulent state is finite for some range of Reynolds numbers. Some experiments suggest that the lifetime diverges at a finite Reynolds number, whereas other experiments suggest that the lifetime diverges only at infinite Reynolds number. We present measurements of the turbulent state lifetimes for the flow between concentric, rotating cylinders in the regime where the transition to turbulence is subcritical. The streamwise periodicity of this flow allows for arbitrarily long observation times, a feature lacked by previous experimental implementations, which used open flows. Our study also allows us to test whether the transient nature of the turbulence observed in previous experiments is specific to those flow geometries or is present in a more general class of shear flows. [Preview Abstract] |
Sunday, November 23, 2008 11:48AM - 12:01PM |
BD.00007: Highly-symmetric travelling waves in pipe flow Chris Pringle, Yohann Duguet, Rich Kerswell The recent theoretical discovery of finite-amplitude travelling waves in pipe flow has re-ignited interest in the transitional phenomena that Osborne Reynolds studied 125 years ago. Despite all being unstable, these waves are providing fresh insight into the flow dynamics. We describe two new classes of travelling wave which while possessing more restrictive symmetries than the previously found travelling waves of Faist \& Eckhardt (2003) and Wedin \& Kerswell (2004) seem to be more fundamental to the hierarchy of exact solutions. They exhibit much higher wall-shear stresses and appear at notably lower Reynolds numbers. [Preview Abstract] |
Sunday, November 23, 2008 12:01PM - 12:14PM |
BD.00008: Intermittent and equilibrium puffs in transitional pipe flow Dwight Barkley, David Moxey We report on numerical simulations of flow in pipes at Reynolds numbers 1800 to 3000 - near the minimum Reynolds numbers that supports turbulence. The computational domains are periodic in the streamwise direction with lengths up to 150 pipe diameters. We find both intermittent and equilibrium puffs. More particularly we find that, just as with other shear flows near the transition to turbulence, there are well defined transitions between uniform turbulence, intermittent states of turbulent and laminar flow, and spatially periodic states of turbulent and laminar flow. [Preview Abstract] |
Sunday, November 23, 2008 12:14PM - 12:27PM |
BD.00009: ABSTRACT WITHDRAWN |
Sunday, November 23, 2008 12:27PM - 12:40PM |
BD.00010: Momentum balance in a transitional boundary layer O. Ramesh, P. Desai The mechanism of bypass transition at elevated turbulence levels is believed to be fundamentally different from the so-called canonical transition. We hypothesise that the difference between bypass and canonical transition should show up even in a gross balance as the momentum integral equation. Momentum integral equation for a zero pressure gradient boundary flow is $\frac{d\theta }{dx}=\frac{C_f }{2}-\frac{\partial}{\partial x}\int\limits_0^{\infty} {\frac{\overline {v'^2-u'^2}}{U^2}} dy$. The last term is usually neglected except near separation. We argue that it may not be really negligible for transitional boundary layers under constant pressure as it is conceivable that the stream-wise velocity gradient of the turbulence quantities could be substantial at least for some increased values of free-stream turbulence levels. By conducting experiments in a constant pressure boundary layer, we find that the stream-wise derivative term involving turbulence quantities can be quite substantial at least near the initial part of the transition region. This is indicative of a different mechanism at work in bypass transition compared to canonical transition. One may speculate that bypass transition perhaps has a flavour of flow separation.This study also indicates that skin friction estimate in transitional boundary layer experiments using momentum balance should be carefully performed and use all the three terms shown above in the equation. [Preview Abstract] |
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