Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N33: Focus Session: Instabilities & Turbulence in Complex Fluids |
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Sponsoring Units: GSNP Chair: Daniel Lathrop, University of Maryland Room: Baltimore Convention Center 336 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N33.00001: Nonlinear dynamics and flow transitions in viscoelastic shear flows Invited Speaker: Dynamical explorations of viscoelastic flows are of fundamental and practical interest. Elastic forces cause flow instability even in the absence of inertia (creeping flow) and greatly modify the onset and ensuing sequence of flow transitions in flows with finite inertia. While past research has yielded much progress, literature on several intriguing nonlinear phenomena has been only slowly emerging. These include: (i) nonlinear transitions in \textit{linearly} \textit{stable}, parallel shear flows, (ii) influence of elastic instability on \textit{pressure-flow rate }relationship under creeping flow conditions, (iii) effect of elasticity on \textit{pattern formation} in \textit{curved} shear flows and (iv) novel instabilities caused by thermal effects induced by \textit{viscous heating}. The recent advances and challenges in the abovementioned areas will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N33.00002: Low-dimensional models for coherent states in viscoelastic turbulent shear flows Anshuman Roy, Alexander Morozov, Wim van Saarloos, Ronald Larson We present low-dimensional models for the sustenance of turbulence in shear flows of viscoelastic liquids. We develop these models by systematically investigating the effect of incremental amounts of elasticity on the self-sustaining process maintaining turbulence in shear flows. The recently proposed (Waleffe, 1997) self-sustaining process for shear flows consists of streamwise rolls leading to redistribution of the mean shear into spanwise streaks. A Kelvin-Helmholtz instability of the spanwise streaky flow then results in the regeneration of the streamwise rolls via nonlinear interactions. With the help of our low-dimensional model, we are able to identify which part of the cycle is interrupted or enhanced by the presence of elasticity. Additionally, we explore the effect of fluid rheology on the flow kinematics, particularly the role played by the first and second normal stress differences. For Newtonian liquids, such low-dimensional models have demonstrated their utility by helping to understand the features of full numerical solutions of turbulent flows. We believe that our low dimensional model for viscoelastic turbulent flow will help interpret experiments and direct numerical simulations of turbulent drag reduction by polymers. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N33.00003: Instabilities in the oscillatory flow of a complex fluid Jordi Ortin, Mireia Torralba, Alfonso A. Castrejon-Pita, Gabriela Hernandez, Guadalupe Huelsz, Jose Antonio del Rio The dynamics of both a Newtonian and a viscoelastic shear- thinning fluid, subjected to an oscillatory pressure gradient in a vertical tube, is studied experimentally. PIV is used to determine the 2d velocity fields in the vertical plane of the tube axis, for driving amplitudes from 0.8 to 2.5 mm and driving frequencies from 2.0 to 11.5 Hz. The Newtonian fluid exhibits always a laminar flow regime, independent of the axial position. For the complex fluid, instead, the parallel shear flow regime exhibited at low amplitudes [Torralba et al., Phys. Rev. E {\bf 72}, 016308 (2005)] becomes unstable at higher drivings against the formation of symmetric vortices, equally spaced along the tube. At even higher drivings the vortex structure itself becomes unstable, and complex nonsymmetric structures develop. The system studied represents an interesting example of the development of shear-induced instabilities in nonlinear complex fluids in purely parallel shear flow. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N33.00004: Instability of a Sheared Fluid-Gel Interface Satish Kumar, Sheng Lin-Gibson, Erik K. Hobbie The planar interface between a viscous fluid and an elastic gel is known to be unstable to simple steady shear flow [V. Kumaran and R. Muralikrishnan, Phys. Rev. Lett. \textbf{84}, 3310 (2000)]. By embedding a small number of micron-sized Latex particles at the planar interface between a Newtonian fluid and a soft viscoelastic gel, we use stroboscopic particle tracking to study the onset of this instability in the limit of large gel-to-fluid thickness ratios. The mean-square displacement of the interface and the power spectrum of displacement fluctuations are measured as a function of applied shear rate and gel modulus. Long-wavelength fluctuations with a periodic component are observed in the plane of flow and vorticity, with limited motion normal to the plane of the interface. By relating the power spectrum of fluctuations to the viscoelasticity of the gel, we discuss potential applications in the area of non-Brownian microrheology, where one exploits this instability to optically infer the rheological properties of an otherwise inaccessible soft phase. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N33.00005: DNS of Viscoelastic Turbulent Channel Flow at High Drag Reduction Antony Beris, Kostas Housiadas, Luo Wang A new method has been developed to enable Direct Numerical Simulations (DNS) of viscoelastic turbulent channel flow with high accuracy spectral methods at high values of drag reduction (HDR), when the polymer molecules undergo high extensional deformation. To faithfully represent that we have expressed the conformation tensor, c, as the exponential of another tensor a, c=exp(a) and we solve for a instead of c. Thus, by construction, the positive definite property of c is always preserved. In addition, a stabilizing artificial diffusion has been added to the viscoelastic constitutive model and efficiently implemented numerically using a multigrid method. The Finite-Elasticity Non-Linear Elastic Dumbbell model with the Peterlin approximation (FENE-P) is then used to represent the effect of polymer molecules in solution. To achieve HDR we used high values of the key model parameters: (a) the maximum extensional viscosity, which for the FENE-P constitutive model is proportional to the quantity (1-$\beta )$*L\^{}2, where $\beta $ is the solvent viscosity ratio and L is the maximum extensibility parameter and (b) the friction Weissenberg number, We$\tau $. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N33.00006: Time Dependent Drag Reduction by Long Chain Polymers in Taylor-Couette Flow Daniel D. Lanterman, Mathew Ferguson , Daniel P. Lathrop The addition of small amounts of long chain polymers has been shown to dramatically reduce the drag in some aqueous turbulent flows. We examined this effect in flow between concentric rotating cylinders (Taylor-Couette flow). The apparatus is instrumented to measure torque on the inner cylinder and can achieve Reynolds numbers up to $Re=1.4\cdot 10^6$. Reductions in drag of up to 47\% are seen immediately after the addition of the polymer (typical concentrations 10-20 ppm), but this value decays over a time scale of tens of minutes. While the scission of individual polymer molecules may also be important, light scattering measurements, performed on liquid samples, suggest the formation of entangled aggregates of polymer molecules. The polymers used are polyacrylamide with mean molecular weights of 5.5 and 18 MDaltons. Tested concentrations range from 0.5 to 100 parts per million by mass. We examine the dependence on concentration and shear rate (Reynolds number). [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N33.00007: Nonlinear traveling waves as a framework for understanding turbulent drag reduction Wei Li, Li Xi, Michael Graham Nonlinear traveling waves that are precursors to laminar-turbulent transition and capture the main structures of the turbulent buffer layer have recently been found in all the canonical parallel flow geometries. We study the effect of polymer additives on these ``exact coherent states" (ECS), in the plane Poiseuille geometry. Many key aspects of the turbulent drag reduction phenomenon are found, including: delay in transition to turbulence; drag reduction onset threshold; diameter and concentration effects. The examination of the ECS existence region leads to a distinct prediction, consistent with experiments, regarding the nature of the maximum drag reduction regime. Specifically, viscoelasticity is found to completely suppress the normal (i.e. streamwise-vortex-dominated) dynamics of the near wall region, indicating that the maximum drag reduction regime is dominated by a distinct, and perhaps intrinsically elastic, flow structure. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N33.00008: Complex dynamics in simple models of shear banding Suzanne Fielding, Helen Wilson, Peter Olmsted Complex fluids commonly undergo flow instabilities and flow-induced transitions that result in spatially heterogeneous ``shear banded'' states. Often, these banded states display oscillatory or chaotic dynamics, measured in the bulk rheological signals and in the motion of the interface between the bands. Until recently, however, theory predicted a steady state comprising stationary bands separated by a flat interface. We discuss recent theoretical progress in capturing complex dynamics of the banded state: first in a model in which the interface (or interfaces) remains flat but moves in a chaotic way; second in a model that explicitly allows for undulations along the interface. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N33.00009: Elastic Instabilities of Polymer Solutions in Extensional Flows Paulo Arratia, Jerry Gollub When flexible polymer molecules (in dilute solution) pass near the hyperbolic point of a microchannel cross flow, they are strongly stretched. As the strain rate is varied at low Reynolds number $<$0.01, tracer and particle-tracking experiments show that molecular stretching produces two flow instabilities, one in which the velocity field becomes strongly asymmetric, and a second in which it fluctuates non- periodically in time. The flow is strongly perturbed even far from the region of instability, and this phenomenon can be used to produce mixing. Bulk flow instabilities are not observed in dilute solutions of rigid polymers or Newtonian fluids under similar conditions. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N33.00010: Delay of Disorder by Diluted Polymers Christian Wagner, Andriy Kityk We study the effect of diluted flexible polymers on a disordered capillary wave state. The waves are generated at an interface of a dyed water sugar solution and a low viscous silicon oil. This allows for a quantitative measurement of the spatio-temporal Fourier spectrum. The primary pattern after the first bifurcation from the flat interface consists of squares. With increasing driving strength we observe a melting of the square pattern. It is replaced by a weak turbulent cascade. The addition of a small amount of polymers to the water layer does not affect the critical acceleration but shifts the disorder transition to higher driving strengths and the short wave length - high frequency fluctuations are suppressed. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N33.00011: Shear thickening, shear localization and elastic turbulence. Daniel Bonn The vast majority of complex fluids is shear thinning. The mechanisms of shear thinning are relatively well understood, and the phenomenon is widely used to tailor the rheology of complex fluids. Shear thickening is the exception to this rule, is incompletely understood and hardly ever used to tailor fluid properties. We study shear thickening in granular pastes (cornstarch), and show that shear localization (banding) is an essential ingredient for shear thickening. For high flow rates, the shear banding is followed by â€˜elastic turbulenceâ€™. Our measurements provide us with the mechanism of both shear thickening and the flow instabilities that result from it. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N33.00012: A Transitional Pathway to Turbulence in Elastic Fluids Bruce Schiamberg, Laura Shereda, Hua Hu, Ronald Larson Multiple scenarios have been discovered by which laminar flow transitions to turbulence, where transitions are caused by inertia or temperature, in Newtonian fluids. Here we show in non-Newtonian fluids a transition sequence that is due to elasticity from polymers, with negligible inertia. Multiple states are found linking the stable base flow to ``elastic turbulence'' in the flow between a rotating and stationary disk, including circular and spiral rolls, and stationary and time-dependent modes. Also, a surprising progression from apparently ``chaotic'' flow to periodic flow and then to ``elastic turbulence'' is found. In these experiments, either shear stress or shear rate is incrementally increased and then held at fixed values. The modes we discover have distinct rheological signatures, and we also image the accompanying secondary-flow field kinematic structures. Finally, we have explored how polymer concentration and gap-to-radius ratio affect (and possibly limit) the transitional pathway. The most concentrated solution tested appears to stabilize an additional, time-periodic mode. In conclusion, we have studied an unexplored route, which we hope, in time, will make it possible to compare experimentally and theoretically the routes to purely elastic turbulence with those for inertial turbulence, leading to a richer understanding of both. [Preview Abstract] |
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