Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L37: NonNewtonian Flows: Rheology & MixingNonNewtonian

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Chair: Jonathan Freund, University of Illinois Room: 303 
Monday, November 20, 2017 4:05PM  4:18PM 
L37.00001: Fieldsensitivity To Rheological Parameters Jonathan Freund, Randy Ewoldt We ask this question: where in a flow is a quantity of interest $Q$ quantitatively sensitive to the model parameters $\vec \theta$ describing the rheology of the fluid? This field sensitivity is computed via the numerical solution of the adjoint flow equations, as developed to expose the target sensitivity $\delta Q/\delta\vec\theta(\bf{x})$ via the constraint of satisfying the flow equations. Our primary example is a sphere settling in Carbopol, for which we have experimental data. For this Carreaumodel configuration, we simultaneously calculate how much a {\it local} change in the fluid intrinsic timescale $\lambda$, limitviscosities $\eta_o$ and $\eta_\infty$, and exponent $n$ would affect the drag $D$. Such field sensitivities can show where different fluid physics in the model (time scales, elastic versus viscous components, {\it etc.})\ are important for the target observable and generally guide model refinement based on predictive goals. In this case, the computational cost of solving the local sensitivity problem is negligible relative to the flow. The Carreaufluid/sphere example is illustrative; the utility of field sensitivity is in the design and analysis of less intuitive flows, for which we provide some additional examples. [Preview Abstract] 
Monday, November 20, 2017 4:18PM  4:31PM 
L37.00002: Effect on NonNewtonian Rheology on Mixing in TaylorCouette Flow Neil Cagney, Stavroula Balabani Mixing processes within many industry applications are strongly affected by the rheology of the working fluid. This is particularly relevant for pharmaceutical, food and waste treatment industries, where the working fluids are often strongly nonNewtonian, and significant variations in rheology between batches may occur. We approach the question of how rheology affects mixing by focussing on a the classical case of TaylorCouette flow, which exhibits a number of instabilities and flow regimes as a function of Reynolds number. We examine TaylorCouette flow generated for a range of aqueous solutions of xantham gum or corn starch, such that the rheology varies from shearthinning to shearthickening. For each case, we measure the power consumption using a torque meter and the flow field using high speed, timeresolved ParticleImage Velocimetry. The mixing characteristics are quantified using a number of Lagrangian and Eulerian approaches, including the coarse grained density method and vortex strength. By comparing these metrics to the power number, we discuss how the mixing efficiency (ratio of mixing effectiveness to power input) varies with the flow index of the fluid. [Preview Abstract] 
Monday, November 20, 2017 4:31PM  4:44PM 
L37.00003: Experimental and computational fluid dynamics studies of mixing of complex oral health products Marti CortadaGarcia, Simona Migliozzi, Weheliye Hashi Weheliye, Valentina Dore, Luca Mazzei, Panagiota Angeli Highly viscous nonNewtonian fluids are largely used in the manufacturing of specialized oral care products. Mixing often takes place in mechanically stirred vessels where the flow fields and mixing times depend on the geometric configuration and the fluid physical properties. In this research, we study the mixing performance of complex nonNewtonian fluids using Computational Fluid Dynamics models and validate them against experimental laserbased optical techniques. To this aim, we developed a scaleddown version of an industrial mixer. As test fluids, we used mixtures of glycerol and a Carbomer gel. The viscosities of the mixtures against shear rate at different temperatures and phase ratios were measured and found to be well described by the Carreau model. The numerical results were compared against experimental measurements of velocity fields from Particle Image Velocimetry (PIV) and concentration profiles from Planar Laser Induced Fluorescence (PLIF). [Preview Abstract] 
Monday, November 20, 2017 4:44PM  4:57PM 
L37.00004: Simulation of `cavern’ formation in the mixing of viscoplastic fluids Karan Mirpuri, Lyes Kahouadji, Jalel Chergui, Damir Juric, Seungwon Shin, Patrick Piccione, Omar K. Matar This work focuses on elucidating the effects of impeller size and speed on `cavern’ formation in HerschelBulkley fluids using CFD simulations. `Caverns’ are defined as the wellmixed regions within the fluid usually encasing the impeller where shear stress imparted by the impeller overcomes the material yield stress. The caverns are often surrounded by zones of stagnant fluid isolated from bulk flow, wherein mass transfer is mainly restricted to diffusion, making them adverse to mixing quality. Numerous models have been developed to predict cavern size including the spherical (Solomon {\it et al}., 1981), cylindrical (Elson {\it et al.}, 1986) and toroidal (Amanullah {\it et al}., 1998) models. Due to its prevalence as a means of comparison in modern experiments, the Elson {\it et al.} experiment is replicated for a number of rotational speeds (4, 8 and 12 Hz) and three geometricallysimilar Rushton turbines using the code {\it Blue} which facilitates the `measurement’ of cavern size and depth among other parameters. [Preview Abstract] 
Monday, November 20, 2017 4:57PM  5:10PM 
L37.00005: Designing with nonlinear viscoelastic fluids Jonathon Schuh, Yong Hoon Lee, James Allison, Randy Ewoldt Material design is typically limited to hard materials or simple fluids; however, design with more complex materials can provide ways to enhance performance. Using the CriminaleEricksenFilbey (CEF) constitutive model in the thin film lubrication limit, we derive a modified Reynolds Equation (based on asymptotic analysis) that includes shear thinning, first normal stress, and terminal regime viscoelastic effects. This allows for designing nonlinear viscoelastic fluids in thinfilm creeping flow scenarios, i.e. optimizing the shape of rheological material properties to achieve different design objectives. We solve the modified Reynolds equation using the pseudospectral method, and describe a case study in fullfilm lubricated sliding where optimal fluid properties are identified. These materialagnostic property targets can then guide formulation of complex fluids which may use polymeric, colloidal, or other creative approaches to achieve the desired nonNewtonian properties. [Preview Abstract] 
Monday, November 20, 2017 5:10PM  5:23PM 
L37.00006: Shear Rheology of a Suspension of Deformable Solids in Viscoelastic Fluid via Immersed Boundary Techniques Christopher Guido, Eric Shaqfeh The simulation of fluids with suspended deformable solids is important to the design of microfluidic devices with soft particles and the examination of blood flow in complex channels. The fluids in these applications are often viscoelastic, motivating the development of a highfidelity simulation tool with general constitutive model implementations for both the viscoelastic fluid and deformable solid. The Immersed Finite Element Method (IFEM) presented by Zhang et al. (2007) allows for distinct fluid and solid grids to be utilized reducing the need for costly remeshing when particles translate. We discuss a modified version of the IFEM that allows for the simulation of deformable particles in viscoelastic flows. This simulation tool is validated for simple Newtonian shear flows with elastic particles that obey a NeoHookean Law. The tool is used to further explore the rheology of a dilute suspension of NeoHookean particles in a Giesekus fluid. The results show that dilute suspensions of soft particles have viscosities that decrease as the Capillary number becomes higher in both the case of a Newtonian and viscoelastic fluid. A discussion of multiple particle results will be included. [Preview Abstract] 
Monday, November 20, 2017 5:23PM  5:36PM 
L37.00007: Rheology of corn stover slurries during fermentation to ethanol Sanchari Ghosh, Brenden Epps, Lee Lynd In typical processes that convert cellulosic biomass into ethanol fuel, solubilization of the biomass is carried out by saccharolytic enzymes; however, these enzymes require an expensive pretreatment step to make the biomass accessible for solubilization (and subsequent fermentation). We have proposed a potentiallylessexpensive approach using the bacterium Clostridium thermocellum, which can initiate fermentation without pretreatment. Moreover, we have proposed a “cotreatment” process, in which fermentation and mechanical milling occur alternately so as to achieve the highest ethanol yield for the least milling energy input. In order to inform the energetic requirements of cotreatment, we experimentally characterized the rheological properties of corn stover slurries at various stages of fermentation. Results show that a corn stover slurry is a yield stress fluid, with shear thinning behavior well described by a power law model. Viscosity decreases dramatically upon fermentation, controlling for variables such as solids concentration and particle size distribution. To the authors’ knowledge, this is the first study to characterize the changes in the physical properties of biomass during fermentation by a thermophilic bacterium. [Preview Abstract] 
Monday, November 20, 2017 5:36PM  5:49PM 
L37.00008: Mechanistic constitutive model for wormlike micelle solutions with flowinduced structure formation Sarit Dutta, Michael Graham We present a tensor constitutive model for stress and flowinduced structure formation in dilute wormlike micellar solutions. The fluid is treated as a dilute suspension of rigid Brownian rods whose length varies dynamically. Consistent with the mechanism of Turner and Cates [J.~ Phys.:~Condens. Matter 4, 3719 (1992)], flowinduced alignment of the rods is assumed to promote increase of rod length that corresponds to the formation of flowinduced structures observed in experiments. At very high deformation rate, hydrodynamic stresses cause the rod length to decrease. These mechanisms are implemented in a phenomenological equation governing the evolution of rod length, with the number density of rods appropriately modified to ensure conservation of surfactant mass. The model leads first to an increase in both shear and extensional viscosity as deformation rate increases and then to a decrease at higher rates. If the rate constant for flowinduced rod growth is sufficiently large, the model predicts a multivalued relation between stress and deformation rate in both shear and uniaxial extension in agreement with experimental results. By design, the model is simple enough to serve as a tractable constitutive relation for computational fluid dynamics studies. [Preview Abstract] 
Monday, November 20, 2017 5:49PM  6:02PM 
L37.00009: Controlled formation of cyclopentane hydrate suspensions via capillarydriven jet breakup Michela Geri, Gareth McKinley Clathrate hydrates are crystalline compounds that form when a lattice of hydrogenbonded water molecules is filled by guest molecules sequestered from an adjacent gas or liquid phase. Being able to rapidly produce and transport synthetic hydrates is of great interest given their significant potential as a clean energy source and safe option for hydrogen storage. We propose a new method to rapidly produce cyclopentane hydrate suspensions at ambient pressure with tunable particle size distribution by taking advantage of the RayleighPlateau instability to form a monodisperse stream of droplets during the controlled breakup of a water jet. The droplets are immediately frozen into ice particles through immersion in a subcooled reservoir and converted into hydrates with a dramatic reduction in the nucleation induction time. By measuring the evolution of the rheological properties with time, we monitor the process of hydrates formation via surface crystallization and agglomeration with different droplet size distributions. This new method enables us to gain new insights into hydrate formation and transport which was previously hindered by uncontrolled droplet formation and hydrate nucleation processes. [Preview Abstract] 
Monday, November 20, 2017 6:02PM  6:15PM 
L37.00010: Transfer of Ratethinning and Ratethickening Liquids Between Separating Plates and Cavities JyunTing Wu, Marcio S. Carvalho, Satish Kumar One promising technology for largescale fabrication of printed electronics is rolltoroll gravure, which involves transfer of inks from microscale cavities to a second surface. Although printing inks usually exhibit nonNewtonian behavior, the influence of ink rheology on liquid transfer is not yet well understood. To address this issue, an axisymmetric model is used to develop fundamental understanding of how ink rheology affects liquid transfer between vertically separating surfaces. Deformationratedependent liquids described by a Carreau model are considered, inertial and gravitational forces are neglected, and the nonlinear governing equations are solved with the Galerkin finiteelement method. For liquid transfer between two flat surfaces, the results reveal that ratethinning (ratethickening) rheology allows more (less) liquid to be transferred from the less wettable surface to the more wettable one. For liquid transfer between a flat surface and a trapezoidal cavity, the influence of ratedependent rheology is found to primarily occur near the flat surface. This behavior is attributed to the presence of the cavity wall, which reduces the interfacial curvature gradients, the associated capillary pressure gradients, and thus the influence of ratedependent rheology. [Preview Abstract] 
Monday, November 20, 2017 6:15PM  6:28PM 
L37.00011: Designing shearthinning Arif Z. Nelson, Randy H. Ewoldt Design in fluid mechanics often focuses on optimizing geometry (airfoils, surface textures, microfluid channels), but here we focus on designing fluids themselves. The dramatically shearthinning ``yieldstress fluid'' is currently the most utilized nonNewtonian fluid phenomenon. These rheologically complex materials, which undergo a reversible transition from solidlike to liquidlike fluid flow, are utilized in pedestrian products such as paint and toothpaste, but also in emerging applications like directwrite 3D printing. We present a paradigm for yieldstress fluid design that considers constitutive model representation, material property databases, available predictive scaling laws, and the many ways to achieve a yield stress fluid, flipping the typical structuretorheology analysis to become the inverse: rheologytostructure with multiple possible materials as solutions. We describe case studies of 3D printing inks and other flow scenarios where designed shearthinning enables performance remarkably beyond that of Newtonian fluids. [Preview Abstract] 
Monday, November 20, 2017 6:28PM  6:41PM 
L37.00012: Development of a new continuous process for mixing of complex nonNewtonian fluids Simona Migliozzi, Luca Mazzei, Bob Sochon, Panagiota Angeli Design of new continuous mixing operations poses many challenges, especially when dealing with highly viscous nonNewtonian fluids. Knowledge of complex rheological behaviour of the working mixture is crucial for development of an efficient process. In this work, we investigate the mixing performance of two different static mixers and the effects of the mixture rheology on the manufacturing of novel nonaqueousbased oral care products using experimental and computational fluid dynamic methods. The two liquid phases employed, i.e. a carbomer suspension in polyethylene glycol and glycerol, start to form a gel when they mix. We studied the structure evolution of the liquid mixture using timeresolved rheometry and we obtained viscosity rheograms at different phase ratios from pressure drop measurements in a customized minichannel. The numerical results and rheological model were validated with experimental measurements carried out in a specifically designed setup. [Preview Abstract] 
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