Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A35: Non-Newtonian Flows: Instabilities and Applications |
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Chair: Christel Hohenegger, University of Utah Room: Georgia World Congress Center B407 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A35.00001: Stability of viscoelastic flow past rigid and flexible square posts Anita P. Dey, Nicholas Uvanovic, Jonathan Philip Rothstein Viscoelastic fluid flow can become unstable even at infinitesimal Reynolds numbers due to a purely elastic flow instability resulting from the combination of streamline curvature and fluid elasticity at large Weissenberg numbers. We have recently shown that elastic flow instabilities can drive the motion of different flexible structures including sheets and circular cylinders. The fluctuating fluid forces exerted on the structures from the elastic flow instabilities can grow large enough to result in significant motion of the flexible structure. Here we investigate the stability of the flow of a high molecular weight polymer solution past both a rigid and a flexible micropost with a square cross section. We will present streak images and PIV vector fields characterizing the stability of the base flow around the rigid post and study the impact of fluid rheology and flow geometry on the nature and onset criteria of the elastic flow instability. With the base flow established a flexible post will be mounted across the channel and the viscoelastic fluid structure interactions will be investigated to determine the oscillation frequency and amplitude and to look for a lock in behavior. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A35.00002: Scaling of the elastic regime in turbulent flow with polymer additives Yi-Bao Zhang, Heng-Dong Xi We present an experimental study of the effect of polymer additives on the turbulent energy cascade in Von Karman swirling (VKS) system. The three components of the velocity at the center of the VKS in a vertical plane along a diameter of the system are measured by stereoscopic Particle Image Velocimetry (PIV) in pure water (Newtownian) case and polymer solution case. It is found that the inertial range of the velocity structure function is “truncated” at the smaller scale due to the addition of the polymer additives. At scales less that the “truncation scale”, the second order structure function exhibits a new scaling with exponent around 1.38. This new scaling range is dominated by polymer’s elasticity and its range is extending with increasing polymer concentration. The three regimes (dissipation, elastic and inertial ranges) can be well described by the modified Batchelor’s parametrization. With this parametrization, we successfully determined the “truncation scale”. Scaling exponents of high order velocity structure functions are also presented. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A35.00003: Weissenberg Number Dependence of Linear Mechanisms in Polymer Drag-Reduced Turbulent Channel Flow Ryan McMullen, Ashwin Shekar, Michael David Graham, Beverley J McKeon It is well-known that small amounts of high-molecular weight polymers can drastically reduce turbulent drag in a liquid. Furthermore, recent work has shown that studying polymers in turbulence can shed light on the nature of the self-sustaining mechanisms of wall turbulence. This work analyzes the linear mechanisms present in polymer drag-reduced turbulence, with an emphasis on how these mechanisms change with the level of drag reduction. In particular, we extend resolvent analysis to the study of viscoelastic turbulence in order to identify the most amplified velocity and conformation tensor modes at Weissenberg numbers representative of low drag reduction and the recently discovered elastoinertial turbulence. The results predicted by the resolvent analysis are in good qualitative agreement with observations from direct numerical simulations and complement existing work for laminar viscoelastic flows. Furthermore, they provide a starting point for efficient low-order models of viscoelastic turbulence. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A35.00004: Hemorheology of dense suspension of red blood cells in a wall-bounded shear flow Naoki Takeishi, Marco Edoardo Rosti, Yohsuke Imai, Shigeo Wada, Luca Brandt We numerically investigate the rheology of a suspension of red blood cells (RBCs) in a wall-bounded shear flow for a wide range of viscosity ratios between the cytoplasm and plasma. The behavior of RBCs, modeled as a biconcave capsule whose membrane follows the Skalak’s constitutive law, is simulated for volume fractions of RBCs up to 0.41 and different Capillary numbers (Ca). Our numerical results show that, an RBC subjected to lower Ca tends to show stable rolling motion with higher intrinsic viscosity, and sifts to the stable swinging or tank-treading motion as increasing Ca, with lower intrinsic viscosity. Hydrodynamic interaction allows the RBCs to deform into swinging or tank-treading like motion, and hence the intrinsic viscosity starts to decreases at a semi-dilute condition, but regains for further higher volume fractions. Because of such mode change from dilute to semi-dilute condition, a polynomial approach of volume fraction for the suspension of RBCs cannot be simply applied. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A35.00005: Emptying of gravure cavities containing shear-thinning and shear-thickening liquids Jyun-Ting Wu, Marcio S. Carvalho, Satish Kumar Roll-to-roll gravure is widely used for creating uniform coatings on a large scale and involves transfer of liquid from micron-scale cavities to a second surface. To advance fundamental understanding of the influence of rheology on liquid transfer, two-dimensional numerical simulations are used to study liquid emptying from a model configuration where liquid is confined between a stationary trapezoidal cavity and a horizontal substrate. Liquid is driven out of the cavity by a combination of horizontal substrate motion and an imposed pressure gradient. For Newtonian liquids, the fraction of liquid left in the cavity collapses onto a master curve with three distinct regimes. The fraction of liquid left in the cavity is highly dependent on surface wettability in regime I, is characterized by a power-law relationship in regime II similar to that observed for liquid-film withdrawal, and approaches a plateau in regime III as the influence of surface wettability vanishes. Three distinct regimes are also observed for shear-thinning (shear-thickening) liquids. Shear-thinning (shear-thickening) is found to improve (worsen) cavity emptying compared to the Newtonian case by aiding (hindering) contact-line motion through reduced (enhanced) viscosities near the dynamic contact line. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A35.00006: Cavity Expansion for Measuring the Viscoelastic Behavior of Soft Materials Shabnam Raayai, Tal Cohen Cavitation Rheology (CR) is a method of measuring the local elastic modulus of soft materials; by pressure-controlled injection of a Newtonian fluid into the sample, one can identify a maximum pressure, corresponding to the onset of cavitation instability in the material [1]. This peak pressure can then be compared with theoretical predictions of the cavitation pressure to determine the elastic modulus. In this work, we expand the CR method to capture a wider range of material behaviors. Using a custom-designed setup attached to a universal testing machine, we perform volume-controlled CR experiments using incompressible fluids that are immiscible in the tested material. First, we discuss the use of this technique for materials that fracture much earlier than the strains required for capturing the cavitation instability limit. Next, we will discuss the use of cavity expansion to measure the viscoelastic response such as stress-relaxation. Lastly, we will discuss the effect of the size of the material used in the tests and how the current method can offer a novel approach for performing CR experiments with samples of different sizes. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A35.00007: Effects of viscoelasticity on droplet-based bioprinting Daulet Izbassarov, Mohammad Nooranidoost, Savas Tasoglu, Metin Muradoglu We computationally study effects of viscoelasticity of bio-inks on the cell viability during the deposition of cell-laden droplets on a substrate using a front tracking method. The inner droplet representing the cell and the encapsulating droplet are modeled as viscoelastic liquids, while the ambient fluid is Newtonian. Following Nooranidoost et al. [1], the viscoelasticity of the fluids is modeled using the FENE-CR model and is characterized by the Weissenberg number and the polymeric viscosity ratio. We demonstrate that these parameters significantly impact the cell viability and can be useful to control and improve droplet-based bioprinting systems. In particular, viscoelasticity of the encapsulating droplet fluid generally enhances the cell viability, favorable for bioprinting systems. A universal correlation is proposed to relate the cell viability and the viscoelasticity of the encapsulating droplet and is found to fit the computationally estimated values well. The effects of the cell viscoelasticity are also examined, and it is shown that the Newtonian cell models may significantly overpredict the cell viability. [1] M. Nooranidoost, D. Izbassarov, and M. Muradoglu, Phys. Fluids 28, 123102 (2016) |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A35.00008: Cavern Formation in Stirred Vessels Containing Herschel-Bulkley Fluids: Experiments and Simulations Andrew W. Russell, Lyes Kahouadji, Karan Mirpuri, Patrick M. Piccione, Andrew Quarmby, Omar K Matar, Paul Luckham, Christos N Markides The aim of the experimental aspect of this study was to achieve flow similarity over scales in stirred vessels containing viscoplastic fluids in which a ‘cavern’ is formed when they are agitated by a central impeller. Outside of this region, the material is stagnant because the stresses the material is experiencing are not large enough to overcome the material’s yield stress. Although the flow of these fluids in vessels has been widely researched, limited studies have looked at the flow of viscoplastic fluids over scales. Aqueous Carbopol solutions were used and through agitation with Rushton turbine impellers, the resulting caverns were highlighted using food and UV fluorescent dyes. Fluid flow was assumed to be matched if geometrically similar caverns were produced over three geometrically-similar scales. Numerical simulations elucidated the effects of impeller size and speed on cavern formation in Herschel-Bulkley fluids using a 3-D CFD solver. Numerous models have been developed to predict cavern size [Solomon et al(1981), Elson et al(1986), Amanullah et al(1998)]. Comparisons were made between the experimental and numerical results. It was found that the cavern sizes were in relatively good agreement at pre-defined impeller speeds and Reynolds numbers. |
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