Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session H08: Flow Instability: Elastic and Complex Fluids and Multiphase Flows |
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Chair: Arezoo Ardekan, Purdue Room: North 123 |
Monday, November 22, 2021 8:00AM - 8:13AM |
H08.00001: Linear instability of viscoelastic interfacial Hele-Shaw flows: Newtonian fluid displacing an upper-convected Maxwell fluid Prabir Daripa, Zhiying Hai We theoretically study linear stability of two-phase interfacial flow problem where a viscous Newtonian fluid displaces an Upper convected Maxwell (UCM) fluid in a rectilinear Hele-Shaw cell. The dispersion relation is found to be a root of a cubic polynomial $\mathcal{F}$ with coefficients depending on wavenumber along with several other dimensionless groups as parameters. Through Routh–Hurwitz stability criterion, we found the viscosity contrast $\eta^r/\eta^l$ still plays a decisive role in determining the stability (stable if $\eta^r/\eta^l \leq 1$). If $\eta^r/\eta^l>1$, the flow is more unstable than an identical Newtonian-Newtonian setup and the most unstable wavenumber is larger. Increasing Deborah number $De$, capillary number $Ca$ or flow speed worsens the instability. Elasticity has a variety of effects and can give rise up to three types of singular behaviors: (i) there exists infinitely many distinct wavenumbers at which the velocity becomes singular, and (ii) stress becomes singular when the wavenumber exceeds a certain value; and (iii) a resonance phenomenon is discovered when $\eta^r/\eta^l$ is large, where the growth rate increases very rapidly near a certain wavenumber and eventually becomes singular if displacing fluid becomes inviscid. |
Monday, November 22, 2021 8:13AM - 8:26AM |
H08.00002: Linear instability of viscoelastic interfacial Hele-Shaw flows: Upper convected Maxwell (UCM) fluid displacing another UCM fluid Zhiying Hai, Prabir Daripa In continuation of the talk by Dr. Daripa, the displacing fluid is replaced with another UCM fluid. $\mathcal{F}$ now becomes a quartic polynomial rather than cubic with one additional parameter involved, namely the relaxation time contrast $\lambda^r/\lambda^l$. Qualitatively, many features and conclusions found in the previous Newtonian-UCM setup are still valid in this case. The long wave stability is still determined by the viscosity contrast $\eta^r/\eta^l$, however always unstable to short waves. The critical wavenumber beyond which instability occurs depends only on the relaxation time of the displacing fluid $\lambda^l$ and flow speed. The resonance can still happen but only over a relatively narrow region in the parameter domain and may be avoided in multiple ways. |
Monday, November 22, 2021 8:26AM - 8:39AM Not Participating |
H08.00003: Experimental Studies of Liquid-Liquid Displacement in Microchannels with Pure Viscoelastic Fluids Seng Hoe (Billy) Hue, Panagiota Angeli, Loïc Chagot Displacement of one liquid by another is very common in a number of applications including cleaning of pipes and EOR. Previous studies have considered displacement of Newtonian fluids, while often in applications the fluids involved are viscoelastic. |
Monday, November 22, 2021 8:39AM - 8:52AM |
H08.00004: Tristability in the elastic wake of confined cylinders Manish Kumar, Arezoo M Ardekani Polymeric flow through confined geometries is relevant in industrial applications such as enhanced oil recovery, microbial mining, groundwater remediation, and biological processes such as drug delivery, particles and cells transport during respiration and fertilization. Large elastic stresses induced due to confined geometries can lead to elastic instability for the viscoelastic fluid flow. We have studied viscoelastic instabilities between two cylinders confined in a channel. We have discovered three distinct flow states (tristability) in the region between the cylinders induced by the elastic wake of the cylinder. Strands of high polymeric stress act as a barrier for the flow crossing these regions, which leads to flow separation and the formation of multiple flow states inside the channel. We investigate the effect of fluid rheology and cylinders’ separation on the flow instability. The strands of high polymeric stress also impact particle transport in realistic porous media. |
Monday, November 22, 2021 8:52AM - 9:05AM |
H08.00005: Chiral domains in a flowing achiral nematic liquid crystal Qing Zhang, Rui Zhang, Shuang Zhou, Irmgard Bischofberger An achiral nematic liquid in a microfluidic cell could be expected to form achiral structures in a pressure-driven flow. Surprisingly, we find the spontaneous emergence of chiral structures when an achiral lyotropic chromonic liquid crystal (LCLC) in the nematic phase relaxes from a high velocity flow to a steady-state lower velocity flow. The chirality results from a periodic double-twist deformation of the liquid crystal and leads to striking stripe patterns vertical to the flow direction. We demonstrate that this occurrence of chiral structures can be rationalized by the disparate elastic constants of the LCLC; the peculiarly low twist elastic constant compared to the bend and splay elastic constants of LCLCs allows for twist deformations, which lead to spontaneous symmetry breaking. We show that the period of the chiral structures depends on the local flow velocity and is set by the competition between the elastic torque and viscous torque acting on the structures. |
Monday, November 22, 2021 9:05AM - 9:18AM |
H08.00006: Instabilities in viscoelastic parallel shear flows with free-slip boundary conditions Martin Lellep, Moritz F Linkmann, Bruno Eckhardt, Alexander N Morozov Elastic turbulence is a strongly non-linear, turbulent-like flow state recently observed in polymer solutions at vanishing Reynolds numbers. Despite its relevance for industrial processing of viscoelastic materials and our general understanding of flows of complex fluids, available numerical results on elastic turbulence are scarce due to numerical instabilities. To ease the numerical complexity of such simulations, here we study free-slip plane Couette (pCF) and plane Poiseuille flows (pPF) of Oldroyd-B fluids. We perform a temporal linear stability analysis of free-slip pCF and pPF. Although their no-slip counterparts are generally linearly stable, we find that both geometries exhibit linear instabilities. By performing a boundary conditions homotopy, we show that these instabilities are caused by the least stable modes of no-slip flows loosing their stability under free-slip conditions. We perform direct numerical simulations to study the states that appear at the instability. We report a sequence of coherent structures from 2D to 3D periodic orbits to flows of yet higher complexity. We speculate that the 3D structures we observe in free-slip flows could be traced back to the no-slip setup, where they would form a dynamical scaffold for purely elastic turbulence. |
Monday, November 22, 2021 9:18AM - 9:31AM |
H08.00007: Experimental Observation of Inclined Waves in a Continuous Double Gradient of Salt and Sediment Patrick H Bunton, Abby Ledford, Eckart H Meiburg Linear stability analysis of stratified fluids wherein the density depends on two scalar fields has predicted the existence of inclined waves arising from a phase shift between the peak amplitudes of two scalars when one of the scalars has a settling velocity [ A. Alsinan, E. Meiburg, P. Garaud, JFM (217) 816 243-267]. Here are reported what to our knowledge are the first experimental observations of candidate inclined waves. The settling scalar was 1200 grit (approximately 3 micron) silicon carbide and the diffusing scalar was sodium chloride salt. A continuously-stratified double gradient was established in a nominally 30 cm x 20 cm x 7.5 cm cell using a continuously stirred, gravity-flowed, double-bucket technique. Typical parameters were 0 – 2.5% by mass particles and 0 – 2% by mass salt with the salt denser at the bottom of the cell and the particles denser at the top. Imaging consisted of backlighting and a 25 MPixel Nikon Z6 mirrorless camera and Nikkor 24-70 lens. Images were typically acquired once per minute over a period of 30 minutes or more and later combined to form video. Results are compared to predictions of linear stability theory. |
Monday, November 22, 2021 9:31AM - 9:44AM |
H08.00008: Mixing dynamics of bilgewater emulsions in Taylor Couette flows Vishal Panwar, Cari S Dutcher Taylor-Couette (TC) flows between two concentric, rotating cylinders, is ideal for studying the mixing dynamics and stability of emulsions due to the availability of wide variety of hydrodynamic flows. The control of oil discharge concentration at sea from the marine vessels require a better understanding of the stability and formation of Navy standard bilge mix emulsion. In this study, a pre-prepared concentrated oil-water emulsion is directly injected into the annulus of the TC cell containing surfactant-water solution at varied flow conditions to determine the emulsion formation dynamics. The optical properties of the TC cell enables us to visually study the mixing dynamics of the emulsion in solution. It was observed that the dispersion coefficient showed an approximately linear response to increasing inner cylinder speed of TC cell. Samples were collected at different mixing stages from the TC cell and laser diffraction particle analyzer was used to characterize droplet size distribution for these stages. The measurements indicate an initial droplet breakup followed by shear induced coalescence to form a larger median droplet size with time at lower mixing speed. Whereas at higher mixing speeds, the droplet breakup continues under shear and droplet coalescence was delayed. |
Monday, November 22, 2021 9:44AM - 9:57AM |
H08.00009: Vortex-Induced Interfacial Instability As A Means to Improve Droplet Entrainment Matthew Schulwitz, Mae Sementilli, James Chen Modern laboratory experiments for hybrid rocket motors are concerned with increasing the fuel regression rate to improve the overall impulse of the engine. The simulations developed in this study demonstrate a novel method to promote droplet formation, atomization, and entrainment of liquid fuel into the gaseous oxidizer flow. A pair of two-dimensional isothermal cases, both with and without the ignition source for the combustion reaction, are conducted to compare its hydrodynamic influence on the oxidizer-fuel interactions. By exploiting the geometry of the ignition source upstream of the fuel, it has been found that the shedding vortices are capable of inducing instabilities at the fuel/oxidizer interface without increasing oxidizer flow rate. These investigations are performed using Volume of Fluid method and were inspired by a slab motor experiment conducted at CHREST at the University at Buffalo. The results indicate that these vortex-induced instabilities are generated faster and cause improved fuel entrainment in the case where the ignition source is present. This vortex-induced method of improved fuel mixing can provide new avenues for the development of future hybrid rocket motors. |
Monday, November 22, 2021 9:57AM - 10:10AM |
H08.00010: Impact of dynamic surface deformations on the flow instability in high-Prandtl-number liquid bridges Mario Stojanovic, Hendrik C Kuhlmann, Francesco Romano The linear stability of the axisymmetric thermocapillary flow in a in high-Prandtl-number liquid bridge is investigated numerically. We address the full two-phase problem in which both, the concentric support rods and the liquid bridge are surrounded by an annular gas duct. While many previous investigations were based on a hydrostatically determined shape of the liquid bridge assuming a vanishingly small capillary number, dynamic surface deformations caused by the axisymmetric liquid and gas flows are fully taken into account. The steady basic flow is then subjected to a linear stability analysis. Since the dynamic surface defomations are of minor importance for weak mean axial gas flow, surface deformations due to the three-dimensional perturbation flow are neglected. Critical Reynolds numbers will be presented and the effect of the dynamic surface deformations in the basic state will be assessed by comparison with the stability boundaries for statically deformed bridges. Dynamic surface deformations become more important the larger the imposed axial gas flow rate is, significantly affecting the critical Reynolds number. Different critical modes are analyzed regarding their structure and the instability mechanisms. |
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