Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session G26: Flow Instability: Complex Fluids II |
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Chair: Emilie Dressaire, University of California Santa Barbara Room: 151A |
Sunday, November 19, 2023 3:00PM - 3:13PM |
G26.00001: Instabilities, Traveling Waves, and Connections Between Viscoelastic Channel Flow and Kolmogorov Flow Jeffrey Nichols, Becca Thomases, Robert Guy Mixing at low Reynolds number is difficult, but viscoelasticity is known to enhance mixing via instabilities and turbulence, and these transitions are not well understood. 2D inertialess viscoelastic channel flow is known to be linearly stable and some evidence suggests that it cannot exhibit elastic turbulence. However, large perturbations can lead to stable traveling wave solutions known as “narwhals.” These narwhals resemble traveling waves that occur in viscoelastic Kolmogorov flow (doubly periodic flow driven by a unidirectional sinusoidal force). Kolmogorov flow does exhibit elastic turbulence, and these waves appear to be involved in the transition to turbulence. To explore the connection between channel flow and Kolmogorov flow, we study the dynamics of Kolmogorov flow with a low wave number driving force, which has not been examined thoroughly in the past, as well as wall-bounded flows driven by sinusoidal forcing. We find that the stability properties and ability to trigger elastic turbulence depend on both the wave number of the forcing and the presence of walls. |
Sunday, November 19, 2023 3:13PM - 3:26PM |
G26.00002: The Role of Fluid Elasticity and Surfactant Chemistry on Kinetics of Shear Banding Flow Formation in Wormlike Micelles Alfredo Scigliani, Hadi Mohammadigoushki Shear banding, i.e., discontinuities in velocity gradients, has been best documented in self-assembled wormlike micellar solutions. We report experiments on the kinetics of shear banding flow formation on two wormlike micellar solutions based on CTAB/NaSal and CPCl/NaSal that exhibit similar rheological properties. We examine the effect of fluid elasticity and surfactant chemistry on the spatiotemporal evolution of flow profiles in these two systems in Taylor-Couette geometries (i.e., flow between two concentric cylinders). The fluid elasticity is changed over a range of 1.3×10^{5} to 1.3×10^{7} through variation of the gap size of the measuring flow geometry. Our results indicate that, unlike CTAB/NaSal system, the wormlike micellar fluid based on CPCl/NaSal does not show any signs of transient flow reversal despite having similar rheological properties suggesting a vital role of surfactant chemistry on shear banding dynamics. In addition, beyond a critical elasticity number, the transient flow reversal appears in CTAB/NaSal system, and the strength of the flow reversal depends on the fluid elasticity. Concurrently, we adopt advanced rheo-NMR techniques to examine diffusion coefficients in 50μm voxels across the gap of the Taylor-Couette cell. This microscale analysis allows us to compare the diffusive dynamics of shear bands in a spatially resolved manner, shedding light on their distinct properties and behavior, such as the micelles' physical structure, concentration, and orientation. |
Sunday, November 19, 2023 3:26PM - 3:39PM |
G26.00003: Linear instability of a viscoelastic Poiseuille flow confined between porous walls Elmira Taheri, Elmira Taheri, Harunori N Yoshikawa, Parisa Mirbod This research investigates the linear instability of an Oldroyd-B fluid flow confined between porous walls. The study focuses on analyzing the impact of dimensionless parameters, including permeability parameter (α=L/√K), Weissenberg number (Wi=λU_{max}/L), and depth ratio (δ=H/L), on the flow stability. Here L represents the channel half-width, K is the permeability, λ is the relaxation time, U_{max }is the maximum velocity, and H is the thickness of the porous layer. Throughout the entire range of Weissenberg numbers (0≤Wi≤10), increasing the permeability parameter up to certain value leads to an increase in the critical Reynolds number, indicating enhanced flow stability. Notably, this behavior deviates from that of Newtonian flow, as Weissenberg numbers increase, multimodal instabilities for smaller values of α occur. Remarkably, larger Wi numbers also trigger multimodal instability in larger permeability parameters. Furthermore, our study uncovers that at small permeability parameters, increasing the depth ratio has a destabilizing effect on the flow, resulting in a decrease in the critical Reynolds number. These observations suggest the potential of controlling flow instability in viscoelastic fluids by varying the permeability and the thickness for a fixed porosity of porous walls. |
Sunday, November 19, 2023 3:39PM - 3:52PM |
G26.00004: Linear stability analysis of viscoelastic flow around a confined cylinder Alexandros Spyridakis, John Tsamopoulos, Pantelis Moschopoulos, Yannis Dimakopoulos, Stylianos Varchanis Increasing elasticity in a viscoelastic fluid, turns the symmetric and 2D flow around a confined cylinder either into a laterally asymmetric flow when the fluid is also shear-thinning ^{[1]} or into a 3D flow with periodic fluctuations along the axial direction ^{[2]}. In this work, we solve the steady 2D governing equations with our new stabilized Finite Elements method ^{[3]}. Then, we perform linear stability analysis by imposing a perturbation on the flow variables. We solve the resulting eigenvalue problem using the SLEPc toolkit ^{[4]}. We validate the lateral flow asymmetry and detect a pitchfork bifurcation leading to the new stable solution, as reported in experiments and transient simulations ^{[1]}. We allow for linear perturbations in the neutral direction and reproduce the periodic instability, which has previously been observed only experimentally. Finally, we examine the effect of the material properties and the geometry on the induced instabilities. |
Sunday, November 19, 2023 3:52PM - 4:05PM |
G26.00005: Magnetic torque-induced wave propagation on a ferrofluid thin film Zongxin Yu, Ivan C Christov When a ferrofluid is subjected to a fast-varying magnetic field, a phase lag arises between the magnetization and the external field due to the magnetization's relaxation time scale. Consequently, a magnetic torque density will emerge, which means that the spin velocity of the iron particles and the vorticity of the ferrofluid no longer coincide. To study this new physical feature, we develop a long-wave model for a ferrofluid thin film subjected to a rotating magnetic field in a 2D Cartesian configuration. Separating the slow flow time scale from the fast magnetization-relaxation time scale allows the decoupling of the flow equations from Maxwell's equations, and thus for an approximation of the magnetic torques and forces. A traveling-wave Dirichlet boundary condition is imposed on the magnetic scalar potential, which gives rise to the desired locally rotating magnetic field. Its spatial variation with the evolving thin film is found by solving Maxwell's equations with an interface condition. The derived model reveals a surface boundary condition featuring a shear stress originating from the surface torque. Through linear stability analysis, we identify the rotating field as a new mechanism that can be both destabilizing and lead to wave propagation along the film. The linear stability predictions are verified through nonlinear simulations. The observed behaviors hint at the emergence of complex and highly nonlinear wave phenomena, such as the formation of self-sustained propagating fronts and the transition to long-lasting potentially chaotic states. |
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