Session F46: General Fluid Dynamics

Numerical and Analytical Solutions for Viscous Flow between Parallel disks in Different Nanofluids

We investigate the incompressible boundary layer viscous flow of fluids with nanoparticles between parallel disks. In this article, the set of the governing PDEs is reduced into system of coupled ODEs, which is then numerically solved by using Runge-kutta method. Effects of the various parameters such as thermal conductivity, volume fraction, and Prandtl number on velocity and heat transfer characteristics for different nanofluids containing Cu, Al2O3, TiO2 are analyzed. Also a comparative thermal behavior of the fluids has been discussed and presented presented through tables. It is found that the velocity profiles are enhanced by increasing the volume concentration of the nanoparticles and also the heat transport is improved. As a result, the momentum boundary layer thickness increases while the thermal boundary layer thickness becomes thinner.   

Optimizing Micromixer Surfaces to Deter Biofouling

A surface patterned with angled ridges can be used to generate vortices in microfluidic devices. We investigate an application of this technology for biofouling mitigation, making use of the characteristics of the flow to break up clusters of cells as they form. We represent such a system computationally using a hybrid of bulk fluid simulated via the lattice Boltzmann method, and deformable vesicles, representing cells, simulated via the lattice spring method. This simulation methodology allows us to rapidly implement and test different surface patterns, and explore how their parameters can most effectively deter the accumulation of biofilms. By adjusting the shape of these ridges, we can increase the effectiveness of the surface across a range of shear values.   

Inertial Focusing in Triangular Cross-sectional Microchannel with Varying vertex angle and a/H.

Inertial microfluidics provides high-throughput, passive micro-particle manipulation methods which are widely used for various biomedical applications. We examined inertial focusing in isosceles triangular channels having various vertex angle (45°, 60°, 90°, 120°) and dimension with varying Reynolds number by changing flow rate. In particular, we investigated the top focusing positions shifting as a function of Re and a/H (a=particle diameter, H=hydraulic diameter). In case of 90° channels, the total number of focusing positions were changed from two to three as the Re is increased or a/H is decreased. Interestingly, three focusing positions become five in case of 120° triangular channel under high Re or low a/H conditions. The direction of shifting is differed whether vertex angle is acute or obtuse. Equilateral triangular channels do not show much changes due to 120° rotational symmetry. These results suggest various applications for particle manipulation and the understanding of roles of different lift forces.   

Theory of the Radius Dependence of Water Flow through Carbon and Boron Nitride Nanotubes

Secchi, et. al. observed that the flow velocity of water in carbon nanotubes increases rapidly as the tube radius decreases below 50nm, but they observed that the same flow velocity enhancement with decreasing radius does not occur in boron nitride nanotubes, despite the fact that they have the same crystal structure. Scaling arguments show that in order for such large flow velocity enhancement to occur at such large tube radii compared to the interatomic spacing in the wall and the spacing between water molecules, the diffusion rate of water molecules near the wall would have to be much larger than that found by molecular dynamics for a flat carbon surface, which should be applicable to nanotubes of such large radii. It will be shown that the enhancement of the flow velocity when the radius drops from 50m to 15nm, observed by Secchi, et. al., that occurs in carbon, but not boron nitride, nanotubes can be accounted for (for semiconducting nanotubes) by a reduction in the contribution to the friction from electron excitations in the wall with decreasing radius, resulting from the fact that the electron energy band gap in carbon, but not boron nitride, nanotubes varies from less than to greater than the product of Boltzmann’s constant and the temperature over this radius range.   

Equilibrium structures of water molecules confined inside a multiply-connected carbon nanotube: A molecular dynamics study

The water confinement inside a carbon nanotube (CNT) is a well-known phenomenon and has been a subject of both experimental and theoretical interests. However, only simple CNT structures with cylindrical shapes have been considered for the study. In this talk, we report a classical molecular dynamics study of equilibrium structures of water molecules confined inside a multiply-connected carbon nanotube (MCCNT), where a larger (14, 14) CNT is branched off into two smaller (7, 7) CNT arms and then they merge into one. The arrangements of water molecules in the MCCNT are investigated both in terms of water density and an average number of hydrogen bonds per water molecule. Our results demonstrate that the arrangement transition of water molecules occurs several angstroms ahead of the junction, not at the Y-shaped junction. Since a single chain of water molecules can be formed in the (7, 7) tube, that the oscillation wavelength (~ 2. 5 Å) of the number density of water inside the (7, 7) CNT arms is almost same as the so-called hydrogen bond length (2. 8 Å).   

Three-dimensional Flow Profile in the Ventral Third Ventricle of the Brain

A complex transport network driven by coordinated motile cilia inside the ventral third ventricle (v3V) of mammalian brain was recently reported. This network generates cerebrospinal fluid (CSF) flow patterns such as a separatrix and a whirl that establish intraventricular boundaries. The CSF flow in the overall three-dimensional v3V cavity was studied numerically via the Lattice Boltzmann Method and Immersed Boundary Method. In particular, the experimental trajectory data obtained by tracking fluorescent beads were converted to velocity vectors and processed by the Smoothed Particle Hydrodynamics. The velocity maps were refined by considering divergence-free and projected onto a curved virtual surface representing cilia tips, with a constant gap from the v3V wall. Three-dimensional flow features with likely physiological consequences were uncovered numerically.   

Characterizing Pulsatile Tinnitus in the Internal Jugular Vein using Hemodynamic Data from CFD and 4D Flow MRI

Pulsatile Tinnitus (PT) occurs when a sound is perceived in the ear that is synchronous with the heartbeat and has no extracorporeal source. The etiology of PT is sometimes traced to abnormalities in the Internal Jugular Vein (IJV). The work herein utilizes new tools from 4D Flow Magnetic Resonance Imaging (MRI) and computational fluid dynamics (CFD) to study hemodynamics in the IJV region in an effort to better characterize potential sources of sound generation in PT. Recent developments in 4D Flow MRI can provide non-invasive qualitative and quantitative characterization of blood flow in complex patient specific vascular geometries. In this work we use a commercially available 4D Flow MRI sequence as well as a novel, in-house, accelerated sequence to measure the velocity field in a PT patient’s IJV. We next employ CFD over the same region with consistent boundary conditions. Velocity fields obtained in-vivo by 4D Flow MRI, in-vitro by MRI of a flow phantom with the same geometry, and by CFD are compared and analyzed to identify important hemodynamic features that may play a role in sound generation in PT.
  

A new model for thin film flow down an incline

An accurate modeling of stationary waves on a thin film of viscous, incompressible, Newtonian fluid has been developed. A system of evolution equations for the film height h and flow rate
Q has been obtained using the depth averaged kinetic energy balance, namely the energy integral method with an ellipse type profile. The study is motivated by
the success of energy integral method with ellipse type profile in effectively and accurately predicting the squeeze film force in squeeze flow problems and in predicting the
inertial effects in the performance of squeeze film dampers. The linear stability of the basic flow is analyzed and the results are compared with the existing models.
  

Effects of the Convective Flows on Enzymatic Chemical Oscillations

Chemical oscillations are ubiquitous in nature and have a variety of promising applications. Usually, oscillating chemical systems are analyzed within the context of a reaction-diffusion framework. Here, we examine how convective flows carrying the reactants can be utilized to modulate the negative feedback loops and time delays that promote chemical oscillations. We consider a model where a chemical reaction network involves two species, X and Y, which undergo transformations catalyzed by respective enzymes immobilized at the walls of a fluid-filled microchamber. The reactions with the enzymes provide a negative feedback in the chemically oscillating system. The first enzyme, localized on the first patch, promotes production of chemical X, while the second enzyme, immobilized on the second patch, promotes production of chemical Y, which inhibits the production of chemical X. The separation distance between the enzyme-coated patches sets the time delay required for the transportation of X and Y. The chemical transport is significantly enhanced if convective fluxes accompany the diffusive ones. Therefore, the parameter region where oscillations are present is modified. The findings provide guidance to designing micro-scale chemical reactors with improved functionalities.   

Laminar flow drag reduction on soft porous media

While researches have focused on drag reduction of various coated surfaces such as superhydrophobic structures and polymer brushes, the insights to understand the fundamental physics of the laminar skin friction coefficient and the related drag reduction due to the formation of finite velocity at porous surfaces is still relatively unknown. Herein, we quantitatively investigated the flow over a porous medium by developing a framework to model flow of a Newtonian fluid in a channel where the lower surface was replaced by various porous media. We showed that the flow drag reduction induced by the presence of the porous media depends on the values of the permeability parameter α = L/(MK)^1/2 and the height ratio d=H/L, where L is the half thickness of the free flow region, H is the thickness and K is the permeability of the fiber layer, and M is the ratio of the fluid effective dynamic viscosity μ_e in porous media to its dynamic viscosity μ. We also examined the velocity and shear stress profiles for flow over the permeable layer for the limiting cases of α goes to zero or infinity. The model predictions were compared with the experimental data for specific porous media and good agreement was found.   

Experimental demonstration of perfect water wave absorption

We report an experimental demonstration of perfect wave absorption applied, for the first time, in the case of surface water waves. We used a Helmholtz-type resonator coupled to a mono-mode waveguide, which generates radiation damping. By tuning the geometry of the resonator, we found the equilibrium between the radiation damping and the intrinsic (viscous) damping of the system, which yields perfect absorption. Additionally, by varying the incident wave amplitude, we tuned the nonlinear resistance and consequently the intrinsic damping. In this case, we also obtained a perfect absorption, now as a function of the nonlinearity. The accuracy of the experimental set-up allowed us to measure absorption around 99%.