# Latest papers in fluid mechanics

### Characterization of peristaltic flow during the mixing process in a model human stomach

Numerical simulations are performed to investigate the characteristics of peristaltic flow in a model stomach during the mixing and digestion process. The geometrical model for the stomach consists of an axisymmetric tube of varying diameter with a wall at one end, representing the antrum and closed pylorus. The antral contraction waves (ACWs) that produce the peristaltic flow are modeled as traveling waves that deform the boundary and consequently the computational mesh. This geometrical model is implemented into the open source code OpenFOAM. A parametric study is performed in which the fluid viscosity, wave speed, wave width, and maximum relative occlusion are varied. The effect of these parameters on the retropulsive jet induced near the pylorus and the recirculation between pairs of consecutive ACWs is investigated. Both of these flow features contribute to the mixing and digestion process. The retropulsive jet is quantified by its peak velocity and length along the centerline. For each wave geometry, these quantities are found to be independent of the Reynolds number for low Reynolds numbers, while for Reynolds numbers exceeding one, the peak centerline velocity decreases and the jet length increases as the Reynolds number increases. Moreover, the velocity and pressure curves are found to scale with the wave speed at low Reynolds numbers. Between different wave geometries, scaling laws are proposed and tested for the peak centerline velocity and jet length. Particle tracking and vorticity plots show that mixing efficiency increases when the relative occlusion increases, as well as when the viscosity or wave width decreases.

### Impact of airflow on the heat transfer conditions inside an oven cavity, characterized using particle imaging velocimetry

The complex airflow in convection ovens directly influences the heat transferred to the product placed inside, thereby affecting product quality. Characterization of related airflow profiles can provide scientific understanding for improvement of oven designs as well as important parameters for simulation of involved thermal processes. In this study, the particle-imaging velocimetry (PIV) technique was applied to visualize airflow inside a household convection oven with samples placed at three different locations on a baking tray. The oven cavity was modified for optical access, and airflow was measured at room temperature. A 30 mW green laser was used for illuminating tracer particles in a laser sheet that were generated using incense sticks. The flow patterns were captured using a high-speed camera at 1000 fps. The vorticity and turbulent kinetic energy parameters derived from velocity fields reflected adequate mixing of air inside the cavity. The computed heat transfer coefficient distribution from the boundary layer flow fields to the sample surface ranged between 2.0 and 18.3 W m−2 K−1. The results showed separation of the laminar boundary layer from the object surface at angles of 85°–90°. The PIV-algorithms and boundary layer flow derived parameters developed in this study can be used for refined characterization of complex air or gas flows and related heat transfer characteristics in closed cavity convection ovens and the like arrangements.

### Effect of bolus viscosity on carbohydrate digestion and glucose absorption processes: An in vitro study

Digestion is the process of breaking down food into smaller nutrient components which can be easily absorbed in the intestinal tract. The aim of this study was to experimentally investigate the influence of bolus (gastric content) viscosity on digestion and nutrient absorption processes, using an in vitro gastrointestinal model, the TIM-1 system. Two types of simple carbohydrates, namely, glucose and maltodextrin, were used as model foods. The initial bolus viscosity was varied (∼1 mPa·s, ∼15 mPa·s, and ∼100 mPa·s) using different glycerol-water proportions. A fluorescent molecular rotor compound (Fast Green For Coloring Food) was used to monitor viscosity changing patterns of the gastrointestinal content during digestion in the in vitro stomach and small intestinal sections. The digested-nutrient absorption data indicated that the initial bolus viscosity did not significantly affect the glucose absorption process in the small intestine. However, an increase in the initial bolus viscosity from ∼1 mPa·s to ∼15 mPa·s reduced the maltodextrin to glucose conversion by 35%. A further increase in the initial bolus viscosity from ∼15 mPa·s to ∼100 mPa·s did not significantly reduce the maltodextrin to glucose conversion.

### Retention and infiltration of bacteria on a plant leaf driven by surface water evaporation

Evaporation-driven internal flows within a sessile droplet can transport microorganisms close to the leaf surface and facilitate their infiltration into the available openings, such as stomata. Here, using microfabricated surfaces out of polydimethylsiloxane, the sole effects of evaporation of sessile droplets in contamination of plant leaves was studied. These surfaces were patterned with stomata, trichomes, and grooves that are common surface microstructures on plant leaves. Evaporation of sessile droplets, containing bacterial suspensions, on real leaves and fabricated surfaces was studied using confocal microscopy. To provide insight about the effects of leaf hydrophobicity and surface roughness on the bacterial retention and infiltration, variations of contact angle of sessile droplets at these surfaces were measured during evaporation. The results showed that evaporation-driven flow transported bacteria close to the surface of spinach leaves and fabricated surfaces, leading to distinct infiltration into the stomata. Larger size and wider spacing of the micropores, and a more hydrophilic surface, led bacteria to spread more at the droplet base area and infiltrate into more stomata. Evaporation-driven movement of contact line, which can sweep bacteria over the leaf surface, was shown to lead to bacterial infiltration into the stomatal pores. Findings should help improve microbial safety of leafy greens.

### Numerical and experimental investigation of dough kneading in a three-dimensional spiral kneader

This work reports on the first three-dimensional viscoelastic dough kneading simulation performed in a spiral kneader. Unstructured tetrahedral grids were generated using ICEM CFD 17.1. Viscoelastic volume-of-fluid simulations were performed using OpenFOAM v.4.0 in combination with the RheoTool package v.2.0. The White-Metzner model with a Bird-Carreau type of shear-rate dependency of the viscosity and relaxation time was utilized to describe the rheology of the dough matrix. We validated our numerical method by simulating the viscoelastic rod climbing benchmark problem in a cylindrical bowl. The temporal evolution of the dough surface was compared with screenshots obtained with a high-speed video camera during laboratory kneading. We found that the curvature of the free surface matches the experimental data well. With our numerical approach, we were able to predict the formation, extension, and breakup of dough pockets. The dough is convected around the inner stationary rod by the rotation of the outer cylindrical bowl, whereas the spiral arm located in between these two parts produces spiral flow patterns. Vertical mixing is not as good as radial mixing and may be enhanced by utilizing two spiral arms similar to hand kneading. Industrial kneading geometries and processes may be further optimized by performing such types of simulations.

### Contraction of a shear-thinning axisymmetric cavity

We investigate the capillary driven collapse of a small contracting cavity or hole in a shear-thinning fluid. We find that shear-thinning effects accelerate the collapse of the cavity by decreasing the apparent liquid viscosity near the cavity’s moving front. Scaling arguments are used to derive a power-law relationship between the size of the cavity and the rate of collapse. The scaling predictions are then corroborated and fully characterized using high-fidelity simulations. The new findings have implications for natural and technological systems including neck collapse during microbubble pinch-off, the integrity of perforated films and biological membranes, the stability of bubbles and foams in the food industry, and the fabrication of nanopore based biosensors.

### Numerical study on electrohydrodynamic multiple droplet interactions

Author(s): P. S. Casas, M. Garzon, L. J. Gray, and J. A. Sethian

We present a numerical study of inviscid multiple droplet coalescence and break-up under the action of electric forces. Using an embedded potential flow model for the droplet hydrodynamics, coupled with an unbounded exterior electrostatic problem, we are able to perform computations through various ...

[Phys. Rev. E 100, 063111] Published Mon Dec 30, 2019

### Flagellated bacteria swim in circles near a rigid wall

Author(s): Yunyoung Park, Yongsam Kim, and Sookkyung Lim

The rotation of bacterial flagella driven by rotary motors enables the cell to swim through fluid. Bacteria run and reorient by changing the rotational direction of the motor for survival. Fluid environmental conditions also change the course of swimming; for example, cells near a solid boundary dra...

[Phys. Rev. E 100, 063112] Published Mon Dec 30, 2019

### Numerical forcing scheme to generate passive scalar mixing on the centerline of turbulent round jets in a triply periodic box

Author(s): Kyupaeck Jeff Rah and Guillaume Blanquart

A forcing technique is devised to generate centerline scalar mixing of round jets in a triply periodic box. It is derived from the scalar transport equation using a Reynolds-like decomposition of the scalar field. The result is a combination of a mean gradient term and a linear scalar term.

[Phys. Rev. Fluids 4, 124504] Published Mon Dec 30, 2019

### Impact of capillary drops of complex fluids on a solid surface

The drop impact on a solid surface is studied in the context of complex fluids that exhibit viscoplastic, viscoelastic, and thixotropic behavior. The effects of rheology and surface tension are investigated for a range of corresponding dimensionless numbers associated with each phenomenon. Two usual quantities are employed to understand the drop dynamics, namely, the maximum spreading diameter and the time the drop remains in contact with the solid. Another result is the drop shape evolution, captured by displaying selected instants. The first part of the work is dedicated to examine the influence of capillary effects for more real fluids, in the present case, solutions of Carbopol, kaolin, and bentonite whose mechanical properties are taken from experimental measurements reported in the literature. In the second part, we conduct parametric studies varying the dimensionless numbers that govern the problem. We have shown that the influence of surface tension in yield stress materials is less significant and can be negligible when real parameters are input in the model. On the other hand, Newtonian and viscoelastic fluids are more susceptible to surface tension effects. This quantity tends to decrease maximum spreading diameter and decrease contact time due to its resistance in the spreading stage. While inertia, elasticity, and plastic effects favor the drop to spread and to increase its contact time with the solid substrate, a more thixotropic behavior leads to the opposite trend.

### Solute transport in porous media studied by lattice Boltzmann simulations at pore scale and x-ray tomography experiments

Author(s): Chunwei Zhang, Tetsuya Suekane, Kosuke Minokawa, Yingxue Hu, and Anindityo Patmonoaji

With the aid of nondestructive microfocus x-ray computed tomography (CT), we performed three-dimensional (3D) tracer dispersion experiments on randomly unconsolidated packed beds. Plumes of nonreactive sodium iodide solution were point injected into a sodium chloride solvent as a tracer for the eval...

[Phys. Rev. E 100, 063110] Published Thu Dec 26, 2019

### Viscoelasticity of liquid water investigated using molecular dynamics simulations

Author(s): Timothy J. O'Sullivan, Sridhar K. Kannam, Debadi Chakraborty, Billy D. Todd, and John E. Sader

Nonequilibrium and equilibrium classical molecular dynamics simulations of the linear viscoelastic response of water are performed. Comparison with available measurements are reported. Anomalous behavior for frequencies above 2 THz is observed, which is yet to be understood.

[Phys. Rev. Fluids 4, 123302] Published Thu Dec 26, 2019

### Shape stability of deflated vesicles in general linear flows

Author(s): Charlie Lin and Vivek Narsimhan

A simulation of deflated vesicles in linear flows finds that their shape stability is unaffected by moderately rotational flows, where the results collapse to those of extensional flow by a simple scaling factor. Vesicles in near shear flows develop an asymmetrical shape that increases their critical capillary number.

[Phys. Rev. Fluids 4, 123606] Published Thu Dec 26, 2019

### Stability of a static liquid bridge knowing only its shape

Author(s): P. Lin, X. Lin, L. E. Johns, and R. Narayanan

We show that the shape and stability of any static liquid bridge can be solved from a master problem obtained from a model of a pressure-controlled experiment.

[Phys. Rev. Fluids 4, 123904] Published Thu Dec 26, 2019

### Stability of leapfrogging vortex pairs: A semi-analytic approach

Author(s): Brandon M. Behring and Roy H. Goodman

We develop an alternate method for investigating the stability of a one-parameter family of periodic solutions of the four-vortex problem known as ‘leapfrogging’ orbits and confirm the value of a bifurcation parameter. This approach could be applied to other related and more general problems.

[Phys. Rev. Fluids 4, 124703] Published Thu Dec 26, 2019

### Upper bound on angular momentum transport in Taylor-Couette flow

Author(s): Zijing Ding and Elena Marensi

We investigate the upper bound on angular momentum transport in Taylor-Couette flow theoretically and numerically by a one-dimensional background field method. The flow is bounded between a rotating inner cylinder of radius Ri and a fixed outer cylinder of radius Ro. A variational problem is formula...

[Phys. Rev. E 100, 063109] Published Tue Dec 24, 2019

### Coupling of vibro-acoustic waves with premixed flame

Author(s): Basile Radisson, Juliette Piketty-Moine, and Christophe Almarcha

The variation in the wrinkling of a flame burning down inside a vertical Hele-Shaw cell is related to flexure modes of the walls of the cell and a Faraday-like instability of the the flame front.

[Phys. Rev. Fluids 4, 121201(R)] Published Tue Dec 24, 2019

### Surfing the capillary wave: Wetting dynamics beneath an impacting drop

Author(s): John M. Kolinski, Ramin Kaviani, Dylan Hade, and Shmuel M. Rubinstein

Experiments show that prior to impact upon a solid surface, a droplet compresses the air beneath it into a nanometer-scale film. This air film is destroyed by the local initiation of liquid-solid contact, forming capillary bridges. A capillary wave leads the advancing wetting front, limiting its speed.

[Phys. Rev. Fluids 4, 123605] Published Tue Dec 24, 2019

### Modeling chemo-hydrodynamic interactions of phoretic particles: A unified framework

Author(s): Akhil Varma and Sébastien Michelin

A theoretical framework based on the method of reflections for Laplace’s and Stokes’ equations is developed to systematically determine multibody chemohydrodynamic interactions of autophoretic colloids. This approach is an efficient alternative to full numerical simulations of suspension dynamics.

[Phys. Rev. Fluids 4, 124204] Published Tue Dec 24, 2019

### Secondary vortex dynamics in the cylinder wake during laminar-to-turbulent transition

Author(s): Jeffrey McClure, Colin Pavan, and Serhiy Yarusevych

A detailed analysis of cylinder wake dynamics is presented for characteristic instabilities spanning laminar-to-turbulent transition regimes. A new three-dimensional vortex tracking algorithm is introduced to elucidate the vorticity transport and relate it to structural loads.

[Phys. Rev. Fluids 4, 124702] Published Tue Dec 24, 2019