Jung Yul Yoo

Streamline upwind numerical simulation of two-dimensional confined impinging slot jets

Abstract In the present paper, flow and heat transfer characteristics of confined impinging slot jets have been numerically investigated using a SIMPLE-based segregated streamline upwind Petrov–Galerkin finite element method. For laminar jets, it is shown that the skin friction coefficient approaches the grid-independent Galerkin solution and that the present simulation induces negligible false diffusion in the flow field. For turbulent jets, the k–ω turbulence model is adopted. The streamwise mean velocity and the heat transfer coefficient respectively agree very well with existing experimental data within limited ranges of parameters.

Droplet merging in a straight microchannel using droplet size or viscosity difference

In the present study, a novel droplet-merging technique is proposed, which enables two nanoliter or picoliter droplets to merge in a straight microchannel with high precision. Two dispersed phase fluids are supplied respectively from two side channels in a certain range of flow rates to generate droplets of different sizes or different viscosities in a regularly alternating mode at a cross-channel intersection prior to the straight microchannel, in such a manner that the droplets are spontaneously synchronized by themselves due to the competition of the two interfaces formed at the cross-channel intersection. Then they are autogeneously merged in the downstream straight microchannel with or without a sudden expansion of the cross section, due to their velocity difference which is induced as they are transported aboard the continuous phase fluid. This droplet-merging method has no desynchronization or secondary-merging problems. Thus, it can be applied efficiently to mixing or encapsulating one target sample with another material for the purposes of nanoparticle synthesis, hydrogel-bead production, cell transplantation and so forth.

Three-dimensional focusing of red blood cells in microchannel flows for bio-sensing applications

Three-dimensional (3D) focusing of particles in microchannels has been a long-standing issue in the design of biochemical/biomedical microdevices. Current microdevices for 3D cell or bioparticle focusing involve complex channel geometries in view of their fabrication because they require multiple layers and/or sheath flows. This paper proposes a simple method for 3D focusing of red blood cells (RBCs) in a single circular microcapillary, without any sheath flows, which is inspired from the fluid dynamics phenomenon in that a spherical particle lagging behind a Poiseuille flow migrates toward and along the channel axis. More explicitly, electrophoresis of RBCs superimposed on the pressure-driven flow is utilized to generate an RBC migration mode analogous to this phenomenon. A particle-tracking scheme with a sub-pixel resolution is implemented to spatially position red blood cells flowing through the channel, so that a probability density function (PDF) is constructed to evaluate the tightness of the cell focusing. Above a specific strength of the electric field, approximately 90% of the sheep RBCs laden in the flow are tightly focused within a beam diameter that is three times the cell dimension. Particle shape effect on the focusing is discussed by making comparisons between the RBCs and the spherical particles. The lateral migration velocity, predicted by an existing theoretical model, is in good agreement with the present experimental data. It is noteworthy that 3D focusing of non-spherical particles, such as RBCs, has been achieved in a circular microchannel, which is a significant improvement over previous focusing methodologies.

Microfluidic monitoring of Pseudomonas aeruginosa chemotaxis under the continuous chemical gradient

This study presents a microfluidic approach for the rapid analysis of bacterial chemotaxis in response to chemical gradients. The diffusional mixing of laminar flow continuously generates a stable chemical gradient in a microfluidic device. For the proof of concept, we have investigated the effects of the attractant peptone and repellent trichloroethylene (TCE) on chemotactic responses of wild type Pseudomonas aeruginosa PAO1 and chemotactic mutant PC4. The microfluidic method clearly demonstrates that P. aeruginosa PAO1 is attracted to peptone and repelled from TCE, whereas PC4 shows non-chemotactic behavior. In addition, the analysis of PAO1 chemotaxis on 20 amino acids revealed the effective concentration range of each amino acid as a chemoeffector. Not only does the microfluidic approach facilitate the quantitative information of chemotaxis, which gives an insight into understanding the mechanism of P. aeruginosa motility, but it also provides a useful tool for the rapid monitoring of bacterial chemotaxis in a reproducible experimental manner.

Forces Acting on a Single Particle in an Evaporating Sessile Droplet on a Hydrophilic Surface

The evaporating sessile droplet of a mono/didisperse colloid on a plate is a very useful and handy technique in micro/nano/bioapplications to separate, pattern, and control the particles. Although the fundamental nature of the evaporation phenomena and its applications have been extensively proposed, the crucial forces affecting a single particle motion in an evaporating droplet are not reported yet. To elucidate the impact of various forces including the drag, electrostatic, van der Waals, and surface tension forces on the particle motion in suspension, the magnitudes of them are compared using the scale analysis. In the early stage of the evaporation, in which the contact line is fixed, the motion of a single particle suspended in liquid are mostly affected by drag force. Later, with the incidence of the contact line recession, the surface tension force takes over the control of the single particle motion.

Time-accurate computation of unsteady free surface flows using an ALE-segregated equal-order FEM

Abstract Arbitrary Lagrangian–Eulerian (ALE) finite element formulations based on segregated equal-order interpolation are presented with the aim of computing unsteady free surface flows time-accurately. A standing vortex problem is solved using both fixed and moving grids to design a solution method which is time-accurate in the sense that it conserves vortex kinetic energy. It turns out that the ‘Chorin type SIMPLE algorithm’ serves this purpose satisfactorily when it is used in conjunction with the Galerkin spatial discretization and the Crank–Nicolson temporal discretization. Then, a small amplitude sloshing problem is solved to assure that the Crank–Nicolson/central difference scheme among others used for discretizing the kinematic condition preserves the oscillating amplitude of the free surface. Lastly, the most time-accurate numerical technique thus designed is applied to solve a solitary wave propagation problem, which shows that the predicted maximum run-up heights for various initial heights are in good agreement with existing experiment.

Direct numerical simulation of vortex synchronization due to small perturbations

Direct numerical simulation (DNS) is performed to investigate the vortex synchronization phenomena in the wake behind a circular cylinder at the Reynolds numbers, Re = 220 (mode-A regime) and 360 (mode-B regime). To generate vortex synchronization, a sinusoidal streamwise velocity perturbation, the frequency of which is about twice the natural shedding frequency, is superimposed on the free stream velocity. At Re = 360, vortex synchronization occurs when the perturbation frequency is exactly twice the natural shedding frequency. However, at Re = 220, it does not occur when the same perturbation frequency condition is imposed. Instead, it occurs when the perturbation frequency is near twice the hypothetical two-dimensional laminar vortex shedding frequency as if there were no wake transition at Re = 220. It is elucidated that, as a result of vortex synchronization, the trajectory of the Karman vortices and the vortex structure are changed. The Karman vortices are formed along the mean separating streamline slightly inside the mean wake bubble at Re = 220, but slightly outside at Re = 360. Thus, the Reynolds shear stress force has different contribution to the streamwise force balance of the mean wake bubble depending on the Reynolds numbers: its magnitude is negligible at Re = 220, compared to other force components, while it reverses its sign at Re = 360. More importantly, at Re = 220, the mode-A instability is suppressed into two-dimensional laminar flow with strong Karman vortices. At Re = 360, the dominant instability mode changes from mode B to mode A.

Finite element simulation of thin liquid film flow and heat transfer including a hydraulic jump

A numerical simulation has been performed to investigate planar and radial flows of thin liquid film subject to constant wall temperature or constant wall heat flux, considering the surface tension effect. To simulate the variation of the film height including a hydraulic jump, an Arbitrary Lagrangian–Eulerian (ALE) method is adopted in describing the governing equations. An iterative split algorithm is used to improve the continuity constraint in time marching of the governing equations which are discretized by Streamline Upwind Petrov–Galerkin (SUPG) finite element method. It has been shown clearly that the surface tension has to be considered in order to describe realistically a hydraulic jump preceded by a capillary ripple. The variation of the film height is in good agreement with the existing experimental data. Physical aspects of how the flowrate as well as temperature-dependent fluid properties affect the formation of the hydraulic jump and the variation of the Nusselt number are discussed rationally. Copyright

Bidirectional inward migration of particles lagging behind a Poiseuille flow in a rectangular microchannel for 3D particle focusing

Electrophoretic mobility of particles dispersed in an electrolyte solution induces the particles to lag behind a Poiseuille flow in a rectangular microchannel, which causes bidirectional inward migration of particles to the central axis of the channel. As a result, in the present theoretical and experimental study, three-dimensional (3D) particle focusing is clearly realized in such a manner that the particles are aligned in a single file along the axis of the channel as they are transported downstream. Theoretical prediction on the particle migration time provides an excellent assessment of the experimental results. The method proposed in the present study has potential for development of low-cost rapid manufacturing process of sheathless monolayer microchips for 3D particle focusing.

Flow Visualization and Performance Measurements of a Flagellar Propeller

A new type of propeller that is optimized for low Reynolds numbers is required to propel a small object in a medium where the flow is dominated by viscous rather than inertial forces. A propeller in the shape of a bacterial flagellum seems an appropriate choice for driving a small object. Accordingly, in this study, we visualized the velocity field induced by a spring-like propeller inspired by the Escherichia coli flagellum, using a macroscopic model and applying stereoscopic particle image velocimetry. We also experimentally evaluated the effect of pitch and rotational speed on the performance of this flagellar propeller. Silicone oil, which has a kinematic viscosity 100,000 times that of water, was used as the working fluid to generate a low Reynolds number for the macroscopic model. Thrust, torque, and velocity were measured as functions of pitch and rotational speed, and the efficiency of the propeller was calculated from the measured results. We found that the flagellar propeller reached a maximum efficiency when the pitch angle was approximately 53°. Compared to pitch, rotational speed had a relatively small effect on the efficiency, and the pitch altered the flow pattern behind the rotating propeller.