Jung-Yul Yoo | Seoul National University (original) (raw)

Papers by Jung-Yul Yoo

Transactions of the Korean Society of Mechanical Engineers B

International Journal of Fluid Mechanics Research

Transactions of the Korean Society of Mechanical Engineers B, 2007

ASME 3rd International Conference on Microchannels and Minichannels, Part B cont’d, 2005

Effects of cross-section geometry of capillary on the evaporation from the meniscus have been inv... more Effects of cross-section geometry of capillary on the evaporation from the meniscus have been investigated by adopting several circular and rectangular capillaries. The evaporating meniscus shape, evaporation rate and flow near the evaporating meniscus of various liquids such as water, ethanol and methanol are determined. The shapes of water and ethanol menisci in circular capillary are quite different from each other due to the difference in surface tension. But the difference in meniscus shapes is relatively small in rectangular channel. The averaged evaporation fluxes in rectangular channel are much larger than that in circular capillary. The rotating vortex motion is observed near the evaporating menisci of ethanol and methanol except for the case of methanol in 200 × 20-μm capillary. The reason for this is considered to be the existence of the corner menisci at the four corners.

ASME-JSME-KSME 2011 Joint Fluids Engineering Conference: Volume 1, Symposia – Parts A, B, C, and D, 2011

ABSTRACT To drive a small object which swims in low Reynolds number situation, we need a new type... more ABSTRACT To drive a small object which swims in low Reynolds number situation, we need a new type of propeller which is optimized for low Reynolds number usage since the flow at low Reynolds numbers is dominated by viscous force instead of inertia force. Propeller in a shape of bacterial flagellum can be a strong candidate for propeller of small swimming object. In this paper, we visualized velocity field induced by flagellar shaped propeller using stereoscopic particle image velocimetry. We also have experimentally evaluated the effect of pitch and rotational speeds on the performance of flagellar shaped propeller inspired by flagellum of E.coli using macroscopic model. Silicone oil whose viscosity is 100 times larger than water is used as working fluid to make low Reynolds number situation using macroscopic model. Thrust, torque and velocity were measured as a function of pitch and rotational speed, and efficiency was calculated using measured results. We found that the maximum efficiency of flagellar propeller reaches where the pitch angle is about 40°. However, the effect of rotational speed on the efficiency is relatively smaller than that of pitch. And the flow pattern behind the rotating propeller was altered by pitch of the propeller.

ASME 2009 7th International Conference on Nanochannels, Microchannels and Minichannels, 2009

ABSTRACT It is well known that the liquid and the nanoparticles in an evaporating colloid droplet... more ABSTRACT It is well known that the liquid and the nanoparticles in an evaporating colloid droplet on the hydrophilic surface move radially outward for the contact line to maintain its position. However, the motion of micro-/nano-particles in an evaporating di-dispersed colloid droplet has not been reported to date. In this study, an experiment on an evaporating di-dispersed colloid droplet on the hydrophilic surface is carried out. It is found that nano-particles move radially outward and remain at the contact line while micro-particles move inward toward the center of the droplet. Further the mechanism of the micro-particles moving toward the center of the droplet is found to be due to the surface tension force of the liquid.

Volume 1: Symposia, Parts A and B, 2005

As far as the author knows, few papers have been reported for the level set simulation of a two-p... more As far as the author knows, few papers have been reported for the level set simulation of a two-phase (bubble-in-liquid or liquid droplet-in-gas) flow based on unstructured background mesh. Almost all existing studies are based on structured mesh using finite difference or finite volume method as spatial discretization. Therefore, the application of existing various level set methods based on structured mesh to a bubble-in-liquid flow problem in a complex geometry is not straightforward. In the present study, a level set based two-phase flow code has been developed using finite element discretization, which can be utilized for the analysis of a bubble-in-liquid flow problem in a complex geometry. Since the finite element method has been employed for the spatial discretization of governing equations, unstructured mesh can be naturally adopted for the level set simulation of a bubble-in-liquid flow without an additional load for the code development except that solution methods of the...

European Journal of Mechanics B-fluids - EUR J MECH B-FLUID, 2002

The turbulent flow in a pipe of annular cross section is studied for the first time through a dir... more The turbulent flow in a pipe of annular cross section is studied for the first time through a direct numerical simulation (DNS) using the Navier–Stokes equations written in cylindrical coordinates. To this aim a novel numerical method is developed, which extends to the cylindrical coordinate system an existing, efficient method designed for cartesian coordinates, and allows us to eliminate the pressure and formulate the problem in two scalar unknowns. The unnecessary increase of resolution at smaller radius typically brought about by polar coordinates, with its consequent stability limitations, is avoided by changing the number of azimuthal Fourier modes with the radial coordinate itself. In addition, the azimuthal extension of the computational domain is reduced, for the cases with lowest curvature, by considering only a part of the annulus, without loss of physical significance of the results. A computer code based on this method is run on a desktop PC for the simulation (with up ...

Journal of Mechanical Science and Technology, 2007

Circulatory disease is the number two cause of death next to cancer in Korea, while cardiovascula... more Circulatory disease is the number two cause of death next to cancer in Korea, while cardiovascular disease alone is the number one cause of death in the US. Among circulatory diseases, the most intriguing feature of atherosclerosis is that it occurs prevalently at the sites where the endothelium cells are deformed by low wall shear stress (WSS), causing damage to the tunica intimae. Thus, it is shown how computational t1uid dynamics (CFD) and medical imaging technique (MIT) can contribute to the diagnosis and the treatment of circulatory diseases in such atherosclerosis-prone sites of blood vessels. On the other hand, people have still suffered from many diseases caused by abnormal proliferation of cancerous cells or bacteria. Thus, it is discussed how human tissue cells and bacteria react to their external fluid media and how their motilities can be analyzed quantitatively by using micro-optical technique and microfluidics.

Journal of Fluids Engineering, 1992

An experimental study has been performed to investigate the redeveloping turbulent boundary layer... more An experimental study has been performed to investigate the redeveloping turbulent boundary layer beyond separation-reattachment for a transitional separated flow. By considering the distribution of the intermittency, it has been confirmed that the turbulent structure changes gradually from a mixing layer to a turbulent boundary layer downstream of reattachment. The balances of the respective terms in the turbulent kinetic energy transport equation are evaluated from the energy dissipation rate obtained through numerical integration of the second moment of the energy spectrum. These terms together with those in the shear stress transport equation indicate the recovery process of the redeveloping boundary layer from nonequilibrium to equilibrium.

Journal of Computational Physics, 2004

The suitability of high-order accurate, centered and upwind-biased compact difference schemes for... more The suitability of high-order accurate, centered and upwind-biased compact difference schemes for large eddy simulation (LES) is evaluated through the static and dynamic analyses. For the static error analysis, the power spectra of the finite-differencing and aliasing errors are evaluated in the discrete Fourier space, and for the dynamic error analysis LES of isotropic turbulence is performed with various dissipative and non-dissipative schemes. Results from the static analysis give a misleading conclusion that both the aliasing and finite-differencing errors increase as the numerical dissipation increases. The dynamic analysis, however, shows that the aliasing error decreases as the dissipation increases and the finite-differencing error overweighs the aliasing error. It is also shown that there exists an optimal upwind scheme of minimizing the total discretization error because the dissipative schemes decrease the aliasing error but increase the finite-differencing error. In addition, a classical issue on the treatment of nonlinear term in the Navier-Stokes equation is revisited to show that the skew-symmetric form minimizes both the finite-differencing and aliasing errors. The findings from the dynamic analysis are confirmed by the physical space simulations of turbulent channel flow at Re ¼ 23000 and flow over a circular cylinder at Re ¼ 3900.

Previous studies suggest that most theoretical models for drop evaporation based on an ideally si... more Previous studies suggest that most theoretical models for drop evaporation based on an ideally simplified domain in which evaporation occurs do not correlate well with experimental results. The present paper proposes a novel empirical model f(h) as a simple function of the contact angle h, which is to be used to predict temporal evolution of a sessile water drop volume when the drop evaporates on surfaces of various wettabilities widely adopted in microelectronic engineering. For hydrophilic and hydrophobic surfaces, the evolution of the drop volume during the evaporation process can be predicted more accurately by representing f(h) as an empirical linear function rather than by using previous theoretical models. Furthermore, the proposed model can account for the increased evaporation rate on smooth surfaces, thus providing a wide applicability to various substrate surfaces. For cases involving a superhydrophobic surface, f(h) can be represented by an empirical constant because the contact angle remains constant during evaporation, which is in excellent agreement with the experimental observation.

Journal of Micromechanics and Microengineering, 2008

The lateral migration of neutrally-buoyant particles transported through square microchannels has... more The lateral migration of neutrally-buoyant particles transported through square microchannels has been experimentally investigated over a Reynolds number range of 0.06 <= Re <= 58.65 at the ratio of channel hydraulic diameter to particle size, lambda ≈ 14. Flow Reynolds numbers are determined by applying a conventional particle-tracking algorithm to small tracer particles, while novel imaging techniques have been proposed

AIAA Journal, 2002

ABSTRACT

Serious heat-transfer deterioration may occur in fluids at supercritical pressure owing to the ef... more Serious heat-transfer deterioration may occur in fluids at supercritical pressure owing to the effects of buoyancy, flow acceleration, and significant variations in thermophysical properties. Although there have been numerous experimental studies on this subject, no single heat-transfer correlation has been found to be capable of describing this phenomenon accurately. Relatively few experimental studies have been carried out on the fluid mechanics of supercritical flows because of the technical difficulties of dealing with turbulent flows and high heat fluxes simultaneously. Conversely, many computational fluid dynamics studies have examined a number of low-Reynolds-number turbulence models. However, none have reproduced the buoyancy production of turbulence reasonably well because of limitations with the use of a constant turbulent Prandtl number. Direct numerical simulations may provide more insight into the physics of fluids at supercritical pressure within a limited range of flow and heat-transfer conditions. 495 Annu. Rev. Fluid Mech. 2013.45:495-525. Downloaded from www.annualreviews.org by Seoul National University on 01/07/13. For personal use only.