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Jinil Park

Bio: Jinil Park is an academic researcher from Ajou University. The author has contributed to research in topics: Fuel efficiency & Boundary layer. The author has an hindex of 11, co-authored 45 publications receiving 603 citations. Previous affiliations of Jinil Park include Brown University & Kia Motors.

Papers
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Journal ArticleDOI
TL;DR: In this paper, total internal reflection velocimetry (TIRV) images are illuminated with the evanescent field of an incident laser pulse and contain only particles that are very close to the channel surface.
Abstract: Total internal reflection velocimetry (TIRV) is used to measure particle motion in the near-wall region of a microfluidic system. TIRV images are illuminated with the evanescent field of an incident laser pulse and contain only particles that are very close to the channel surface. Sub-micron-sized fluorescent particles suspended in water are used as seed particles and their images are analyzed with a particle tracking velocimetry (PTV) algorithm to extract information about apparent slip velocity. At relatively low shear rates (less than 2,500 s-1), a velocity proportional to the shear rate was observed. The statistical difference between velocities measured over hydrophilic and hydrophobic surfaces was found to be minimal. The results suggest that the slip length, if present, is less than 10 nm, but uncertainty regarding the exact character of the illumination field prevents a more accurate measurement at this time. Numerical simulations are presented to help understand the results and to provide insight into the mechanisms that result in the experimentally observed distributions. Issues associated with the accuracy of the experimental technique and the interpretations of the experimental results are also discussed.

104 citations

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TL;DR: In this article, the authors used spanwise aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction.
Abstract: Results concerning the design and fabrication of electromagnetic actuators, and their application to affect the wall shear stress in a fully turbulent channel flow are discussed. The actuators utilize a Lorentz force to induce fluid motion due to the interaction between a magnetic field and a current density. The actuators are comprised of spanwise-aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction. Problems commonly associated with electromagnetic flow control—electrolysis, bubble formation, and electrode corrosion are substantially reduced, and in most cases eliminated by the use of a conductive polymer coating. The actuators generate velocity profiles with a penetration depth into the flow of approximately 1 mm (set by the electrode/magnet pitch) and maximum velocities of approximately 4 cm/s. The actuation velocities are found to scale linearly with forcing voltage and frequency. The electrical to mechanical efficiency is found to be very low (≈10−4), primarily due to the limitations on the magnetic field strength and the low conductivity of the working fluid (saltwater). The actuators are used in a fully turbulent low Reynolds number channel flow and their effect on the turbulent skin friction is measured using a direct measurement of drag. Maximum drag reductions of approximately 10% are measured when the flow is forced using a spanwise oscillating Lorentz force. A scaling argument for the optimal amplitude of the current density is developed. The efficiency of this method for drag reduction, and its application at higher Reynolds numbers is also discussed.

101 citations

Journal ArticleDOI
TL;DR: In this paper, it is shown from the surface pressure measurement, surface oil-flow visualization and near-wall streamwise velocity measurement that the disturbances from the high-frequency forcing grow inside the boundary layer and delay the first separation while maintaining laminar separation, and they grow further along the separated shear layer and high momentum in the free stream is entrained toward the sphere surface, resulting in the reattachment of the flow.
Abstract: Active control of flow over a sphere at $). For the forcing frequencies smaller than this critical frequency, the drag is either nearly the same as, or slightly smaller than, that without forcing. The critical forcing frequency is found to be closely associated with the onset of the boundary-layer instability. It is shown from the surface-pressure measurement, surface oil-flow visualization and near-wall streamwise velocity measurement that the disturbances from the high-frequency forcing grow inside the boundary layer and delay the first separation while maintaining laminar separation, and they grow further along the separated shear layer and high momentum in the free stream is entrained toward the sphere surface, resulting in the reattachment of the flow (thus forming a separation bubble above the sphere surface) and the delay of the main separation. The reverse flow region in the wake is significantly reduced and the motion in that region also becomes weak owing to the forcing. Finally, the variation of drag by the present forcing with respect to the Reynolds number is very similar to that by dimples on the surface, but is different from that by surface roughness.

68 citations

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TL;DR: In this paper, the authors developed a macro-scale particle image velocimetry (PIV) system to measure the full-field velocity distribution for rotating rigid helices and rotating flexible helices.
Abstract: Escherichia coli (E. coli) and other bacteria are propelled through water by several helical flagella, which are rotated by motors embedded at random points on the cell wall. Depending on the handedness and rotation sense, the motion of the flagella induces a flow field that causes them to wrap around each other and form a bundle. Our objective is to understand and model the mechanics of this process. Full-scale flagella are 10 μm in length, 20 nm in diameter, and turn at a rate of 100 Hz. To accurately simulate bundling at a more easily observable scale, we built a scale model in which 20-cm-long helices are rotated in 100,000 cp silicone oil (Poly-di-methyl-siloxane). The highly viscous oil ensures an appropriately low Reynolds number. We developed a macro-scale particle image velocimetry (PIV) system to measure the full-field velocity distribution for rotating rigid helices and rotating flexible helices. In the latter case, the helices were made from epoxy-filled plastic tubing to give approximately the same ratio of elastic to viscous stresses as in the full-scale flagella. Comparison between PIV measurements and slender-body calculations shows good agreement for the case of rigid helices. For the flexible helices, we find that the flow field generated by a bundle in the steady state is well approximated by the flow generated by a single rigid helix with twice the filament radius.

62 citations

Patent
Woo-Seok Kim1, Jonghwa Lee1, Jinil Park1, Kyoung-Seok Park1, Hyoseong Wi1 
21 Nov 2011
TL;DR: A cooling system of a vehicle may include a high and low temperature radiators, a cooling fan that blows air to the high temperature radiator and the low temperature radiator, a high temperature coolant pump that pumps the high-temperature coolant, a low-temperature coolant pumps that pump the lowtemperature coolingant, and a control portion that controls the cooling fan according to driving conditions of the vehicle and environmental conditions as mentioned in this paper.
Abstract: A cooling system of a vehicle may include a high and low temperature radiators that cool a high and low temperature coolants respectively circulating an engine and passing a water cooled intercooler and a low exhaust gas recirculation cooler of a turbo charger, a cooling fan that blows air to the high temperature radiator and the low temperature radiator, a high temperature coolant pump that pumps the high temperature coolant, a low temperature coolant pump that pumps the low temperature coolant, and a control portion that controls the high temperature coolant pump, the low temperature coolant pump, and the cooling fan according to driving conditions of the vehicle and environmental conditions. A controlling method may include detecting driving conditions of the vehicle and environmental conditions, setting an operating target for the cooling system and/or a lubrication system, and determining operating conditions for the cooling system and/or the lubrication system.

46 citations


Cited by
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Journal ArticleDOI
TL;DR: The biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below are reviewed, with emphasis on the simple physical picture and fundamental flow physics phenomena in this regime.
Abstract: Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

2,274 citations

Journal ArticleDOI
TL;DR: A review of the use of the combination of surface roughness and hydrophobicity for engineering large slip at the fluid-solid interface is given in this paper, with an eye toward implementing these surfaces in a wide range of applications.
Abstract: This review discusses the use of the combination of surface roughness and hydrophobicity for engineering large slip at the fluid-solid interface. These superhydrophobic surfaces were initially inspired by the unique water-repellent properties of the lotus leaf and can be employed to produce drag reduction in both laminar and turbulent flows, enhance mixing in laminar flows, and amplify diffusion-osmotic flows. We review the current state of experiments, simulations, and theory of flow past superhydrophobic surfaces. In addition, the designs and limitations of these surfaces are discussed, with an eye toward implementing these surfaces in a wide range of applications.

1,013 citations

Journal ArticleDOI
TL;DR: A review of experimental studies regarding the phenomenon of slip of Newtonian liquids at solid interfaces is provided in this article, with particular attention to the effects that factors such as surface roughness, wettability and the presence of gaseous layers might have on the measured interfacial slip.
Abstract: For several centuries fluid dynamics studies have relied upon the assumption that when a liquid flows over a solid surface, the liquid molecules adjacent to the solid are stationary relative to the solid. This no-slip boundary condition (BC) has been applied successfully to model many macroscopic experiments, but has no microscopic justification. In recent years there has been an increased interest in determining the appropriate BCs for the flow of Newtonian liquids in confined geometries, partly due to exciting developments in the fields of microfluidic and microelectromechanical devices and partly because new and more sophisticated measurement techniques are now available. An increasing number of research groups now dedicate great attention to the study of the flow of liquids at solid interfaces, and as a result a large number of experimental, computational and theoretical studies have appeared in the literature. We provide here a review of experimental studies regarding the phenomenon of slip of Newtonian liquids at solid interfaces. We dedicate particular attention to the effects that factors such as surface roughness, wettability and the presence of gaseous layers might have on the measured interfacial slip. We also discuss how future studies might improve our understanding of hydrodynamic BCs and enable us to actively control liquid slip.

985 citations

Journal ArticleDOI
TL;DR: This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications and attempts to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.
Abstract: Actuators are transducers that convert an electrical signal to a desired physical quantity. Active flow control actuators modify a flow by providing an electronically controllable disturbance. The field of active flow control has witnessed explosive growth in the variety of actuators, which is a testament to both the importance and challenges associated with actuator design. This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications. Actuator fundamentals are discussed, and various popular actuator types used in low-to-moderate speed flows are then described, including fluidic, moving object/surface, and plasma actuators. We attempt to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.

915 citations

Journal ArticleDOI
TL;DR: A hydrodynamic model is provided for near a solid boundary, Escherichia coli swims in clockwise circular motion and the radius of curvature of the trajectory is observed to increase with the length of the bacterium body.

890 citations