Inwon Lee

Time-Resolved PIV Investigation on the Unsteadiness of a Low Reynolds Number Confined Impinging Jet

The flow characteristics in a confined slot jet impinging on a flat plate were investigated in low Reynolds number regime by using time-resolved Particle Image Velocimetry technique. The jet Reynolds number was varied from 404 to 1026, where it is presumed that the transient regime exists. We found that the vortical structures in the shear layer are developed with increase of Reynolds number and that the jet becomes remains steady at the Reynolds number of 404. Vortical structures and their temporal evolution are verified and the results were compared with previous numerical studies.

Polyurethane-based Actuators with Various Polyols

This study dealt with the electrostrictive responses of polyurethane (PU) actuators with different microphase separation structure, which was a promising candidate for a material used in polymer actuators. In order to construct PUs with different higher-order structure, PUs with various types of polyol were synthesized: poly(neopentyl glycol adipate) (PNAD), poly(tetramethylene glycol) (PTMG), and poly(dimethyl siloxnae) (PDMS). Synthesized PU was characterized by Fourier transform-infrared (FT-IR) spectroscopy and gel permeation chromatography (GPC). Thermal analysis and mechanical properties of PU films were carried out with differential scanning calorimetry (DSC) and UTM (universal testing machine), respectively. And PU actuator was formed in a type of monomorph, which was made by carbon black electrodes on the both surfaces of PU film by spin coating method. Actuation behavior was mainly influenced on microphase separation structure and mechanical property of PU. In result, PU actuator with PNAD, polyester urethane, had the largest field-induced displacement.

Wave properties of coating for skin friction reduction

Toward the understanding of the drag reduction mechanism, theoretical as well as experimental investigations have been made of the wave properties of compliant coatings such as the wave velocity and the decay parameters. The compliant coating consisted of a homogeneous layer of viscoelastic material attached to a rigid substrate. Based on two-dimensional elastic wave analysis, wave properties such as the dispersion of wave velocity, the ratio between the amplitudes of two wave components, and decay characteristics have been calculated as a function of Poisson’s ratio σ and the loss tangent μ. It was found that the wave parameters are strongly affected by Poisson’s ratio σ. A new experimental technique is devised for direct measurement of the wave parameters of the coating. The wave velocity and the rate of decay were measured based on the amplitude and phase of coating deformation with varying distances from the excitation point. Two kinds of silicon rubber were tested in the frequency range from 500 to 1...

Hydroquinone as a single precursor for concurrent reduction and growth of carbon nanotubes on graphene oxide

Effective reduction and inhibition of restacking are critical steps in realizing the full potential of chemically derived graphene. In this manuscript, we report a one-step, all solid-state microwave procedure for simultaneous reduction and concurrent growth of carbon nanotubes ‘spacers’ on graphene from a single precursor, namely hydroquinone. Our newly developed technique not only effectively reduces graphene oxide but also results in vertically anchored carbon nanotubes on graphene substrate to give unique mesoporous, hierarchical carbon nano-architectures. When applied as a negative electrode in lithium-ion batteries, our 3D graphene–carbon nanotube hybrids exhibit a high capacity of 1016 mA h g−1 with a columbic efficiency of 98% even after prolonged cycling.

The response of compliant coating to nonstationary disturbances

The amplitude and phase lag of surface deformation were determined for a compliant coating under the action of turbulent pressure fluctuations. For this purpose, pressure fluctuations were measured experimentally. The amplitude and duration of coherent wave train of pressure fluctuations were investigated using digital filtration. The transient response was calculated for stabilization of forced oscillations of the coating in approximation of local deformation. The response of coating was analyzed with considerations of its inertial properties and limited duration of coherent harmonics action of pressure fluctuations. It is shown that a compliant coating interacts only with a frequency range near the first resonance. According to the analysis, with increasing elasticity modulus of the coating material E, deformation amplitude decreases as 1∕E, and dimensionless velocity of the coating surface decreases as 1∕E. For sufficiently hard coatings, deformation amplitude becomes smaller than the thickness of the ...

Actuation behavior of waterborne polyurethane/conductive filler nanocomposite electrode

This study dealt with the Maxwell stress effect of waterborne polyurethane (WPU)/conductive filler nanocomposite, which was a promising candidate for a material to be used in a dielectric elastomer actuator electrode. Conductive nanocomposites were produced by using three types of conductive filler: carbon black (CB), vapor grown carbon fiber (VGCF), and silver powder (Ag). Among them, conductive nanocomposite containing VGCF exhibited the lowest threshold concentration; and the mixture of CB and VGCF (CB/VGCF) filler had a synergistic effect to electrical conductivity. Actuation test revealed that CB/VGCF nanocomposite electrode had the largest displacement. Then it could be stated that the improvement of the displacement in CB/VGCF nanocomposite electrode originated from the increase in relative dielectric constant. In addition, a unique feature for the hysteresis of bending deformation was observed. This feature is that the prior application of an electric field significantly improves the bending speed in the successive application. Also, the effect of electrode thickness on the displacement and breakdown strength was examined.

Self-polishing behavior of zinc-based copolymer with different monomer composition

The zinc methacrylate copolymers with various monomer compositions were prepared for investigating the effect of zinc methacrylate (ZMA), 2-methoxyethyl acrylate (2-MTA) contents and compositions of ethyl acrylate (EA) and methyl methacrylate (MMA) as hydrophobic monomers on self-polishing behavior. The effect of molecular weight of the self-polishing copolymer (SPC) was also investigated. The characterizations of copolymers were performed by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The leaching rate of SPC was determined from the reduction of dry film thickness after seawater immersion in a dynamic condition. The experimental results indicate the leaching rate of SPC can be controlled by monomer composition and molecular weight.

Epitaxial growth and metallicity of rutile MoO2 thin film

Molybdenum oxides have various crystal structures and physical properties due to the multiple valence states of the 4d molybdenum. Among them, MoO2 has a distorted rutile structure with chemical stability and metallic behavior. In this study we grew epitaxial (100) MoO2 thin films on (0001) Al2O3 substrates. Through careful control of the Ar-partial pressure and growth temperature, we determined the optimal growth condition. From our structural assessments, MoO2 epitaxial thin films with high crystallinity can only be achieved in very narrow growth conditions such as 500 °C and 7 mTorr. The thin film prepared under optimal condition showed good metallic behavior, which was confirmed by electronic transport and optical reflectance measurements. A detailed electronic structure was also investigated by spectroscopic ellipsometry.

Effect of riblets on the streaky structures excited by free stream tip vortices in boundary layer

In this study, experimental investigations were made regarding the effect of riblets on the streak instability in boundary layer. The streak instability is now regarded as a major source of the self-regeneration mechanism for the hairpin type coherent structures in turbulent boundary layer flow. Thus, it is important to control the instability to suppress the drag-inducing vortical structure in terms of drag reduction. Toward enhancing the measurement accuracy and spatial resolution, an enlarged version of riblets was applied to a streak which was artificially induced by a microwing in a laminar boundary layer. It is found that the riblets have attenuation effect on the streak instability, i.e., to reduce the spanwise velocity gradient of the quasi-streamwise streak in boundary layer.

An experimental study of the effect of mooring systems on the dynamics of a SPAR buoy-type floating offshore wind turbine

Abstract An experimental study of the effect of mooring systems on the dynamics of a SPAR buoy-type floating offshore wind turbine is presented. The effects of the Center of Gravity (COG), mooring line spring constant, and fair-lead location on the turbine’s motion in response to regular waves are investigated. Experimental results show that for a typical mooring system of a SPAR buoy-type Floating Offshore Wind Turbine (FOWT), the effect of mooring systems on the dynamics of the turbine can be considered negligible. However, the pitch decreases notably as the COG increases. The COG and spring constant of the mooring line have a negligible effect on the fairlead displacement. Numerical simulation and sensitivity analysis show that the wind turbine motion and its sensitivity to changes in the mooring system and COG are very large near resonant frequencies. The test results can be used to validate numerical simulation tools for FOWTs.