Seung Jae Yi

Phosphorescence-based multiphysics visualization: a review

Phosphorescence-based visualization techniques have been successfully developed for the optical measurement of physical phenomena, such as particle image velocimetry (PIV) for velocity measurement, pressure-sensitive paint (PSP) for pressure measurement, and laser-induced fluorescence (LIF) or thermographic phosphors (TP) for temperature measurement. Multiphysics measurement techniques have been developed to analyze the correlations between each physical parameter. For multiphysics measurement, some researchers have combined two measurement techniques, including the application of functional particles which are sensitive to pressure or temperature to the conventional PIV technique for simultaneous measurement. The present review introduces some recent results of phosphorescence-based multiphysics visualization. The basics of phosphorescence are described, and the techniques of PIV, PSP, LIF, and TP are also discussed because these techniques were used for multiphysics measurement. Finally, the remarkable results of multiphysics measurement are summarized in detail, especially for simultaneous measurement techniques.Graphical Abstract

Visualization study on the transient liquid film behavior and inner gas flow after rupture of a soap bubble

This study examined the transient behavior of liquid films and the flow of inner gas. Olive oil particles were inserted into a soap bubble through a Laskin nozzle for visualization, and the inner gas flow fields were measured by time-resolved particle image velocimetry. A pulse laser was used for contactless rupturing of the soap bubble. The transient behavior of the liquid film after the soap bubble ruptured was captured using a high-speed camera at 3,600 frames per second. After rupturing the soap bubble, the inner gas flowed out to the outside through the crack. This is called the primary flow. The removal velocity of the upper liquid film was faster than that of the bottom liquid film. The Kelvin–Helmholtz vortex was generated at the upper and bottom boundaries of the liquid film. A series of Kelvin–Helmholtz vortices, which arise in shear flow along a contact discontinuity, were formed around the bubble sphere. Secondary flow was generated due to a change in momentum after impinging the soap film to a point, and was faster than primary flow.Graphical Abstract

Dynamic analysis of bubble-driven liquid flows using time-resolved particle image velocimetry and proper orthogonal decomposition techniques

AbstractAn experimental study to evaluate dynamic structures of flow motion and turbulence characteristics in bubble-driven water flow in a rectangular tank with a varying flow rate of compressed air is conducted. Liquid flow fields are measured by time-resolved particle image velocimetry (PIV) with fluorescent tracer particles to eliminate diffused reflections, and by an image intensifier to acquire enhanced clean particle images. By proper orthogonal decomposition (POD) analysis, the energy distributions of spatial and temporal modes are acquired. Time-averaged velocity and turbulent kinetic energy distributions are varied with the air flow rates. With increasing Reynolds number, bubble-induced turbulent motion becomes dominant rather than the recirculating flow near the side wall. Detailed spatial structures and the unsteady behavior of dominant dynamic modes associated with turbulent kinetic energy distributions are addressed.Graphical AbstractGraphical Abstract text

Characteristics of Bubble-driven Flow by Using Time-resolved PIV and POD Technique

In this paper, the recirculation flow motion and mixing characteristics driven by air bubble stream in a rectangular water tank is studied. The time-resolved PIV technique is adopted for the quantitative visu- alization and analysis. 488 nm Ar-ion CW laser is used for illumination and orange fluorescent (λex=540 nm, λem=560 nm) particle images are acquired by a PCO 10bit high-speed CCD camera (1280×1024). To obtain clean particle images, 545 nm long pass optical filter and an image intensifier are employed and the flow rates of compressed air is 3 l/min at 0.5 MPa. The recirculation and mixing flow field is further investigated by time-resolved POD analysis technique. It is observed that the large scale recirculation resulting from the interaction between rising bubble stream and side wall is the most dominant flow structure and there are small scale vortex structures moving along with large scale recirculation flow. It is also verified that the sum of 20 modes of velocity field has about 67.4% of total turbulent energy.

Visualization study on the interactions between water droplet and elastic film

The interactions between water droplets and elastic films were examined by acquiring droplet and PDMS film images using a high-speed visualization technique. The contact angle with a water droplet and the deflection of a PDMS thin polymer film were measured from the images and compared with the values calculated from modified cantilever theory. The contact angle and deflection of the PDMS thin plate increased with increasing droplet volume. The static behavior of the elasto-capillarity effect was confirmed by theory and experiment. The dynamic behavior of an elastic film was also examined using the AC electro-wetting condition. The elastic polymer with a droplet was vibrating at double the frequency of the excited AC frequency. The amplitude of vibration increased with increasing AC voltage. These results can be used to design practical microactuators.Graphical Abstract.

Visualization of Instantaneous Temperature Field Using Thermographic Phosphor Thermometry

Thermographic phosphorescence method is superior quantitative temperature field visualization technique compared with IR camera in terms of accuracy and the range of temperature. The instantaneous temperature field was measured using manganese-activated magnesium fluorogermanate (Mg4FGeO6:Mn, MFG) as the thermographic phosphor at the high temperature condition. A pulsed UV-LED (385nm) was used for the light source, and a band-pass filter (655±20nm) installed high speed camera was used to capture the phosphorescence decay of MFG with 4000 fps. The lifetime calibration method was employed to reveal a temperature field for the wide range of temperature from ambient to 550 oC. The uncertainty in temperature measurement was less than 2%. The temporal variation of surface temperature field was obtained when the oblique impinging air jet was issued to the hot plate. [This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012R1A2A4A01008749)].

A Visualization Study on the Characteristics of Droplets Impinging on a Hot Surface

Abstract. Hydrophobic characteristics of high temperature metal surface were investigated by high-speedvisualization of water droplet impact. An aluminum plate was used as the sample plate and the initialdiameter of a water droplet was 2 mm. Transient behavior of a single droplet impinging on the surfacewith and without heating was captured by using a high speed camera running at 4,000 frames per second.The Leidenfrost phenomenon was demonstrated for the case of 300 o C surface temperature, however therewas no rebounding of droplet on the cold plate due to hydrophilic nature. The experimental results showthat the shape evolution of a droplet impinging on the surface varies with the Weber number, i.e. the ratioof impact inertia to capillary force. The overall water-repellent characteristics of the heated surface wasvery similar to that of the super hydrophobic surfaces. Key Words:Super hydrophobic characteristics (초소수성 현상), High temperature metal surface (고온금속표면), Weber number (웨버 수), Leidenfrost phenomenon (라이덴프로스트 현상), Rebound height (재반동 높이), Single droplet (단일 액적)

Velocity and Concentration Field Measurements in Bubble-Driven Turbulent Liquid Flows

In this study, simultaneous measurements of velocity and concentration fields using the time-resolved particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) methods have conducted to investigate mixing characteristics in turbulent water flows driven by air bubbles in a cylindrical water tank. The flow rates of compressed air is changed from 1 to 5 L/min at 0.5 MPa and the corresponding range of bubble based Reynolds number is from 8,320 to 22,100. PLIF measurement results demonstrate that the mixing efficiency is enhanced with increase of gas flow rate. The sloshing motion of the free surface is also effective to the scalar mixing process since the vertical motion can be correlated with concentration fluctuation and increase turbulent dispersion process.Copyright

Dissolved Oxygen Concentration Field Measurement in Microfluidics Using PtOEP/PS Film Sensor

A planar optode system based on an oxygen quenchable luminophore platinum (II) octaethyporphrin (PtOEP) bound with thin polystyrene (PS) film and UV light-emitting diodes (UV-LEDs) was developed to measure the dissolved oxygen (DO) concentration field in microscale water flows. An intensity-based method adopting a pixel-to-pixel in situ calibration technique was used to visualize DO concentration fields in a Y-shaped microchannel. The achievable spatial resolution of the acquired concentration map could be as high as 2.94 μm. The diffusion process of DO through the interface between two parallel water flows having different DO concentrations was quantitatively analyzed. We found that the thickness of the concentration gradient of DO increased as the Reynolds number decreased. The ratio of diffusion length scales coincided with the ratio of inner scales of viscous shear layers in the microchannel for two different Reynolds numbers.Copyright