Jaeyong Sung

High-definition PIV Analysis on Vortex Shedding in the Cylinder Wake

A high-definition analysis based on flow topology is made on the vortex motions under natural and lock-on conditions in the near-wake region of a circular cylinder, where two-dimensional flow fields in the wake-transition regime are measured by a time-resolved PIV system. The Reynolds stress distributions are examined in view of the mean separation streamline and the trajectory ofthe vortex center. It is shown that, by the lock-on, the Reynolds stresses become stronger and their dispositions match well with the shortened wake bubble, indicating perfect synchronization of shedding to the oscillatory forcing flow in the near field, which causes increased lift and drag forces.

On the Characteristics of the Droplet Formation from an Inkjet Nozzle Driven by a Piezoelectric Actuator

Abstract. The present study has focused on the characteristics of droplet formation from an inkjet nozzledriven by a piezoelectric actuator. As an operating fluid, ethylene glycol was used and the physical prop-erties of it such as viscosity, surface tension, contact angle and shear stress were measured. During theexperiments, various temperatures and driving voltages are imposed on a capillary tube. These conditionsresult in a proper drive condition or an overdrive condition. In case of the proper drive condition, animage processing technique is applied to measure the diameter of a single free drop. As a result, the sizeof droplet is increased when the driving voltage is increased from 160 V to 190 V at 25 o C. In the over-drive condition where temperature or driving voltage becomes higher than the proper drive condition, sat-ellites and the misdirected jets happen. Key Words: Piezoelectric Actuation(피에조 구동), Capillary Tube(모세관), Inkjet(잉크젯), DropletFormation(액적 형성) 1. 서론 잉크젯(inkjet) 기술은 프린터 외에도 MEMS, 반도체 공정 등에서 폭넓게 사용되고 있으며, 특히 금속 잉크를 이용한 마이크로 회로 패터닝 및 전자 패키징 공정에서의 마이크로 범프(bump) 제작에서 중요한 역할을 한다. 전자 패키징 공정에서는 잉크젯을 이용하면공정을 획기적으로 줄일 수 있는 장점이 있으나 범퍼의 크기와 피치간격을 미세하게 제어하기 위해서는 많은 연구가 필요하다. 따라서 잉크젯 기술이 더욱 더 발전되어지기 위해서는 다양한 물질에 대응할 수 있는잉크젯 노즐의 설계와 제작에 대한 체계적인 접근이무엇보다 시급하다. 잉크젯 기술은 노즐의 구동방식에 따라 Fig. 1과 같이 continuous 방식과 drop-on-demand(DOD) 방식으로 나누어진다. Continuous 방식은 액체분사(liquid jet)를 형성하기 위해서 잉크가 펌프에 의해서 노즐까지이송되어지는 반면에 DOD 방식은 용기(reservoir)에있는 유체가 모세관 힘(capillary force)에 의해서 노즐로 이송되는 것이 주된 차이점이다. 이 중에서 DOD방식은 지금까지 열적(thermal) 구동 방식과 피에조(piezoelectric) 구동 방식을 주로 사용하여 왔다. 열적구동 방식에서는 노즐 안에 열을 가하면 기포(bubble)

Ventilation Performance According to Outdoor and Operating Conditions of the Vertical Exhaust Duct System in High Riser Public Houses

The ventilation performance of a vertical exhaust duct system in the high riser public house has been evaluated by a commercial software, Fluid Flow, which solves pressure losses through the duct system including bathroom fans and a hybrid roof fan. During the numerical simulations, outdoor wind condition and stack effects in summer and winter were considered as well as the operating conditions of a basement damper and the roof fan. The results show that the bathroom ventilation in summer is the most unsatisfactory. The opening of the basement damper has a problem that the polluted air in the lower floors is exhausted to the underground parking lot, not to the rooftop. If the basement damper is closed, the exhaust flow rate in the lower floors is not sufficient due to the strong flow resistance in the long vertical duct even though the roof fan is under operating.

Change of Vortex Dynamics in the Cylinder Wake by the Lock-on to Oscillatory Incident Flow

When vortex shedding is locked-on to a single frequency oscillatory flow, the variations of vortex dynamics are investigated using a time-resolved PIV system. Wake regions of recirculation and vortex formation, dynamic behavior of the shed vortices and the Reynolds stress fields are measured in the wake-transition regime at the Reynolds number 360. In the lock-on state, reduction of the wake region occurs and flow energy distributed downstream moves upstream being concentrated near the cylinder base. To observe the dynamic behavior of the shed vortices, the trajectory of the vortex center extended to the inside of the wake bubble is considered, which describes well the formation and evolution processes. The Reynolds stresses and their contributions to overall force balance on the wake bubble manifest the increase of the drag force by the lock-on.

Dynamics Response of a Micro Bubble under Temporal Pressure Variations

Abstract. The growth of micro bubble has been simulated under the variation of ambient pressure. The Rayleigh-Plesset equation governs the dynamic growth and collapse of a bubble according to pressure and temperature conditions. The Rayleigh-Plesset equation was solved by 4th-order Runge-Kutta method for wide range of pressure variations. As numerical parameters, the pressure difference between initial and final pressures, and the temporal pressure gradient are changed. The results show that the pressure difference has little effect on the growth rate of the micro bubble in the inertia controlled growth region. On the other hand, the growth rate increases linearly with the increase of the pressure gradient. Key Words: Micro Bubble(마이크로 기포), Pressure Variation(압력변화), Rayleigh-Plesset Equation(레일리 플리젯 방정식) 1. 서 론 2) 마이크로 기포는 의료기기 및 산업 현장에 적용 등 다양한 가능성과 무한한 응용성을 가지고 있다. 따라서 유체시스템에서 기포 크기와 체적을 자유자재로 제어하고 시스템을 유용하게 하기 위해서는 기포의 역학을 이해하는 것이 매우 중요하다. 기포에 대한 기초적인 연구로는, Rayleigh (1) 가 기포 경계를 통한 질량이동이 없고 표면장력과 점성력을 무시한다는 가정하에 단일 기포의 역학에 대한 방정식을 제안했다. 그 후, Plesset

Droplet Formation of a Piezoelectric Inkjet Nozzle According to the Variation of Pulse Widths in Bipolar Waveform

In this study, a piezoelectric inkjet nozzle with a rectangular shaped channel has been developed, and the characteristics of droplet formation have been investigated according to the variation of pulse widths in bipolar waveform. The channel of the nozzle was fabricated transparently by a precision machining technique. A tantalum membrane which was attached to a piezoelectric material covers the channel. By applying two types of bipolar waveforms to the piezoelectric actuators, droplet formation through the nozzle was monitored by a CCD camera. For the variety of the first and second pulse widths in the bipolar waveforms, the regimes of single and double droplet formations are presented. The change of droplet velocity which depends on the pulse width and the type of waveform is also discussed.

A Similarity of the Velocity Profiles According to Water Depth in Partially Filled Circular Pipe Flows

Contrary to the flow rate in fully filled pipe flows, the flow rate in partially filled pipe flows is significantly influenced by the variation of water level, channel slop, and so on. The major difference in these two flows results from the existence of a free surface. To make it clear, in the present study, a similarity of the velocity profile in a partially filled circular pipe has been investigated according to the water level. A particle image velocimetry (PIV) technique was applied to measure the three-dimensional velocity profiles. As a result, there is found a similarity of the velocity profile near the central region. However, near the side wall, the similarity is broken due to the interaction between the wall and the free surface.