• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.8
• /
• pp.839-847
• /
• 2012

Numerical simulations for channel flows with $Re_{\tau}$ = 180, 395 and 590 have been performed to investigate the hairpin packet formation process in wall-bounded turbulent flows. Using direct numerical simulation databases, the initial flow fields are given by the conditionally averaged flow field with the second quadrant event specified at the buffer layer. By tracking the initial vortex development, the change in the initial vortex to an ${\Omega}$-shaped vortex and th generation of a secondary hairpin vortex were found to occur with time scales based on the wall units. In addition, at the time when the initial vortex has grown to the channel center, the inclination angle of the hairpin vortex packet is approximately $12{\sim}14^{\circ}$, which is similar for all three Reynolds numbers. Finally, numerical simulations of the evolution of two adjacent hairpin vortices have been performed to examine the interaction between the adjacent vortex packets.

• 한국연소학회:학술대회논문집
• /
• 2001.06a
• /
• pp.17-22
• /
• 2001

The flow induced by a circular jet vertically impinging under a horizontal plate is investigated by visualization technique, using kerosene smoke in nitrogen gas to visualize the flow in the vortex as well as under the plate. The light source was the sheet beam of Ar-Ion laser. The vertical and 3-dimensional images of vortices were recorded by the digital video camera.

• 한국전산유체공학회:학술대회논문집
• /
• 1999.05a
• /
• pp.79-84
• /
• 1999

The shock wave diffracted behind a finite wedge is partially scattered after interacting with a starting vortex originated from the sharp vertex of the wedge. The shock is divided into the accelerated and decelerated shocks. The decelerated shock then interacts with the small vortexlets brought about by the vortex instability, producing weak compression waves. The shock-shock interaction produces Mach stems. Through this successive process, the shock attenuated. In this study, these complicated shock phenomena are computed using Euler equations and compared with experimental results obtained by the authors.

• Journal of Mechanical Science and Technology
• /
• v.15 no.5
• /
• pp.665-670
• /
• 2001

The interaction of a travelling shock with the shear layer of a flat plate is studied computationally. The Euler and Navier-Stokes equations are solved numerically on quadrilateral unstructured adaptive grids. The flat plate is installed horizontally on the central axis of a shock tube. The shear layer is first created by two shock waves at different speeds splitted by a flat plate. A series of small vortices is developed as a consequence in the shear layer. The shock wave reflected at the end wall impinges the shear layer. The complicated shock dynamics in the evolution to the pseudo-steady state is represented with the morphological transformation of a planar shock into an oblique shock.

• Journal of Advanced Marine Engineering and Technology
• /
• v.22 no.3
• /
• pp.371-380
• /
• 1998

The flow patterns and dynamic properties of ship's propulsion mechanism of two-stage Weis-Fogh type are studied by the discrete vortex method. In order to study the effects of the interaction of the two wings two cases of the phase differences of the wing's motion are considered the same phase and the reverse phase. The flow patterns by simulations correspond to the photographs obtained by flow visualization and flowfield of the propulsion mechanism which is unsteady and complex is clearly visualized by numerical simulations. The time histories of the thrust an the drag coefficients on the wings are also calculated and the effects of the interaction of the two wings are numerically clarified.

• 한국연소학회:학술대회논문집
• /
• 1999.10a
• /
• pp.47-54
• /
• 1999

A method is developed to include the effect of volume expansion in the description of the flame dynamics using G-equation. Line volume-source is used to represent the effect of the exothermic process of combustion with source strength assigned by the density difference between the burned and the unburned region. The present model provides good agreement with the experimental results by using realistic volume expansion ratio which was not reached in the previous researches. Including volume expansion, the flow predicts the same behavior of measured velocity field qualitatively. The flame propagation in varying flow field due to volume expansion provides a promising way to represent the wrinkled turbulent premixed flames in a numerically efficient manner.

• 한국전산유체공학회:학술대회논문집
• /
• 1998.11a
• /
• pp.204-210
• /
• 1998

A method is developed to include the effect of volume expansion in the description of the flame dynamics using G-equation. Line volume-source is used to represent the effect of the exothermic process of combustion with source strength determined by the density difference between the burned and the unburned region. Volume expansion adjusts the flow field to accommodate the increased volume flow rate crossing the flame front. Test result predicted the measured velocity field qualitatively. The method was applied to study the interaction of vortex and premixed flame. Increased volume expansion did not change the initial growth rate of flame area. However, the residence time and flame surface area increased with higher expansion ratios.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.22 no.12
• /
• pp.1669-1680
• /
• 1998

A method is developed to include the effect of volume expansion in the description of the flame dynamics using G-equation. Line volume-source is used to represent the effect of the exothermic process of combustion with source strength assigned by the density difference between the burned and the unburned region. The present model provides good agreement with the experimental results. Including volume expansion, the flow field is adjusted to accommodate the increased volume flow rate which crossing the flame front and the result predicts the same behavior of measured velocity field qualitatively. The effect of increasing volume expansion does not change the initial growth rate of flame area but increase the residence time. Consequently this effect increases the maximum area of flame front. The flame propagation in varying flow field due to volume expansion provides a promising way to represent the wrinkled turbulent premixed flames in a numerically efficient manner.

• Journal of Biomedical Engineering Research
• /
• v.21 no.5
• /
• pp.505-512
• /
• 2000

기계식 인공심장판막을 통한 혈액의 유동과 이 유동에 관련된 판첨의 거동특성을 수치해석기법을 이용하여 연구하였다. 혈액은 맥동류, 층류, 비압축성 유동으로 가정하였으며 유체-고체의 상호작용을 고려하기 위하여 혈액의 유동방정식과 고체의 운동방정식이 동시에 계산되었다. 심실과 대동맥에서의 압력파형을 경계조건으로 사용하였다. 연구의 결과로서 혈액유동과 판첨의 거동이 예측되었으며, 판막을 통한 3개의 제트가 발견되었으며 vortex가 판첨의 끝단에서 발생하여 하부로 흘러가는 것이 관찰되었다. 판첨의 닫힘 거동은 열림 거동에 비하여 2배정도 빠르게 진행되었으며 sinus에서 2개의 큰 vortex가 관찰되었다. 유체-고체 상호작용을 고려하는 본 연구방법은 향후 판막의 연구와 개발에 매우 유용할 것으로 판단된다.

• Aerospace Engineering and Technology
• /
• v.8 no.1
• /
• pp.197-206
• /
• 2009

To improve the aerodynamic efficiency of Smart Unmanned Aerial Vehicle (SUAV), vortex generators and flow fence are applied on the surface and the tip of wing. The initially applied vortex generator increased maximum lift coefficient and delayed the stall angle while it produced excessive increase in drag coefficient. It turns out reduction of the airplane's the lift/drag ratio. The new vortex generators with L-shape and two different height, 3mm and 5mm, were used to TR-S4 configuration to maintain the desired level of maximum lift coefficient and drag coefficient. Flow fence was also applied at the end of both wing tip to reduce the interaction between nacelle and wing when nacelle tilting angles are large enough and produce flow separation. To examine the effect of flow fence, flow visualization and force and moment measurements were done. The variation of the aerodynamic characteristics of SUAV after applying flow control devices are summarized.