• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.7
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• pp.905-914
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• 2001

A large eddy simulation(LES) is performed for turbulent flow around a bluff body inside a sudden expansion cylinder chamber, a configuration which resembles a premixed gas turbine combustor. To promote turbulent mixing and to accommodate flame stability, a flame holder is installed inside the combustion chamber. The Smagorinsky model is employed and the calculated Reynolds number is 5,000 based on the bulk velocity and the diameter of the inlet pipe. The simulation code is constructed by using a general coordinate system based on the physical contravariant velocity components. The predicted turbulent statistics are evaluated by comparing them with the laser-doppler velocimetry (LDV) measurement data. The agreement of LES with the experimental data is shown to be satisfactory. Emphasis is placed on the time-dependent evolutions of turbulent vortical structure behind the flame holder. The numerical flow visualizations depict the behavior of large-scale vortices. The turbulent mixing process behind the flame holder is analyzed by visualizing the sectional views of vortical structure.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.472-478
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• 2000

An experimental study is performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer, The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about $Re_{\theta}=1700$. The effects of local forcing are scrutinized by altering the forcing frequency $(0.011{\leq}f^+{\leq}0.044)$. The forcing amplitude is fixed at $A_0=0.4$. It is found that a small local forcing reduces the skin friction, and this reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the coherent structures. Velocity signals are decomposed into a periodic part and a fluctuating part. An organized spanwise vortical structure is generated by the local forcing. The larger reduction of skin friction for the higher forcing frequencies is attributed to the diminished adverse effect of the secondary vortex. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures.

• Journal of the Korean Society of Combustion
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• v.11 no.4
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• pp.27-35
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• 2006

The impacts of equivalence ratio on flow structure and flame dynamic in a model gas turbine combustor are investigated using large eddy simulation(LES). Dynamic k-equation model and G-equation flamelet model are employed as LES subgrid model for flow and combustion, respectively. As a result of mean flow field for each equivalence ratio, the increase of equivalence ratio brings about the decrease of swirl intensity through the modification of thermal effect and viscosity, although the same swirl intensity is imposed at inlet. The changes of vortical structure and turbulent intensity etc. near flame surface are occurred consequently. That is, the decrease of equivalence ratio can leads to the increase of heat release fluctuation by the more increased turbulent intensity and fluctuation of recirculation flow. In addition, the effect of inner vortex generated from vortex breakdown on the heat release fluctuation is increased gradually with the decrease of equivalence ratio. Finally, it can be identified that the variations of vortical structure play an important role in combustion instability, even though the small change of equivalence ratio is occurred.

• 한국연소학회:학술대회논문집
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• 2006.10a
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• pp.242-250
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• 2006

The impacts of equivalence ratio on the flow structure and flame dynamics in a model gas turbine combustor are investigated using large eddy simulation(LES). Dynamic k-equation model and G-equation flamelet model are employed as LES subgrid model for flow and combustion, respectively. As a result of mean flow field for each equivalence ratio, the increase of equivalence ratio brings about the decrease of swirl intensity through the modification of thermal effect and viscosity, although the same swirl intensity is imposed at inlet. The changes of vortical structure and turbulent intensity etc. near flame surface are occurred consequently. That is, the decrease of equivalence ratio can leads to the increase of heat release fluctuation by the more increased turbulent intensity and fluctuation of recirculation flow. In addition, the effect of inner vortex generated from vortex breakdown on the heat release fluctuation is increased gradually with the decrease of equivalence ratio. Finally, it can be identified that the variations of vortical structure play an important role in combustion instability, even though the small change of equivalence ratio is occurred.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.12
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• pp.1750-1757
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• 2003

This paper describes the flow measurements inside the blade passage of an axial flow fan by using a rotating hot-wire probe sensor from a relative flame of reference fixed to the rotor blades. The validity of fan rotor designed by a streamline curvature equation was performed by the measurement of the three-dimensional flow upstream and downstream of the fan rotor using a 5-hole pitot tube. The vortical flow structure near the rotor tip can be clearly observed by the measurements of a relative velocity and its fluctuation on quasi-orthogonal planes to a tip leakage vortex. Larger vortical flow, which results in higher blockage in the main flow, is formed according to decrease a flow rate. The vortical flow spreads out to the 30 percent span from the rotor tip at near stall condition. In the design operating condition, the tip leakage vortex is moved downstream while the center of the vortex keeps constant in the spanwise direction. Detailed characteristics of a velocity fluctuation with relation to the vortex were also analyzed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.6
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• pp.672-681
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• 2004

Direct numerical simulation was carried out to study the vortical structures of the flow around a wall-mounted cube in a channel at Re=1,000 and Re=3,500 based on cubic height and bulk mean velocity. The cubic obstacle is situated in the entrance region of the channel flow where the boundary layers are developing. Upstream of the obstacle, steady and unsteady laminar horseshoe vortex systems are observed at Re=1,000 and Re=3,500, respectively; the near-wake flow is turbulent in both cases. The flow separates at each leading sharp edge of the cube, and subsequent vortex roll-up is noticed in the corresponding free-shear layer. The vortex shedding from the upper leading edge (upper vortices) and that from the two lateral leading edges (lateral vortices) are both quasi-periodic and their frequencies are computed. The upper and lateral vortices further develop into hairpin and Λ vortices, respectively. A series of instantaneous contours of the second invariant of velocity gradient tensor helps us identify spatial and temporal behaviors of the vortices in detail. The results indicate that the length and time scales of the vortical structures at Re=3,500 are much shorter than those at Re:1,000. Correlations between the upper and lateral vortices are also reported.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.12
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• pp.1861-1868
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• 2001

Vortical structures in the near-wake region of a cubic obstacle are studied using numerical simulation. We consider flow between two parallel plates with a cube mounted on one of the plates. In the turbulent near-wake region of the flow, coherent structures such as hairpin vortices are found. Quasi-periodic behavior of the hairpin vortices is noticed; its dimensionless frequency at Re=1,000 is about 0.82 which is consistent with the result of Elavarasan of et al.'s experiment [Fluid Dyn. Research, 2000, 27] although their geometry is somewhat different from on. In the case of Re=3,500, the dimensionless frequency of the hairpin vortex is about 1.60. It is observed that the translating speed of the head of the hairpin vortex is lower than the streamwise mean velocity at that location. In the vicinity of the lower plate downstream of the cube, various length scales are identified thus less coherent. However, it is noticed that the vortical structures become gradually elongated downstream of the new reattachment.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.284-287
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• 2008

Flow over a sphere under forced oscillation at Re=300 is simulated for various frequency ratios which are defined as excitation frequency over natural frequency of stationary sphere. The results of oscillating sphere are compared with those of stationary sphere and an oscillating cylinder. Detailed vortical structures, hydrodynamic forces and frequencies of the wake are prescribed as a function of frequency ratio. For oscillating sphere, planar symmetry of the wake is kept and two nearly symmetric hair pin vortices are induced by oscillation for one period of oscillation when the frequency ratio is bigger than 0.5. Modulation phenomenon which can be found in an oscillating cylinder were not seen for an oscillating sphere.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.845-850
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• 2003

Unsteady nature of a tip leakage vortex in an axial flow fan operating at a design and off-design operating conditions has been investigated by measuring the velocity fluctuation in a blade passage with a rotating hotwire probe sensor. Two hot-wire probe sensors rotating with the fan rotor were also introduced to obtain the cross-correlation coefficient between the two sensors located in the vortical flow as well as the fluctuating velocity. The results show that the vortical flow structure near the rotor tip can be clearly observed at the quasi-orthogonal planes to a tip leakage vortex. The leakage vortex is enlarged as the flow rate is decreased, thus resulting in the high blockage to main flow. The spectral peaks due to the fluctuating velocity near the rotor tip are mainly observed in the reverse flow region at higher flow rates than the peak pressure operating condition. However, no peak frequency presents near the rotor tip for near stall condition.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.223-230
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• 2005

The unsteady nature of vortex structures has been investigated by a large eddy simulation (LES) in an axial flow fan with a shroud covering only the rear region of its rotor tip. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the fan. The movements of the vortex structures induce high-pressure fluctuations on the rotor blade and in the blade passage. Frequency characteristics of the fluctuating pressure on the rotor blade are analyzed using wavelet transform. The dominant frequency of the real-time pressure selected at the high pressure fluctuation region corresponds well to that of the fluctuating rotor torque and the experimental result of fan noise. It is mainly generated due to the unsteady behavior of the vortical flow, such as the tip vortex and the leading edge separation vortex.