• Proceedings of the KSME Conference
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• 2001.11b
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• pp.312-317
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• 2001

Modifying effects of the rectangular forward step for suppressing the unsteady pressure fluctuation during interaction between the upstream vortical flow with the edge are studied numerically. The vortical flow is modeled by a point vortex, and the unsteady pressure coefficient is obtained from the velocity and the potential field. To investigate the effects of the edge shape the rectangular forward step is chamfered with various angles. Calculation show that the pressure peaks become decreased by increasing the vortex height as well as the chamfering angle. The pressure amplitudes are very sensitive to the change of the initial vortex height. From this study we can find out that the chamfered edge has two effects; the one is that it suppresses the pressure amplitude generated from the edge, and the other is that it decreases the time variation of unsteady pressure fluctuation. These modifying concepts can be applied to attenuate the self-sustained oscillation mechanism at the open cavity flow.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.7
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• pp.105-112
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• 2002

An experimental study of the vortical flow characteristics around a yawed delta wing with the leading edge extension at high incidence angle is undertaken by upper surface pressure measurements. A special emphasis has been put on analyzing the basic physics of vortical flows, concerning the effects of incidence and sideslip angle on the aerodynamic characteristics of the wing, especially under high angle of attack. The experimental data has been dearly demonstrated the beneficial effect of the LEX vortex on the wing vortex. It leads to an essential stabilization of the wing vortex against its breakdown until at much higher incidence angle under small sideslip. An interesting flow feature is occurrence of the rolling moment reversal at a certain range of angle of attack and sideslip angle.

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.294-298
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• 2003

The complex, unsteady, self-sustained pressure oscillations induced by supersonic flow past a rectangular cavity is investigated using numerical simulations. The present numerical study is performed using a parallel, multiblock solver for the two-dimensional, compressible Navier­Stokes equations. Open cavities with length-to-depth (L / D) ratio in the range 0.5 - 3.3 are considered. This paper sheds light on the cavity physics, cavity oscillatory mechanism, and the organisation of vortical structures inside the cavity. The vortex shedding phenomenon, the shear layer impingement event at the aft wall and the movement of the acoustic/compression wave within the cavity are well predicted. The vortical structures· and the source of the acoustic disturbances are found to be located near the aft wall of the cavity. With the increase in the cavity length, strong recompression of the flow near the aft wall leading to a sudden jump in the cavity form drag is observed. The estimated cavity tones are in good agreement with the available semi­empirical relation. Multiple peaks are noticed in deep and long cavities. For the present free­stream Mach number 1.71, it is observed that around L/D=2.0, the cavity oscillatory mechanism changes from the transverse to longitudinal oscillatory mode. The effects of this transition on various fluid dynamics and acoustic properties are also discussed.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.3
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• pp.233-237
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• 2011

In the present experiments, vortical structures behind the hydrofoil trailing edge are visualized and analyzed as an elementary study for propeller singing phenomena. Two sorts of hydrofoil are selected for the measurement of shedding vortices. One was KH45 hydrofoil section and the other is KH45 with the truncated trailing edge that is positioned at X/C = 0.9523(C=chord length). Assuming the Strouhal number of 0.23, the shedding frequencies of vortices are extracted by analyzing the boundary layer thickness and the flow speed. The frequency distribution of shedding vortices is obtained with the variation of angle-of-attack while the flow speed is fixed to 8m/s. The truncation of the trailing edge makes the frequency of shedding vortices about 120Hz lower than that of original trailing edge and makes the vorticity value higher than the original trailing edge.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.71-74
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• 2007

Large Eddy Simulation(LES) has been conducted to insight interaction effects of turbulent flow and chemical reaction of a lean-Premixed swirl combustor. The unsteady turbulent flame is carefully simulated so that the motion of flow and flame can be characterized in detail. Fuel lumps escaping from the primary combustion zone move downstream and consequently produce local hot spots conveying large vortical structures in the azimuthal direction. The correlation between pressure oscillation and unsteady heat release is examined by the spatial and temporal Rayleigh parameter.

• Journal of the Korea Institute of Military Science and Technology
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• v.2 no.2
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• pp.99-109
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• 1999

This paper is concerned with instability of flow fields which are dominated by the entropy mode with the presence of usual acoustic and vortical modes. These combined modes lead to nonlinear unstable waves which may occur in automobile, aircraft, or rocket engines. In this study instability in a side-burning rocket is investigated. It is shown that the energy growth rate parameters increase with an increase of the energy growth factor. The energy growth rate parameters for turbulent flows are larger than those for laminar flows. It is further shown that unstable wave motions for the high-temperature side-burning rocket are dictated mostly by the entropy mode, somewhat by the vortical mode, and least by the acoustic mode.

• International Journal of Aeronautical and Space Sciences
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• v.4 no.1
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• pp.63-74
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• 2003

The vortical flow of a 65-deg flat plate delta wing with a leading edge extension(LEX) was examined through off-surface visualization, 5-hole probe and hot-film measurements. The off-surface flow visualization technique used micro water droplets generated by a home-style ultrasonic humidifier and a laser beam sheet. The angles of attack ranged from 10 to 30 degrees, and the sideslip angles ranged from 0 to -15 degrees. The Reynolds number was $1.82{\times}10^5$ for the flow visualization, and $1.76{\times}10^6$ for the 5-hole probe and hot-film measurements. The comparison of the visualization photos and the flow field measurement showed that the two results were in a good agreement for the relative position and the structure of the wing and LEX vortices, even though the flow Reynolds numbers of the two results were much different. The wing vortex and the LEX vortex coil each other while maintaining a comparable strength and identity at zero sideslip. Neither a looping of the wing vortex around the strake vortex, nor the lopsided coiling of the stronger strake and the weaker wing vortices was observed. At non-zero sideslip, the downward movement of the LEX vortex when going downstream was enhanced on the windward side, and the downward and inboard movement of the LEX vortex when going downstream was suppressed on the leeward side. The counterclockwise coiling of the wing and LEX vortices was decreased significantly on the leeward side.

• The KSFM Journal of Fluid Machinery
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• v.13 no.4
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• pp.45-50
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• 2010

The goal of paper is to investigate the flow and scalar distribution through the HP Nozzle Guide Vane (NGV) passage. Flow and scalar distribution measurement are conducted by using 5-hole pressure probe and $CO_2$ tracing technique, respectively. Three different experimental cases are considered depending on cooling flow condition. The result shows that the vortical secondary flow patterns are observed clearly and these flow characteristics maintain through the NGV passage regardless of cooling flow injection. Compared to center region, the high axial velocity flow is observed near wall region due to cooling flow injection. Without cooling flow, the $CO_2$ (scalar) distribution becomes to be uniform quickly due to the strong flow mixing phenomenon. However, in cases of cooling flow, scalar distribution is significantly non-uniform.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.3.2-3.2
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• 2005

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.270-271
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• 2003