• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3209-3214
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• 2007

This study numerically investigates the unsteady flow and acoustic characteristics of a flapping wing using a hydrodynamic/acoustic splitting method. The Reynolds number based on the maximum translation velocity of the wing is Re=8800 and Mach number is M=0.0485. The flow around the flapping wing is predicted by solving the two-dimensional incompressible Navier-Stokes equations (INS) and the acoustic field is calculated by the linearized perturbed compressible equations (LPCE), both solved in moving coordinates. Numerical results show that the hovering sound is largely generated by wing translation (transverse and tangential), which have different dipole sources with different mechanisms. As a distinctive feature of the flapping sound, it is also shown that the dominant frequency varies around the wing.

• 한국전산유체공학회지
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• 제13권2호
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• pp.8-13
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• 2008

A supersonic viscous flow over a five-degree half-angle cone is studied computationally with three-dimensional Navier-Stokes equations. Steady asymmetric solutions show that the asymmetric flow separation is caused by convective instability. The effects of angle of attacks, Reynolds numbers, and Mach numbers have been investigated and it is found that those factors affect the generation of the side force. The side force has the maximum value at ${\alpha}=22^{\circ}$, while over ${\alpha}=22^{\circ}$, asymmetric vortex becomes transient, which results in the unsteady shedding. At the angle of attack of 22 degrees, the side force increases with Reynolds number and decreases with Mach number. The increase of the side force stops over the critical Reynolds number for the present configuration.

• 한국추진공학회지
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• 제8권1호
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• pp.27-37
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• 2004

예조건화된 오일러 방정식의 수렴특성에 대한 연구를 수행하였다. 지배방정식의 거동을 이해하기 위하여 섭동 해석을 수행하였다. 중앙부에 10％ 원호를 가진 2차원 관을 통과하는 다양한 마하수의 비점성 유동장에 대해 수치 계산을 수행하였다. 공간차분은 Roe의 FDS를 사용하고 시간적분은 LU-SGS 기법을 사용하였다. 압력 및 속도의 수렴특성은 마하수와 상관없이 일정하게 유지되었으나, 온도의 수렴성은 마하수가 작아질수록 악화되는 것으로 나타났다. 섭동 해석을 통해 이러한 지배방정식의 수렴특성을 설명할 수 있었으며, 수렴특성이 예조건화 행렬의 거동 특성에 의해 결정된다는 사실을 알 수 있었다.

• 한국항공우주학회지
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• 제38권6호
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• pp.586-595
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• 2010

초음속에서 극초음속 영역까지 작동 가능한 이중램제트(이중연소/이중모드) 추진기관의 성능특성을 파악하고 설계인자를 도출하기 위하여 램제트/스크램제트 추진기관의 작동원리에 대한 물리적인 이해를 바탕으로 공기 및 열역학적 관점에서 각 추진기관의 성능특성에 대한 이론적인 연구를 수행하였다. 사이클 해석을 기반으로 하여 각 추진기관의 연소기 입구 마하수와 연소기 형상에 따른 성능특성 및 작동영역의 한계를 연구하였다. 이를 바탕으로 이중램제트의 작동천이 마하수를 파악하고 효율적인 작동영역분배특성을 살펴보았다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2005년도 제25회 추계학술대회논문집
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• pp.141-146
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• 2005

본 논문은 램제트/스크램제트 엔진에 대한 이론적인 해석 결과를 보여준다. 본 연구의 주요 목적은 램제트/스크램제트 엔진의 성능변수에 대하여 해석적 연구를 통해 물리적인 이해와 기초를 제시하는 것이다. 램제트/스크램제트 성능에 있어서 비행 마하수와 흡입구, 연소특성 등의 영향에 대하여 해석적 연구를 수행하였으며, 일정한 압력 또는 일정한 단면적을 가진 연소기에 대하여 사이클 해석을 통해 비교/분석하였다. 또한 연소기 입구에서의 최적 마하수에 대하여 연구하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.151-154
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• 2002

The twin impulse wave leads to very complicated flow fields, such as Mach stem, spherical waves, and vortex ring. The twin impulse wave discharged from the exits of the two tubes placed in parallel is investigated to understand detailed flow physics associated with the twin impulse wave, compared with those in a single impulse wave. In the current study, the merging phenomena and propagation characteristics of the impulse waves are investigated using a shock tube experiment and by numerical computations. The Harten-Yee's total variation diminishing (TVD) scheme is used to solve the unsteady, two-dimensional, compressible, Euler equations. The Mach number $M_{s}$, of incident shock wave is changed below 1.5 and the distance between two-parallel tubes, L/d, is changed from 1.2 to 4.0. In the shock tube experiment, the twin impulse waves are visualized by a Schlieren optical system for the purpose of validation of computational work. The results obtained show that on the symmetric axis between two parallel tubes, the peak pressure produced by the twin-impulse waves and its location strongly depend upon the distance between two parallel tubes, L/d and the incident shock Mach number, $M_{s}$. The predicted Schlieren images represent the measured twin-impulse wave with a good accuracy.

• 대한기계학회논문집B
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• 제31권9호
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• pp.814-821
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• 2007

A hybrid method is presented for efficient computation of turbulent flow noise at low Mach numbers. In this method, the turbulent flow field is computed by incompressible large eddy simulation (LES), while the acoustic field is computed with the linearized perturbed compressible equations (LPCE) derived in this study. Since LPCE is computed on the rather coarse acoustic grid with the flow variables and source term obtained by the incompressible LES, the computational efficiency of calculation is greatly enhanced. Furthermore, LPCE suppress the instability of perturbed vortical mode and therefore secure consistent and stable acoustic solutions. The proposed LES/LPCE hybrid method is applied to three low Mach number turbulent flow noise problems: i) circular cylinder, ii) isolated flat plate, and iii) interaction between cylinder wake and airfoil. The computed results are closely compared with the experimental measurements.

• 한국항공우주학회지
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• 제35권7호
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• pp.586-591
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• 2007

예조건화 오일러 방정식의 수렴특성에 미치는 계산 오차의 영향을 해석하였다. 다양한 마하수의 원형 턱이 있는 이차원 관내 유동을 계산하였다. 마하수가 감소함에 따라 차분오차는 증가하는데, 에너지 방정식의 계산 오차는 다른 방정식의 계산 오차보다 빠르게 증가하는 것으로 나타났다. 그리고 예조건 행렬의 역행렬을 곱하여 변형된 지배방정식 형태를 이용하면 계산 오차 문제를 완화할 수 있음을 보였다

• 대한기계학회논문집B
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• 제29권2호
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• pp.197-204
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• 2005

The under-expanded jet discharged from a nozzle or an orifice has been extensively employed in industrial applications and aerospace technologies. A number of studies have been made to investigate the under-expanded jet structures such as Mach disk, barrel shock wave, jet boundary configuration, etc. In the current study, a computational work is performed to investigate the effect of non-equilibrium condensation of moist air on the under-expanded jet, which is discharged from a sonic nozzle. The results obtained are compared with an available experimental data. It is found that non-equilibrium condensation of moist air alleviates the oscillations of the under-expanded jet, and can increase Mach disk diameter, without changing the location.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 1992년도 한국우주과학회보 제2권1호
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• pp.16-16
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• 1993

The Cosmic Mach number M is the ratio of the bulk flow velocity of the galaxrvelocity field on some scale R to the unall scale velocity dispersion within refcions of scale R. Because M is the ratio of two velocities, it is inn-dimansionat and the Here, independent of the amplitude of the power specHim and of the biasplnmeter in the linear theory. We have measured the Mach rnlmber for two observational samples: a spiral galaxy sample(AHM) of Aaronson and hiscoBlaborators with absolute distances measured by the infrared Ttillr-Fisher relatioa and an elliptical galaxy sample(EGALS) of Faber or 0, with distances determined by the relation. The effective depths distances of galaxies from the Local Group of these samples are 1639 km/s and 2862 e/s, respectivelr. The Machnumbers from these observed peculiar velocity Selds He fund as M=0.95 for AHMand M=0.59 for EGALS. We comPBre these calculated Mach numbers with thosefrom meck surweys drawn fuom three cosnulogical medels: the stand8rd biased nh=0.5 CDM modet an open CDM rrudel with gh=0.2, and a medd with thepower-law power specelm P(k)-k-1 and n=1. The Mach rnlmber test can give robust constraints on these cosmelogical nudels whose power spectra have very different shapes at large scales.