• International Journal of Aeronautical and Space Sciences
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• v.5 no.1
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• pp.1-5
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• 2004

The discharge performance of an impinging-type injector for cavitating flow has been evaluated. The predicted discharge coefficient for cavitating flow agree s well with the measured data showing less than 2% discrepancy. For the case of non- cavitating flow analysis, the disagreement between CFD resu lts and the experimental data is 8%. The discharge coefficient for the cavitating flow decreases with decrea se in the Reynolds number. On the other hand, it increases slightly as the Reynolds number increases for the non-cavitating flow because of the reduced viscous effect. The incipience of cavitation is predicted to occur around the cavitation number of 1.3 for fixed Reynolds number flow. In this environment, the discharge performance is proportional to the cavitation number for cavitating flow while it is independent to the cavitation number for non-cavitating flow regime.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.6
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• pp.860-869
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• 2000

Numerical and experimental investigations are performed for the molecular transition and slip flows in pumping channels of a disk-type drag pump. The flow occurring in the pumping channel develops from the molecular transition to the slip flow traveling downstream. Two different numerical methods are used in this analysis: the first one is a continuum approach in solving the Navier-Stokes equations with slip boundary conditions, and the second one is a stochastic approach through the use of the direct simulation Monte Carlo method. In the experimental study, the inlet pressures are measured for various outlet pressures in the range of 0.1{\sim}4Torr. From the present study, the numerical results of predicting the performance, obtained by both methods, agree well with the experimental data for the range of Knudsen number $Kn{\leq}0.1$ (i.e., the slip flow regime). But the results from the second method only agree with the experimental data for Kn>0.1(i.e., the molecular transition regime)

• The KSFM Journal of Fluid Machinery
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• v.3 no.3 s.8
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• pp.12-18
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• 2000

In this study, experimental and numerical analyses are carried out to investigate the performance of centrifugal pump with various air admitting conditions. Experiments on the pump performance under air-water two-phase flow are accomplished using a centrifugal pump with semi-open type impeller having three, five and seven blades, respectively. Also, the numerical analysis of turbulent air-water two-phase flow using the finite volume method has been carried out to obtain the pressure, velocities and void fraction on the basis of a so-called bubbly flow model with the constant size and shape of cavity. The results obtained through this study show the reasonable agreements within the range of bubbly flow regime. There are promising developments concerning application of the present study for the flow in a centrifugal pump with two-phase flow conditions and efforts must be followed to improve the turbulence model and two-phase flow model for turbomachinery.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.4
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• pp.375-384
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• 1984

본 연구에서는 종횡비가 낮은 직사각형 밀폐용기 내에 Rayleigh수가 충분히 커서 흐름의 운동력이 용기 양단에 존재하는 경우에 PartⅠ에서 개발된 해석적인 모델 을 근거한 scaling analysis를 통해 그 내부 흐름 형태를 정성적으로 예측, 기존의 결 과와 비교, 검토하였다. 해석결과, Prandtl수에 따라 여러가지 내부 흐름 형태가 존 재할 수 있음이 밝혀졌으며 용기 내 뚜렷한 경계층 흐름이 존재하기 위한 필요조건도 아울러 얻어졌다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.3
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• pp.274-287
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• 1984

밀폐용기 내의 자연대류에 의한 흐름현상에 고유하며 아직까지 해결되지 않고 있는 중요한 문제 는 그 내부흐름의 형태를 주어진 물리적인 조건으로부터 미리 예측할 수 없다는 것이다. 이는 이 문제의 해석적인 해결에 가장 큰 난점이 되고 있다. 본 논문에서는 multiple scales method를 이 용한 해석적인 모델을 개발하여 종횡비가 낮은 직각형 밀폐용기 내에서 운동력이 코어에 존재하 는 경우 그 내부 흐름형태를 scaling analysis를 통해 정성적으로 예측, 기존의 결과와 비교 검토 하였다.

• Korea-Australia Rheology Journal
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• v.19 no.1
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• pp.35-42
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• 2007

The effects of particle size distribution on the magnetorheological response of inverse ferrofluids was investigated using controlled mixtures of two monodisperse non-magnetisable powders of sizes $4.6\;{\mu}m\;and\;80{\mu}m$ at constant volume fraction of 30%, subjected to large amplitude oscillatory shear flow. In the linear viscoelastic regime (pre-yield region), it was found that the storage and loss moduli were dependent on the particle size as well as the proportion of small particles, with the highest storage modulus occurring for the monodisperse small particles. In the nonlinear regime (post yield region), Fourier analysis was used to compare the behaviour of the $1^{st}\;and\;3^{rd}$ harmonics ($I_{1}\;and\;I_{3}\;respectively$) as well as the fundamental phase angle as functions of the applied strain amplitude. The ratio of $I_{3}/I_{1}$ was found to become more pronounced with decreasing particle size as well as with increasing proportion of small particles in the bidisperse mixtures. Furthermore, the phase angle was able to clearly show the transition from solid-like to viscous behaviour. The results suggested that the nonlinear response of a bidisperse IFF is dependent on particle size as well as the proportion of small particles in the system.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.12
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• pp.1196-1204
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• 2001

The rivulet is a narrow stream of liquid flowing down a solid surface. When the rivulet\`s flow rate exceeds a certain limit, it tends to meander exhibiting the instability of its interface. This analysis performs a perturbation analysis of this meandering rivulet assuming an inviscid flow possessing contact angle hysteresis at the contact line. The analysis reveals that the contact angle hysteresis as well as the velocity difference across the inter-face, strongly induces the instability of the liquid interface. Moreover, when the rivulet veto-city is low, it is predicted that the axisymmetric disturbance amplifies more rapidly than the anti-axisymmetric disturbance, which explains the emergence of the droplet flow at the low velocity regime.

• 한국전산유체공학회:학술대회논문집
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• 1998.05a
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• pp.138-144
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• 1998

The BGK equation, which is the kinetic model equation of Boltzmann equation, is solved using FDDO(finite difference with the discrete-ordinate method) to compute the rarefied flow of monatomic gas. Using reduced velocity distribution and discrete ordinate method, the scalar equation is transformed into a system of hyperbolic equations. High resolution ENO(Essentially Non-Oscillatory) scheme based on Harten-Yee's MFA(Modified Flux Approach) method with Strang-type explicit time integration is applied to solve the system equations. The calculated results are well compared with the experimental density field of NACA0012 airfoil, validating the developed computer code. Next. the computed results of circular cylinder flow for various Knudsen numbers are compared with the DSMC(Direct Simulation Monte Carlo) results by Vogenitz et al. The present scheme is found to be useful and efficient far the analysis of two-dimensional rarefied gas flows, especially in the transitional flow regime, when compared with the DSMC method.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.4 s.247
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• pp.298-305
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• 2006

The flow around a circular cylinder which oscillates rotationally with a relatively high forcing frequency has been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), oscillation amplitude $({\theta}_A)$, and frequency ratio $F_R=f_f/f_n$, where $f_f$ is the forcing frequency and $f_n$ is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14{\times}10^3,\;{\theta}_A={\pi}/6$, and $0{\leq}F_R{\leq}2$. The effect of frequency ratio $F_R$ on the flow structure of wake was evaluated by measuring wake velocity profile and spectral analysis of hot-wire signal. Depending on the frequency ratio $F_R$, the cylinder wake has 5 different flow regimes. The vortex formation length and vortex shedding frequency are changed significantly before and after the lock-on regime. The drag coefficient was reduced under the condition of $F_R<1.0$ and the maximum drag reduction is about 33% at $F_R=0.8$. However, the drag is increased as $F_R$ increases beyond $F_R=1.0$. This active flow control method can be effective in aerodynamic applications, if the forcing parameters are selected optimally.

• Journal of Korea Water Resources Association
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• v.46 no.12
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• pp.1209-1220
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• 2013

Roughness coefficients calibrated by flow modeling using the 1-dimensional numerical model were analyzed for the downstream section of Naesung Stream in this study. Also, the bedform configuration at the Hyangseok Station was predicted for measured and simulated hydraulic conditions of flows and total flow roughness was estimated with the coefficient of grain roughness. The Manning's n coefficients calibrated by numerical modeling and estimated by considering of grain and bedform roughness were compared and examined. As a result, the Manning's n by numerical modeling was greater than the coefficient range estimated by grain and bedform roughness at the low flow regime due to the other factors such as vegetation, sinuosity, and sand bar. However, the Manning's n by numerical modeling was included in the coefficient range by grain and bedform roughness at the transition and high flow regime over $500m^3/s$ of flow discharge.