• Nuclear Engineering and Technology
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• 제47권5호
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• pp.513-522
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• 2015

The present paper aims at improving the modeling of turbulence for the upward turbulent bubbly flow through the use of experimental databases that contain data on small and large vertical ducts. First, the role of bubble-induced turbulence was analyzed, which indicated the dominant role of the bubble-induced turbulence in the duct center for relatively high void fraction cases. Therefore, the turbulence therein was mainly focused on, which indicated that the stronger turbulence could be induced by bubbles in large ducts with similar void fractions as compared to that in small ducts. Next, the turbulence of upward turbulent bubbly flow near the wall is discussed to understand the interaction between the wall-induced and bubble-induced turbulence. It showed that the existence of a wall could suppress the bubble-induced turbulence given the same void fraction, and the existence of bubbles could also suppress the solely wall-induced turbulence as compared to the single-phase turbulent flow, even though the total turbulence is enhanced. The above characteristics indicated that the current turbulence modeling method needs to be modified, especially when the bubble-induced turbulence plays a dominant role.

• 대한기계학회논문집B
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• 제30권9호
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• pp.891-897
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• 2006

The SRS method is applied to a turbulent flame with radiation/turbulence interaction to invert the temperature and concentration profile. The flame is conditioned as optically thin per each fluctuation length and the flame spectral intensity is measured for inversion. From inversion result, we find that SRS can successfully invert the coupled temperature/concentration fluctuation amplitudes. For two cases of experiments, inverted values are within approximately 1% over the full range of fluctuation amplitude. However, SRS cannot find the detailed local fluctuation parameters such as pattern and phase, etc. as far as they do not affect the resulting radiation intensity. Important available parameters are the mean temperature and the temperature fluctuation amplitude. The radiation/turbulence interaction effect is verified to play an important role in the radiation.

• 한국전산유체공학회지
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• 제18권2호
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• pp.56-65
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• 2013

Two compression ramp problems and an impinging shock problem are computed to investigate influence of turbulence models and eddy viscosity on the shock-wave / boundary layer interaction. A Navier-Stokes boundary layer generation code was applied to the generation of inflow boundary conditions. Computational results are validated well with the experimental data and effects of turbulence models are investigated. It is shown that the behavior of turbulence (eddy) viscosity directly affects both the extent of the separation and shock-wave positions over the separation.

• 한국추진공학회지
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• 제15권4호
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• pp.57-64
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• 2011

펄스 데토네이션 엔진에서와 같이 탄화수소를 연료로 하는 데토네이션 파는 강한 불안정성을 가지며 난류 연소 효과를 고려한 연구를 수행하여야 함이 제시된 바 있다. 본 연구에서는 강한 불안정성을 가지는 데토네이션 파의 구조를 이해하기 위하여 비점성 해석, 점성 해석, 난류 모델 및 간단한 난류 연소 모델을 고려한 수치 해석 연구를 수행하였다. 모델링 수준에 따른 연구를 통하여 점성 및 난류는 저주파 특성에는 거의 영향이 없으나, 고주파 특성을 강화하는 경향이 있는 것으로 보인다. 한편, 데토네이션 연구를 위한 난류-연소 상호 작용 모델에는 활성화 에너지의 영향이 고려되어야 하는 것으로 여겨진다.

• Wind and Structures
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• 제35권2호
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• pp.131-146
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• 2022

Tornadic wind flow is inherently turbulent. A turbulent wind flow is characterized by fluctuation of the velocity in the flow field with time, and it is a dynamic process that consists of eddy formation, eddy transportation, and eddy dissipation due to viscosity. Properly modeling turbulence significantly increases the accuracy of numerical simulations. The lack of a clear and detailed comparison between turbulence models used in tornadic wind flows and their effects on tornado induced pressure demonstrates a significant research gap. To bridge this research gap, in this study, two representative turbulence modeling approaches are applied in simulating real-world tornadoes to investigate how the selection of turbulence models affects the simulated tornadic wind flow and the induced pressure on structural surface. To be specific, LES with Smagorinsky-Lilly Subgrid and k-ω are chosen to simulate the 3D full-scale tornado and the tornado-structure interaction with a building present in the computational domain. To investigate the influence of turbulence modeling, comparisons are made of velocity field and pressure field of the simulated wind field and of the pressure distribution on building surface between the cases with different turbulence modeling.

• 대한기계학회논문집B
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• 제23권10호
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• pp.1229-1239
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• 1999

A computational study on unsteady compressible flows has been performed by adopting a low Reynolds number $k-{\omega}$ turbulence model in conjunction with dual time stepping scheme. An explicit four-stage Runge-Kutta scheme for the Navier-Stokes equations and an approximate factorization scheme for the $k-{\omega}$ turbulence model equations are used. Computational results obtained for blade surface pressure distributions in the process of rotor-stator interaction in a turbine stage are in good agreement with extant experimental data. The effects of the wake from the stator on the boundary-layer transition over the rotor blade surface are discussed by showing that high intensity turbulence of the stator wake induces an early transition.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2013년도 춘계학술대회 논문집
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• pp.353-358
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• 2013

Present paper deals with turbulence-airfoil interaction noise and mainly investigates the effects of airfoil thickness on the broadband noise spectrum. The acoustic power radiation from an airfoil is predicted using high-order time-domain method, which is based on the computational aeroacoustic technique solving the linear Euler equations. The homogeneous and isotropic turbulence is generated by utilizing the synthetic turbulence modeling based on random particle method. The airfoils taken into consideration are a flat-plate and a NACA0012 airfoil aligned with uniform mean flow. The effects of airfoil thickness on the radiated inflow turbulence noise are investigated by comparing acoustic power spectrum predicted for each airfoil. The comparison of acoustic power spectrum reveals that the airfoil thickness significantly contributes the high frequency noise reduction.

• 한국소음진동공학회논문집
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• 제16권3호
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• pp.263-269
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• 2006

The objective of the present work is to develop a time-domain numerical method of broadband noise in a cascade of airfoils. This paper focuses on dipolar broadband noise sources, resulting from the interaction of turbulent inflows with the flat-plate airfoil cascade. The turbulence response of a two-dimensional cascade is studied by solving both of the linearised and the full nonlinear Euler equations employing accurate higher order spatial differencing, time stepping techniques and non-reflecting inflow/outflow boundary condition. The time-domain result using the linearised Euler equations shows good agreement with the analytical solution using the modified LINSUB code. Through the comparison of the nonlinear time-domain result using the full nonlinear Euler equations with the linear, it is found that the acoustic mode amplitude of the nonlinear response is less than that of the linear response due to the energy cascade from low frequency components to the high frequency ones. Considering the merits of the time-domain methods over the typical time-linearised frequency-domain analysis, the current method is expected to be promising tools for analyzing the effects of the airfoil shapes, non-uniform background flow, linear-nonliear regimes on the broadband noise due to turbulence-cascade interaction.

• 한국해안해양공학회지
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• 제8권1호
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• pp.28-36
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• 1996

조류와 취송류의 비선형 상호작용에 대한 연직확산계수의 영향을 점모형을 이용하여 살펴보았다. q$^2$-q$^2$1 난류모형을 난류운동에너지의 생성과 감쇄가 균형을 이룬다는 가정 하에 단순화한 0-방정식 난류모형을 도입하고 마찰수심의 영향을 적절히 반영하도록 수정하여 연직확산계수를 산정하였다. 0-방정식 난류모형의 유도과정과 전향력 항이 포함됨으로써 공진이 발생될 수 있음을 제시하였다. 왕복성 수면경사력과 바람응력이 복합된 경우, 고려된 바람응력에 의한 조류 진폭의 변화는 무시할만한 크기였으나, 왕복성 수면경사력만에 의해서는 발생되지 않았던 2배조의 조류 성분이 파생되었다. 취송류의 연직구조는 조류에 의한 배경난류가 지배적인지의 여부에 따라 상당한 차이를 보였다. 따라서 황해와 같이 강한 조류가 존재하는 해역의 취송순환을 파악함에 있어 조류에 의한 배경난류를 무시할 경우는 실제와는 상당히 다른 결과가 나타날 수 있다.

• Wind and Structures
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• 제29권1호
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• pp.55-64
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• 2019

The effect of vortex impingement on the fluid dynamics around a cylinder submerged in the wake of another of different diameters is numerically investigated at a Reynolds number Re = 200. While the diameter (D) of the downstream cylinder is fixed, impinging vortices are produced from the upstream cylinder diameter (d) varied as d/D = 0.24, 0.4, 0.6, 0.8 and 1.0, with a spacing ratio L=5.5d, where L is the distance between the center of the upstream cylinder to the front stagnation point of the downstream cylinder. Two-dimensional simulations are carried out using the finite volume method. Fluid forces acting on the two cylinders are correlated with impinging vortices, vortex shedding, and wake structure. Different facets of wake formation, wake structure, and flow separation and their connections to fluid forces are discussed.