Numerical Investigation on Flow Characteristics of Forest Fire Smoke under 3D Atmospheric Dispersion Modeling

Riyandwita B. W.,Myung-whan Bae,Kichang Im 한국자동차공학회 2008 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-

Smoke from a forest fire is creating a variety of environmental problems in the atmosphere. Numerical simulation for a smoke dispersion in large-scale atmospheric is performed using a spectral finite difference method. An incompressible viscous fluid with a buoyancy effect in a three-dimensional domain is assumed. The fire phenomenon as the source of a heat flux and a smoke inflow is taken in the center of the analytical domain. Three-dimensional Navier-Stokes equations with the Boussinesq assumption, the continuity equation, the energy equation, and the diffusion equation are used as the governing equations. Unknown variables are expanded using the Fourier series in the periodicity directions (x and z axes) to obtain good computation results and the finite difference method is used in the direction normal to the ground (y axis) for the fast computation speed. The Navier-Stokes equations, the Poisson equation for pressure, the energy equation, and the diffusion equation are discretized and solved directly. The continuity equation is served as a conditional equation to move the next time step. This method is based on a direct numerical simulation (DNS). A low value of the Reynolds number with the natural convection condition is investigated to test the nonlinear term of the Navier-Stokes equation. It is found that the nonlinear term of the Navier-Stokes equations start to affect the flow pattern at the Reynolds number of 2. The results also show that a dimensionless velocity increases with a Reynolds number, while a dimensionless temperature, a dimensionless concentration, and a dimensionless pressure stan to change after the Reynolds number of 2.

Direct numerical simulations on the migration of particles in tube flow

Choeng Ryul Choi(최청렬),Chang Nyung Kim(김창녕),Yong Jun An(안용준) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

The cross-stream migration of a single neutrally buoyant rigid particle in tube flow is simulated by a direct numerical simulation. Numerical experiments using the method of constrained simulation generate data for the lift force and velocities of a freely rotating sphere in steady flows arising from initial-value problems in which the sphere is constrained to move at a fixed radius. The simulations give results in good agreement with previous results. Constrained simulations are very efficient. The lift and all velocities are obtained for different radii at each specified Reynolds number. The equilibrium position (the Segre?Silberberg radius) moves towards the wall as Re increases at each fixed radius.

Simulation and Experiment on Direct Continuous Casting Process of Lead Frame Copper Alloy

Huang Guojie,Xie Shuisheng,Cheng Lei 한국소성가공학회 2010 기타자료 Vol.2010 No.6

Direct Continuous Casting (D.C.C) is an important method in casting lead frame copper alloy. In this paper, numerical simulation is adopted to investigate the casting process in order to optimize the D.C.C technical parameters, such as the casting temperature, casting speed and cooling intensity. According to the numerical results, the reasonable parameters are that the casting temperature is between 1413K~1413K, the casting speed is between 8m/h~10m/h and the speed of cooling water is between 4.2m/s~4.6m/s. And the depth of liquid-solid boundary is measured in different casting temperature and casting speed by experiments. The results show the actual measurements have a little deviation with the numerical simulation. The results of numerical simulation provide the significant reference to the actual experiments.

난류 파이프 유동 내 응집 구조의 횡 방향 성장

안준선(Junsun Ahn),이진영(Jinyoung Lee),황진율(Jinyul Hwang) 한국가시화정보학회 2021 한국가시화정보학회지 Vol.19 No.2

The spanwise growth of turbulence structures in turbulent pipe flow was investigated using the direct numerical simulation data of Re<SUB>τ</SUB> = 544, 934 and 3008. Two-point correlations and pre-multiplied energy spectra of streamwise velocity fluctuations were examined along the spanwise direction. The arclength direction is defined as rθ, which is useful for an analogy with the spanwise direction for channels or boundary layers; here, r and θ are the radial distance from the core and the azimuthal angles, respectively. Both analyses showed that the arclength scales increased with increasing the wall-normal distance. It showed that the coherent structures were confined in the core region due to the crowding effect of a circular pipe geometry. The pipe flow simulation could describe a realistic geometrical flow along the azimuthal direction, unlike the simulations of turbulent channel or boundary layer flow using periodic boundary conditions along the spanwise direction. The present results provided the spanwise organization of energy-containing motions over a broad range of scales in turbulent pipe flow.

직접 수치 모사법을 이용한 섬유 강화 복합재료의 열팽창계수 예측

남윤식(Youn-Sic Nam),오민환(Min Hwan Oh),김광식(Kwang Sik Kim),조진연(Jin Yeon Cho) 한국항공우주학회 2007 韓國航空宇宙學會誌 Vol.35 No.9

본 논문에서는 직접 수치 모사 기법을 이용해 섬유 강화 복합재료의 열팽창계수를 예측하였다. 직접 수치 모사 기법을 통해 구한 열팽창계수 예측치와 실험치의 비교를 통해 본 논문에서 제안한 직접 수치 모사 기법을 이용하면 인위적인 가정을 최소화하면서 기존의 방법과 유사하게 복합재료 열팽창계수를 예측할 수 있음을 확인하였다. 또한 섬유 체적비 변화에 따른 열팽창계수의 변화를 예측하고 그 경향성을 고찰하였다. In this paper, thermal expansion coefficients of fiber-reinforced composite materials are predicted by direct numerical simulation. From comparing the predicted results with experimental results, it is confirmed that direct numerical simulation gives similar results to the previously proposed methods while minimizing artificial assumptions. Additionally trend of variation in thermal expansion coefficients is investigated according to the fiber volume fraction.

Ignition characteristics of a temporally evolving <i>n</i>-heptane jet in an <i>iso</i>-octane/air stream under RCCI combustion-relevant conditions

Yu, Gwang Hyeon,Luong, Minh Bau,Chung, Suk Ho,Yoo, Chun Sang Elsevier 2019 Combustion and flame Vol.208 No.-

<P><B>Abstract</B></P> <P>The ignition characteristics of a temporally-evolving <I>n</I>-heptane jet in an <I>iso</I>-octane/air stream under reactivity controlled compression ignition (RCCI) combustion-relevant conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species primary reference fuel (PRF)/air reduced mechanism. For the DNSs of RCCI combustion, <I>iso</I>-octane and <I>n</I>-heptane are chosen as two different fuels delivered by the port-fuel and direct-fuel injections, respectively. Therefore, the ignition characteristics of both fuels can be investigated by simulating the ignition of a temporally-evolving <I>n</I>-heptane jet with relative jet velocity, <I>U</I> <SUB>0</SUB>, within <I>iso</I>-octane/air charge. It is found that the first-stage ignition kernels governed by the low-temperature chemistry first develop primarily within the <I>n</I>-heptane jet near the mixing layer regardless of <I>U</I> <SUB>0</SUB>, and evolve into low-temperature flames, propagating into relatively fuel-rich mixtures in the <I>n</I>-heptane jet. High-temperature flames also develop first in the <I>n</I>-heptane jet, following the trajectories of low-temperature flames, and then, propagate towards both relatively fuel-lean mixtures of the <I>iso</I>-octane/air charge and fuel-rich mixtures of the <I>n</I>-heptane jet. They keep propagating into fuel-lean mixtures and finally end-gas auto-ignition occurs. It is also found that the first-stage ignition occurs more quickly with increasing <I>U</I> <SUB>0</SUB> due to enhanced mixing between relatively cold <I>n</I>-heptane jet and hot <I>iso</I>-octane/air charge, and consequently, the second-stage ignition also advances in time with increasing <I>U</I> <SUB>0</SUB>, which are opposite to the results found in previous DNSs of RCCI combustion. Such ignition characteristics are more likely to prolong the overall combustion duration and reduce the peak of heat release rate with increasing <I>U</I> <SUB>0</SUB>. In addition, chemical explosive mode analysis (CEMA) identifies important variables and reactions for the low-, intermediate-, and high-temperature chemistries under such RCCI conditions.</P>

Direct-Numerical Simulation with the Stability Theory for Turbulent Transition in Hypersonic Boundary Layer

배하준,임지섭,김민우,지솔근 한국항공우주학회 2023 International Journal of Aeronautical and Space Sc Vol.24 No.4

Laminar-to-turbulent transition in hypersonic boundary layer is numerically investigated using the direct-numerical simulation (DNS) method combined with the linear stability theory (LST). The DNS-LST framework is validated first for 2D hypersonic boundary layer. The growth of the Mack second mode is matched well to previous DNS data. A complete 3D turbulent transition at Mach 6 is computed in the current DNS to demonstrate the capability of the current method for a whole 3D turbulent transition scenario. Two modes are assigned at the DNS inlet for the fundamental breakdown in the hypersonic boundary layer: the Mack second mode (the fundamental mode) and the pair of oblique waves of the fundamental frequency. These instability modes are obtained from the stability analysis. The current DNS successfully resolves the 3D turbulent transition in the hypersonic boundary layer. Computational data are investigated to identify major flow features associated with the fundamental breakdown phenomena. Major instability modes are analyzed in the late transient stage.

하이브리드 로켓의 저주파불안정성에 미치는 당량비 영향 직접수치해석

최효상,이창진,강상훈 한국추진공학회 2019 한국추진공학회지 Vol.23 No.2

To understand the low frequency instability(LFI) characteristics in hybrid rockets combustion, effects of equivalence ratio variations on the phase shift between pressure and heat release oscillations were investigated by using the direct numerical simulation. The change in the equivalence ratio of the main chamber was simulated by the temperature and composition variation of the combustion gas introduced into the post-combustion chamber. In the results, additional combustion appeared along with vortex generation at the backward step, and combustion pressure and heat release oscillations were observed as the vortex moved. In addition, the results confirmed that the phase difference between the pressure and heat release oscillation shifts because of the changes in the propagation velocity of pressure wave as the temperature of combustion gas changes. 하이브리드로켓의 저주파수 연소불안정(LFI) 특성을 이해하기 위해, 주연소실의 연소 당량비 변화가 500 Hz대역의 압력 및 열방출 진동의 위상변화에 미치는 영향에 대해 직접수치해석을 수행하였다. 주연소실의 당량비 변화는 후연소실로 유입되는 연소가스의 온도 및 조성 변화로 모사하였다. 수치해석 결과, 후향 계단 하류에 와류 생성과 함께 추가적인 연소가 나타나며, 와류가 이동함에 따라 연소 압력 및 반응률의 진동이 관찰되었다. 또한, 유입유동의 온도가 변화하면 압력파의 전파속도도 함께 변화하므로 압력 및 반응률 진동 사이의 위상차가 천이하게 됨을 확인하였다.

회전하는 원형단면 실린더 주위의 난류유동 물질전달에 대한 직접수치모사

황종연(Jong-Yeon Hwang),양경수(Kyung-Soo Yang) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5

Characteristics of turbulent flow and mass transfer around a rotating circular cylinder are investigated by direct numerical simulation. Mass-transfer results are presented at a high Schmidt number (Sc=1,670). The concentration field is computed for three different case of Reynolds number, Re*R=161, 348, 623, based on cylinder radius and friction velocity. Results confirm that the thickness of Nernst diffusion layer is very small compared with that of viscous sub-layer in the case of high Sc mass transfer. A strong correlation of the concentration field with streamwise and vertical velocity components is noticed. However, that is not the case with the spanwise velocity component. The correlation between Sherwood number and Reynolds number is consistent with other experiments currently available.

A-mode 영역에서 교란유동장에 놓인 원형실린더 후류의 유동공진 현상

김수현(Soo Hyeon Kim),배중헌(Joong Hun Bae),유정열(Jung Yul Yoo) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

Lock-on phenomenon in the wake of a circular cylinder is investigated at the Reynolds number of 220 (Amode instability regime) using direct numerical simulation. For lock-on, the sinusoidal perturbation, whose frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. In order to analyze the vortex structure, the power spectra of the streamwise velocity signal and orthogonal enstrophies are investigated. It is observed that the three-dimensional vortex structure behind the circular cylinder turns into two-dimensional one due to the perturbed velocity. And this change causes the jump of the Strouhal number from the stable regime (A-mode instability regime) to the unstable regime (extrapolated from twodimensional shedding regime) in the Strouhal-Reynolds number relationship. As a result, the vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.