Flow and dispersion in an urban cubical cavity

Ryu, Y.H.,Baik, J.J. Pergamon Press ; Elsevier [distribution] 2009 Atmospheric environment Vol.43 No.10

Flow and dispersion in an urban cubical cavity are numerically investigated using a Reynolds-averaged Navier-Stokes equations (RANS) model with the renormalization group (RNG) k-@? turbulence closure model. The urban cubical cavity is surrounded by flank walls that are parallel to the streamwise direction, called end-walls, as well as upstream and downstream walls. A primary vortex and secondary vortices including end-wall vortices are formed in the cavity. Because of the end-wall drag effect, the averaged mean-flow kinetic energy in the cavity is smaller than that in an urban street canyon that is open in the along-canyon direction. A trajectory analysis shows that the end-wall vortices cause fluid particles to move in the spanwise direction, indicating that flow in the cavity is essentially three-dimensional. The iso-surfaces of the Okubo-Weiss criterion capture cavity vortices well. The pollutant concentration is high near the bottom of the upstream side in the case of continuous pollutant emission, whereas it is high near the center of the primary vortex in the case of instantaneous pollutant emission. To get some insight into the degree of pollutant escape from the cavity according to various meteorological factors, extensive numerical experiments with different ambient wind speeds and directions, inflow turbulence intensities, and cavity-bottom heating intensities are performed. For each experiment, we calculate the time constant, which is defined as the time taken for the pollutant concentration to decrease to e<SUP>-1</SUP> of its initial value. The time constant decreases substantially with increasing ambient wind speed, and tends to decrease with increasing inflow turbulence intensity and cavity-bottom heating intensity. The time constant increases as the ambient wind direction becomes oblique. High ambient wind speed is found to be the most crucial factor for ventilating the cavity, thus improving air quality in an urban cubical cavity.

에어컨 실내기의 공력소음 예측을 위한 RANS 난류모델의 성능 평가

민윤홍(Y.H. Min),강성원(S. Kang),허남건(N. Hur),이창훈(C. Lee),박정택(J. Park) 한국전산유체공학회 2012 한국전산유체공학회지 Vol.17 No.4

The objective of the present study is to investigate the effects of various turbulence models on the aerodynamic noise of an air-conditioner (AC) indoor unit. The results from URANS (unsteady Reynolds-averaged Navier-Stokes) simulations with the standard k-ε, k-ω shear stress transport (SST) and Spalart-Allmaras (S-A) turbulence models were analyzed and compared with the noise data from the experiments. The frequency spectra of the far-field acoustic pressure were computed using the Farrasat equation derived from the Ffowcs Williams-Hawkings (FW-H) equation based on the acoustic analogy model. Two fixed fan casings and the rotating cross-flow fan were used as the source surfaces of the dipole noise in the Farrasat equation. The result with the standard k-ε model showed a much better agreement with the experimental data compared to the k-w SST and S-A models. The differences in the pressure spectra from the different turbulence models were discussed based on the instantaneous vorticity fields. It was found that the over-estimated power spectra with the k-w SST and S-A models are related to the emphasized small-scale vortices produced with these models.

Analysis of the impact of flow characteristics on the separation efficiency and pressure drop of a cyclone-type oil separator

Guwon Seon,Joon Ahn,Wontae Hwang 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.1

The effects of flow structure on the separation efficiency and pressure drop in a cyclone-type oil separator were investigated via Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES), and the results were compared with experiments. Compared with the RANS simulations, the LES results were more similar to the experimental data as they simulated the complex flow structure more realistically. Swirling flow with strong turbulent kinetic energy (TKE) at the top section hindered the flow of particles toward the separator wall. In addition, a decrease in tangential velocity along the wall at the bottom reduced the centrifugal force, resulting in a decreased separation efficiency as particles were able to flow directly toward the outlet. The LES also predicted the pressure drop slightly better than the RANS simulation did, due to increased pressure drop caused by collision of the flow with the helix and outlet tube, which led to the formation of vortical flow structures with strong TKE.

CFD 경계층 파라미터 인공신경망을 이용한 익형 자체소음 예측

이재원,주병규,정용수 한국항공우주학회 2024 韓國航空宇宙學會誌 Vol.52 No.5

본 연구에서는 2차원 Reynolds-Averaged Navier-Stokes (RANS)를 통해 예측된 경계층 파라미터를 학습하여 인공신경망 기반의 경계층 파라미터 예측 모델을 개발하였다. 개발된 경계층 모델은 다양한 익형의 뒷전 경계층 및 배제 두께를 예측하며 실험식 기반의 익형 자체소음을 예측하였다. 이 경계층 모델을 이용하여 익형의 최대 두께 및 캠버의 크기에 따른 NACA 익형을 통해 익형 형상에 따른 자체소음 민감도를 분석하였다. 그 결과 받음각, 양력계수, 양항비에 따른 익형의 최대 두께 및 캠버의 경향성을 확인할 수 있었다. 모델을 UH-1B 제자리 비행 회전익 로터에 적용하여 팁 마하수, 블레이드의 수, solidity, 익형의 최대 두께 및 캠버 등 로터의 파라미터를 변화함에 따라 톤 소음 및 익형 자체소음을 측정하였고 그 경향성을 확인할 수 있었다. This study presents the development of an artificial neural network-based boundary layer parameter prediction model, trained using two-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations. The model accurately predicts boundary layer and displacement thicknesses on the trailing edge of diverse airfoil shapes, alongside estimating airfoil self-noise using empirical formulations. Employing this boundary layer model, the study analyzes the self-noise sensitivity of airfoil shapes, exploring variations in maximum thickness and camber across NACA airfoils. The findings revealed discernible trends in maximum thickness and camber of the airfoils with respect to angle of attack, lift coefficient, and lift-to-drag ratio. Furthermore, the model is extended to assess the UH-1B hovering rotor, predicting both tonal noise and airfoil self-noise across parameteric sweeps of tip Mach number, number of blades, rotor solidity, maximum thickness, and camber. The observed trends confirm the influence of these rotor parameters on tonal noise and self-noise levels.

하이드라진(N₂H₄) 아크젯 추력기의 수치적 모델링

신재렬(Jae-Ryul Shin),이대성(Dae-Sung Lee),오세종(Se-Jong Oh),최정열(J.-Y. Choi) 한국항공우주학회 2008 韓國航空宇宙學會誌 Vol.36 No.9

하이드라진(N₂H₄) 아크젯 추력기의 열화학 유동장 해석을 위한 전산유체해석을 수행하였다. 열복사와 전기장이 고려된 압축성 유동해석을 위해서 RANS 방정식을 수정하여 사용하였다. 로렌츠 힘과 Ohm 가열효과를 고려한 Maxwell 방정식이 유동방정식과 결합되어 전기 방전으로 인한 전기장해석을 위해 이용되었다. 아크젯 추력기 내부에서 유동장은 충분히 고온상태이기에 화학평형 해석이 이용되었으며, 광학 두께를 이용한 열복사 모델이 유동방정식에 적용되었다. 계산 결과들은 아크젯 추력기유동이 동결유동에 비해 추력은 180%증가되며, 비추력은 200%가까이 상승됨을 보여준다. 또한 유동장 해석 결과들은 아크젯 추력기 내부의 열적 물리적 특성에 대한 이해를 돕는다. The computational fluid dynamic analysis has been conducted for the thermo-chemical flow field in an arcjet thruster with mono-propellant Hydrazine (N₂H₄) as a working fluid. The Reynolds Averaged Navier-Stokes (RANS) equations are modified to analyze compressible flows with the thermal radiation and electric field. the Maxwell equation, which is loosely coupled with the fluid dynamic equations through the Ohm heating and Lorentz forces, is adopted to analyze the electric field induced by the electric arc. The chemical reactions of Hydrazine were assumed to be infinitely fast due to the high temperature field inside the arcjet thruster. The chemical and the thermal radiation models for the nitrogen-hydrogen mixture and optical thick media respectively, were incorporated with the fluid dynamic equations. The results show that performance indices of the arcjet thruster with 1㎾ arc heating are improved by amount of 180% in thrust and 200% in specific impulse more than frozen flow. In addition thermo-physical process inside the arcjet thruster is understood from the flow field results.

Free Surface Flow in a Trench Channel Using 3-D Finite Volume Method

Lee Kil-Seong,Park Ki-Doo,Oh Jin-Ho 한국수자원학회 2011 한국수자원학회논문집 Vol.44 No.6

In order to simulate a free surface flow in a trench channel, a three-dimensional incompressible unsteady Reynolds-averaged Navier-Stokes (RANS) equations are closed with the model. The artificial compressibility (AC) method is used. Because the pressure

레일리-버나드 대류에 대한 비정상 레이놀즈­평균 모사 기법의 분석

주다솔,유동현 한국전산유체공학회 2024 한국전산유체공학회지 Vol.29 No.2

This study investigates the capability of unsteady Reynolds-averaged Navier-Stokes(URANS) simulations in predicting turbulent Rayleigh-Bénard convection, with a particular focus on the prediction of convective cells and turbulent heat transfer. Previous researchers have posited that URANS can accurately simulate convective rolls, which in turn may enable reliable predictions of heat transfer in turbulent regimes. The analysis distinguishes between the effects modeled and resolved by URANS—the latter capturing the unsteady fluid motions’ contributions to heat transport. By simplifying the RANS model equations, the study estimates the turbulence diffusivities for momentum and heat, and derives scaling relationships for the Rayleigh, Prandtl, and Nusselt numbers that demonstrate the influence of large-scale convection cells. This approach qualitatively explains previous observations on the diminishing modeled effects on heat flux at higher Rayleigh numbers as observed in prior studies by Kenjereš and Hanjalić [2006, Int. J. Heat. Fluid. Fl., Vol. 27, No.5.]. Additionally, the study estimates the velocity scale of convective cells predicted by URANS, factoring in the modeled turbulent viscosity and thermal diffusivity alongside Rayleigh and Prandtl numbers. Findings suggest that the velocity scale, consistent across various turbulence models, can be approximated using the free-fall velocity concept, irrespective of the modeled turbulent viscosity.

Detached-Eddy Simulation of Vortex Breakdown Over Onera 70-Degree Delta Wing

Miso Son,Soo Hyung Park,Yung Hwan Byun 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.10

Unsteady simulations using Reynolds-Averaged Navier-Stokes (URANS) and Spalart-Allmaras delayed detached-eddy simulation (D-DES) were performed to investigate the vortex breakdown over the ONERA70 delta wing at an angle-of-attack of 27 degrees. A low-diffusive preconditioned Roe-scheme with third-order MUSCL interpolation is applied and the second-order dual-time stepping combined with the diagonalized alternating direction implicit method is used for the present unsteady simulations. Vortex breakdown was not captured by using RANS approaches, whereas DES results showed a good agreement with the experimental data. Vortex breakdown is investigated through total pressure loss, axial velocity and axial vorticity by tracing the center of the primary vortex.

다양한 부채꼴 핀휜 형상의 열성능 평가

문미애(Mi-Ae Moon),김광용(Kwang-Yong Kim) 대한기계학회 2014 大韓機械學會論文集B Vol.38 No.7

본 연구에서는 삼차원 RANS 방정식을 이용하여 냉각 유로 내에 부착하는 새로운 핀휜의 다양한 부채꼴 형상에 대해 열전달, 압력강하, 열성능을 평가하였다. 레이놀즈수가 5,000부터 100,000인 경우에 대하여 수치해석을 수행하였으며, 난류모델로는 Low-Re SST 모델을 사용하였다. 수치해석의 정당성을 확보하기 위하여 실험과 동일한 조건에서 면적 평균 누셀트수에 대한 실험값과 계산값을 비교하였다. 앞전 각도와 뒷전 각도를 매개변수로 하여 세 종류의 부채꼴 핀휜의 형상 변화에 따른 열성능을 평가하였다. The heat transfer, pressure loss, and thermal performance in a cooling channel were evaluated for various new fan-shaped pin-fin geometries using three-dimensional Reynolds-averaged Navier?Stokes equations. The turbulence was modeled using the low-Reynolds-number SST turbulence model in the Reynolds number range of 5,000?100,000. The numerical results for the area-averaged Nusselt numbers were validated by comparing them with the experimental data under the same conditions. A parametric study for three types of fan-shaped pin-fin geometries was performed with two parameters, namely, the leading and trailing reduction angles.

Free Surface Flow in a Trench Channel Using 3-D Finite Volume Method

이길성,박기두,오진호 한국수자원학회 2011 한국수자원학회논문집 Vol.44 No.6

In order to simulate a free surface flow in a trench channel, a three-dimensional incompressible unsteady Reynolds-averaged Navier-Stokes(RANS) equations are closed with the model. The artificial compressibility(AC) method is used. Because the pressure fields can be coupled directly with the velocity fields, the incompressible Navier-Stokes(INS) equations can be solved for the unknown variables such as velocity components and pressure. The governing equations are discretized in a conservation form using a second order accurate finite volume method on non-staggered grids. In order to prevent the oscillatory behavior of computed solutions known as odd-even decoupling, an artificial dissipation using the flux- difference splitting upwind scheme is applied. To enhance the efficiency and robustness of the numerical algorithm, the implicit method of the Beam and Warming method is employed. The treatment of the free surface, so-called interface-tracking method, is proposed using the free surface evolution equation and the kinematic free surface boundary conditions at the free surface instead of the dynamic free surface boundary condition. AC method in this paper can be applied only to the hydrodynamic pressure using the decomposition into hydrostatic pressure and hydrodynamic pressure components. In this study, the boundary-fitted grids are used and advanced each time the free surface moved. The accuracy of our RANS solver is compared with the laboratory experimental and numerical data for a fully turbulent shallow-water trench flow. The algorithm yields practically identical velocity profiles that are in good overall agreement with the laboratory experimental measurement for the turbulent flow.