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  • Title/Summary/Keyword: RELATIVE MOMENTUM

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Mixing Performance of Unlike Doublet Impinging Liquid Jets (이중 충돌제트의 혼합 성능 연구)

  • Jo, Yong Ho;Lee, Seong Ung;Yun, Ung Seop
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.31 no.4
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    • pp.82-91
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    • 2003
  • Experiments to investigate the mixing performance of unlike doublet impinging jets are conducted. Reynolds number of simulants used in this study rages from 1.0 to 1.5 Cold flow test is performed to investigate the hydrodynamic effect and spray of the impinging jets are collected locally and calculated by using Rupe's mixing efficiency equation. Momentum exchanges and relative velocity ratio between two jets are taken as the main parameter to represent the effect of enlargement of the orifice diameter. As diameter ratio increases, the corresponding momentum ratio where maximum mixing efficiency occurs and relative velocity at the maximum mixing efficiency ranges 0.6 to 0.7, respectively. Penetration depth can be taken as a prominent parameter to estimate the mixing efficiency.

Uncertainty Analysis of the Eddy-Covariance Turbulent Fluxes Measured over a Heterogeneous Urban Area: A Coordinate Tilt Impact (비균질 도시 지표에서 측정된 에디 공분산 난류 플럭스의 불확실성 분석: 좌표계 편향 영향)

  • Lee, Doo-Il;Lee, Jae-Hyeong;Lee, Sang-Hyun
    • Atmosphere
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    • v.26 no.3
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    • pp.473-482
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    • 2016
  • An accurate determination of turbulent fluxes over an urban area is a challenging task due to its morphological diversity and associated flow complexity. In this study, an eddy covariance (EC) method is applied over a highly heterogeneous urban area in a small city (Gongju), South Korea to investigate the quantitative influence of 'coordinate tilt' in determining the turbulent fluxes of sensible heat, latent heat, momentum, and carbon dioxide mass. Two widely-used coordinate transform methods are adopted and applied to eight directional sections centered on the site to analyze a 1-year period EC measurement obtained from the urban site: double rotation (DR) and planar fit (PF) transform. The results show that mean streamline planes determined by the PF method are distinguished from the sections, representing morphological heterogeneity of the site. The sectional pitch angles determined by the DR method also compare well with those in the PF method. Both the PF and DR methods show large variabilities in the determined streamline planes at each directional section, implying that flow patterns may form in a complicate way due to the surface heterogeneity. Resulting relative differences of the turbulent fluxes, defined by (FDRFPF)/FDR, are found on average +13% in sensible heat flux, +21% in latent heat flux, +37% in momentum flux, and +26% in carbon dioxide mass flux, which are larger values than those reported previously for fairly homogeneous natural sites. The fractional differences depend significantly on wind direction, showing larger differences in northerly winds at the measurement site. It is also found that the relative fractional differences are negatively correlated with the mean wind speed at both stable/unstable atmospheric conditions. These results imply that EC turbulent fluxes determined over heterogeneous urban areas should be carefully interpreted with considering the uncertainty due to 'coordinate tilt' effect in their applications.

Turbulent Fluid Flow and Heat Transfer in Concentric Annuli with Square-Ribbed Surface Roughness (사각돌출형 표면거칠기가 있는 이중동심원관 내의 난류유동과 열전달)

  • 안수환;이윤표;김경천
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.5
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    • pp.1294-1303
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    • 1993
  • The fully developed turbulent momentum and heat transfer induced by the square-ribed roughness elements on the inner wall surface in concentric annuli is studied analytically based on a modified turbulence model. The analytical results of the fluid flow is verified by experiment. The resulting momentum and heat transfer are discussed in terms of various parameters, such as the radius ratio, the relative roughness, the roughness density, fluid Reynolds number and for heat transfer, fluid Prandtl number. The study demonstrates that certain artificial roughness elements may be used to enhance heat transfer rates with advantages from the overall efficiency point of view.

Experimental Study of Spray Characteristics of Liquid jet in Cross-flow (횡단류를 이용한 액체제트의 분무 및 분열 특성 실험)

  • Ko Jung-Bin;Lee Kwan-Hyung;Moon Hee-Jang;Koo Ja-Ye
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • v.y2005m4
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    • pp.155-158
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    • 2005
  • The spray characteristics of liquid jet minted in subsonic cross-flow were investigated numerically and experimentally. The behaviors of column, penetration and breakup of plain liquid jet in non-swirling cross-flow of air have been studied. Numerical and physical models are based on a modified KIVAII code. The primary atomization is represented by a wave model based on the KH(Kelvin-Helmholtz) instability that is generated by a high interface relative velocity between the liquid and gas flows. CCD camera has been utilized in oder to capture the spray trajectory. The nozzle diameter was 0.5 mm and its L/D ratios were between 1 and 5. Numerical and experimental results indicate that the breakup point is delayed by increasing gas momentum ratio, the penetration decreases by increasing Weber number and the turbulent or nonturbulent liquid jet is obtained at different L/D ratio.

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A numerical study on the vaporization of a droplet considering internal circulating flow in the presence of an oscillating flow (진동하는 유동장하에서 내부 순환 유동을 고려한 액적의 증발에 관한 수치적 연구)

  • Ha, Man-Yeong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.5
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    • pp.1700-1716
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    • 1996
  • The two-dimensional, unsteady, laminar conservation equations for mass, momentum, energy and species transport in the gas phase and mass, momentum and energy in the liquid phase are solved simultaneously in spherical coordinates in order to study heating and vaporization of a droplet entrained in the oscillating flow. The numerical solution gives the velocity and temperature distribution in both gas and liquid phase as a function of time. When the gas flow oscillates around an vaporizing droplet, the liquid flow circulates in the clockwise or counterclockwise direction and the temperature distribution in the liquid phase changes its shapes, depending on the gas fow direction. When the gas flow changes its direction of circulating liquid flow is opposite to the gas flow, forming two vortex circulating in the opposite direction. During the heating period, the difference in the maximum and minimum temperature is large, followed by the almost uniform temperature slightly below the boiling temperature. The mass and heat transfer from the droplet depend on the droplet temperature, droplet diameter and the magnitude of relative velocity, giving the droplet lifetime different from the d2-law.

Calculation Method and Influence Factor for Speed Change of a Vehicle Impacting Small Sign Post (소형지주에 충돌하는 차량의 속도변화 산정방법과 영향인자)

  • Ko, Man-Gi;Kim, Kee-Dong;Jun, Sung-Min;Sung, Jung-Gon
    • Journal of the Korean Society of Hazard Mitigation
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    • v.8 no.4
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    • pp.47-52
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    • 2008
  • Important factor in designing a breakaway sign support is the velocity change of the impact vehicle. It is measured from the crash test or can be calculated by 3-D Finite Element Analysis. It can also be calculated with relative ease utilizing energy and momentum conservation. In this paper a formula to calculate the velocity change of a car during the time of impact against a small sign is derived utilizing the energy and momentum balance. Using the formula, parametric studies were conducted to find that impact speed, separation force and Breakaway Fracture Energy(BFE) of the posts which represent the degree of fixedness to the foundation are the important factor to vehicle's speed change. It is shown that speed change is larger in the lower speed impact and to the posts with large separation force and BFE.

Aero-acoustic Performance Analysis Method of Regenerative Blower (재생형 송풍기의 공력음향학적 성능 해석 방법)

  • Lee, Chan;Kil, Hyun Gwon;Kim, Gang Chun;Kim, Jun Gon;Ma, Jae Hyun;Chung, Kyung Ho
    • The KSFM Journal of Fluid Machinery
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    • v.16 no.2
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    • pp.15-20
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    • 2013
  • An aero-acoustic performance analysis method of regenerative blower is developed as one of the FANDAS codes. The aerodynamic performance of regenerative blower is predicted by using momentum exchange theory coupled with pressure loss and leakage flow models. Based on the performance prediction results, the noise level and spectrum of regenerative blower are predicted by discrete frequency and broadband noise models. The combination of the performance and the noise prediction methods gives aero-acoustic performance map and noise spectrum analysis results, which are well-agreed with the actual measurement results within a few percent relative error.

Experimental Study of High-Altitude Simulation using Small-Scale Supersonic Diffuser (소형 초음속 디퓨저를 이용한 고고도환경 모사에 대한 시험적 연구)

  • Lee Ji-Hyung;Oh Jong-Yun;Byun Jong-Ryul
    • Journal of the Korea Institute of Military Science and Technology
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    • v.7 no.4 s.19
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    • pp.138-145
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    • 2004
  • Experimental study was conducted on cylindrical supersonic diffuser in order to investigate the effects of the ratios of diffuser area to nozzle throat area (Ad/At), diffuser area to nozzle exit area (Ad/Ae), nozzle exit area to its throat area (Ae/At), and diffuser length to its diameter (L/D), the free volume of vacuum chamber, and the relative distance between nozzle exit and diffuser inlet on the diffuser performance. The study showed that the minimum diffuser starting pressure (Po/Pa)st increased monotonically with increase in (Ad/At) as predicted by the normal shock and momentum theory models and the volume of vacuum chamber affected vacuum pressure level during diffuser operation at lower value of (Ad/Ae). The results of this investigation will be utilized in the design of real-scale high-altitude simulation test facility.

Reduction of Grid Size Dependency in DME Spray Modeling with Gas-jet Model (가스 제트 모델을 이용한 DME 분무 해석의 격자 의존성 저감)

  • Oh, Yun-Jung;Kim, Sa-Yop;Lee, Chang-Sik;Park, Sung-Wook
    • Journal of ILASS-Korea
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    • v.15 no.4
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    • pp.170-176
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    • 2010
  • This paper describes the grid-size dependency of the conventional Eulerian-Lagrangian method to spray characteristics such as spray penetration and SMD in modeling DME sprays. In addition, the reduction of the grid-size dependency of the present Gas-jet model was investigated. The calculations were performed using the KIVA code and the calculated results were compared to those of experimental result. The results showed that the conventional Eulerian-Laglangian model predicts shorter spray penetration for large cell because of inaccurate calculation of momentum exchange between liquid and gas phase. However, it was shown that the gas-jet model reduced grid-size dependency to spray penetration by calculating relative velocity between liquid and ambient gas based on gas jet velocity.

Determination of the number of 235U target nuclei in the irregular target using a fission time projection chamber

  • Jiajun Zhang;Jun Xiao;Junjie Sun;Mingzhi Zhang;Taiping Peng;Pu Zheng
    • Nuclear Engineering and Technology
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    • v.56 no.2
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    • pp.444-450
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    • 2024
  • Based on multiple measurements of ionization loss, the Time Projection Chamber (TPC) combines strong tracking ability with particle identification ability in a large momentum range, which is an important advantage of TPC detection technology over traditional ionization measurement technology. According to these two characteristics of TPC, applying it to the measurement of fission cross-section can greatly improve the measurement accuracy. During the measurement of the fission cross-section, the number of target nuclei is required to be accurately measured. So this paper introduces a method for measuring the number of 235U target nuclei using a fission TPC system. The measurement result agrees with the reference value, and relative error is around 1 %.