液體 噴流의 安定 曲線에서의 臨界 레이놀즈 수의 豫測

황용하,임성빈 忠州大學校 2003 한국교통대학교 논문집 Vol.38 No.4

The first maximum point in the stability curve of liquid jet, ie, the critical point is associated with the critical Reynolds number. This critical Reynolds number should be predicted by simple means. In this work, the critical Reynolds number in the stability curve of liquid jet are predicted using the empirical correlations and the experimental data reported in the literatures. The critical Reynolds number was found to be a function of the Ohnesorge number, nozzle length-to-diameter ratio, ambient Weber number and nozzle inlet type. An empirical correlation for the critical Reynolds number as a function of the Ohnesorge number and nozzle length-to-diameter ratio is newly proposed here. Although an empirical correlation proposed in this work may not be universal because of excluiding the effects of ambient pressure and nozle inlet type, it has resonably agrees with the measured critical Reynolds number.

An experimental study on fully developed laminar flow and heat transfer in rectangular microchannels

Elsevier 2016 The International journal of heat and fluid flow Vol.62 No.2

<P><B>Abstract</B></P> <P>An empirical study has been performed to explore the validity of theoretical correlations based on conventional sized channels for predicting fluid flow and heat transfer characteristics in microchannels. The flow resistance and thermal behavior of laminar flow through 10 different rectangular microchannels with hydraulic diameters of 155–580 µm and aspect ratios of 0.25–3.8 at Reynolds numbers ranging from 30 to 2500 were investigated. The single-phase laminar friction factors in the microchannels conformed to the conventional Poiseuille flow theory. The critical Reynolds number increased from 1700 to 2400 with a decrease in the aspect ratio from 1.0 to 0.25. The existence of two heat transfer regimes was verified. At <I>Re</I>< 180, the experimental Nusselt numbers were lower than the theoretical values and increased rapidly with the Reynolds number. The Nusselt numbers began to exceed the theoretical values for <I>Re</I>> 180; however, its rate of increase with the Reynolds number slowed considerably. In this region, the theoretical value of the Nusselt number was assessed to be reasonable as an estimate for rectangular microchannels when the aspect ratio was greater than 1.0.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The laminar fluid flow and heat transfer characteristics in rectangular microchannels were investigated. </LI> <LI> The single-phase laminar friction factors in the microchannels conformed to the conventional Poiseuille flow theory. </LI> <LI> The critical Reynolds number increased from 1700 to 2400 with a decrease in the aspect ratio from 1.0 to 0.25. </LI> <LI> The existence of two heat transfer regimes in the rectangular microchannels was verified. </LI> <LI> For <I>Re</I> > 180, the theoretical value of the Nusselt number was assessed to be reasonable as an estimate for rectangular microchannels when the aspect ratio was greater than 1.0. </LI> </UL> </P>

Effect of diffuser angle on the discharge coefficient of miniature critical nozzles

김재형,김희동 대한기계학회 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.9

Many studies on critical nozzles have been made to accurately measure the mass flow rate of gas and standardize its performance as a flow meter. Recently, much interest has been given to measuring very small mass flow rates in industrial fields, such as MEMS applications. However, the design and performance data of the critical nozzles obtained thus far have been applied mainly to critical nozzles with comparatively large diameters, and available studies on miniature critical nozzles are lacking. In this study, computational fluid dynamics (CFD) method was applied to investigate the influence of the diffuser angle on the discharge coefficient of miniature critical nozzles. In computations, the throat diameter of a critical nozzle varied from 0.2 to 5.0 mm, and the diffuser angle changed from 2° to 8°. The computational results were validated with some available experimental data. The present computational results accurately predicted the discharge coefficient of gas flows through miniature critical nozzles. The discharge coefficient is considerably influenced by the diffuser angle as the throat diameter of the nozzle becomes smaller below a certain value. This implies that miniature critical nozzles should be designed with careful consideration of its effects.

Spatial flow structure around a smooth circular cylinder in the critical Reynolds number regime under cross-flow condition

Raeesi, Arash,Cheng, Shaohong,Ting, David S.K. Techno-Press 2008 Wind and Structures, An International Journal (WAS Vol.11 No.3

The spanwise flow structure around a rigid smooth circular cylinder model in cross-flow has been investigated based on the experimental data obtained from a series of wind tunnel tests. Surface pressures were collected at five spanwise locations along the cylinder over a Reynolds number range of $1.14{\times}15^5$ to $5.85{\times}10^5$, which covered sub-critical, single-bubble and two-bubble regimes in the critical range. Separation angles were deduced from curve fitted to the surface pressure data. In addition, spanwise correlations and power spectra analyses were employed to study the spatial structure of flow. Results at different spanwise locations show that the transition into single-bubble and two-bubble regimes could occur at marginally different Reynolds numbers which expresses the presence of overlap regions in between the single-bubble regime and its former and later regimes. This indicates the existence of three-dimensional flow around the circular cylinder in cross-flow, which is also supported by the observed cell-like surface pressure patterns. Relatively strong spanwise correlation of the flow characteristics is observed before each transition within the critical regime, or formation of first and second separation-bubbles. It is also noted that these organized flow structures might lead to greater overall aerodynamic forces on a circular cylinder in cross-flow within the critical Reynolds number regime.

CFD에 의한 2차원 밀폐캐비티의 임계레이놀즈수에 관한 연구

김진구(Jin-Goo Kim),조대환(Dae-Hwan Cho),이영호(Young-Ho Lee) 한국해양공학회 1997 韓國海洋工學會誌 Vol.11 No.4

Flow characteristics of two-dimensional closed square cavities near unsteady critical Reynolds numbers were studied numerically at five Reynolds numbers : 8 × 10³, 8.5 × 10³, 9 × 10³, 9.5 × 10³ and 10⁴ were investigated.<br/> A convection conservative difference scheme based upon SOLA to maintain the nearly 2nd-order spatial accuracy was adopted on irregular grid formation. Irregular grid number is 80 × 80 and its minimum size is about 1/400 of the cavity height(H) and its maximum is about 1/53 H. The result shows that the critical Reynolds number indicating the emergence of flow unsteadiness exists near Re=8.5×10³ and their flow patterns reveal periodic fluctuation during transient and fully-developed stages.

실린더 내부 유동에서 반경비 및 각속도의 변화에 따른 임계수에 관한 수치해석

배강열(Kang-Youl Bae),김형범(Hyoung-Bum Kim) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11

This paper represents the numerical analysis on critical Taylor number according to the variation of radius ratio and angular velocity on flow in cylinder. The numerical model consisted of two cylinder which inner cylinder is rotating and outer cylinder is fix, and the axial direction is used the cyclic condition because of the length for axial direction is assumed infinite. The diameter of inner cylinder is assumed 86.8mm, the numerical parameters are angular velocity and radius ratio. The numerical method is compared with the experimental results by Wereley, and the results are very good agreement. As the results, the vortex size is from 0.8 to 1.1 for TVF at various  , and the traveling distance for wavy vortex have the critical traveling distance for each case. Also, the critical Taylor number is calculated by theoretical and numerical analysis, and the results is showed the difference about ±2~3%.

암반단열에서 비선형유동이 발생하는 임계 레이놀즈수

김다혜,여인욱 대한자원환경지질학회 2019 자원환경지질 Vol.52 No.4

단열을 통한 유체의 유동은 선형유동이 우세하다는 가정아래 Navier-Stokes 방정식에서 유도된 Stokes 방정식, Reynolds 식(또는 local cubic law), cubic law 와 같은 방정식을 이용하여 해석되고 있다. 하지만 이러한 방정식은선형 흐름에 국한되며, 비선형 유동영역에 적용하게 되면 오류가 발생한다. 본 연구에서는 레이저 계측기를 이용하여정밀하게 측정한 3차원 단열 자료와 Navier-Stokes 방정식과 Stokes 방정식을 지배방정식으로 한 수치모델링을 수행함으로써 비선형 유동이 일어나는 현상과 임계 레이놀즈수를 제시하였다. 레이놀즈수가 10이상이 되면 유속의 제곱에비례하는 관성력이 점성력을 충분히 압도할 정도로 커지면서 지하수 유동이 선형영역에서 비선형 유동영역으로 전환되는 것으로 분석되었다. 이는 평균 간극과 거친 정도가 다른 두 단열에서 모두 동일하게 나타났다. 비선형 유동의 발생기작은 소용돌이 구조의 발생과 성장에 의한 것으로 알려져 있지만, 본 연구결과 단순히 소용돌이 구조가 비선형유동을 일으키는 아니라 유속이 증가하면서 관성력의 영향이 훨씬 큰 영향을 끼치게 되어 비선형 유동이 발생하는 것으로 나타났다.

Numerical and experimental investigation of vortex breaker effectiveness on the improvement in launch vehicle ballistic parameters

Mahdi N. Mahyari,Hasan Karimi,Hasan Naseh,Mehran Mirshams 대한기계학회 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.10

The focus of the present study is to investigate the effectiveness of installing vortex breakers at the outlet of launch vehicle tanks on postponing vortex formation and decreasing the critical height of propellants while discharging. Analytical results in the absence of a vortex breaker show that the effects of the Weber and Reynolds numbers in the flow field can be ignored for values greater than 720 and 1.1 × 105, respectively; and critical height can be considered as a function of Froude number under aforementioned conditions. The analytical criteria are verified by two-dimensional, axis symmetrical, transient and two-phase numerical model. Eventually, some experiments are conducted to examine the effectiveness of the applied vortex breakers in reduction of the critical height of propellant. Experimental results show that a 30% decrease can be achieved in critical height by using a particular type of vortex breaker. Additionally, the carried out simulations for an existing two-stage launch vehicle indicate a 13% increase in orbital altitude, which in turn proves the considerable improvement in launch vehicle mass/energetic capabilities.

Galloping analysis of stranded electricity conductors in skew winds

J. H. G. Macdonald,P. J. Griffiths,B. P. Curry 한국풍공학회 2008 Wind and Structures, An International Journal (WAS Vol.11 No.4

When first commissioned, the 1.6 km span 275kV Severn Crossing Conductor experienced large amplitude vibrations in certain wind conditions, but without ice or rain, leading to flashover between the conductor phases. Wind tunnel tests undertaken at the time identified a major factor was the lift generated in the critical Reynolds number range in skew winds. Despite this insight, and although a practical solution was found by wrapping the cable to change the aerodynamic profile, there remained some uncertainty as to the detailed excitation mechanism. Recent work to address the problem of dry inclined cable galloping on cablestayed bridges has led to a generalised quasi-steady galloping formulation, including effects of the 3D geometry and changes in the static force coefficients in the critical Reynolds number range. This generalised formulation has been applied to the case of the Severn Crossing Conductor, using data of the static drag and lift coefficients on a section of the stranded cable, from the original wind tunnel tests. Time history analysis has then been used to calculate the amplitudes of steady state vibrations for comparison with the full scale observations. Good agreement has been obtained between the analysis and the site observations, giving increased confidence in the applicability of the generalised galloping formulation and providing insight into the mechanism of galloping of yawed and stranded cables. Application to other cable geometries is also discussed.

Galloping analysis of stranded electricity conductors in skew winds

Macdonald, J.H.G.,Griffiths, P.J.,Curry, B.P. Techno-Press 2008 Wind and Structures, An International Journal (WAS Vol.11 No.4

When first commissioned, the 1.6 km span 275kV Severn Crossing Conductor experienced large amplitude vibrations in certain wind conditions, but without ice or rain, leading to flashover between the conductor phases. Wind tunnel tests undertaken at the time identified a major factor was the lift generated in the critical Reynolds number range in skew winds. Despite this insight, and although a practical solution was found by wrapping the cable to change the aerodynamic profile, there remained some uncertainty as to the detailed excitation mechanism. Recent work to address the problem of dry inclined cable galloping on cable-stayed bridges has led to a generalised quasi-steady galloping formulation, including effects of the 3D geometry and changes in the static force coefficients in the critical Reynolds number range. This generalised formulation has been applied to the case of the Severn Crossing Conductor, using data of the static drag and lift coefficients on a section of the stranded cable, from the original wind tunnel tests. Time history analysis has then been used to calculate the amplitudes of steady state vibrations for comparison with the full scale observations. Good agreement has been obtained between the analysis and the site observations, giving increased confidence in the applicability of the generalised galloping formulation and providing insight into the mechanism of galloping of yawed and stranded cables. Application to other cable geometries is also discussed.