• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.611-614
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• 2002

Rotating stall in vaneless diffusers of centrifugal compressor occurs in the diffuser wall due to flow separation at large inlet flow angle. For this reason, the critical inlet flow angles are suggested by several researchers. Beyond this critical angle, flow separates in the diffuser, and develops into rotating stall. This paper studied this critical flow angle. Rotating stall is measured through eight fast-response pressure transducers which are equally spaced around the circumference at the inlet and exit of a vaneless diffuser. Experiments are done from 20000rpm to 60000rpm for the diffuser stall. Two-cell structure which rotates at $6{\~}l0{\%}$ of impeller speed is fully developed at $20000{\~}40000rpm$, and three-cell structure which rotates at $7{\~}9{\%}$ of impeller speed is fully developed at $50000{\~}60000rpm$. This paper shows that the critical inlet flow angle is not constant but related with tip speed of impeller. As tip speed increases, so does the critical inlet flow angle.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.131-134
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• 2002

Compressible gas flow through a convergent-divergent nozzle is choked at the nozzle throat under a certain critical pressure ratio, and then being no longer dependent on the pressure change in the downstream flow field. In practical, the flow field at the divergent part of the critical nozzle can affect the effective critical pressure ratio. In order to investigate details of flow field through a critical nozzle, the present study solves the axisymmetric, compressible, Wavier-Stokes equations. The diameter of the nozzle throat is D=8.26mm and the half angle of the diffuser is changed between $2^{\circ}\;and\;10^{\circ}$ Computational results are compared with the previous experimental ones. The results obtained show that the divergence angle is significantly influences the critical pressure ratio and the present computations predict the experimented discharge coefficient and critical pressure ratio with a good accuracy. It is also found that a nozzle with the half angle of $4^{\circ}$ nearly predicts the theoretical critical pressure ratio.

• 오토저널
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• 제9권5호
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• pp.41-48
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• 1987

The aerodynamic critical phenomenon of notch-back type automobile-like bodies was investigated experimentally. The aerodynamic forces were measured for the various bodies of different back-light rake angle at R$_{e}$0.8*10$^{5}$ , 1.0*10$^{6}$ and 1.4*10$^{6}$ . Also, surface flow visualization was effected by the oil mixture. It was found that the critical phenomenon for the notch-back type bodies was milder than for the hatch-back type bodies; the drag vs. slant angle curve for the notch-back type bodies exhibiting local maximum was much smoother. Surface oil flow visualization revealed that the flow pattern associated with the critical phenomenon was characterized by the reattachment of the separated flow on the boot. The effect of the forebody on the critical phenomenon was seen to be negligible.e.

• 로봇학회논문지
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• 제2권2호
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• pp.109-118
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• 2007

The thruster is the crucial factor of an underwater vehicle system, because it is the lowest layer in the control loop of the system. In this paper, we propose an accurate and practical thrust modeling for underwater vehicles which considers the effects of ambient flow velocity and angle. In this model, the axial flow velocity of the thruster, which is non-measurable, is represented by ambient flow velocity and propeller shaft velocity. Hence, contrary to previous models, the proposed model is practical since it uses only measurable states. Next, the whole thrust map is divided into three states according to the state of ambient flow and propeller shaft velocity, and one of the borders of the states is defined as Critical Advance Ratio (CAR). This classification explains the physical phenomenon of conventional experimental thrust maps. In addition, the effect of the incoming angle of ambient flow is analyzed, and Critical Incoming Angle (CIA) is also defined to describe the thrust force states. The proposed model is evaluated by comparing experimental data with numerical model simulation data, and it accurately covers overall flow conditions within 2N force error. The comparison results show that the new model's matching performance is significantly better than conventional models'.

• 한국추진공학회지
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• 제19권4호
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• pp.1-7
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• 2015

캐비테이션 벤츄리는 후단 압력에 상관없이 액체의 유량을 일정하게 유지시켜주는 장치로, 일정한 추진제 유량의 공급을 필요로 하는 액체로켓엔진 시스템에 성공적으로 사용되고 있다. 본 연구에서는 축소되는 유입각과 팽창하는 출구각 만이 다른 네 개의 캐비테이션 벤츄리를 설계, 제작하였다. 압력을변경시켜가며 벤츄리를 통과하는 유량과 전/후단의 압력을 측정하였다. 실험결과로부터 각 벤츄리에 대한 유량계수와 임계 압력비를 계산하였다. 캐비테이션 벤츄리의 입구각과 출구각은 유량계수에 영향을 주었으며, 출구각은 임계 압력비에도 영향을 주는 것을 확인하였다.

• 설비공학논문집
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• 제13권8호
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• pp.674-681
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• 2001

An experiment is conducted to investigate the effect of the inclination angle on convective boiling heat transfer of a uniformly heated tube. The test section used is a stainless steel tube with10.7mm in inner diameter. The hating length is 3m and is heated directly by an AC current. The test fluid is R-113. Experiment are carried out with mass flow rates of 300, 500 and $700\;kg/m^{2}s$, and heat fluxes varying from 5 to 65 kW/$m^2$. The inclination angles of the tube are $0^{\circ},\;5^{\circ},\;11^{\circ}\;and\;25^{\circ}$. the circumferential temperature variation at low quality region and the location of dryout at high quality region are mainly observed. Circumferential anisothermality occurring at low mass flow rate and low quality conditions is gradually reduced with the increase in the inclination angle and finally disappears at the inclination angle of $25^{\circ}$. Critical quality where dryout is initiated is seriously influenced by the inclination angle. Wall temperature after critical quality is also affected by the inclination angle.

• Wind and Structures
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• 제11권3호
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• pp.221-240
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• 2008

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.

• 대한기계학회논문집
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• 제13권1호
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• pp.178-189
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• 1989

본 연구에서는 우선 승용차의 1/10 모형의 형상을 더욱 단순화시킨 2차원 자동차형 물체를 대상으로 하여 그 주위의 압력, 속도 및 난류량의 분포를 측정하고, 또한 울터프트(wool tuft)를 이용한 유동가시화법으로 후류구조를 파악함으로써 향후 계속될 실험적 및 수치해석적 연구의 기초를 마련하고자 한다.

• 설비공학논문집
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• 제6권2호
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• pp.140-154
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• 1994

Experimental studies were performed to determine the characteristics of flow structure and pressure drop in 15 : 1 scale models of multi-louvered fin heat exchanger in a wide range of variables($L_P/F_P=0.5{\sim}1.23$, ${\theta}=27^{\circ}{\sim}37^{\circ}$, $Re_{LP}=50{\sim}2000$). Flow structure inside the louvered fin was analyzed by smoketube method and new correlations on flow efficiency and drag coefficient were suggested. The new definition for flow efficiency, which modifies the existing flow efficiency, can predict the flow efficiency in the range above mentioned and is represented as a function of Reynolds number, louver pitch to fin pitch ratio, louver angle at low Reynolds number. Drag coefficient which is defined here is a function of Reynolds number, louver pitch to fin pitch ratio, louver angle below critical Reynolds number, and can be represented by a function of louver pitch to fin pitch ratio only above the critical Reynolds number.

• 한국전산유체공학회지
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• 제13권2호
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• pp.8-13
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• 2008

A supersonic viscous flow over a five-degree half-angle cone is studied computationally with three-dimensional Navier-Stokes equations. Steady asymmetric solutions show that the asymmetric flow separation is caused by convective instability. The effects of angle of attacks, Reynolds numbers, and Mach numbers have been investigated and it is found that those factors affect the generation of the side force. The side force has the maximum value at ${\alpha}=22^{\circ}$, while over ${\alpha}=22^{\circ}$, asymmetric vortex becomes transient, which results in the unsteady shedding. At the angle of attack of 22 degrees, the side force increases with Reynolds number and decreases with Mach number. The increase of the side force stops over the critical Reynolds number for the present configuration.