• Title/Summary/Keyword: Unsteady Flow

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Steady/unsteady Flow Analysis for Industrial Mixer (산업용 교반기 내부 정상/비정상 유동특성해석)

  • Chang, J.;Hur, N.
    • 유체기계공업학회:학술대회논문집
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    • 2001.11a
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    • pp.460-465
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    • 2001
  • In the present study, steady and unsteady flow characteristics inside an industrial mixer with flat turbine type impeller are studied. For the flow analysis, STAR-CD is used with an automatic mesh generator developed in the present study. flow results are compared to the an available experimental data to show validity or the present simulation.

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Study of the Unsteady Gas Flow in a Critical Nozzle (임계노즐에서 발생하는 비정상유동에 관한 연구)

  • Kim, Jae-Hyung;Kim, Heuy-Dong;Park, Kyung-Am
    • 유체기계공업학회:학술대회논문집
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    • 2002.12a
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    • pp.337-345
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    • 2002
  • The present study addresses a computational result of unsteady gas flow through a critical nozzle. The axisymmetric, unsteady, compressible, Wavier-Stokes equations are solved using a finite volume method that makes use of the second order upwind scheme for spatial derivatives and the multi-stage Runge-Kutta integral scheme for time derivatives. The steady solutions of the governing equation system are validated with the previous experimental data to ensure that the present computational method is valid to predict the critical nozzle flows. In order to simulate the effects of back pressure fluctuations on the critical nozzle flows, an excited pressure oscillation with an amplitude and frequency is assumed downstream of the exit of the critical nozzle. The results obtained show that for low Reynolds numbers, the unsteady effects of the pressure fluctuations can propagate upstream of the throat of critical nozzle, and thus giving rise to the applicable fluctuations in mass flow rate through the critical nozzle, while for high Reynolds numbers, the pressure signals occurring at the exit of the critical nozzle do not propagate upstream beyond the nozzle throat. For very low Reynolds number, it is found that the sonic line near the throat of the critical nozzle remarkably fluctuateswith time, providing an important mechanism for pressure signals to propagate upstream of the nozzle throat, even in choked flow conditions. The present study is the first investigation to clarify the unsteady effects on the critical nozzle flows.

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Rotordynamic Forces Due to Rotor Sealing Gap in Turbines (비대칭 터빈 로터 실에 기인한 축 가진력)

  • Kim Woo June;Song Bum Ho;Song Seung Jin
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.545-548
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    • 2002
  • Turbines have been known to be particularly susceptible to flow-induced self-excited vibration. In such vibrations, direct damping and cross stiffness effects of aerodynamic forces determine rotordynamic stability. In axial turbines with eccentric shrouded rotors, the non-uniform sealing gap causes azimuthal non-uniformities in the seal gland pressure and the turbine torque which destabilize the rotor system. Previously, research efforts focused solely on either the seal flow or the unshrouded turbine passge flow. Recently, a model for flow in a turbine with a statically offset shrouded rotor has been developed and some stiffness predictions have been obtained. The model couples the seal flow to the passage flow and uses a small perturbation approach to determine nonaxiymmetric flow conditions. The model uses basic conservation laws. Input parameters include aerodynamic parameters (e.g. flow coefficient, reaction, and work coefficient); geometric parameters (e.g. sealing gap, depth of seal gland, seal pitch, annulus height); and a prescribed rotor offset. Thus, aerodynamic stiffness predictions have been obtained. However, aerodynamic damping (i.e. unsteady aerodynamic) effects caused by a whirling turbine has not yet been examined. Therefore, this paper presents a new unsteady model to predict the unsteady flow field due to a whirling shrouded rotor in turbines. From unsteady perturbations in velocity and pressure at various whirling frequencies, not only stiffness but also damping effects of aerodynamic forces can be obtained. Furthermore, relative contributions of seal gland pressure asymmetry and turbine torque asymmetry are presented.

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Comparative study between TVD and MOC methods for the analysis of Unsteady compressible flow in pipe network (배관망의 비정상상태 압축성 유동해석을 위한 TVD 와 MOC 방법의 비교 연구)

  • Shin Young-Seob;Sah Jong-Youb
    • 한국전산유체공학회:학술대회논문집
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    • 2000.10a
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    • pp.101-108
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    • 2000
  • Pipe network analysis is analyze all of it about pressure and volume flow rate through that are pipeline, junction, regulator and valve etc. In this study is compare TVD with MOC method for analysis of unsteady compressible flow in pipelines. Then, we calculated unsteady compressible flow for pipe network that periodic volume flow rate conditions.

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Numerical simulation of unsteady flow field behind bluff body (Bluffbody 비정상 유동장에 대한 수치해석)

  • Ryu, Myeong-Seok;Gang, Seong-Mo;Kim, Yong-Mo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.21 no.3
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    • pp.350-357
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    • 1997
  • The transient incompressible flow behind the axisymmetric bluff body is numerically simulated using the random vortex method(RVM). Based on the vorticity formulation of the unsteady Navier-Stokes equations, the Lagrangian approach with a stochastic simulation of diffusion using random walk technique is employed to account for the transport processes of the vortex elements. The numerical solutions for 2-dimensional recirculating flow behind a backward-facing step in the laminar range of Reynolds number are compared with experimental data. The present simulation focuses on the transitional flow regime where the recirculation zone behind the bluff body becomes highly unsteady and large-scale vortex eddies are shed from the bluff body wake due to intrinsic shear layer instabilities. The unsteady vertical flow structures and the mixing characteristics behind the bluff body are discussed in detail.

Development of a 3-D Incompressible Flow Solver Based on an Artificial Compressibility Method (가상 압축성 기법을 이용한 삼차원 비압축성 유동해석 코드 개발)

  • Jung, Mun-Seung;Kwon, Oh-Joon
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03b
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    • pp.614-617
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    • 2008
  • An unstructured hybrid mesh flow solver has been developed for the simulations of three dimensional steady and unsteady incompressible flow fields. The incompressible Navier-Stokes equations with an artificial compressibility method were discretized by using a node-based finite-volume method. For the unsteady time-accurate computation, a dual-time stepping method was adopted to satisfy a divergence free flow field at each physical time step. The one equation Spalart-Allmaras turbulence model has been adopted to solve the high-Reynolds number flow fields. This method has been applied to calculate the steady flow fields around submarine configurations and unsteady flow fields around a 3-D infinite cylinder.

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Three Dimensional Heat Transfer Analysis of a Thermally Stratified Pipe Flow (열성층 배관 유동에 대한 3차원 열전달 해석)

  • Jo Jong Chull;Kim Byung Soon
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.103-106
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    • 2002
  • This paper presents an effective numerical method for analyzing three-dimensional unsteady conjugate heat transfer problems of a curved pipe subjected to infernally thermal stratification. In the present numerical analyses, the thermally stratified flows in the pipe are simulated using the standard $k-{\varepsilon}$turbulent model and the unsteady conjugate heat transfer is treated numerically with a simple and convenient numerical technique. The unsteady conjugate heat transfer analysis method is implemented in a finite volume thermal-hydraulic computer code based on a non-staggered grid arrangement, SIMPLEC algorithm and higher-order bounded convection scheme. Numerical calculations have been performed far the two cases of thermally stratified pipe flows where the surging directions are opposite each other i.e. In-surge and out-surge. The results show that the present numerical analysis method is effective to solve the unsteady flow and conjugate heat transfer in a curved pipe subjected to infernally thermal stratification.

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Thermal Instability of Natural Convection in a Glass Melting Furnace (유리 용융로에서 자연대류의 열적 불안정성)

  • Lim, Kwang-Ok;Lee, Kwan-Soo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.22 no.12
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    • pp.1774-1783
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    • 1998
  • The transition from steady laminar to chaotic convection in a glass melting furnace specified by upper surface temperature distribution has been studied by the direct numerical analysis of the two and three-dimensional time dependent Navier-Stokes equations. The thermal instability of convection roll may take place when modified Rayleigh number($Ra_m$) is larger than $9.71{\times}10^4$. It is shown that the basic flows in a glass melting furnace are steady laminar, unsteady periodic, quasi-periodic or chaotic flow. The dimensionless time scale of unsteady period is about the viscous diffusion time, ${\tau}_d=H^2/{\nu}_0$. Through primary and secondary instability analyses the fundamental unsteady feature in a glass melting furnace is well defined as the unsteady periodic or weak chaotic flow.

A Twin Impulse Turbine for Wave Energy Conversion -The Performance under Unsteady Airflow-

  • Alam, M M Ashraful;Sato, Hideki;Takao, Manabu;Okuhara, Shinya;Setoguchi, Toshiaki
    • International Journal of Fluid Machinery and Systems
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    • v.9 no.4
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    • pp.300-306
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    • 2016
  • A twin unidirectional impulse turbine for wave energy conversion has been suggested in our previous study, and the performance under unsteady flow has been investigated by quasi-steady analysis. In the present study, the performance of twin impulse turbine under unsteady flow condition has been investigated by unsteady analysis of Computational fluid dynamics. As a result, the mean efficiency of twin unidirectional impulse turbine under unsteady flow is lower than the maximum efficiency of unidirectional impulse turbine. Moreover, it is verified that airflow goes backward in the reverse turbine in low flow rates.

Calculation of Probability of System Failure for Pipe Network with Surge Tank regarding Unsteady Flow (Surge Tank가 설치된 상수도관망에서 부정류를 고려한 불능확률 산정)

  • Kwon, Hyuk Jae;Lee, Cheol-Eung
    • Journal of Korean Society of Water and Wastewater
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    • v.23 no.3
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    • pp.295-303
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    • 2009
  • In the present study, a reliability analysis calculating the probability of system failure has been performed using cut set and results of numerical analysis for unsteady flow in pipe. Especially, the probability of system failure has been evaluated regarding the effect of valve closure which is a really important activity in operation of piping system. In spite of small amount of demand, it was found that fast valve closure can generate high probability of system failure. Furthermore, it was confirmed that surge tank can reduce the unsteady effects and probability of system failure in water distribution system. From the results, it was found that the unsteady flow has a significant effect on the probability of system failure Furthermore, it was able to find which pipe or cut set has high probability of system failure. So it could be used to determine which pipe or cut set has a priority of repair and replacement. Therefore, reliability analysis regarding unsteady flow has to be performed for the planning, designing, maintenance, and operation of piping system.