• International Journal of Fluid Machinery and Systems
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• v.2 no.4
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• pp.295-302
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• 2009

The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresses the following fundamental question: is it possible to compute the circumferentially averaged flow field induced by the precessing vortex rope by using an axisymmetric turbulent swirling flow model? In other words, instead of averaging the measured or computed 3D velocity and pressure fields we would like to solve directly the circumferentially averaged governing equations. As a result, one could use a 2D axi-symmetric model instead of the full 3D flow simulation, with huge savings in both computing time and resources. In order to answer this question we first compute the axisymmetric turbulent swirling flow using available solvers by introducing a stagnant region model (SRM), essentially enforcing a unidirectional circumferentially averaged meridian flow as suggested by the experimental data. Numerical results obtained with both models are compared against measured axial and circumferential velocity profiles, as well as for the vortex rope location. Although the circumferentially averaged flow field cannot capture the unsteadiness of the 3D flow, it can be reliably used for further stability analysis, as well as for assessing and optimizing various techniques to stabilize the swirling flow. In particular, the methodology presented and validated in this paper is particularly useful in optimizing the blade design in order to reduce the stagnant region extent, thus mitigating the vortex rope and expending the operating range for Francis turbines.

• The KSFM Journal of Fluid Machinery
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• v.10 no.4
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• pp.39-46
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• 2007

In large pump station, vortex generation such as free-surface vortex and submerged vortex occurring around pump intake, or at bell-mouth inlet has been an important flow characteristics which should be considered always to keep away the suction of air-entrained or cavitated flow. In this study, a commercial CFD code was used to predict accurately the vortex generation for the specified intake design. These result shows the preliminary result of submerged vortex prediction for the Turbo-machinery Society of Japan Sump Test CFD standard model. At bottom wall, air volume fraction (red color) was found in a large scale to explain the submerged vortex generation at particular operation and configuration condition. And these indicate the free surface formation behind the bell mouth. Particularly, non-uniform approaching flow is a major parameter to govern the occurrence of the free-surface vortex. Futhermore the comparison between turbulence ($k-{\epsilon}$ & $k-{\omega}$ model) mode were executed in this study.

• Journal of computational fluids engineering
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• v.8 no.2
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• pp.49-56
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• 2003

Three-dimensional endwall flow within a linear cascade passage of high performance turbine blade is simulated with a 3-D Navier-Stokes CFD code (MOSA3D), which is based on body-fitted coordinate system, pressure-correction and finite volume method. The endwall flow characteristics, including the development and generation of horseshoe vortex, passage vortex, etc. are clearly simulated, consistent with the generally known tendency. The effects of both turbulence model and convective differencing scheme on the prediction performance of endwall flow are systematically analyzed in the present paper. The convective scheme is found to have stronger effect than the turbulence model on the prediction performance of endwall flow. The present simulation result also indicates that the suction leg of the horseshoe vortex continues on the suction side until it reaches the trailing edge.

• Journal of computational fluids engineering
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• v.8 no.1
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• pp.8-15
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• 2003

Three-dimensional endwall flow within a linear cascade passage of high performance turbine blade is simulated with a 3-D Wavier-Stokes CFD code (MOSA3D), which is based on body-fitted coordinate system, pressure-correction and finite volume method. The endwall flow characteristics, including the development and generation of horseshoe vortex, passage vortex, etc. are clearly simulated, consistent with the generally known tendency. The effects of both turbulence model and convective differencing scheme on the prediction performance of endwall flow are systematically analyzed in the present paper. The convective scheme is found to have stronger effect than the turbulence model on the prediction performance of endwall flow. The present simulation result also indicates that the suction leg of the horseshoe vortex continues on the suction side until it reaches the trailing edge.

• Wind and Structures
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• v.19 no.1
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• pp.89-111
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• 2014

The dynamics of a tornado-like vortex with touching down is investigated by using the LES turbulence model. The detailed information of the turbulent flow fields is provided and the force balances in radial and vertical directions are evaluated by using the time-averaged axisymmetric Navier-Stokes equations. The turbulence has slightly influence on the mean flow fields in the radial direction whereas it shows strong impacts in the vertical direction. In addition, the instantaneous flow fields are investigated to clarify and understand the dynamics of the vortex. An organized swirl motion is observed, which is the main source of the turbulence for the radial and tangential components, but not for the vertical component. Power spectrum analysis is conducted to quantify the organized swirl motion of the tornado-like vortex. The gust speeds are also examined and it is found to be very large near the center of vortex.

• Journal of Mechanical Science and Technology
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• v.17 no.4
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• pp.606-616
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• 2003

A numerical analysis has been conducted in order to simulate the characteristics of complex flow through linear cascades of high performance turbine blade with/without tip clearance by using a pressure-correction based, generalized 3D incompressible Wavier-Stokes CFD code. The development and generation of horseshoe vortex, passage vortex, leakage vortex, tip vortex within tip clearance, etc. are clearly identified through the present simulation which uses the RNG k-$\varepsilon$ turbulent model with wall function method and a second-order linear upwind scheme for convective terms. The present simulation results are consistent with the generally known tendency that occurs in the blade passage and tip clearance. A 3D model for secondary and leakage flows through turbine cascades with/without tip clearance is also suggested from the present simulation results, including the effects of tip clearance height.

• Journal of the Society of Naval Architects of Korea
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• v.43 no.2 s.146
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• pp.177-185
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• 2006

Cavitation is the dominant noise source of the marine vehicle. Of the various types of cavitation , tip vortex cavitation is the first appearance type of marine propeller cavitation and it generates high frequency noise. In this study, tip vortex cavitation behavior and noise are numerically investigated. A numerical scheme using Eulerian flow field computation and Lagrangian particle trace approach is applied to simulate the tip vortex cavitation on the hydrofoil. Vortex flow field is simulated by combined Moore and Saffman's vortex core radius equation and Sculley vortex model. Tip vortex cavitation behavior is analyzed by coupled Rayleigh-Plesset equation and trajectory equation. The cavitation nuclei are distributed and released in the vortex flow result. Vortex cavitation trajectories and radius variations are computed according to nuclei initial size. Noise is analyzed using time dependent cavitation bubble position and radius data. This study may lay the foundation for future work on vortex cavitation study and it will provide a basis for proper underwater propeller noise control strategies.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.2162-2167
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• 2003

It is well-known that high anisotropic characteristic of turbulent flow field is dominant inside tip leakage vortex. This anisotropic nature of turbulence invalidates the use of the conventional isotropic eddy viscosity turbulence model based on the Boussinesq assumption. In this study, to check whether an anisotropic turbulence model is superior to the isotropic ones or not, the results obtained from steady-state Reynolds averaged Navier-Stokes simulations based on the RNG ${\kappa}-{\varepsilon}$ and the Reynolds stress model in two test cases, such as a linear compressor cascade and a forward-swept axial-flow fan, are compared with experimental data. Through the comparative study of turbulence models, it is clearly shown that the Reynolds stress model, which can express the production term and body-force term induced by system rotation without any modeling, should be used to predict the complex tip leakage flow, including the locus of tip leakage vortex center, quantitatively.

• Journal of Mechanical Science and Technology
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• v.16 no.4
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• pp.532-548
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• 2002

A hybrid model consisting of a modified TAB (Taylor Analogy Breakup) model and DVM (Discrete Vortex Method) is proposed for numerical analysis of the evaporating spray phenomena in diesel engines. The simulation process of the hybrid model is divided into three steps. First, the droplet breakup of injected fuel is analyzed by using the modified TAB model. Second, spray evaporation is calculated based on the theory of Siebers'liquid length. The liquid length analysis of injected fuel is used to integrate the modified TAB model and DVM. Lastly, both ambient gas flow and inner vortex flow of injected fuel are analyzed by using DVM. An experiment with an evaporative free spray at the early stage of its injection was conducted under in-cylinder like conditions to examine an accuracy of the present hybrid model. The calculated results of the gas jet flow by DVM agree well with the experimental results. The calculated and experimental results all confirm that the ambient gas flow dominates the downstream diesel spray flow.

• Nuclear Engineering and Technology
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• v.51 no.3
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• pp.665-675
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• 2019

The wire-wrapped fuel bundle is an assembly design in a sodium-cooled fast reactor. A wire spacer is used to maintain a constant gap between rods and to enhance the mixing of coolants. The wire makes the flow complicated by creating a sweeping flow and vortex flow. The vortex affects the flow field and heat transfer inside the subchannels. However, studies on vortices in this geometry are limited. The purpose of this research is to investigate the vortex flow created in the wire-wrapped fuel bundle. For analysis, a RANS-based numerical analysis was conducted for a 37-pin geometry. The sensitivity study shows that simulation with the shear stress transport model is appropriate. For the case of Re of 37,100, the mechanisms of onset, periodicity, and rotational direction were analyzed. The vortex structures were reconstructed in a three-dimensional space. Vortices were periodically created in the interior subchannel three times for one wire rotation. In the edge subchannel, the largest vortex occurred. This large vortex structure blocked the swirl flow in the peripheral region. The small vortex formed in the corner subchannel was negligible. The results can help in understanding the flow field inside subchannels with sweeping flow and vortex structures.