• Journal of Korean Society of Environmental Engineers
• /
• v.22 no.7
• /
• pp.1173-1182
• /
• 2000

The rapid mixing process has been considered as an important step in water treatment. Since the coagulant dispersion into raw water by rapid mixer can influence on the flocculation and filtration efficiency, many researchers have developed various devices and mixing methodologies. Until now, they focused attention on only coagulant dose, pH. rotating velocity and G value but overlooked the real turbulent flow and mixer geometry in rapid mixer. Therefore this paper questions the significance of turbulent flows in rapid mixer and focuses on the analysis of turbulent fluid in various mixer geometry with CFD(Computational Fluid Dynamics). The results of the jar-tests using various geometries indicate that the turbidity removal rate in a circular jar without baffle is higher than that of a circular with baffle. And the turbidity removal rate in Hudson jar is also founded to be higher than in the circular jar with baffle. The CFD simulation of velocity fields in jar demonstrates that the differences of removal rates among the various geometries are largely due to the formation of the different turbulent fluids fields with different geometries.

• Journal of the Korean Society of Safety
• /
• v.33 no.1
• /
• pp.7-15
• /
• 2018

The friction factors according to the flow regimes in helical coil tubes depend on the coil diameter, the tube diameter, and the coil pitch. In previous studies, correlations for the laminar flow regime in helical coil tubes have been proposed. However, studies on the transition flow regime and the turbulent flow regime are insufficient and further researches are necessary. In this study, characteristics of the friction factors for the laminar, transition and turbulent flow regimes in helical coil tubes were experimentally investigated. The helical coil tubes used in the experiments were made of copper. The curvature ratios of the helical coil tubes, which means the ratio of helical coil diameter to inner diameter of the helical coil tube are 24.5 and 90.9. Experiments were carried out in the range of $529{\leq}Re{\leq}39,406$ to observe the flows from the laminar to the turbulent regime. The friction factors were obtained by measuring the differential pressures according to the flow rates in the helical coil tubes while varying the curvature ratios of the helical coil tubes. Experimental data show that the friction factors for the helical coil tube with 24.5 in the curvature ratio of the helical coil tube were larger than those in the straight tube in all flow regimes. As the curvature ratio of the helical coil tube increases, the friction factor in turbulent flow regime tends to be equal to that of the straight tube. In addition, it was confirmed that the transition flow regimes in the helical coil tubes were much wider than those in the straight tube, also the critical Reynolds numbers were larger than those in the straight tube. The results obtained in this experimental study can be used as basic data for studies on the water hammer phenomenon in helical coil tubes.

• Proceedings of the Korean Society of Computational and Applied Mathematics Conference
• /
• 2003.09a
• /
• pp.1-1
• /
• 2003

In this talk, a reduced-order modeling methodology based on centroidal Voronoi tessellations (CVT's)is introduced. CVT's are special Voronoi tessellations for which the generators of the Voronoi diagram are also the centers of mass (means) of the corresponding Voronoi cells. The discrete data sets, CVT's are closely related to the h-means clustering techniques. Even with the use of good mesh generators, discretization schemes, and solution algorithms, the computational simulation of complex, turbulent, or chaotic systems still remains a formidable endeavor. For example, typical finite element codes may require many thousands of degrees of freedom for the accurate simulation of fluid flows. The situation is even worse for optimization problems for which multiple solutions of the complex state system are usually required or in feedback control problems for which real-time solutions of the complex state system are needed. There hava been many studies devoted to the development, testing, and use of reduced-order models for complex systems such as unsteady fluid flows. The types of reduced-ordered models that we study are those attempt to determine accurate approximate solutions of a complex system using very few degrees of freedom. To do so, such models have to use basis functions that are in some way intimately connected to the problem being approximated. Once a very low-dimensional reduced basis has been determined, one can employ it to solve the complex system by applying, e.g., a Galerkin method. In general, reduced bases are globally supported so that the discrete systems are dense; however, if the reduced basis is of very low dimension, one does not care about the lack of sparsity in the discrete system. A discussion of reduced-ordering modeling for complex systems such as fluid flows is given to provide a context for the application of reduced-order bases. Then, detailed descriptions of CVT-based reduced-order bases and how they can be constructed of complex systems are given. Subsequently, some concrete incompressible flow examples are used to illustrate the construction and use of CVT-based reduced-order bases. The CVT-based reduced-order modeling methodology is shown to be effective for these examples and is also shown to be inexpensive to apply compared to other reduced-order methods.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.34 no.3
• /
• pp.813-820
• /
• 2014

This study presents a numerical modeling of mean flow and turbulence structures of partly-vegetated open-channel flows. For this, Reynolds-averaged Navier-Stokes equations with vegetation drag terms are solved numerically using the non-linear k-${\varepsilon}$ model. The numerical model is applied to laboratory experiments of Nezu and Onitsuka (2001), and simulated results are compared with data from measurement and computations by Kang and Choi's (2006) Reynolds stress model. The simulation results indicate that the proposed numerical model simulates the mean flow well. Twin vortices are found to be generated at the interface between vegetated and non-vegetated zones, where turbulence intensity and Reynolds stress show their maximums. The model simulates the pattern of the Reynolds stress well but under-predicts the intensity of Reynolds stress slightly.

• Journal of Mechanical Science and Technology
• /
• v.20 no.8
• /
• pp.1256-1265
• /
• 2006

The supersonic flows around tandem cavities were investigated by two-dimensional and three-dimensional numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) equation with the k- ω turbulence model. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves, and the acoustic effect transmitted from wake flow to upstream. The upwind TVD scheme based on the flux vector split with van Leer's limiter was used as the numerical method. Numerical calculations were performed by the parallel processing with time discretizations carried out by the 4th-order Runge- Kutta method. The aspect ratios of cavities are 3 for the first cavity and 1 for the second cavity. The ratio of cavity interval to depth is 1. The ratio of cavity width to depth is 1 in the case of three dimensional flow. The Mach number and the Reynolds number were 1.5 and $4.5{\times}10^5$, respectively. The characteristics of the dominant frequency between two- dimensional and three-dimensional flows were compared, and the characteristics of the second cavity flow due to the first cavity flow was analyzed. Both two dimensional and three dimensional flow oscillations were in the 'shear layer mode', which is based on the feedback mechanism of Rossiter's formula. However, three dimensional flow was much less turbulent than two dimensional flow, depending on whether it could inflow and outflow laterally. The dominant frequencies of the two dimensional flow and three dimensional flows coincided with Rossiter's 2nd mode frequency. The another dominant frequency of the three dimensional flow corresponded to Rossiter's 1st mode frequency.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.28 no.6B
• /
• pp.627-634
• /
• 2008

In a numerical simulation of open channel turbulent flows using RANS (Reynolds averaged Navier-Stokes) equations model equipped with VOF (Volume of Fluid) scheme, the determination of wall roughness for wall function was studied. The roughness constant, based on the law-of-the-wall for flow on rough walls, obtained by experimental works for pipe flows is employed in general wall functions. However, this constant of wall function is the function of Froude number in open channel flows. Thus, the wall roughness should be determined by taking into account the effect of Froude number. In addition, the wall roughness should be corresponding to Manning's roughness coefficient widely used for open channels. In this study, the relation between wall roughness height as an input condition and Manning's roughness coefficient was investigated, and an equation for effective wall roughness height considering the characteristics of numerical models was proposed as a function of Manning's roughness coefficient.

• Journal of Korean Port Research
• /
• v.13 no.1
• /
• pp.175-182
• /
• 1999

The ocean wave is hydrodynamically investigated to get more reliable solution. To improve the computational accuracy more fine grids are used with relatively less computer storage on the free surface. One element of the free surface is discretized into more fine grids because the free-surface waves are much affected by the grid size in the finite difference scheme. Here the multi-grid method is applied to confirm the efficiency for the S103 ship model by solving the Navier-Stokes equation for the turbulent flows. According to the computational result approximately 30% can be improved in the free surface generation, Finally the limiting streamlines show numerical result is similar to the experiment by twin tuft.

• Proceedings of the KSME Conference
• /
• 2001.06e
• /
• pp.754-759
• /
• 2001

This paper presents unsteady computational investigations and wind tunnel tests on the flow field around a square cylinder with a gap between the body and the ground plane. Two-dimensional unsteady, incompressible Navier-Stokes codes are developed for the computation of the viscous turbulent flows. By computing the flow around a square cylinder without ground effect, three two-equation turbulence models are evaluated and the developed code is validated. The results show a good agreement with experimental values and other computational results. Critical gap height at which the formation of Karman vortex streets is interrupted, is demonstrated and another transition regime is pointed out

• Journal of the KSME
• /
• v.34 no.9
• /
• pp.666-677
• /
• 1994

이 글에서는 난류유동의 개략적인 특성을 살펴보았고, 환경 및 산업에서 등장하는 난류유동 문 제의 종류들과 그 해결방법들을 살펴보았다. 기술의 역사를 반추해보면 과학적 진보로부터 얻은 연구결과가 응용됨으로써 많은 분야의 사람들에게 넓고 보편적인 개념으로 확장 적용될 때 기 술적 진보가 이루어졌음을 우리는 알고 있다. 난류는 더 이상 복잡하고 어려운 학문이나 과학 으로 인식되어서는 안되며 누구나에게 보편적인 상식으로 파급되어야 한다. 현재까지 개발된 난류유동의 지식으로서도 충분히 해결가능한 문제들이 도처에 산재되어 있으며, 시급한 해결을 요구하고 있는 중요한 문제들이 대부분이기 때문이다. 특히, 전지구적인 문제로 부각된 환경문 제를 해결하고 산업기술의 획기적인 진보를 달성하여 지속가능한 기술적 미래를 창조하는 엔지 니어들은 더욱 큰 책임의식을 가지고 난류를 공부해야 할 것이다.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.3
• /
• pp.653-661
• /
• 1994

In this paper the CSCM type upwind flux difference splitting Navier-Stokes method has been applied to study the ARL-SL19 supersonic/transonic compressor cascade flow. H-type grid was chosen for its simplicity in applying cyclic tridiagonal matrix algorithm along with conventional slip/no-slip boundary conditions. The thin-layer algebraic model of Baldwin-Lomax was employed for the calculation of turbulent flows. The test case inlet Mach No. was 1.612 and inlet/exit pressure ratio($P_2/P_1$) was 2.15. The results were compared with experimental results from current method were compared well in suction surface with the experiments and other computational results; however, not well in pressure surface. It might be due to the complex flowfields such as shock/boundary layer interaction, turbulence, and flow separation, etc. In the future, a proper turbulence modelling and adaptive grid system will be studied to improve the solution quality.