• 대한기계학회논문집B
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• 제29권6호
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• pp.687-695
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• 2005

On the notion that the Reynolds stresses are transported with different time scale depending on the transport direction, the third order velocity correlations are represented by a new turbulent gradient transport model with tonsorial Lagrangian time scale. In order to verify the proposed model, DNS data are first obtained in a turbulent channel flow at Re = 180 and tonsorial Lagrangian time scales are computed. The present model predictions are compared with DNS data and those predicted by the third-order turbulent transport model of Hanjalic and Launder that uses a scalar time scale. The result demonstrates that the Reynolds stresses are indeed transported with different time scale depending on the transport direction.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1801-1805
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• 2003

The intermittent characteristics of fluid particle accelerations near the wall are investigated with the higher-order statistics and the probability density functions (PDF) by using a direct numerical simulation of turbulent channel flow. Also, the behaviors of acceleration associated with the coherent structures are discussed. The flatness factor of wall-normal acceleration is extremely high near the wall and it exceeds the previously reported value obtained in isotropic turbulence. The presence of the wall seems to make the accelerations more intermittent and the associated mechanism is explained with the PDFs. The skewness factor of wall-normal acceleration indicates that accelerations are associated with the streamwise vortices.

• 대한기계학회논문집B
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• 제27권7호
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• pp.859-866
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• 2003

The Lagrangian dispserion of fluid particles in inhomogeneous turbulence is investigated by a direct numerical simulation of turbulent channel flow. Fluid particle velocity and acceleration along a particle trajectory are computed by employing several interpolation schemes such as linear interpolation, high-order Lagrange polynomial interpolation and the Hermite interpolation schemes. The performances of the schemes are evaluated through comparison of errors in computed particle positions, velocities and accelerations against spectral interpolation. Adopting the four-point Hermite interpolation in the homogeneous directions and Chebyshev polynomials in the wall-normal direction appears to produce most reliable Lagrangian statistics including acceleration correlations with a reasonable amount of computational overhead.

• 한국물환경학회지
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• 제22권1호
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• pp.150-158
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• 2006

Over the past few years, many studies have been conducted on the flow, sediment movement, pollution transportation and scour etc. However, very few attempts have been made at the hydraulic studies reflecting upon the ecological function. The objective of this study is to examine the structures of the flow and turbulence in an open circular channel and their relationship to distribution of the organisms and chironomids. Under different flow conditions, the organic matter and some chironomids were injected into the channel. Using the obtained velocity data, the flow mechanisms and the turbulent shear stresses were analyzed. Organic matters and chironomids were distributed on the region that the velocity was slower and the turbulent shear stresses were smaller. Some habitat moved even though chironomids were inhabited. This phenomenon has relationship with the flow mechanism. Some chironomids have distributed around the habitat structure of a hemisphere. The secondary flow has affected the deposition of the organic matters and the distribution of chironomids.

• 대한기계학회논문집
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• 제10권1호
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• pp.86-95
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• 1986

본 연구에서는 폭이 일정한 급경사 개수노상(0.15mW*1.6mL)을 상온의 물이 유동할 때 유동율과 수로 급경사에 따른 명입치에서의 유동깊이 변화를 실측을 통하여 밝히고 이념치와 비교하였다.

• Journal of Advanced Marine Engineering and Technology
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• 제30권3호
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• pp.369-375
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• 2006

Large eddy simulation has been applied to simulate turbulent flow around a cubic obstacle mounted on a channel surface for a Reynolds number of 40000(based on the incoming bulk velocity and the obstacle height) using a Smagorinsky model and a Lagrangian dynamic model. In order to develop the LES to the practical engineering application, the effect of upwind scheme, turbulent sub-grid scale model were investigated. The computed velocities. turbulence quantifies, separation and reattachment length were evaluated by compared with the previous experimental results.

• 대한조선학회논문집
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• 제40권3호
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• pp.9-15
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• 2003

An experimental study has been carried out as a basic research for development of the friction drag reduction technology for ships by polymer injection. Experimental apparatus and procedures have been devised and prepared to measure the changes of the wall friction with injection of a polymer solution and basic experimental data on the friction drag reduction are obtained for a turbulent fiat-plate boundary layer and fully-developed channel flows. Variations of the friction drag reduction with some important parameters of polymer injection, such as the concentration of polymer solution, its injection flow rate and the measuring position downstream from the injection slot, are also investigated. Important experimental data and results obtained in the present study are presented. The amount of friction drag reduction up to 50% is observed.

• Journal of Mechanical Science and Technology
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• 제19권9호
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• pp.1801-1810
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• 2005

The effect of longitudinal thin rectangular riblets aligned with the flow direction on turbulent channel flow has been investigated using direct numerical simulation. The thin riblets have been modeled using the immersed boundary method (IBM) where the velocities at only one set of vertical nodes at the riblets positions are enforced to be zeros. Different spacings, ranging between 11 and 43 wall units, have been simulated aiming at getting the optimum spacing corresponding to the maximum drag reduction while keeping the height/spacing ratio at 0.5. Reynolds number based on the friction velocity ${\mu}_\tau$ and the channel half depth $\delta$ is set to 150. The flow is driven by adjusted pressure gradient so that the mass flow rate is kept constant in all the simulations. This study shows similar trend of the drag ratio to that of the experiments at the different spacings. Also, this research provides an optimum spacing of around 17 wall units leading to maximum drag reduction as experimental data. Explanation of drag increasing/decreasing mechanism is highlighted.

• 대한기계학회논문집B
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• 제28권9호
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• pp.1022-1031
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• 2004

The commutation errors by the filtering process in the large eddy simulation are considered. It is compared the conventional filter with the inhomogeneous filter that is devised to reduce the commutation errors. The weighting factor of the inhomogeneous filter suggested by Vasilyev is adopted. Also, using the optimizing function that estimates test filter width to eliminate the dissipations in the region excluding the vicinity of the wall, the flow patterns are analyzed. It is evaluated in simulations of the turbulent channel flow at Reynolds number of 1020, based on friction velocity and channel half height. Results show that the commutation errors can be significantly reduced by using the inhomogeneous filter and the optimized test filter width.

• 한국전산유체공학회지
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• 제12권4호
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• pp.74-84
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• 2007

The flowfield characteristics in a straight rectangular channel have been investigated through a numerical model to analyze the regenerative cooling system that is used in rocket engine cooling. The supercritical hydrogen coolant introduces strong property variations that have a major influence on the developing flow and heat transfer characteristics. Of particular interest is the improved understanding of the physical characteristics of such flows through parametric studies. The approach used is a numerical solution of the full Navier-Stokes equations in the three dimensional form including the arbitrary equation of state and property variations. The present study compares constant and variable property solutions for both laminar and turbulent flow. For laminar flow, the variation of aspect ratio is examined, while for turbulent flow, the effects of variation of channel length and Reynolds number are discussed.