• Journal of Power System Engineering
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• v.5 no.3
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• pp.31-37
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• 2001

A study to obtain the enhancement of turbulence at low Reynolds number is carried out by adding ultrasonic force into the jet flow field of a coaxial circular pipe which can afford the sufficient data of flow characteristics with the shear flow and turbulence flow in spite of its simple shape. A coaxial circular flow field is made vertically in a large and transparent acryl tank. The time mean velocity vector, distribution, kinetic energy and turbulence intensity formed in the complex flow field of turbulence enhancement are investigated, observed and discussed at Reynolds number of 2,000, 3,000 and 5,000 by using PIV measurement, in results, the validity of ultrasonic to obtain the enhancement of turbulence is certified.

• Journal of Korean Society for Atmospheric Environment
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• v.13 no.1
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• pp.65-77
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• 1997

A theoretical and numerical investigation on the boundary-layer flow over a two- or three-dimensional hill is presented. The numerical model is based on the finite volume method with boundary-fitted coordinates. The k-$\varepsilon$ turbulence model with modified wall function and the low-Reynolds-number model are employed. The hypothesis of Reynolds number independency for the atmospheric boundary-layer flow over aerodynamically rough terrain is confirmed by the numerical simulation. Comparisons of the mean velocity profiles and surface pressure distributions between the numerical predictions and the wind-tunnel experiments on the flow over a hill show good agreement. The linear theory provides generally good prediction of speed-up characteristics for the gentle-sloped hills. The flow separation occurs in the hill slope of 0.5 and the measured reattachment points are compared with the numerical prediction. It is found that the k- $\varepsilon$ turbulence model is reasonably accurate in predicting the attached flow, while the low- Reynolds-number model is more suitable to simulate the separated flows.ows.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.5
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• pp.1661-1670
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• 1996

Incompressible flow over a backward-facing step is computed by low Reynolds number turbulence models in order to compare with direct simulation results. In this study, selected low Reynolds number 1st and 2nd (Algebraic Stress Model : ASM) moment closure turbulence models are adopted and compared with each other. Each turbulence model predicts different flow characteristics, different re-attachment point, velocity profiles and Reynolds stress distribution etc. Results by .kappa.-.epsilon. turbulence models indicate that predicted re-attachment lengths are shorter than those by standard model. Turbulent intensity and eddy viscosity by low Reynolds number .kappa.-.epsilon. models are still greater than DNS results. The results by algebraic stress model (ASM) are more reasonable than those by .kappa.-.epsilon. models. The convective scheme is QUICK (Quadratic Upstream Interpolation for Convective Kinematics) and SIMPLE algorithm is adopted. Reynolds number based on step height and inlet free stream velocity is 5100.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2423-2430
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• 1994

Several factors of the efficient combustion process are shape of combustion chamber, position of spark plug, turbulence flow and so on. the shape of combustion chamber and position of spark plug are constrained to geometrically, and then it could not make a change the shape easily. But the turlence flow in combustion chamber have a great influence on combustion phenomena, and which is much easier to control relatively. And since characteristics of turbulence flow would be very important to the stability of combustion and performances, This study is also essential to future engine-low emission and lean burn engine. This paper shows that the visualization of the turbulence flow of single cylinder engine by using 2way, $45^{\circ}$ inclined and 2 channel hot wire probe through the park plug hole. We also study the characteristics of turbulence flow by means of ensemble averaged mean velocity, turvulence intensity and integral length scale.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1203-1217
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• 1994

In the present study, in order to analyze a turbulent flow in a square sectiond 180.deg. bend, Kim's low Reynolds number second moment turbulence closure is adopted. In this model, turbulence model constants in the wall region are modified as functions of turbulent Reynolds number by use of near wall turbulent universal properties based on Laufer's experimental results of Reynolds stress distriburions. Algebraic stress model and Reynolds stress equation model are used to verify the low Reynolds number second moment closure. The application of the present low Reynolds number algebraic stress model to the prediction of a square sectioned 180.deg. bend flow gives improved velocities and Reynolds stresses profiles compared with those obtained by using the van Driest mixing length model and present low Reynolds number Reynolds stress equation model.

• Journal of computational fluids engineering
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• v.8 no.2
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• pp.1-7
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• 2003

The V2F turbulence model, which has shown very good performance in several test cases at low speeds, has been applied to supersonic ramp flow with 20. corner angle at the free stream Mach number of 2.79. The flow is known to manifest strong shock wave/turbulent boundary layer interactions. As a comparative study, low-Reynolds k-ε models are also considered. While the V2F model predicts wall-pressure distribution well, it relatively predicts larger separation bubble and higher skin-friction after the reattachment than the experimental data. Although the ellpticity of f equation is the characteristics of incompressible flows, the converged solutions are acquired in the compressible flow with shock waves. The effect of the realizability constraints used in the model is also examined. In contrast to the result of impinging jet flows, the realizability bounds proposed by Durbin deterioate the overall solutions of the supersonic ramp flow.

• Journal of Mechanical Science and Technology
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• v.19 no.6
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• pp.1347-1357
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• 2005

Effects of combustor-level high free-stream turbulence on the blade-surface heat/mass transfer have been investigated in the three-dimensional flow region near the endwall within a high-turning turbine rotor cascade passage. Free-stream turbulence intensity and integral length scale in the high turbulence case are 14.7 percents and 80 mm, respectively. The result shows that there is no considerable discrepancy in the blade heat/mass transfer near the endwall between the low and high turbulence cases. As departing from the endwall, however, the deviation between the two cases becomes larger, particularly in the region where flow separation and re-attachment occur. Under the high turbulence, flow disturbances such as boundary-layer separation and re-attachment seem to be suppressed, which makes the blade heat/mass transfer more uniform. Moreover, there are some evidences that endwall vortices tend to be weakened under the high turbulence.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.10
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• pp.2699-2709
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• 1995

This investigation reports on the study of the ambient turbulent effects on the droplet vaporization in the fuel spray combustion. For tractability, this discussion considers a single droplet in an infinite turbulent flow. In this numerical study, the low-Reynolds-number version of k-.epsilon. turbulence model was used to represent the turbulence effects. The set of two-dimensional conservation equations which describe the transport phenomena in turbulent flow using the mean flow quantities including the droplet internal laminar motion, are solved numerically with the finite difference procedure of Patankar(SIMPLER). The evaluation of the computational model is provided by two limiting cases: turbulent flow over the solid sphere and the laminar flow over a liquid drop. The results show that the turbulence effects are noticeable for the vaporization at high turbulence intensity (10-50%) which is encountered in a typical spray. The magnitude of turbulence effects mainly depends on the turbulent intensity. These effects are not sensitive to the Reynolds number in the range of 50 to 200, ambient temperature in the range of 700 to 1000.deg. K and the volatility.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1596-1608
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• 1993

Low Reynolds number second moment turbulence model which be applicable to the fine gird near the wall region was developed. In this model, turbulence model coefficients in the pressure strain model of the Reynolds stress equation was expressed as functions of turbulence Reynolds number $R_{t}\equivk^{2}/(\nu\varepsilon)).$ In the derivation procedure of the present low Reynolds number algebraic stress model, Laufer's near wall experimental data on Reynolds stresses were curve fitted as functions of R$_{t}$ and the resulting simultaneous equations of the model coefficients were solved by using the boundary conditions at wall and high Reynolds number limiting conditions. Predicted Reynolds stresses and dissipation rate of turbulent kinetic energy etc. in the 2 dimensional parallel, plane channel flow and pipe flow were compared with the preditions obtained by employing the Launder-Shima model, standard algebraic stress model and several experimental data. Results show that all the Reynolds stresses and dissipation rate of turbulent kinetic energy predicted by the present low Reynolds number algebraic stress model agree better with the experimental data than those predicted by other algebraic stress models.

• 한국전산유체공학회:학술대회논문집
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• 2001.10a
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• pp.51-57
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• 2001

The eddy viscosity turbulence models were applied to predict the flows through a cascade, and the prediction performances of turbulence models were assessed by comparing with the experimental results for a controlled diffusion(CD) compressor blade. The original $\kappa-\omega$ turbulence model and $\kappa-\omega$ shear stress transport(SST) turbulence model were used as two-equation turbulence model which were enhanced for a low Reynolds number flow and the Baldwin-Lomax turbulence model was used as algebraic turbulence model. Farve averaged Wavier-Stokes equations in a two-dimensional, curvilinear coordinate system were solved by an implicit, cell-centered finite-volume computer code. The turbulence quantities are obtained by lagging when the men flow equations have been updated. The numerical analysis was made to the flows of CD compressor blade in a cascade at three different incidence angles (40. 43.4. 46 degrees). We found the reversion in the prediction performance of original $\kappa-\omega$ turbulence model and $\kappa-\omega$ SST turbulence model when the incidence angie increased. And the algebraic Baldwin-Lomax turbulence model showed inferiority to two-equation turbulence models.