• Journal of the Korean Society of Combustion
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• v.11 no.4
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• pp.27-35
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• 2006

The impacts of equivalence ratio on flow structure and flame dynamic in a model gas turbine combustor are investigated using large eddy simulation(LES). Dynamic k-equation model and G-equation flamelet model are employed as LES subgrid model for flow and combustion, respectively. As a result of mean flow field for each equivalence ratio, the increase of equivalence ratio brings about the decrease of swirl intensity through the modification of thermal effect and viscosity, although the same swirl intensity is imposed at inlet. The changes of vortical structure and turbulent intensity etc. near flame surface are occurred consequently. That is, the decrease of equivalence ratio can leads to the increase of heat release fluctuation by the more increased turbulent intensity and fluctuation of recirculation flow. In addition, the effect of inner vortex generated from vortex breakdown on the heat release fluctuation is increased gradually with the decrease of equivalence ratio. Finally, it can be identified that the variations of vortical structure play an important role in combustion instability, even though the small change of equivalence ratio is occurred.

• Journal of Advanced Marine Engineering and Technology
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• v.21 no.2
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• pp.126-135
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• 1997

Drag reduction in polymer solutions is the phenomenon where by extremely dilute solutions of high molecular weight polymers exhibit frictional resistance to flow much lower than the pure solvent. This effect, largely unexplained as yet, has attracted the attention of polymer scientists and fluid flow specialists. Although applications are beginning to appear, the principle interest to data has been in attempting to relate the effect to the fluid mechanics of turbulent flow. Drag reduction in two phase flow can be applied to the transport of crude oil, phase change system such as chemical reactor, and pool and boiling flow. But the research on drag reduction in two phase flow is not intensively investigated. Therefore, experimental investigations have been carried out to analyze the drag reduction produced by polymer addition in the single phase and two phase flow system. The objectives of the proposed investigation are primarily in identifying and developing high performance polymer additives for fluid transportations with the benefits of turbulent drag. Also we want to is to evaluate the drag reduction in horizontal flow by measuring pressure drop and mean velocity. Experimental results show higher drag reduction using co - polymer(A611P) then using polyacrylamide (PAAM) and faster degradation using PAAM than using A611P under the same superficial velocity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.8
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• pp.2593-2600
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• 1996

A modified k-$\varepsilon$ turbulence model having a generality is proposed in the present study, in which the constant $C_{\varepsilon2}$in the $\varepsilon$-equation is simply changed as a functional form of a new parameter both satisfying the tensor invariant condition and representing the extra straining effect on complex shear flows. With this model turbulent shear flows over two-dimensional obstacles placed in a channel are numerically studied for different blockage ratios and aspect ratios. Comparing with the available experimental data, the predicted results with the present model provide definite improvements over the standard model's results and work fairly well with the experimental data on the size of the recirculation zone, as well as mean velocity, wall static pressure, turbulent kinetic energy and Reynolds stresses.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.224-232
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• 2000

Four turbulent $k-{\varepsilon}$ models (i.e., standard model, modified models with streamline curvature modification and/or preferential dissipation modification) are applied in order to analyze the turbulent flow of wall-attaching offset jet. For numerical convergence, this paper develops a method of slowly increasing the convective effect induced by skew-velocity in skew-upwind scheme (hereafter called Partial Skewupwind Scheme). Even though the method was simple, it was efficient in view of convergent speed, computer memory storage, programming, etc. The numerical results of all models show good prediction in first order calculations (i.e., reattachment length, mean velocity, pressure), while they show some deviations in ·second order (i.e., kinetic energy and its dissipation rate). Like the previous results obtained by upwind scheme, the streamline curvature modification results in better prediction, while the preferential dissipation modification does not.

• 한국가시화정보학회:학술대회논문집
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• 2005.12a
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• pp.28-35
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• 2005

Experiments were conducted to show the characteristics of the flow on triple parallel plane impinging jets. Measurements of mean velocity components and turbulent intensities were carried out with a particle image velocimetry(PIV) to investigate the flow field generated by the air issued from three identical plane parallel nozzles and mixed with the ambient air. The measurements range of these experiments were Reynolds number of 5000 and 1000 based on the nozzle width and the case of nozzle-to-plate distances were two times, six times and ten times the width of the nozzle. Results show that recirculation region of Re=5000 is the stronger than that of Re=1000. Therefore, velocity loss of centerline for Re=5000 that shows strongly recirculation region takes effect greatly.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.11
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• pp.1399-1409
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• 1999

It is known that previous models are unsatisfactory in predicting adverse pressure gradient turbulent flows. In the present paper, a revised low Reynolds number $k-{\varepsilon}$ model is proposed. In this model, a newly developed term is added lo the dissipation rate equation. In order to reflect appropriate effects for an adverse pressure gradient. The added tenn is derived by considering the distribution of mean velocity and turbulent properties in the turbulent flow with, adverse pressure gradient. The new $k-{\varepsilon}$ model was applied to calculations of flat plate flow with adverse pressure gradient, conical diffuser flow and backward facing step flow. It was found that the three numerical results showed better agreement than other models compared with DNS results and experimental ones.

• Journal of the Korean Society of Combustion
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• v.12 no.1
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• pp.1-10
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• 2007

Analytical investigation of acoustic wave scattering from turbulent premixed flames was conducted to evaluate the acoustic energy amplification/damping. Such acoustic energy change is attributed to the acoustic velocity jump due to flame's heat release. Small perturbation method up to second order and stochastic analysis were utilized to formulate net acoustic energy and the energy transfer from coherent to incoherent energy. Randomly wrinkled flame surface is responsible for the energy transfer from coherent to incoherent field. Nondimensional parameters that govern net acoustic energy were determined: rms height and correlation length of flame front, incident wave frequency, incidence angle, and temperature ratio. The dependence of net acoustic energy upon these parameters is illustrated by numerical simulations in case of Gaussian statistics of flame front. Total net energy was amplified and the major factors that affect such energy amplification are incidence angle and temperature ratio. Coherent (incoherent) energy is damped (amplified) with rms height and correlation length of flame front.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.11a
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• pp.237-240
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• 2006

Hybrid propulsion systems provide many advantages in terms of stable operation and safety. However, classical hybrid rocket motors have lower fuel regression rate and combustion efficiency compared to solid propellant rocket motor. Accordingly, the recent research efforts are focused on the improvement of engine efficiency and regressionrate in the hybrid rocket engine. The present study has numerically investigated the combustion processes and the flame structure in the hybrid rocket engine. The turbulent combustion is represented by the flamelet model and Low Reynolds number $k-{\varepsilon}$turbulent model is employed to reduce the uncertainties for convective heat transfer near solid fuel surface having strong blowing effect. Numerical results suggest that the present approach is capable of realistically simulating the combustion characteristics of the hybrid rocket engines.

• Wind and Structures
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• v.7 no.3
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• pp.173-186
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• 2004

Flutter derivatives provide the basis of predicting the critical wind speed in flutter and buffeting analysis of long-span cable-supported bridges. In this paper, one popular stochastic system identification technique, covariance-driven Stochastic Subspace Identification(SSI in short), is firstly presented for estimation of the flutter derivatives of bridge decks from their random responses in turbulent flow. Secondly, wind tunnel tests of a streamlined thin plate model and a ${\Pi}$ type blunt bridge section model are conducted in turbulent flow and the flutter derivatives are determined by SSI. The flutter derivatives of the thin plate model identified by SSI are very comparable to those identified by the unifying least-square method and Theodorson's theoretical values. As to the ${\Pi}$ type section model, the effect of turbulence on aerodynamic damping seems to be somewhat notable, therefore perhaps the wind tunnel tests for flutter derivative estimation of those models with similar blunt sections should be conducted in turbulent flow.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2000.04a
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• pp.75-80
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• 2000

We revisit the arrested Ekman boundary layer problem, using a fully non-linear numerical model with the subgrid dissipation modeled by the large eddy simulation method (LES). The main objective of this study is to find out whether the dynamic balance of the arrested Ekman boundary layer explained by MacCready and Rhines (1991) is valid for high Reynolds number. The model solution indicates that for high Reynolds number and low Richardson number flows, the density anomaly diffusion by near-wall turbulent action may become intense enough to homogenize completely the density structure within the boundary layer, in the direction perpendicular to the sloping wall. Then the buoyancy effect becomes negligible allowing a near-equilibrium Ekman boundary layer flow to persist for a long period.