• 한국자동차공학회논문집
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• 제11권2호
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• pp.126-133
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• 2003

The aerodynamic characteristics of automobiles have received substantial interest recently. Detailed knowledge of the vehicle aerodynamics is essential to improve fuel efficiency and enhance stability at high-speed cruising. In this study, a numerical simulation has been carried out for three-dimensional turbulent flows around a commercial bus body. Also, the effect of rear-spoiler attached at rear end of bus body was investigated. The Wavier-Stokes equation is solved with SIMPLE method in general curvilinear coordinates system. RNG $k-\varepsilon$ turbulence model with the MARS scheme was used for the evaluating aerodynamic forces, velocity and pressure distribution. The results showed details of the three-dimensional wake flow in the immediate rear of bus body and the effect of rear-spoiler on the wake structure. A maximum of 14% reduction in drag coefficient was achieved for a model with a rear-spoiler.

• 한국전산유체공학회지
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• 제13권1호
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• pp.35-40
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• 2008

Generally flight vehicles have many cavities such as wheel wells, bomb bays and windows on their external surfaces and the flow around these cavities makes separation, vortex, shock and expansion waves, reattachment and other complex flow phenomenon. The flow around the cavity makes abnormal and three-dimensional noise and vibration even thought the aspect ratio (L/D) is small. The cavity giving large effects to the flow might make large noise, cause structural damage or breakage, harm the aerodynamic performance and stability, or damage the sensitive devices. In this study, numerical analysis was performed for cavity flows by the unsteady compressible three dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with Wilcox's $\kappa-\omega$ turbulence model. The MPI(Message Passing Interface) parallelized code was used for calculations by PC-cluster. The cavity has the aspect ratios of 2.5, 3.5 and 4.5 with the W/D ratio of 2 for three-dimensional cavities. The Sound Pressure Level (SPL) analysis was done with FFT to check the dominant frequency of the cavity flow. The dominant frequencies were analyzed and compared with the results of Rossiter's formula and Ahuja& Mendoza's experimental datum.

• 동력기계공학회지
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• 제5권1호
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• pp.21-26
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• 2001

This paper represents the turbulent intensity, the turbulent kinetic energy and Reynolds shear stress in the X-Y plane of cone type swirl gas burner measured by using X-probe from the hot-wire anemometer system. The experiment is carried out at flowrate 350 and $450{\ell}/min$ respectively in the test section of subsonic wind tunnel. The turbulent intensity and the turbulent kinetic energy show that the maximum value is formed in the narrow slits distributed radially on the edge of a cone type swirl burner, hence, the combustion reaction is anticipated to occur actively near this region. And the turbulent intensities ${\upsilon}\;and\;{\omega}$ are disappeared faster than the turbulent intensity u due to the inclined flow velocity ejecting from the swirl vanes of a cone type baffle plate of burner. Moreover, the Reynolds shear stress $u{\upsilon}$ is distributed about three times as large as the Reynolds shear stress $u{\omega}$ in the outer region of the cone type gas burner.

• 대한기계학회논문집B
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• 제22권9호
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• pp.1290-1306
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• 1998

An experimental study has been performed on a three-dimensional boundary layer over a rotating disk with an impinging jet at the center of the disk. The objective of the present study is to investigate the turbulence statistics of the three-dimensional turbulent boundary layer, which may be regarded as one of the simplest models for the flow in turbomachinery. Six components of the Reynolds stresses and ten triple products are measured by aligning the miniature X-wire probe to the mean velocity direction. The ratio of the wall-parallel shear stress magnitude to twice the turbulent kinetic energy in the near-wall region is strongly decreased by the impinging jet. In the case of the free rotating disk flow the shear stress vector lags behind the mean velocity gradient vector in the whole boundary layer, while the lag is weakened as the impinging jet speed increases.

• Journal of Advanced Marine Engineering and Technology
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• 제24권1호
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• pp.31-40
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• 2000

This paper describes the numerical computations of the interaction between the streamwise vortex and a flat plate 3-D turbulent boundary layer. In the present study, the main interest is in the behavior of the streamwise vortices introduced in turbulent boundary layers. The flow behind a vortex generator is modeled by the information that is avilable from studies on the dalta winglet. An algorithm of the solution of the incompressible Navier-Stokes equations for three-dimensional turbulent flows, together with a two layer turbulent model to resolve the near-wall flow, is based on the method of artificial compressibility. The present results show boundary layer distortion due to vortices, such as strong spanwise flow divergence and boundary thinning, and have a good agreement with the experimental data.

• Wind and Structures
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• 제10권6호
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• pp.511-532
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• 2007

The present study aims to generate turbulent inflow data to more accurately represent the turbulent flow around a square cylinder when the inflow turbulence level is significant. The modified random flow generation (RFG) technique in conjunction with a previously developed LES code is successfully adopted into a finite element based fluid flow solver to generate the required inflow turbulence boundary conditions for the three-dimensional (3-D) LES computations of transitional turbulent flow around a square cylinder at Reynolds number of 22,000. The near wall region is modelled without using wall approximate conditions and a wall damping coefficient is introduced into the calculation of sub-grid length scale in the boundary layer of the cylinder wall. The numerical results obtained from simulations are compared with each other and with the experimental data for different inflow turbulence boundary conditions in order to discuss the issues such as the synthetic inflow turbulence effects on the 3-D transitional flow behaviour in the near wake and the free shear layer, the basic mechanism by which stream turbulence interacts with the mean flow over the cylinder body and the prediction of integral flow parameters. The comparison among the LES results with and without inflow turbulence and the experimental data emphasizes that the turbulent inflow data generated by the present RFG technique for the LES computation can be a viable approach in accurately predicting the effects of inflow turbulence on the near wake turbulent flow characteristics around a bluff body.

• 대한기계학회논문집
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• 제19권9호
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• pp.2223-2236
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• 1995

0The fields of turbomachinery and electrical generators provide many examples of flow through rotating internal passages. At the practicing Reynolds number, most of the flow motion is three dimensional and highly turbulent. The proper understanding for the characteristics of these turbulent flow is necessary for the design of thermo-fluid machinery of a good efficiency. The flow characteristics in the rotating duct with curvature are very complex in practice due to the curvature and rotational effect of the duct. The understanding of the effect of the curvature on the structure and rotational effect of the duct. The understanding of the effect of the curvature on the structure of turbulence in the curved passage and the characteristics of the flow in a rotating radial straight channel have been well studied separately by many workers. But the combined effects of curvature and rotation on the flow have not been well understood inspite of the importance of the phenomena in the practical design process. In this study, the characteristics of a developing turbulent flow in a square sectioned 90.deg. bend rotating at a constant angular velocity are measured by using hot-wire anemometer to seize the rotational effects on the flow characteristics. As the results of this study, centrifugal forces associated with the curvature of the bend and Coriolis forces and centripetal forces associated with the rotational affect directly both the mean motion and the turbulent fluctuations.

• 동력기계공학회지
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• 제12권5호
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• pp.40-47
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• 2008

The turbulent flow characteristics around a small-size axial fan(SSAF) for a refrigerator are strongly dependent upon the operating points. Four operating points such as $\phi$ =0.1, 0.18, 0.25 and 0.32 were adopted in this study to investigate three-dimensional turbulent flow characteristics around the SSAF by using a fiber-optic type Laser Doppler Anemometer(LDA) system. Downstream mean velocity profiles of the SSAF along the radial distance show that axial and tangential velocity components exist predominantly, except $\phi$ = 0.1, and have a maximum value at $r/R{\fallingdotseq}0.8$, but radial velocity component having a relatively small value only turns flow direction to the outside or the central part of the SSAF. The turbulent intensity shows that the radial component exists most greatly after $r/R{\fallingdotseq}0.5$. Downstream turbulent kinetic energy at $\phi$ = 0.25 and 0.32 together has the largest peak value at $r/R{\fallingdotseq}0.9$.

• 대한기계학회논문집B
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• 제33권8호
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• pp.635-642
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• 2009

The three-dimensional turbulent wingtip vortex flows have been examined in the present study by using the commercial code FLUENT. The standard ${\kappa}-{\varepsilon}$ model is used as a closure relationship. The wing is constructed by using an elliptic body whose aspect ratio is 3.8 and the NACA 16-020 airfoil section. The simulations for various angle attack (${\alpha}=0^{\circ}$, $5^{\circ}$, and $10^{\circ}$) are carried out. The effect of Reynolds number is also investigated in this study. As the angle attack increases, the wingtip vortex becomes stronger. However, the relative vortex strength to inlet velocity decreases as Reynolds number increases.

• 수산해양기술연구
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• 제31권1호
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• pp.107-119
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• 1995

3차원 실내공간의 상부에서 일정유속이 유입하여 양측면 하부쪽으로 유출될 경우, 책상형의 가열 장애물 영향에 의한 유동장의 변화와 온도분포를 수치계산 한 결과 다음과 같은 결론을 얻었다. (1)표준 k-$\varepsilon$ 2 방정식 모델로 난류유동장을 해석한 결과 장애물을 기준으로 상하부에는 독립적인 큰 재순환 영역이 존재함을 알 수 있었다. (2) 유입류의 속도 변화에 따른 전제적인 유동패턴은 큰 변화없이 상사적인 분포를 하는 것을 알 수 있었다. (3) 유입속도가 감소함에 따라서, 공간에 대한 발열체의 온도영향이 커진다. (4) Re수가 작은 범위 (10 상(4)-10 상(5)에서는 열전달이 급격히 상승하여, Re수가 10 상(5)이상이 도면 열전달이 거의 일정하다.