• Title, Summary, Keyword: Unsteady pressure drag coefficient

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Numerical study on the effect of three-dimensional unsteady tunnel entry flow characteristics on the aerodynamic performance of high-speed train (터널진입시 비정상 유동특성이 고속전철의 공력성능에 미치는 영향에 관한 수치해석적 연구)

  • 정수진;김태훈;성기안
    • Journal of Advanced Marine Engineering and Technology
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    • v.26 no.5
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    • pp.596-606
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    • 2002
  • The three-dimensional unsteady compressible Euler equation solver with ALE, CFD code, PAM-FLOW based on FEM method has been applied to analyze the flow field around the high speed train which is entering into a channel. From the present study, the pressure and flow transients were calculated and analyzed. The generation of compression wave was observed ahead of train and the high pressure in the gap between the train and the tunnel was also found due to the blockage effects. It was found that abrupt fluctuation in pressure exists in the region from train nose to shoulder of train corresponding to 10% of total length of train during tunnel entry. Computed time history of aerodynamic forces of train during tunnel entry show that drag coefficient rapidly rises and saturates at about non-dimensional time 0.31. The total increase of drag coefficient before and after tunnel entry is about 1.1%. Transient profile of lift force shows similar pattern to drag coefficient except abrupt drop after saturation and lift force in the tunnel increases 0.08% more than that before tunnel entry.

Unsteady Aerodynamics of Flat Plate with Porous Trailing-edge (다공성 표면 평판 끝 단 위의 비정상 공력 특성에 대한 연구)

  • Jeong, Ye-Eun;Moon, Young-J.
    • 한국전산유체공학회:학술대회논문집
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    • pp.134-137
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    • 2008
  • In this study, a computational analysis is conducted to investigate the effects of porous surfaces on the lift and drag forces of the flat plate. With the porous treatment, it is found that the strength of the Karman vortex as well as its influences over the trailing-edge surface are much weakened, resulting in significant reduction of the pressure fluctuations over the flat plate. The drag and lift coefficients are decreased by 85% and 18%, respectively, compared to the solid surface. The computed results also indicate that the size of the porous surface area does not have much influences but the back side of the flat plate has non-negligible effects on the interaction between the wall and the Karman vortex. As a result, the lift coefficient for the solid back side case is decreased only by 50.5% compared to the solid case and the drag coefficient is even increased by 65%.

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Large Eddy Simulation on the Drag and Static Pressure Acting on the Blade Surface of Three-Dimensional Small-Size Axial Fan with Different Operating Loads (운전부하에 따른 3차원 소형축류홴 날개표면에 작용하는 정압과 항력에 대한 대규모와 모사)

  • Kim, Jang-Kweon;Oh, Seok-Hyung
    • Journal of Power System Engineering
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    • v.21 no.2
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    • pp.57-63
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    • 2017
  • The large-eddy simulation(LES) was carried out to evaluate the drag and static pressure acting on the blade surface of a small-size axial fan(SSAF) under the condition of unsteady-state, incompressible fluid and three-dimensional coordination. The axial component of drag coefficient increases with the increase of operating load, but the radial components have negligible sizes regardless of operating loads. Otherwise, the static pressures acting on the blade surfaces of SSAF show different distributions around the operating point of D equivalent to the stall. Also, with the increase of operating load, the static pressures acting on the pressure and suction surfaces of blade concentrate at the tips and leading-edges as a whole.

Aerodynamic Characteristics of 2-D, Unsteady Flow Past a Square Cylinder (Revaluation of SOLA Scheme) (2차원각주의 비정상 공력특성(SOLA스킴의 재평가))

  • 이영호
    • Journal of Advanced Marine Engineering and Technology
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    • v.14 no.2
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    • pp.48-65
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    • 1990
  • Numerical solutions of the Navier-Stokes equations, governing 2-dimensional, time-dependent, viscous, incompressible fluid flow past a square cylinde in an infinite region, are presented for Reynolds numbers $10^2$, $10^3$and $10^4$. Finite-difference scheme, based on SOLA-VOF is adopted and a discretization of the convection term for irregular grid is newly suggested by altering the original nonconservation form into conservation one. Distribution of finer grids around the body reveals fairly reasonable consistency with the experimental variables : drag coefficient, lift coefficient, Strouhal number, fluctuating pressure coefficient, etc.

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Unsteady Aerodynamic Characteristics depending on Reduced Frequency for a Pitching NACA0012 Airfoil at Rec=2.3×104

  • Kim, Dong-Ha;Chang, Jo-Won;Sohn, Myong Hwan
    • International Journal of Aeronautical and Space Sciences
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    • v.18 no.1
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    • pp.8-16
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    • 2017
  • Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number $2.3{\times}10^4$. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

Numerical Analysis for the Unsteady Laminar Flow and Heat Transfer Around a Circular Cylinder (원주 주위의 비정상 층류유동과 열전달에 대한 수치해석)

  • 조석호;남청도;부정숙
    • Journal of Advanced Marine Engineering and Technology
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    • v.15 no.2
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    • pp.64-72
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    • 1991
  • The unsteady, two-dimensional laminar flow and heat transfer of an incompressible, constant-property fluid flowing around a circular cylinder are numerically analyzed. The Navier-Strokes equation and the energy equation are solved by the finite difference method. The range of the Reynolds number is 10 to 100 and the Prandtl number considered is 0.7. The contours of the flow pattern, equi-vorticity line and isotherm pattern around a circular cylinder are shown. Also, numerical solutions of the surface vorticity, pressure coefficient, drag coefficient, local Nusselt number and mean Nusselt number are obtained. The numerical results for the final time fo calculation are compared with the other available experimental and theoretical results for the steady state and are found to be in good agreement with them.

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Laminar Flow over Two Spheres in a Tandem Arrangement (직렬로 배열된 두 개의 구를 지나는 층류 유동)

  • Kim Dong-Joo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.30 no.5
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    • pp.480-488
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    • 2006
  • Numerical simulations of flow over two spheres placed in a tandem arrangement are conducted to investigate the effect of the inter-sphere spacing on the flow characteristics. The Reynolds numbers considered are 100, 250, 300 and 425, corresponding to steady axisymmetric, steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows, respectively, in the case of a single sphere. For small inter-sphere spacings, the flow past two spheres is more stable than that past a single sphere. For example, with the spacing of the sphere radius, the flow is steady axisymmetric up to Re=300. However, for relatively large spacings, the flow past two spheres becomes unstable and vortex shedding takes place even at Re=250. The drag coefficient of the rear sphere decreases significantly with decreasing inter-sphere spacing due to reduction of the stagnation pressure, thus being smaller than that of the front sphere. Also, the rear sphere shows large fluctuations of the lift force as compared to the front one in the case of unsteady flow.

A Study on Vortex Shedding Characteristics of Rectangular Marine Structure With Aspect Ratio (장방형 해양구조물의 변장비에 따른 와방출 특성에 관한 연구)

  • 김진구;조대환
    • Journal of the Korean Society of Marine Environment & Safety
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    • v.5 no.2
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    • pp.35-44
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    • 1999
  • High negative pressure coefficient is formed in the corner of the bluff body structures. For many curtain wall designers this phenomena is of interest because this high negative pressure coefficient is adopted in structural calculation. The present study is aimed to investigate shedding vortex characteristics of two-dimensional rectangular prism flow. Unsteady calculation by finite difference method based upon SOLA is carried out for three aspect ratios(1:1, 1:2, 1:3) of Re=10$^4$ in viscous incompressible flow within infinite domain. Fluctuation of velocity components at various pick-up points and time variation of drag and lift coefficients are analysed by FFT method to reveal shedding vortex frequency patterns. At aspect ratio 1:1, one primary Strouhal number appears for about all pick-up points. At aspect ratio 1:2, two representative Strouhal numbers are classified by pick-up positions and their flows show two different reattachment patterns. For aspect ratio 1:3, frequency spectrum maintains multiple peaks.

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Numerical Study on the Drag of a Car Model under Road Condition (주행조건에서의 자동차 모델 항력에 대한 수치해석적 연구)

  • Kim, Beom-Jun;Kang, Sung-Woo;Choi, Hyoung-gwon;Yoo, Jung-Yul
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.27 no.8
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    • pp.1182-1190
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    • 2003
  • A parallelized FEM code based on domain decomposition method has been recently developed for large-scale computational fluid dynamics. A 4-step splitting finite element algorithm is adopted for unsteady flow computation of the incompressible Navier-Stokes equation, and Smagorinsky LES model is chosen for turbulent flow computation. Both METIS and MPI Libraries are used for domain partitioning and data communication between processors, respectively. Tiburon model of Hyundai Motor Company is chosen as the computational model at Re=7.5 $\times$ 10$^{5}$ , which is based on the car height. The calculation is carried out under both the wind tunnel condition and the road condition using IBM SP parallel architecture at KISTI Super Computing Center. Compared with the existing experimental data, both the velocity and pressure fields are predicted reasonably well and the drag coefficient is in good agreement. Furthermore, it is confirmed that the drag under the road condition is smaller than that under the wind-tunnel condition.

Computation of Turbulent Flows and Radiated Sound From Axial Compressor Cascade

  • Lee, Seungbae;Kim, Hooi-Joong;Kim, Jin-Hwa;Song, Seung-Jin
    • Journal of Mechanical Science and Technology
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    • v.18 no.2
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    • pp.272-285
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    • 2004
  • The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from -40$^{\circ}$ to +20$^{\circ}$ and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8$^{\circ}$ caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.