• Title/Summary/Keyword: Reattachment

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NUMERICAL ANALYSIS OF THREE DIMENSIONAL SUPERSONIC CAVITY FLOW FOR THE VARIATION OF CAVITY SPANWISE RATIO (공동의 폭 변화에 따른 3차원 초음속 공동 유동연구)

  • Woo, C.H.;Kim, J.S.
    • Journal of computational fluids engineering
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    • v.11 no.4 s.35
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    • pp.62-66
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    • 2006
  • High-speed flight vehicle have various cavities. The supersonic cavity flow is complicated due to vortices, flow separation, reattachment, shock waves and expansion waves. The general cavity flow phenomena includes the formation and dissipation of vortices, which induce oscillation and noise. The oscillation and noise greatly affect flow control, chemical reaction, and heat transfer processes. The supersonic cavity flow with high Reynolds number is characterized by the pressure oscillation due to turbulent shear layer, cavity geometry, and resonance phenomenon based on external flow conditions. The resonance phenomena can damage the structures around the cavity and negatively affect aerodynamic performance and stability. In the present study, we performed numerical analysis of cavities by applying the unsteady, compressible three dimensional Reynolds-Averaged Navier-Stokes(RANS) equations with the ${\kappa}-{\omega}$ turbulence model. The cavity model used for numerical calculation had a depth(D) of 15mm cavity aspect ratio (L/D) of 3, width to spanwise ratio(W/D) of 1.0 to 5.0. Based on the PSD(Power Spectral Density) and CSD(Cross Spectral Density) analysis of the pressure variation, the dominant frequency was analyzed and compared with the results of Rossiter's Eq.

Jet Impingement Heat Transfer on a Pedestal Encountered in Chip Cooling (충돌제트를 이용한 pedestal 형상의 칩 냉각연구)

  • Lee, Dae-Hee;Chung, Seung-Hoon;Chung, Young-Suk;Lee, Joon-Sik
    • Proceedings of the KSME Conference
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    • 2001.11b
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    • pp.124-130
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    • 2001
  • The heat transfer and flow measurements were made on a cylindrical pedestal mounted on a flat plate with a turbulent impinging air jet. The heat transfer coefficient distributions on the flat plate were measured using the shroud-transient technique and liquid crystal was used to measure the surface temperature. The jet Reynolds number (Re) is 23,000, the dimensionless nozzle-to-surface distance (L/d) from 2 to 10, the dimensionless pedestal diameter-to-height (H/D) from 0 to 1.5, the dimensionless 2nd pedestal diameter-to-height ($H/D_2$) from 0 to 0.4 and the distance from the stagnation point to 2nd pedestal (p/D). The results show that for H/D = 0.5 to 1.5, the Nusselt number distributions on the plate surface exhibit a maximum between $r/d\;{\cong}\;1.0$ and 1.5. The presence of the pedestal appears to cause the flow separation and reattachment on the plate surface, which results in the maximum heal transfer coefficient. Also, for p/D = 2.5 and $H/D_2$ = 0.3, the local Nusselt number in the region corresponding to $r/d\;{\cong}\;1.1$ was increased up to 50% compared to that for $H/D_2=0$.

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Unsteady Wall Pressure Fluctuation Generated from the Impinging Vortex on the Chamfered Forward Step (모따기된 전향계단에 부딪치는 와류에 의한 비정상 벽면압력 변동)

  • Ryu, Ki-Wahn;Lee, Jun-Shin
    • Proceedings of the KSME Conference
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    • 2001.11b
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    • pp.312-317
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    • 2001
  • Modifying effects of the rectangular forward step for suppressing the unsteady pressure fluctuation during interaction between the upstream vortical flow with the edge are studied numerically. The vortical flow is modeled by a point vortex, and the unsteady pressure coefficient is obtained from the velocity and the potential field. To investigate the effects of the edge shape the rectangular forward step is chamfered with various angles. Calculation show that the pressure peaks become decreased by increasing the vortex height as well as the chamfering angle. The pressure amplitudes are very sensitive to the change of the initial vortex height. From this study we can find out that the chamfered edge has two effects; the one is that it suppresses the pressure amplitude generated from the edge, and the other is that it decreases the time variation of unsteady pressure fluctuation. These modifying concepts can be applied to attenuate the self-sustained oscillation mechanism at the open cavity flow.

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Effect of Boundary Layer Thickness on the Flow Characteristics around a Rectangular Prism (직사각형 프리즘 주위의 유동특성에 대한 경계층 두께의 영향)

  • Ji, Ho-Seong;Kim, Kyung-Chun
    • Proceedings of the KSME Conference
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    • 2001.11b
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    • pp.306-311
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    • 2001
  • Effect of boundary layer thickness on the flow characteristics around a rectangular prism has been investigated by using a PIV(Particle Image Velocimetry) technique. Three different boundary layers(thick, medium and thin)were generated in the Atmospheric Boundary Layer Wind Tunnel at Pusan National University. The thick boundary layer having 670mm thickness was generated by using spires and roughness elements. The medium thickness of boundary layer$(\delta=270mm)$ was the natural turbulent boundary layer at the test section with fully long developing length(18m). The thin boundary layer with 36.5mm thickness was generated by on a smooth panel elevated 70cm from the wind tunnel floor. The Reynolds number based on the free stream velocity and the height of the model was $7.9{\times}10^3$. The mean velocity vector fields and turbulent kinetic energy distribution were measured and compared. The effect of boundary layer thickness is clearly observed not only in the length of separation bubble but also in the reattachment points. The thinner boundary layer thickness, the higher turbulent kinetic energy peak around the model roof. It is strongly recommended that the height ratio between model and approaching boundary layer thickness should be a major parameter.

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Effects of Bleed Flow and Angled Ribs on Heat Transfer Distributions in a Rotating Square Channel (유출유동 및 각도진 요철이 회전하는 사각덕트 내 열전달분포에 미치는 영향)

  • Park, Suk-Hwan;Jeon, Yun-Heung;Kim, Kyung-Min;Lee, Dong-Hyun;Cho, Hyung-Hee
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.1 s.256
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    • pp.76-82
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    • 2007
  • The present study investigated the effects of channel rotation and bleed flow on heat/mass transfer in a square channel with $45^{\circ}$ rib turbulators. The bleed holes were located between the rib turbulators on the leading surface and those on the trailing surface case by case. The tests were conducted under the conditions of various bleed ratios (0.0, 0.2, 0.4) and rotation numbers (0.0, 0.2, 0.4) at Re=10,000. The results suggested that heat/mass transfer characteristics were influenced by the Coriolis force, decrement of main flow rate, secondary flow by angled ribs and bleed hole location. As the bleed ratio (BR) increased, the heat/mass transfer decreased on both surfaces due to the reduction of main flow rate. With increment of the rotation number, heat/mass transfer also decreased and almost the same because the reattachment of the secondary flow induced by angled ribs was weakened on the leading surface and the secondary flow was disturbed on the trailing surface by the Coriolis force.

Numerical Study of Pulsatile Blood Flow in Stenotic Artery (협착 동맥에서의 맥동 혈류 유동에 대한 수치해석적 연구)

  • Seo, Tae-Won;Buriev, Bahtiyor
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.32 no.11
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    • pp.891-896
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    • 2008
  • In the present computational study, simple stenotic artery models using pulsatile flow condition were investigated. A 1 Hz non-reversing sinusoidal velocity for pulsatile flow was imposed at the flow inlet and the corresponding Womersley number based on the vessel radius is 2.75. The simple stenotic geometries have been used that consist of 25%, 50% and 75% semicircular constriction in a cylindrical tube. In this paper, numerical solutions are presented for a first harmonic oscillatory flow using commercial software ADINA 8.4. As stenosis and Reynolds number increase, the maximum wall shear stress(WSS) increases while the minimum WSS decreases. As the stenotic rate increases, the pressure drop at the throat severely decreases to collapse the artery and plaque. It is found that the fluid mechanical disturbances due to the constriction were highly sensitive with rate of stenosis and Reynolds number. When Reynolds number and stenosis increase, the larger recirculation region exists. In this recirculation region the possibility of plaque attachment is increasingly higher. The present results enhance our understanding of the hemodynamics of a stenotic artery.

Numerical Study for 3D Turbulent Flow in High Incidence Compressor Cascade (고입사각 압축기 익렬 내의 3차원 난류유동에 관한 수치적 연구)

  • 안병진;정기호;김귀순;임진식;김유일
    • Journal of the Korean Society of Propulsion Engineers
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    • v.6 no.3
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    • pp.29-36
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    • 2002
  • A numerical analysis based on two-dimensional and three-dimensional incompressible Wavier-Stokes equations has been carried out for double-circular-arc compressor cascades and the results are compared with available experimental data at various incidence angles. The 2-D and 3-D computational codes based on SIMPLE algorithm adopt pressure weighted interpolation method for non-staggered grid and hybrid scheme for the convective terms. Turbulence modeling is very important for prediction of cascade flows, which are extremely complex with separation and reattachment by adverse pressure gradient. Considering computation times, $\kappa$-$\varepsilon$ turbulence model with wall function is used.

Effects of inflow turbulence and slope on turbulent boundary layer over two-dimensional hills

  • Wang, Tong;Cao, Shuyang;Ge, Yaojun
    • Wind and Structures
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    • v.19 no.2
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    • pp.219-232
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    • 2014
  • The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

Multi-Objective Optimization of a Dimpled Channel Using NSGA-II (NSGA-II를 통한 딤플채널의 다중목적함수 최적화)

  • Lee, Ki-Don;Samad, Abdus;Kim, Kwang-Yong
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03b
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    • pp.113-116
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    • 2008
  • This work presents numerical optimization for design of staggered arrays of dimples printed on opposite surfaces of a cooling channel with a fast and elitist Non-Dominated Sorting of Genetic Algorithm (NSGA-II) of multi-objective optimization. As Pareto optimal front produces a set of optimal solutions, the trends of objective functions with design variables are predicted by hybrid multi-objective evolutionary algorithm. The problem is defined by three non-dimensional geometric design variables composed of dimpled channel height, dimple print diameter, dimple spacing and dimple depth to maximize heat transfer rate compromising with pressure drop. Twenty designs generated by Latin hypercube sampling were evaluated by Reynolds-averaged Navier-Stokes solver and the evaluated objectives were used to construct Pareto optimal front through hybrid multi-objective evolutionary algorithm. The optimum designs were grouped by k-mean clustering technique and some of the clustered points were evaluated by flow analysis. With increase in dimple depth, heat transfer rate increases and at the same time pressure drop also increases, while opposite behavior is obtained for the dimple spacing. The heat transfer performance is related to the vertical motion of the flow and the reattachment length in the dimple.

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THE STUDY ON THE SEPARATED FLOW OF A HUMP USING RANSMODELING (RANS 모델링을 이용한 Hump 형상의 박리 유동에 대한 연구)

  • Lee, J.;Bae, J.H.;Jung, K.J.
    • Journal of computational fluids engineering
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    • v.22 no.1
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    • pp.8-14
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    • 2017
  • In this paper, separated flow characteristics is studied using the RANS(Reynold-averaged Navier-Stokes) modeling. The analysis is performed for the NASA's hump configuration which is the combination of a flat plate and a hump. This configuration was used in NASA's flow control workshop and it was one of validation cases for RANS and LES simulations. The separation occurs at the 65% of model length where a slot is positioned for the flow control. No flow control case and steady suction case are studied using RANS modeling. The Spalart-Allmaras model and the SST(Shear Stress Transport) model are applied and their accuracy are compared. To correlate CFD analysis with experimental data, the optimal boundary condition was investigated and the effect of a cavity around the slot is studied for the no flow case.