• Title/Summary/Keyword: backward-facing step

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Effect of Two-Frequency Forcing on Flow Behind a Backward-Facing Step (이중주파수 가진이 후향계단 유동에 미치는 영향)

  • Yu, Jeong-Yeol;Jin, Song-Wan;Kim, Seong-Uk;Choe, Hae-Cheon;Kim, Sa-Ryang
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.26 no.3
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    • pp.423-431
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    • 2002
  • An experimental study is conducted to investigate the effect of two-frequency forcing on turbulent flow behind a backward-facing step at the Reynolds number of 27000 based on the step height. The forcing is provided from a thin slit located at the edge of the backward-facing step to increase mixing behind the backward-facing step and consequently to reduce the reattachment length. With single frequency forcing, the minimum reattachment length is obtained at the non-dimensional forcing frequency (F) of St$\_$h/ = 0.29. With two-frequency forcing, a subharmonic frequency (F/2) or biharmonic frequency (2F) is combined with the fundamental frequency (F), i.e. (F, F/2) or (F, 2F) forcing is applied. In the case of (F, F/2) forcing, the reattachment length is not much sensitive to the phase difference between F and F/2. However, the reattachment length significantly depends on the phase difference between F and 2F in the case of (F, 2F) forcing. At a certain range of the phase difference, the reattachment length becomes smaller than that of the single frequency forcing.

Design Study of a Dual-Mode Ramjet Engine with Large Backward-Facing Step (큰 후향 계단이 있는 이중 모드 램젯 엔진의 설계 연구)

  • Yang, Inyoung;Lee, Yang-Ji;Lee, Kyung-Jae
    • Journal of the Korean Society of Propulsion Engineers
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    • v.19 no.6
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    • pp.33-41
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    • 2015
  • Scaled model of a dual-mode ramjet engine with large backward-facing step, as a component of the rocket-based combined cycle engine, was designed. Design parameters were derived for this engine with the consideration of application for the rocket-based combined cycle engine. Design methodology was established for these design parameters. The design was partially verified through numerical study. Flow characteristics of the dual-mode ramjet engine with large backward-facing step was investigated experimentally. The design methodology for relevant design parameters established in this study was verified as feasible.

ARX Design Technique for Low Order Modeling of Backward-Facing-Step Flow Field (후향계단 유동장 저차 모델링을 위한 ARX 설계 기법)

  • Lee, Jin-Ik;Lee, Eun-Seok
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.40 no.10
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    • pp.840-845
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    • 2012
  • An ARX(Auto-Regressive eXogenous) modeling technique for vortex dynamics in the BFS(Backward Facing Step) flow field is proposed in this paper. In order for the modeling of the dynamics, the spatial and temporal modes are extracted through POD(Proper Orthogonal Decomposition) analysis. Determining the orders of the inputs and outputs for an ARX structure is carried out by the spectrum analysis and temporal mode analysis, respectively. The order of input delay terms is also determined by the flow velocity. Finally the coefficients of the ARX model are designed by using an artificial neural network.

Effects of the free Stream Turbulence Intensity on the Flow Over an Axisymmetric Backward-Facing Step (축대칭 하향단흐름에서 자유흐름 난류강도의 영향)

  • 양종필;김경천;부정숙
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.9
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    • pp.2328-2341
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    • 1995
  • An experimental study on the flow over the axisymmetric backward-facing step was carried out. The purposes of the present study are to investigate the effect of the free stream turbulence intensity on the reattachment length and to understand the turbulence structure of the recirculating flows. Local mean and fluctuating velocity components were measured in the separated and reattaching axisymmetric turbulent boundary layer over the wall of convex cylinder placed in a water tunnel by using 2-color 4-beam fiber optics laser Doppler velocimetry. As the free stream turbulence intensity increased, the reattachment length became shorter due to the enhanced mixing in the separated shear layer. It was also observed that the reverse flow velocity and turbulent kinetic energy increase with increasing free stream turbulence intensity. Spectral data and flow visualization showed that low-frequency motions occur in the separated flow behind a backward-facing step. These motions have a significant effect on the time-averaged turbulence data.

Performance Evaluation of Large Eddy Simulation for Recirculating and Swirling Flows (재순환 및 선회 유동에 대한 대와동모사(LES)의 성능검토)

  • Hwang Cheol-Hong;Lee Chang-Eon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.30 no.4 s.247
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    • pp.364-372
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    • 2006
  • The objective of this study is to evaluate the efficiency and the prediction accuracy of developed large eddy simulation (LES) program for complex turbulent flows, such as recirculating and swirling flows. To save the computational cost, a Beowulf cluster system consisting 16 processors was constructed. The flows in backward-facing step and dump combustor were examined as representative recirculating and swirling flows. Firstly, a direct numerical simulation (DNS) for laminar backward-facing step flows was previously conducted to validate the overall performance of program. Then LES was carried out for turbulent backward-facing step flows. The results of laminar flow showed a qualitative and quantitative agreement between simulations and experiments. The simulations of the turbulent flow also showed reasonable results. Secondly, LES results for non-swirling and swirling flows in a dump combustor were compared with the results of Reynolds-averaged Navier-Stokes (RANS) using standard $k-{\varepsilon}$ model. The results show that LES has a better performance in predicting the mean axial and azimuthal velocities, comer recirculation zone (CRZ) and center toroidal recirculation zone (CTRZ) than those of RANS. Finally, it was examined the capability of LES for the description of unsteady phenomena.

A Study on Mixed Convection Heat Transfer in Duct Flow behind a Backward-Facing Step by Using Schlieren Interferometer (쉴리렌 간섭계에 의한 사각덕트내 후향계단후 유동에서의 혼합대류 전열에 관한 연구)

  • Baek, B.J.;Pak, B.C.;Kim, J.T.
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.6 no.1
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    • pp.1-10
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    • 1994
  • The flow and heat transfer characteristics behind a backward facing step located in a vertical channel has been studied. In this study, the numerical prediction has been performed by solving the Navier-Stokes equation and energy equation simultaneously with the SIMPLE algorithm embedied in TEACH code. Local heat flux was measured by using Schlieren Interferometer. The flow visualization was performed using the cylindrical lens and the laser beam that is scattered by the supplied glycerine particles. The velocity and temperature distributions, recirculation region, reattachment length, and local heat flux are obtained under the various parameters to investigate the buoyancy effect on the flow and heat transfer characteristics behind the step.

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Cross-Spectral Characteristics of Wall Pressure Fluctuations in Flows over a Backward-Facing Step (후향계단 주위의 난류 박리재부착유동에서의 벽압력변동의 통계적 특징)

  • Lee, In-Won;Sung, Hyung-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.24 no.2
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    • pp.280-287
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    • 2000
  • Laboratory measurements were made of wall pressure fluctuations in a separated and reattaching flow over a backward-facing step. An array of 32 microphones along the streamwise direction was utilized. Various statistical properties of pressure fluctuations were scrutinized. The main emphasis was placed on the flow inhomogeneity along the streamwise direction. One point statistics such as the streamwise distribution of rms pressure and autospectra were shown to be generally consistent with other studies. The coherences and wavenumber spectra in the streamwise directions were indicative of the presence of dual modes in pressure; one is the large-scale vortical structure in low frequency and the other is the boundary-layer-like decaying mode in high frequency.

Effect Of The Separating Shear Layer on the Flow Over an Axisymmetric Backward-Facing Step (박리전단층이 축대칭 하향단흐름에 미치는 영향)

  • 부정숙;김경천;양종필
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.4
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    • pp.1102-1115
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    • 1995
  • An experimental study on the flow over the axisymmetric backward-facing step was carried out. The purpose of the present study is to investigate the effect of the boundary layer thickness at the separation point on the reattachment length and to understand the structure of the recirculating flows. Local mean and fluctuating velocity components were measured in the separating and reattaching axisymmetric turbulent boundary layer over the wall of convex cylinder placed in a water tunnel by using 2-color 4-beam fiber optics laser Doppler velocimetry. The study demonstrated that the reattachment length increases with increasing boundary layer thickness. It was also observed that the reverse flow velocity and turbulent kinetic energy decrease with an increase in the momentum thickness at the separation point. The measured velocity field suggests that the boundary layer thickness at the separation can affect definitely on the formation of corner eddy.

Reduced Order Modeling of Backward-Facing-Step Flow Field (후향계단 유동장 축약모델링 기법)

  • Lee, Jin-Ik;Lee, Eun-Seok
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.40 no.10
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    • pp.833-839
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    • 2012
  • In this paper, we analyze the reconstruction error in the modeling of flow field on BFS(Backward Facing Step). In order for the mathematical modelling of a density on the field, the spatial and temporal modes are extracted by POD(Proper Orthogonal Decomposition) method. After formulating the modeling error, we summarize the relationship between the energy strength and the reconstruction errors. Moreover the allowable modeling error limits in the flow control point of view are confined by analysing in the frequency domain as well as time domain of the reconstructed data.