• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.106-110
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• 2004

Generally, flow around a bluff body such as a circular cylinder is complicated compared with that around a streamlined body because of the existence of separated shear layers. Long bluff body such as a flat blunt plate is more complicated than short bluff body, because of separated-and-reattaching flow on the after bodies.(omitted)

• 대한기계학회논문집B
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• 제24권3호
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• pp.467-476
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• 2000

An unsteady numerical simulation was performed for locally-forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the $k-{\varepsilon}-f_{\mu}$ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at $Re_H=33000$, and the forcing frequency was varied in the range $0{\leq}St_H{\leq}2$. The predicted results were compared and validated with the experimental data of Chun and Sung. It was shown that the unsteady locally-forced separated and reattaching flows are predicted reasonably well with the $k-{\varepsilon}-f_{\mu}$ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

• 대한기계학회논문집B
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• 제25권1호
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• pp.87-95
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• 2001

A numerical study was made of heat transfer in locally-forced turbulent separated and reattaching flow over a backward-facing step. The local forcing was given to the flow by means of sinusoidally oscillating jet from a separation line. A Rhee and Sung version of the unsteady $\kappa$-$\varepsilon$-f(sub)u model and the diffusivity tensor heat transfer model were employed. The Reynolds number was fixed at Re(sub)H=33,000 and the forcing frequency was varied in the range 0$\leq$fH/U(sub)$\infty$$\leq$2. The condition of constant heat flux was imposed at the bottom wall. The predicted results were compared and validated with the experimental data of Chun and Sung and Vogel and Eaton. The enhancement of heat transfer in turbulent separated and reattaching flow by local forcing was evaluated and analyzed.

• 대한기계학회논문집B
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• 제24권2호
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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.

• 대한기계학회논문집
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• 제19권8호
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• pp.2039-2050
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• 1995

The nonlinear low-Reynolds-number k..epsilon. model of park and Sung is extended to predict the turbulent heat transports in separated and reattaching flows. The equations of the temperature variance( $k_{\theta}$ and its dissipation rate(.epsilon.$_{\theta}$ are solved, in concert with the equations of the turbulent kinetic energy(k) and its dissiation rate(.epsilon). In the present model, the near-wall effect and the non-equilibrium effect are fully taken into consideration. The validation of the model is then applied to the turbulent flow behind a backward-facing step and the flow over a blunt body. The predicted results of the present model are compared and evaluated with the relevant experiments.

• 대한기계학회논문집B
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• 제24권2호
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• pp.316-323
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• 2000

A nonlinear low-Reynolds-number heat transfer model is developed to predict turbulent flow and heat transfer in separated and reattaching flows. The $k-{\varepsilon}-f_{\mu}$ model of Park and Sung (1997) is extended to a nonlinear formulation, based on the nonlinear model of Gatski and Speziale (1993). The limiting near-wall behavior is resolved by solving the $f_{\mu}$ elliptic relaxation equation. An improved explicit algebraic heat transfer model is proposed, which is achieved by applying a matrix inversion. The scalar heat fluxes are not aligned with the mean temperature gradients in separated and reattaching flows; a full diffusivity tensor model is required. The near-wall asymptotic behavior is incorporated into the $f_{\lambda}$ function in conjunction with the $f_{\mu}$ elliptic relaxation equation. Predictions of the present model are cross-checked with existing measurements and DNS data. The model preformance is shown to be satisfactory.

• 대한기계학회논문집
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• 제19권8호
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• pp.2051-2063
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• 1995

An improved version of nonlinear low-Reynolds-number k-.epsilon. model is developed. In this model, the limiting near-wall behavior and nonlinear Reynolds stress representations are incorporated. Emphasis is placed on the adoption of Ry(.iden. $k^{1}$2/y/.nu.) instead of $y^{[-10]}$ (.iden. $u_{{\tau}/y/{\nu}}$) in the low-Reynolds-number model for predicting turbulent separated and reattaching flows. The non-equilibrium effect is examined to describe recirculating flows away from the wall. The present model is validated by doing the benchmark problem of turbulent flow behind a backward-facing step. The predictions of the present model are cross-checked with the existing measurements and DNS data. The model performance is shown to be generally satisfactory.

• 대한기계학회논문집
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• 제19권9호
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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.

• 대한기계학회논문집B
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• 제27권12호
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• pp.1708-1715
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• 2003

An experimental study was made of turbulent separated and reattaching flow over a backward-facing step, where unsteady wake was generated by a spoked-wheel type wake generator with cylindrical rods in front of the separated flow. The influence of unsteady wake was scrutinized in terms of the rotating speed of the wake generator (0$\leq$S $t_{H}$$\leq$0.4). A conditional averaging technique in corporation with SBF was employed to elucidate the influence of the unsteady wake on the large-scale vortical structures of the separated flow. Special attention was made during two-dimensional measurements of wall-pressure with or without unsteady wake. The wall-pressure fluctuations were used to predict dipole sound source by Curie's integral formula. It was found that the reduction of the dipole sound source was due to the reduction of turbulent kinetic energy by unsteady wake in the recirculation region.n.

• 대한기계학회논문집
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• 제9권4호
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• pp.403-413
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• 1985

본 논문에서는 수직벽 하류에 형성되는 박이 전단층의 발전과 재부착 그리고 재발전 경계층에 대해 평균 속도, 벽면의 압력 분포, 난류 강도, 레이놀즈 전단 압력 및 아직 수직벽에 대해서는 보고된바 없는 난류 떨림 속도 성분들의 3승곱 통계치를 측정하여 난류 구조의 변화를 분석하고 이를 수치적 계산 모델개발의 자 료로 제공하고자 함이 이 연구의 목적이다.