Gwang Hoon Rhee

Direct numerical simulation of turbulent concentric annular pipe flow

Abstract A direct numerical simulation is performed for a turbulent concentric annular pipe flow at Re D h =8900 for two radius ratios ( R 1 / R 2 =0.1 and 0.5). Main emphasis is placed on the transverse curvature effect on near-wall turbulent structures. Near-wall turbulent structures close to the inner and outer walls are scrutinized by computing the lower-order and higher-order statistics. The Reynolds stress budgets are illustrated to confirm the results of the lower-order statistics. A quadrant analysis of the Reynolds shear stress is explored to develop a sufficiently complete picture of the contribution of flow events to turbulence production (consumption). Probability density functions of the inclination angles of the projected vorticity vectors are investigated to analyze the transverse curvature effects on the orientation of the vorticity field. The present numerical results show that the turbulent structures near the outer wall are more activated than those near the inner wall, which may be attributed to the different vortex regeneration processes between the inner and outer walls.

A nonlinear low-Reynolds-number k-ε model for turbulent separated and reattaching flows—II. Thermal field computations

Abstract An improved version of nonlinear low-Reynolds-number κ-e 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 R y (κ 1 2 y v ) instead of γ+ ( ≡ τ y v ) 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.

Enhancement of heat transfer in turbulent separated and reattaching flow by local forcing

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 a sinusoidally oscillating jet from a separation line. The Rhee and Sung version of the unsteady k I e I f model and the diffusivity tensor heat transfer model were m employed. The Reynolds number was fixed at Re s 33,000 and the forcing frequency was H varied in the range 0 F fH r U F 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.memployed. The Reynolds number was fixed at Re s 33,000 and the forcing frequency was H varied in the range 0 F fH rr rr U F 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.

Numerical prediction of locally forced turbulent boundary layer

An unsteady numerical simulation was performed to analyze flow structure behind a local suction/blowing in a flat-plate turbulent boundary layer. The local forcing was given to the boundary layer flow by means of a sinusoidally oscillating jet. A version of the unsteady k–e–fμ model [Fluid Dyn. Res. 26 (6) (2000) 421] was employed. The Reynolds number based on the momentum thickness was about Reθ=1700. The forcing frequency was varied in the range 0.011⩽f+⩽0.044 with a fixed forcing amplitude Ao=0.4. The predicted results were compared and validated with the experimental data. It was shown that the unsteady locally forced boundary layer flow is predicted well by the k–e–fμ model. The time-dependent numerical flow visualizations were demonstrated during one period of the local forcing. The effect of the pitch angle of local forcing on the reduction of skin friction was examined.

A nonlinear low-Reynolds number heat transfer model for turbulent separated and reattaching flows

Abstract A nonlinear low-Reynolds number heat transfer model is developed to predict turbulent flow and heat transfer in separated and reattaching flows. The k – e – f μ model of Park and Sung (T.S. Park, H.J. Sung, A new low-Reynolds-number model for predictions involving multiple surface, Fluid Dynamics Research 20 (1997) 97–113) is extended to a nonlinear formulation, based on the nonlinear model of Gatski and Speziale (G.B. Gatski, C.G. Speziale, On explicit algebraic stress models for complex turbulent flows, J. Fluid Mech. 254 (1993) 59–78). The limiting near-wall behavior is resolved by solving the f μ 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 λ function in conjunction with the f μ elliptic relaxation equation. Predictions of the present model are cross-checked with existing measurements and DNS data. The model performance is shown to be satisfactory.

Numerical prediction of locally forced turbulent separated and reattaching flow

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–e–fμ 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 ReH = 33 000, and the forcing frequency was varied in the range 0 ≤ StH ≤ 2. The predicted results were compared and validated with the experimental data of Chun and Sung (1996, 1998). It was shown that the unsteady locally forced separated and reattaching flows are predicted reasonably well with the k–e–fμ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

A low-Reynolds-number, four-equation heat transfer model for turbulent separated and reattaching flows

Abstract An algebraic heat flux model is applied to predict turbulent heat transfer in separated and reattaching flows. Based on the prior low-Reynolds-number k-e model of Park and Sung (1995), an improved version of the nonequilibrium heat transfer model is developed. The model performance is examined by solving the equations of the temperature variance kθ and its dissipation rate eθ, together with the equations of k and e. In the present model, the near-wall limiting behaviour close to the wall and the nonequilibrium effect away from the wall are incorporated. A tensor eddy-diffusivity is obtained to implement the orientation of mean temperature gradient in separated and reattaching flows. The validation of the model is applied to the turbulent flow over a backward facing step. The predictions of the present model are cross-checked with the existing measurements and direct numerical simulation (DNS) data. The model performance is shown to be generally satisfactory.

Evaluation of spreading thermal resistance for heat generating multi-electronic components

The Lee et al.s equation has been widely used to predict spreading thermal resistance in a single, centered heat source and axi-symmetric condition. However, the eccentric and multiple heat sources are mounted on the base plate in many electronic applications. Thus, it is necessary to determine the spreading thermal resistance for multiple heat sources. For this purpose, we establish the correlation to predict spreading thermal resistance in this study. The correlation which transforms four heat sources to a single equivalent heat source is proposed and then the spreading thermal resistance can be obtained with the Lee et al.s equation. When the four heat sources are mounted on a square base plate, the correlation is expressed as a function of the heat source size, the length of base plate, and the distance between heat sources. Compared to the results of three-dimensional numerical analysis, the spreading thermal resistance by the proposed correlation is in good agreement within 10 percent accuracy

Numerical Simulation of Turbulent Separated and Reattaching Flows by Local Forcing

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 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 , and the forcing frequency was varied in the range . 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 model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

A Non-linear Low-Reynolds-Number Heat Transfer Model for Turbulent Separated and Reattaching Flows

A nonlinear low-Reynolds number heat transfer model is developed to predict turbulent flow and heat transfer in separated and reattaching flows. The k‐e‐fm model of Park and Sung (T.S. Park, H.J. Sung, A new low-Reynoldsnumber model for predictions involving multiple surface, Fluid Dynamics Research 20 (1997) 97‐113) is extended to a nonlinear formulation, based on the nonlinear model of Gatski and Speziale (G.B. Gatski, C.G. Speziale, On explicit algebraic stress models for complex turbulent flows, J. Fluid Mech. 254 (1993) 59‐78). The limiting nearwall behavior is resolved by solving the fm 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 diAusivity tensor model is required. The nearwall asymptotic behavior is incorporated into the fl function in conjunction with the fm elliptic relaxation equation. Predictions of the present model are cross-checked with existing measurements and DNS data. The model performance is shown to be satisfactory. # 2000 Elsevier Science Ltd. All rights reserved.