本研究採用 OpenFOAM 進行二維方塊尾流之大渦模擬（Large-Eddy Simulation, LES），並於入口處給定隨時間變化之紊流入流條件，以提供 LES 計算所需之非定常入口速度場。數值結果與粒子影像測速法（Particle Image Velocimetry, PIV）實驗結果進行比較，以分析方塊後方尾流流場特性。研究中主要探討正規化時間平均速度、紊流強度、動量通量及紊流動能（Turbulent Kinetic Energy, TKE），用以評估低速尾流區、回流區、分離剪切層發展與尾流恢復特性。
　　結果顯示，入口流向紊流強度對方塊後方尾流發展具有明顯影響。當入口流向紊流強度較低時，尾流混合作用較弱，使低速尾流區與近壁面回流區延伸較長，速度恢復較慢。隨著入口流向紊流強度增加，分離剪切層中的速度擾動增強，流向與垂直方向之紊流強度皆隨之提高，並促進尾流區與外部高動量流體之間的混合，使平均速度場恢復加快，再附著位置向上游移動，且回流區範圍縮短。
　　由動量通量結果可知，較高的入口流向紊流強度會增強尾流區內的垂直動量傳輸與混合效率。紊流動能分析則顯示，方塊上緣後方的分離剪切層為主要 TKE 生成區，其中剪切生成項為主要來源，而平流項、紊流傳輸項與耗散項則分別影響 TKE 的傳輸、重新分配與消散。
　　因此，入口流向紊流強度是影響二維方塊後方尾流發展的重要參數。較高的入口流向紊流強度會增強尾流混合與動量交換，加速平均速度恢復並縮短回流區範圍。

This study employs OpenFOAM to perform large-eddy simulation (LES) of the wake flow behind a two-dimensional block. A time-varying turbulent inflow condition is prescribed at the inlet to provide the unsteady incoming velocity field required for the LES calculation. The numerical results are compared with particle image velocimetry (PIV) experimental data to analyze the wake-flow characteristics downstream of the block. The main quantities investigated include normalized time-averaged velocity, turbulence intensity, momentum flux, and turbulent kinetic energy (TKE), which are used to evaluate the low-velocity wake region, recirculation region, separated shear-layer development, and wake recovery characteristics.
　　The results show that the inlet streamwise turbulence intensity has a significant influence on the wake development behind the block. When the inlet streamwise turbulence intensity is low, wake mixing is weaker, causing the low-velocity wake region and near-wall recirculation region to extend farther downstream and resulting in slower velocity recovery. As the inlet streamwise turbulence intensity increases, the velocity fluctuations in the separated shear layer become stronger, and both the streamwise and vertical turbulence intensities increase. This promotes the mixing between the wake region and the external high-momentum flow, accelerates the recovery of the mean velocity field, shifts the reattachment position upstream, and shortens the recirculation region.

　　The momentum flux results indicate that higher inlet streamwise turbulence intensity enhances vertical momentum transport and mixing efficiency within the wake region. The turbulent kinetic energy analysis further shows that the separated shear layer behind the upper edge of the block is the main region of TKE generation. Among the TKE budget terms, shear production is the dominant source, while advection, turbulent transport, and dissipation affect the transport, redistribution, and dissipation of TKE, respectively.
　　Therefore, inlet streamwise turbulence intensity is an important parameter affecting the wake development behind a two-dimensional block. Higher inlet streamwise turbulence intensity enhances wake mixing and momentum exchange, accelerates mean velocity recovery, and shortens the recirculation region.

[1] Y. Tominaga, L. Wang, Z. Zhai, and T. Stathopoulos, “Accuracy of CFD simulations in urban aerodynamics and microclimate: Progress and challenges,” Building and Environment, vol. 243, p. 110723, 2023.
[2] P. A. Mirzaei, “CFD modeling of micro and urban climates: Problems to be solved in the new decade,” Sustainable Cities and Society, vol. 69, p. 102839, 2021.
[3] M. Shirzadi, P. A. Mirzaei, and M. Naghashzadegan, “Improvement of k-epsilon turbulence model for CFD simulation of atmospheric boundary layer around a high-rise building using stochastic optimization and Monte Carlo sampling technique,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 171, pp. 366–379, 2017.
[4] N. Antoniou, H. Montazeri, M. Neophytou, and B. Blocken, “CFD simulation of urban microclimate: Validation using high-resolution field measurements,” Science of The Total Environment, vol. 695, p. 133743, 2019.
[5] H. L. Gough, M. F. King, P. Nathan, C. S. B. Grimmond, A. G. Robins, C. J. Noakes, Z. Luo, and J. F. Barlow, “Influence of neighbouring structures on building façade pressures: Comparison between full-scale, wind-tunnel, CFD and practitioner guidelines,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 189, pp. 22–33, 2019.
[6] Y. Tominaga, A. Mochida, R. Yoshie, H. Kataoka, T. Nozu, M. Yoshikawa, and T. Shirasawa, “AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 96, no. 10–11, pp. 1749–1761, 2008.
[7] D. A. Lyn, S. Einav, W. Rodi, and J.-H. Park, “A laser-Doppler velocimetry study of ensemble-averaged characteristics of the turbulent near wake of a square cylinder,” Journal of Fluid Mechanics, vol. 304, pp. 285–319, 1995.
[8] G. Hu, K. T. Tse, Z. S. Chen, and K. C. S. Kwok, “Particle Image Velocimetry measurement of flow around an inclined square cylinder,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 168, pp. 134–140, 2017.
[9] I. P. Castro and A. G. Robins, “The flow around a surface-mounted cube in uniform and turbulent streams,” Journal of Fluid Mechanics, vol. 79, no. 2, pp. 307–335, 1977.
[10] R. J. Hearst, G. Gomit, and B. Ganapathisubramani, “Effect of turbulence on the wake of a wall-mounted cube,” Journal of Fluid Mechanics, vol. 804, pp. 513–530, 2016.
[11] H. C. Lim, T. G. Thomas, and I. P. Castro, “Flow around a cube in a turbulent boundary layer: LES and experiment,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 97, pp. 96–109, 2009.
[12] W. Rodi, “Comparison of LES and RANS calculations of the flow around bluff bodies,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 69–71, pp. 55–75, 1997.
[13] C. Diaz-Daniel, S. Laizet, and J. C. Vassilicos, “Direct numerical simulations of a wall-attached cube immersed in laminar and turbulent boundary layers,” International Journal of Heat and Fluid Flow, vol. 68, pp. 269–280, 2017.
[14] R. Vinuesa, P. Schlatter, J. Malm, C. Mavriplis, and D. S. Henningson, “Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions,” Journal of Turbulence, vol. 16, no. 6, pp. 555–587, 2015.
[15] F. X. Trias, A. Gorobets, and A. Oliva, “Turbulent flow around a square cylinder at Reynolds number 22,000: A DNS study,” Computers & Fluids, vol. 123, pp. 87–98, 2015.
[16] W.-C. Cheng and F. Porté-Agel, “Evaluation of subgrid-scale models in large-eddy simulation of flow past a two-dimensional block,” International Journal of Heat and Fluid Flow, vol. 44, pp. 301–311, 2013.
[17] Y. Cao and T. Tamura, “Large-eddy simulations of flow past a square cylinder using structured and unstructured grids,” Computers & Fluids, vol. 137, pp. 36–54, 2016.
[18] G. R. Tabor and M. H. Baba-Ahmadi, “Inlet conditions for large eddy simulation: A review,” Computers & Fluids, vol. 39, no. 4, pp. 553–567, 2010.
[19] N. Jarrin, S. Benhamadouche, D. Laurence, and R. Prosser, “A synthetic-eddy-method for generating inflow conditions for large-eddy simulations,” International Journal of Heat and Fluid Flow, vol. 27, no. 4, pp. 585–593, 2006.
[20] Y. Toparlar, B. Blocken, B. Maiheu, and G. J. F. van Heijst, “A review on the CFD analysis of urban microclimate,” Renewable and Sustainable Energy Reviews, vol. 80, pp. 1613–1640, 2017.
[21] R. Yoshie, A. Mochida, Y. Tominaga, H. Kataoka, K. Harimoto, T. Nozu, M. Yoshikawa, and T. Shirasawa, “Cooperative project for CFD prediction of pedestrian wind environment in the Architectural Institute of Japan,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 95, no. 9–11, pp. 1551–1578, 2007.
[22] B. Blocken, “50 years of Computational Wind Engineering: Past, present and future,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 129, pp. 69–102, 2014.
[23] Y. Tominaga and T. Stathopoulos, “CFD simulation of near-field pollutant dispersion in the urban environment: A review of current modeling techniques,” Atmospheric Environment, vol. 79, pp. 716–730, 2013.
[24] H. F. Wang and Y. Zhou, “The finite-length square cylinder near wake,” Journal of Fluid Mechanics, vol. 638, pp. 453–490, 2009.
[25] J. A. Bourgeois, P. Sattari, and R. J. Martinuzzi, “Alternating half-loop shedding in the turbulent wake of a finite surface-mounted square cylinder with a thin boundary layer,” Physics of Fluids, vol. 23, no. 9, 095101, 2011.
[26] M. Saeedi and B.-C. Wang, “Large-Eddy Simulation of turbulent flow around a finite-height wall-mounted square cylinder within a thin boundary layer,” Flow, Turbulence and Combustion, vol. 97, pp. 513–538, 2016.
[27] M. Z. Yousif and H. C. Lim, “Improved delayed detached-eddy simulation and proper orthogonal decomposition analysis of turbulent wake behind a wall-mounted square cylinder,” AIP Advances, vol. 11, 045011, 2021.
[28] R. J. A. M. Stevens, J. Graham, and C. Meneveau, “A concurrent precursor inflow method for Large Eddy Simulations and applications to finite length wind farms,” Renewable Energy, vol. 68, pp. 46–50, 2014.
[29] Á. Martínez-Sánchez, E. López, S. Le Clainche, A. Lozano-Durán, A. Srivastava, and R. Vinuesa, “Causality analysis of large-scale structures in the flow around a wall-mounted square cylinder,” Journal of Fluid Mechanics, vol. 967, A1, 2023.
[30] H.-C. Kao, “Influence of Thermal Conditions on the Wake Characteristics of a 2D Block: A Particle Image Velocimetry Study,” M.S. thesis, Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan, 2025.