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Wind-Tunnel Experiments of Turbulent Wind Fields over a Two-dimensional (2D) Steep Hill: Effects of the Stable Boundary Layer

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Abstract

Flow separation caused by steep topography remains a significant obstacle in accurately predicting turbulent boundary-layer flows over complex terrain, despite the utilization of sophisticated numerical models. The addition of atmospheric thermal stability, in conjunction with steep topography, further complicates the determination of disrupted turbulent wind patterns. The turbulent separated flows over a two-dimensional (2D) steep hill under thermal stratification has not been extensively addressed in previous experimental studies. Such measurements are crucial for enhancing our comprehension of flow physics and validating numerical models. We measured the turbulent wind flows over a 2D steep hill immersed in a stable boundary layer (of the bulk Richardson Number \(\textrm{Ri}_b\) = 0.256) in a thermally-stratified boundary-layer wind tunnel. The flow separation, re-circulation zone and flow reattachment were characterized by the planar particle image velocimetry technique. Vertical profiles of mean air temperature and its fluctuations are also quantified at representative locations above the 2D steep hill and in the near wake region. Results indicate that the separated shear layer, initiated near the crest of the 2D steep hill, dominates the physical process leading to high turbulence levels and the turbulent kinetic energy production in the wake region for both stable and neutral thermal stability. Although the stable boundary layer does not dramatically change the turbulent flow pattern around the hill, the mean separation bubble is elongated by 13%, and its vertical extent is decreased by approximately 20%. Furthermore, the reduced turbulence intensities and turbulent kinetic energy of the near wake flow are attributed to the relatively low turbulence intensity and low momentum of the stable boundary layer due to buoyancy damping, compared to the neutral boundary layer. Additionally, a distinct low-temperature region—a cold pool—is extended beyond the separation bubble, reflecting the significant sheltering effect of the 2D steep hill on the downwind flow and temperature field.

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Acknowledgements

This research was supported by the NSF CAREER Award 1944776 and NSF ATM-0854766, NASA Grant NNG06GE256 and the Swiss National Foundation Grant 200021-132122. The authors would like to express sincere gratitude to the anonymous reviewers for their insightful comments and suggestions that helped enhance the clarity and coherence of this paper.

Funding

The authors would like to thank the National Science Foundation ECI-1944776, ATM-0854766, NASA NNG06GE256 and the Swiss National Foundation Grant 200021-132122.

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Authors and Affiliations

  1. Mechanical Engineering Department, Cleveland State University, 1960 East 24th Street, Cleveland, OH, 44115, USA

    Wei Zhang

  2. Civil and Environmental Engineering, University of Iowa, Iowa City, IA, 52242, USA

    Corey D. Markfort

  3. School of Architecture, Civil and Environmental, Engineering École Polytechnique Fédérale de Lausanne, CH-1015, Lausanne, Switzerland

    Fernando Porté-Agel

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  1. Wei Zhang
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Contributions

WZ and CM conducted wind tunnel experiments. WZ analyzed the data and wrote the manuscript draft. FP conceptualized the topic and discussed the experimental plan. All authors provided funding sources, reviewed and revised the manuscript.

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Correspondence to Wei Zhang.

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Zhang, W., Markfort, C.D. & Porté-Agel, F. Wind-Tunnel Experiments of Turbulent Wind Fields over a Two-dimensional (2D) Steep Hill: Effects of the Stable Boundary Layer. Boundary-Layer Meteorol 188, 441–461 (2023). https://doi.org/10.1007/s10546-023-00820-2

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