Journal of Korea Water Resources Association (한국수자원학회논문집)

Korea Water Resources Association (한국수자원학회)
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A RANS modeling of backward-facing step turbulent flow in an open channel

개수로에서의 후향단차 난류 흐름 RANS 수치모의

  • Kim, Byungjoo (Department of Civil Engineering, Gangneung-Wonju National University) ;
  • Paik, Joongcheol (Department of Civil Engineering, Gangneung-Wonju National University)
  • 김병주 (강릉원주대학교 토목공학과) ;
  • 백중철 (강릉원주대학교 토목공학과)
  • Received : 2021.11.03
  • Accepted : 2021.12.29
  • Published : 2022.02.28
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Abstract

The backward-facing step (BFS) is a benchmark geometry for analyzing flow separation occurred at the edge and resulting development of shear layer and recirculation zone that are occupied by turbulent flow. It is important to accurately reproduce and analyze the mean flow and turbulence statistics of such flows to design physically stable and performance assurance structure. We carried out 3D RANS computations with widely used, two representative turbulence models, k-ω SST and RNG k-ε, to reproduce BFS flow at the Reynolds number of 23,000 and the Froude number of 0.22. The performance of RANS computations is evaluated by comparing numerical results with an experimental measurement. Both RANS computations with two turbulence models appear to reasonably well reproduce mean flow in the shear layer and recirculation zone, while RNG k-ε computation results in about 5% larger velocity between the outer edge of boundary layer and the free surface above the recirculation zone than k-ω SST computation and experiment. Both turbulence models underestimate the shear stress distribution experimentally observed just downstream of the sharp edge of BFS, while shear stresses computed in the boundary layer downstream of reattachment point are agree reasonably well with experimental measurement. RNG k-ε modeling reproduces better shear stress distribution along the bottom boundary layer, but overestimates shear shear stress in the approaching boundary layer and above the bottom boundary layer downstream of the BFS.

후향단차 수공구조물의 모서리에서는 흐름분리가 발생하며 이로 인해 형성되는 전단층과 재순환 흐름 영역에서의 흐름은 복잡한 난류가 지배적이다. 물리적으로 안정하면서 성능이 보장되는 구조물 설계를 위해서는 이러한 난류 흐름의 거동을 정확하게 예측하고 분석하는 것이 중요하다. 이 연구에서는 공학적으로 널리 이용되고 있는 대표적인 난류 모형인 k-ω SST 모형과 RNG k-ε 모형을 이용한 3차원 RANS 계산을 통해서 개수로에 설치된 후향단차를 통과하는 난류 흐름을 레이놀즈 수 23,400과 후르드 수 0.22의 조건에서 수치모의하고, 해석 결과를 기존 실험자료와 비교 분석하여 수치해석의 성능을 평가하고자 한다. 두 가지 난류 모형을 이용하여 구한 평균유속 분포를 보면 모두 경계층에서 관측된 실험값을 양호하게 잘 재현하는 것으로 나타났다. 재순환 영역 상부에서 계산된 평균유속을 보면 RNG k-ε 모형이 k-ω SST 모형보다 중앙부에서의 유속을 약 5% 정도 크게 계산하는 것으로 나타났다. 난류 통계량 관점에서 보면 두 난류 모형 모두 단차 모서리 직하류에서 흐름 분리로 인해 발생하는 레이놀즈 전단응력을 현저히 과소산정하는 한편, 재부착점 하류에서는 실험값을 상대적으로 양호하게 재현하는 것으로 나타났다. RNG k-ε 모형은 수로 바닥 부근 경계층에서의 전단응력 분포를 k-ω SST 모형보다는 우수한 정확도로 실험값을 계산하는 반면에 접근수로 경계층에서 그리고 단차 하류부에서는 경계층 상부에서 전단응력을 과대 산정하는 것으로 나타났다.

Keywords

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2019R1A2C1090809).

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