KSCE Journal of Civil and Environmental Engineering Research (대한토목학회논문집)

Korean Society of Civil Engeneers (대한토목학회)
권호 표지
DOI QR코드

DOI QR Code

Numerical Investigations of Vorticity Generation in Fully Vegetated Open-Channel Flows

수치모의를 이용한 전단면 식생 수로에서의 와도 생성 분석

  • 강형식 (한국환경정책.평가연구원 환경전략연구본부 물순환연구실, 한국건설기술연구원 수자원.환경연구본부 하천.해안항만연구실)
  • Received : 2009.11.12
  • Accepted : 2010.01.13
  • Published : 2010.04.30
Download PDF
⟨ Previous Next ⟩

Abstract

This paper presents a numerical investigation of vorticity generation in fully vegetated open-channel flows. The Reynolds stress model is used for the turbulence closure. Open-channel flows with rough bed-smooth sidewalls and smooth bed-rough sidewalls are simulated. The computed vectors show that in channel flows with rough bed and rough sidewalls, the free-surface secondary currents become relatively smaller and larger, respectively, compared with that of plain channel flows. Also, open-channel flows over vegetation are simulated. The computed bottom vortex occupies the entire water depth, while the free-surface vortex is reduced. The contours of turbulent anisotropy and Reynolds stress are presented with different density of vegetation. The budget analysis of vorticity equation is carried out to investigate the generation mechanism of secondary currents. The results of the budget analysis show that in plain open-channel flow, the production by anisotropy is important in the vicinity of the wall and free-surface boundaries, and the production by Reynolds stress is important in the region away from the boundaries. However, this rule is not effective in vegetated channel flows. Also, in plain channel flows, the vorticity is generated mainly in the vicinity of the free-surface and the bottom, while in vegetated channel flows, the regions of the bottom and vegetation height are important to generate the vorticity.

본 연구에서는 수치모의를 통하여 전단면 식생 수로에서 와도의 생성을 분석하였다. 지배방정식에서 난류 폐합을 위해 레이놀즈응력모형을 이용하였다. 거친 하상-매끄러운 측벽 및 매끄러운 하상-거친 측벽을 갖는 개수로 흐름을 수치모의하여 서로 다른 형태의 이차흐름 구조가 형성되는 것을 확인하였다. 즉, 거친 하상 조건에서는 자유수면 이차흐름의 규모가 감소되고, 거친 측벽 조건에서는 자유수면 이차흐름의 구조가 더 커지는 것으로 나타났다. 또한 전단면 식생 수로를 수치모의하여 수심 크기의 바닥 이차흐름이 형성되고, 식생 밀도가 증가함에 따라 자유수면 이차흐름이 점차 사라지는 것을 확인하였다. 또한 이차흐름 생성에 중요한 역할을 하는 난류의 비등방성 및 레이놀즈응력 분포를 식생밀도에 따라 살펴보았다. 한편, 와도 방정식을 분석한 결과, 비식생 수로의 경우 벽 및 수면 경계 근처에서는 난류 비등방성에 의한 생성항이, 경계와 떨어진 곳에서는 레이놀즈응력에 의한 생성항이 와도 생성에 중요한 역할을 하는 것으로 나타났다. 그러나 식생 수로에서는 이러한 특성이 사라지는 것으로 확인되었다. 또한 비식생 수로에서는 바닥과 수면에서의 와도 생성이 강하게 발생되지만, 식생 수로에서는 바닥과 식생 높이에서 와도 생성이 크게 발생되는 것으로 나타났다.

Keywords

References

  1. Choi, S.-U. and Kang, H. (2004) Reynolds stress modeling of vegetated open-channel flows. Journal of Hydraulic Research, IAHR, Vol. 42, No. 1, pp. 3-11. https://doi.org/10.1080/00221686.2004.9641178
  2. Choi, S.-U. and Kang, H. (2006) Numerical investigations of mean flow and turbulence structures of partly vegetated open channel flows using the Reynolds stress model. Journal of Hydraulic Research, IAHR, Vol. 44, No. 2, pp. 203-217. https://doi.org/10.1080/00221686.2006.9521676
  3. Choi, Sung-Uk, Park, Moonhyeong, and Kang, Hyeongsik (2007) Numerical simulations of cellular secondary currents and suspended sediment transport in open-channel flows over smoothrough bed strips. Journal of Hydraulic Research, IAHR, Vol. 45, No. 6, pp. 829-840. https://doi.org/10.1080/00221686.2007.9521820
  4. Demuran, A.O. and Rodi, W. (1984) Calculation of turbulence driven secondary motion on non circular ducts. Journal of Fluid Mechanics, Vol. 140, pp. 189-222. https://doi.org/10.1017/S0022112084000574
  5. Dunn, C.J. (1996) Experimental determination of drag coefficients in open-channel with simulated vegetation. M.S. Thesis, University of Illinois at Urbana-Champaign, Urbana, IL.
  6. Gessner, B.F. and Jones, J.B. (1965) On some aspects of fully developed turbulent flow in rectangular channels. Journal of Fluid Mechanics, Vol. 23, pp. 689. https://doi.org/10.1017/S0022112065001635
  7. Ghisalberti, M. and Nepf, H.M. (2002) Mixing layers and coherent structures in vegetated aquatic flows. Journal of Geophysical Research, AGU, Vol. 107(C2), pp. 3-1-3-11.
  8. Ghisalberti, M. and Nepf, H.M. (2005) Mass transport in vegetated shear flows. Environmental Fluid Mechanics, Vol. 5, pp. 527-551. https://doi.org/10.1007/s10652-005-0419-1
  9. Gibson, M.M. and Launder, B.E. (1978) Ground effects on pressure fluctuations in the atmospheric boundary layer. Journal of Fluid Mechanics, Vol. 86, pp. 491-511. https://doi.org/10.1017/S0022112078001251
  10. Hanjalic, K. and Launder, B.E. (1972) A Reynolds stress model of turbulence and its application to thin shear flows. Journal of Fluid Mechanics, Vol. 52, pp. 609-638. https://doi.org/10.1017/S002211207200268X
  11. Huser, A. and Biringen, S. (1993) Direct numerical simulation of turbulent flow in a square duct. Journal of Fluid Mechanics, Vol. 257, pp. 65-95. https://doi.org/10.1017/S002211209300299X
  12. Kang, H. (2005) Reynolds stress modeling of vegetated open-channel flows. Ph.D. Thesis, Yonsei University, Korea.
  13. Kang, H. and Choi, S.-U. (2006a) Reynolds stress modeling of rectangular open channel flows. International Journal for Numerical Methods in Fluids, Vol. 51, No. 11, pp. 1319-1334. https://doi.org/10.1002/fld.1157
  14. Kang, H. and Choi, S.-U. (2006b) Turbulence modeling of compound open-channel flows with and without vegetated floodplains using the Reynolds stress model. Advances in Water Resources, Vol. 29, No. 11, pp. 1650-1664. https://doi.org/10.1016/j.advwatres.2005.12.004
  15. Kang, H. and Choi, S.-U. (2009) Turbulent modeling of solute transport in open-channel flows over submerged vegetation. IAHR-APD Congress, Nanjing, China.
  16. Lopez, F. and Garcia, M. (2001) Mean flow and turbulence structure of open-channel flow through non-emergent vegetation. Journal of Hydraulic Engineering, ASCE, Vol. 127, No. 5, pp. 392-402. https://doi.org/10.1061/(ASCE)0733-9429(2001)127:5(392)
  17. Mellor, G.L. and Herring, H.J. (1973) A survey of mean turbulent field closure. AIAA Journal, Vol. 11, pp. 590-599. https://doi.org/10.2514/3.6803
  18. Nezu, I. and Nakagawa, H. (1993) Turbulence in open-channel flows. Monograph, Balkema, Rotterdam, The Netherland.
  19. Nezu, I. and Nakagawa, H. (1984) Cellular secondary currents in straight conduit. Journal of Hydraulic Engineering, ASCE, Vol. 110, No. 2, pp. 173-193. https://doi.org/10.1061/(ASCE)0733-9429(1984)110:2(173)
  20. Ohmoto, T. and Hayashi, S. (2003) Study of generation mechanism of secondary currents in open-channel flow by direct numerical simulation. Journal of Hydroscience and Hydraulic Engineering, Vol. 21, No. 1, pp. 11-21.
  21. Raupach, M.R. and Shaw, R.H. (1982) Averaging procedures for flow within vegetation canopies. Boundary Layer Meteorology, Vol. 44, No. 1, pp. 1-25.
  22. Shir, C.C. (1973) A preliminary numerical study of atmospheric turbulent flow in the idealized planetary boundary layer. Journal of Atmospheric Science, Vol. 30, pp. 1327-1339. https://doi.org/10.1175/1520-0469(1973)030<1327:APNSOA>2.0.CO;2
  23. Speziale, C.G., Sarkar, S., and Gatski, T. (1991) Modeling the pressure strain correlation of turbulence: an invariant dynamical systems approach. Journal of Fluid Mechanics, Vol. 227, pp. 245-272. https://doi.org/10.1017/S0022112091000101
  24. Tominaga, A., Ezaki, K., and Nakagawa, H. (1989) Three dimensional turbulent structure in straight open-channel flows. Journal of Hydraulic Research, IAHR, Vol. 27, No. 1, pp. 149-173. https://doi.org/10.1080/00221688909499249
  25. Yang, W. (2009) Experimental study of turbulent open-channel flows with submerged vegetation, Ph.D. Thesis, Yonsei University, Korea.