Estimation of drag coefficient of a real tree considering the vertical stand structure of trunk, branches, and leaves

Abstract

Drag coefficients of a real tree trunk and branch and the sheltering effects of an upstream trunk or branch on a downstream one in a linear arrangement with different spacings were investigated in detail. Willow and Robinia pseudoacacia, representative and invasive trees in Japanese rivers, respectively, were selected for the study. The drag coefficient of the real tree trunk started to decrease from a relatively low Reynolds number and was smaller than that of a smooth circular cylinder in the entire Reynolds number range investigated. Leaves increased the drag coefficient by around 40–100% in comparison to that of a branch alone and increased with increasing the area ratio of leaves and branches. The sheltering effects of an upstream branch increased with the existence of leaves. The drag coefficient of a downstream branch approached around 95% of that of a single branch with increasing Reynolds number, regardless of the presence of leaves. The moment for drag force of a whole tree was calculated for a strong wind event that overturned trees in the Arakawa River, Japan. The calculated value was compared with the experimental threshold values and was validated within a reasonable limit.

Keywords:

• surface roughness
• Reynolds number
• turbulent intensity
• drag crisis
• sheltering effect

Acknowledgements

This study was supported in part by JSPS AA Science Platform Program, and research fund by Ministry of Land, Infrastructure, Transport, and Tourism, Japan. Mr. M.H. Phyo, a student at Saitama University, is acknowledged for his help in experiments.