Laboratory investigation of mean drag in a random array of rigid, emergent cylinders

Yukie Tanino, Heidi M. Nepf

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This paper investigates the drag exerted by randomly distributed, rigid, emergent circular cylinders of uniform diameter d. Laboratory measurements are presented for solid volume fraction phi=0.091, 0.15, 0.20, 0.27, and 0.35 and cylinder Reynolds number Re-p U(p)d/nu=25 to 685, where U-p=temporally and cross-sectionally averaged pore velocity and nu=kinematic viscosity. These ranges coincide with conditions in aquatic plant canopies. The temporally and cross-sectionally averaged drag coefficient, C-D, decreased with increasing Re-p and increased with increasing phi under the flow conditions investigated. The dimensionless ratio of the mean drag per unit cylinder length <(f(D)) over bar >(H) to the product of the viscosity, mu, and U-p exhibits a linear Re-p dependence of the form <(f(D)) over bar >(H)/(mu U-p)=alpha(0)+alpha Re-1(p), consistent with Ergun's formulation for packed columns. In the range of experimental conditions, alpha(1), increases monotonically with phi. In contrast, alpha(0) is constant within uncertainty for 0.15 <= phi <= 0.35, which suggests that viscous drag per unit cylinder length is independent of phi in this range.

Original languageEnglish
Pages (from-to)34-41
Number of pages8
JournalJournal of Hydraulic Engineering
Issue number1
Publication statusPublished - Jan 2008


  • vegetation
  • drag
  • aquatic plants
  • two-dimensional flow
  • open channel flow
  • experimental data
  • cylinders

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