Drag coefficient of in-line emergent vegetation in open channel flow

Abstract

Along the banks of rivers, trees and bushes are often planted in a single line. In the case of trees, the trunks are simulated in hydraulic laboratories by a set of cylinders and the drag coefficient can be estimated with use of various different methodologies, including by direct measurement, using the momentum equation, equating turbulence intensity and drag force, numerical modeling, and genetic programming. However, for the sake of simplicity, many equations have been proposed in the scientific literature that allow its immediate estimation. Some of these equations are used in this work to verify their ability to reproduce experimental data obtained for in-line cylinders by Mulahasan and Stoesser (2017), who obtained the drag force by applying the momentum equation. Several statistical descriptors have been used for this purpose. We found that the equations derived from staggered and random arrangements generally overestimate by a large amount the CD values; instead, a few relationships and in particular one derived from a squared arrangement provide much better results.