Abstract
The drag on pairs of square obstacles (of side D) in open channel flow is measured in experiments using a laboratory flume. The depth is uniform across the obstacles and the conditions in the flume are subcritical, with Froude number , Reynolds number Re = 4800 to 21900, and turbulence intensity to 10%. The drag coefficient for an isolated square obstacle is found to be , in agreement with previous studies, and independent of Re (for the range covered here). The root-mean-square variation in the drag coefficient for the single obstacle decreased monotonically with Re, defined in terms of hydraulic radius, and approached 0.241 at high Re in agreement with previous research. For two obstacles, standard tandem and side-by-side arrangements are studied first, followed by a full range of relative positions covering in the downstream direction and in the cross-stream direction. The lowest drag coefficients are observed when the downstream obstacle is shielded directly behind the upstream obstacle (tandem arrangement) when negative drag coefficients are found. The largest drag coefficients are observed for nearly side-by-side arrangements, with the peak values found to be for the slightly upstream obstacle of a pair . The blockage ratio (D/B, the relative size of the obstacle compared to the channel width) is found to be an important factor. For D/B = 12.7% the largest drag coefficient is , while for D/B = 6.3% the largest value is . For tandem obstacles, the drag on one obstacle can largely be accounted for by the change in flow speed induced by the other obstacle, except at small separations . The results will be useful in any applications where the force on multiple obstacles is required, such as the design of marine or riverine structures, or flood flows past buildings and vegetation.
5 More- Received 21 August 2017
- Revised 12 September 2018
DOI:https://doi.org/10.1103/PhysRevFluids.3.123802
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society