Vortex development behind a finite porous obstruction in a channel

Abstract

AbstractThis experimental study describes the turbulent wake behind a two-dimensional porous obstruction, consisting of a circular array of cylinders. The cylinders extend from the channel bed through the water surface, mimicking a patch of emergent vegetation. Three patch diameters ($D$) and seven solid volume fractions ($\Phi $) are tested. Because flow can pass through the patch, directly downstream there is a region of steady, non-zero, streamwise velocity, ${U}_{1} $, called the steady wake. For the patch diameters and solid volume fractions considered here, ${U}_{1} $ is a function of $\Phi $ only. The length of the steady wake (${L}_{1} $) increases as $\Phi $ decreases and can be predicted from the growth of a plane shear layer. The formation of the von-Kármán vortex street is delayed until the end of the steady wake. There are two regions of elevated transverse velocity fluctuation (${v}_{\mathit{rms}} $): directly behind the patch, associated with the wake turbulence of individual cylinders; and at the distance ${L}_{1} $ from the patch, associated with the formation of large-scale wake oscillation. Velocity along the centreline of the wake starts to increase only after the patch-scale vortex street is formed, and it approaches the free-stream velocity over a distance ${L}_{2} $. The dimensionless length of the entire wake, $({L}_{1} + {L}_{2} )/ D$, increases with patch porosity.