Vortex development behind a finite porous obstruction in a channel

Abstract

This 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 (Φ) are tested. Because flow can pass through the patch, directly downstream there is a region of steady, non-zero, streamwise velocity, U[subscript 1], called the steady wake. For the patch diameters and solid volume fractions considered here, U[subscript 1] is a function of Φ only. The length of the steady wake (L[subscript 1]) increases as Φ 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[subscript rms]): directly behind the patch, associated with the wake turbulence of individual cylinders; and at the distance L[subscript 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[subscript 2]. The dimensionless length of the entire wake, L[subscript 1] + L[subscript 2]/ D[subscript 1], increases with patch porosity.