Vortex development behind a finite porous obstruction in a channel

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, U₁ , called the steady wake. For the patch diameters and solid volume fractions considered here, U₁ is a function of Φ only. The length of the steady wake ( L₁ ) 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 ( vᵣₘₛ ): directly behind the patch, associated with the wake turbulence of individual cylinders; and at the distance L₁ 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 L₂ . The dimensionless length of the entire wake, (L₁ + L₂ )/ D , increases with patch porosity.