36th IAHR World Congress, Delft, Netherlands, 28 June - 03 July 2015, pp.458-464
The present experimental study investigates the effect of individual emergent vegetation elements on mean flow characteristics, such as streamwise velocity, turbulent intensity and energy spectra. The flume experiments were conducted in a tilting flume and comprise of measurements with an acoustic Doppler velocimeter (ADV). Four different kinds of vegetation elements were employed with increasing complexity. Firstly, measurements were taken downstream of a rigid cylinder, a widely studied and well-known problem, which serves as benchmark for the subsequent experiments and provides a basis from which the additional relevant mechanisms can be inferred. Secondly, an array of smaller rigid cylinders was utilized, which represents a permeable form of the rigid cylinder since they have equal overall diameter. This array was tested under two different orientations. Finally, measurements were taken for a plastic artificial plant. The measurements were conducted for one to five diameters downstream of each obstacle and at nine different points depthwise, in order to form a profile. The generated streamwise mean velocity and turbulence intensity profiles as well as the energy density spectrum plots show that flow remains disturbed at a distance several times the element's diameter, typically more than five diameters and up to about twice that for higher porosity geometries. From the examined elements, flow behind the cylinder seems to recover faster than he others despite the fact that the lee-wake vortices and the generated turbulence are much stronger, exhibiting increased momentum transfer.