Characteristics of Turbulence in the Downstream Region of a Vegetation Patch

Abstract

In presence of vegetation patches in a channel bed, different flow–morphology interactions in the river will result. The investigation of the nature and intensity of these structures is a crucial part of the research works of river engineering. In this experimental study, the characteristics of turbulence in the non-developed region downstream of a vegetation patch suffering from a gradual fade have been investigated. The changes in turbulent structure were tracked in sequential patterns by reducing the patch size. The model vegetation was selected carefully to simulate the aquatic vegetation patches in natural rivers. Velocity profile, TKE (Turbulent Kinetic Energy), turbulent power spectra and quadrant analysis have been used to investigate the behavior and intensity of the turbulent structures. The results of the velocity profile and TKE indicate that there are three different flow layers in the region downstream of the vegetation patch, including the wake layer, mixing layer and shear layer. When the vegetation patch is wide enough (Dv/Dc > 0.5, termed as the patch width ratio, where Dv is the width of a vegetation patch and Dc is the width of the channel), highly intermittent anisotropic turbulent events appear in the mixing layer at the depth of z/Hv = 0.7~1.1 and distance of x/Hv = 8~12 (where x is streamwise distance from the patch edge, z is vertical distance from channel bed and Hv is the height of a vegetation patch). The results of quadrant analysis show that these structures are associated with the dominance of the outward interactions (Q1). Moreover, these structures accompany large coherent Reynolds shear stresses, anomalies in streamwise velocity, increases in the standard deviation of TKE and increases in intermittent Turbulent Kinetic Energy (TKEi). The intensity and extents of these structures fade with the decrease in the size of a vegetation patch. On the other hand, as the size of the vegetation patch decreases, von Karman vortexes appear in the wake layer and form the dominant flow structures in the downstream region of a vegetation patch.

Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

“…Environmental aquatic currents are rarely free of vegetative effects. Moreover, vegetation over the bed or banks exercises a dominant influence on the fluvial hydraulic studies, including drag coefficient, stable channel design, river plan and bed-forms characteristics [1][2][3][4][5][6][7]. Despite considerable investigations on the influence of vegetation in hydraulic projects, there are not many comparative studies for submerged and emergent vegetation in literature.…”

Drag Coefficient of Submerged Flexible Vegetation Patches in Gravel Bed Rivers

“…Using different parameters to estimate the drag coefficient of vegetation [2], researchers have claimed that there is a significant difference in velocity and Reynolds stress distributions for emergent and submerged vegetation cover [3][4][5][6][7][8]. Some studies have focused on specific aspects of vegetation such as foliage impacts [9], effects of stem flexibility on turbulence [10], and the mechanical behavior of vegetation [11]. Certain models have been proposed to take into account the effect of vegetation on the flow resistance [12] and total hydraulic resistance [13].…”

Flow characteristics of open channels based on patch distribution of partially discontinuous rigid combined vegetation

“…Previous studies have mostly focused on the overall and continuous vegetation distribution, and even experimental and numerical simulation studies of patch vegetation have primarily focused on a single vegetation patch (Takemura and Tanaka, 2007;Nicolle and Eames, 2011;Chang et al, 2017;Li et al, 2018;Tny et al, 2019) or the interaction between two patches (Vandenbruwaene et al, 2011;Meire et al, 2014;Ghani et al, 2019a). For example, Kazem et al (2021a); Kazem et al (2021b) studied the vortex structure in the channel of vegetation patch and the characteristics of turbulence gradually subsiding in the undeveloped area downstream of the patch by changing the patch size, and its research results showed that the presence of patches has a great influence on the flow structure inside and around the patches, and there are three different flow layers downstream of vegetation patches: wake layer, mixed layer and shear layer. However, studies on the patch distribution of partially discontinuous vegetation on open channel flow is not systematic enough, although this vegetation distribution pattern is more common in natural channels (Anjum and Tanaka, 2020;Li et al, 2020).…”

“…For example, Kazem et al. (2021a) ; Kazem et al. (2021b) studied the vortex structure in the channel of vegetation patch and the characteristics of turbulence gradually subsiding in the undeveloped area downstream of the patch by changing the patch size, and its research results showed that the presence of patches has a great influence on the flow structure inside and around the patches, and there are three different flow layers downstream of vegetation patches: wake layer, mixed layer and shear layer.…”