Anisotropy in the Free Stream Region of Turbulent Flows through Emergent Rigid Vegetation on Rough Beds

Abstract

Most of the existing works on vegetated flows are based on experimental tests in smooth channel beds with staggered-arranged rigid/flexible vegetation stems. Actually, a riverbed is characterized by other roughness elements, i.e., sediments, which have important implications on the development of the turbulence structures, especially in the near-bed flow zone. Thus, the aim of this experimental study was to explore for the first time the turbulence anisotropy of flows through emergent rigid vegetation on rough beds, using the so-called anisotropy invariant maps (AIMs). Toward this end, an experimental investigation, based on Acoustic Doppler Velocimeter (ADV) measures, was performed in a laboratory flume and consisted of three runs with different bed sediment size. In order to comprehend the mean flow conditions, the present study firstly analyzed and discussed the time-averaged velocity, the Reynolds shear stresses, the viscous stresses, and the vorticity fields in the free stream region. The analysis of the AIMs showed that the combined effect of vegetation and bed roughness causes the evolution of the turbulence from the quasi-three-dimensional isotropy to axisymmetric anisotropy approaching the bed surface. This confirms that, as the effects of the bed roughness diminish, the turbulence tends to an isotropic state. This behavior is more evident for the run with the lowest bed sediment diameter. Furthermore, it was revealed that also the topographical configuration of the bed surface has a strong impact on the turbulent characteristics of the flow.

Flow Resistance in Open Channel Due to Vegetation at Reach Scale: A Review

“…Experimental studies of velocity profiles, like those shown in Figures 3-5, are of much interest; in some cases also the shear stress profiles are studied by means of velocity fluctuant components and Reynolds stress theory [19,[107][108][109][110][111].…”

“…The author gives, for every class of channels, the minimum, average, and maximum values of Manning n coefficient, warning that when the channel is artificial, the average values should be used in case of good maintenance only. In Tables 5 and 6 of Chow's book ( [47], p. [110][111][112][113], one can observe that the Manning coefficient is 0.018 sm −1/3 in case of the excavated channel, straight, clean, uniform cross-section with no vegetation, and 0.035 sm −1/3 in case of dense weeds. In natural streams, its values are 0.030 sm −1/3 when the cross-section is clean, and 0.045 sm −1/3 in case of weeds.…”

Turbulent Flow through Random Vegetation on a Rough Bed

“…Moreover, the Reynolds shear stresses present very small values, practically vanishing in the region dominated by the vegetation [16,22]. Very recently, Penna et al [24,25] studied the flow field around a rigid cylinder in three different rough bed conditions with a uniform pattern of stems that are regularly aligned. Penna et al [24] analyzed the velocity, shear stress distributions, TKE and the energy spectra, showing that, in the region near the free surface, the flow is deeply affected by the stems.…”

“…Moving toward the bed surface, the flow is influenced by both the vegetation and bed roughness effects. Penna et al [25], using the so-called Anisotropy Invariant Maps (AIMs), investigated for the first time the turbulence anisotropy through uniform emergent rigid vegetation on rough beds. The study of the AIMs indicated that, approaching the bed surface, the combined impact of vegetation and bed roughness affects the turbulence evolution from the quasi-three-dimensional isotropy to axisymmetric anisotropy.…”

“…Starting from the results of Penna et al [24,25] in the case of a uniform vegetation pattern, the aim of this work is to study the impact of bed roughness and random vegetation pattern distribution on turbulence. In particular, in order to describe the flow domain, the time-averaged approaching flow velocity field, TKE, normal shear stresses and energy spectra of the velocity components were computed in an area centered on a single stem.…”