This article is aimed at quantifying and discussing the relative magnitude of key terms of the equation of conservation of turbulent kinetic energy (TKE) in the inter-stem space of a flow within arrays of vertical cylinders simulating plant stems of emergent and rigid vegetation. The spatial distribution of turbulent quantities and mean flow variables are influenced by two fundamental space scales, the diameter of the stems and the local stem areal number-density. Both may vary considerably since the areal distribution of plant stems in natural systems is generally not homogeneous; they are often arranged in alternating sparse and dense patches. The magnitude of the terms of the budget of TKE in the inter-stem space has seldom been quantified experimentally and is currently not well known. This work addresses this research need. New databases, consisting of three-component LDA velocity series and two-component PIV velocity maps, obtained in carefully controlled laboratory conditions, were used to calculate the terms of the TKE budget. The physical system comprises random arrays of rigid and emergent cylinders with longitudinally varying areal number-density. It is verified that the main source of TKE is vortex shedding from individual cylinders. The rates of production and dissipation are not in equilibrium. Regions with negative production, a previously unreported feature, are identified. Turbulent transport is particularly important along the von K arm an vortex street. Convective rate of change of TKE and pressure diffusion are most relevant in the vicinity of the cylinders.
The role of turbulence in the dislodgment of benthic stream invertebrates from the riverbed was investigated experimentally in a laboratory flume. For the first time, technological advances allowed measuring the spatio-temporal patterns of turbulent flow around two free-moving invertebrates (Aeshna cyanea and Somatochlora flavomaculata). A specific methodology was developed for the analysis of turbulence around benthic invertebrates. The results confirmed two hypotheses: (i) on the contrary to sediment particles, invertebrates are not only sensitive to the peak values of the turbulent flow forcing but also to the temporal fluctuations in this flow forcing; and (ii) the dominant temporal fluctuations are not due to local turbulent structures of the size of the invertebrate, but to turbulent structures that scale with the flow depth and are inherited from upstream. In 15 of the 17 conducted tests, important turbulent events that scale with the flow depth accompanied by rapid temporal flow fluctuations occurred at the moment of dislodgement. The dominant forcing was consistently a threefold increase in shear stress, and was related to a sweep event in 12 of the 17 tests. Thereby, the increase in longitudinal velocity was typically about 40%, which led to a 100% increase in drag force in comparison with the time-averaged drag force. These results enable a new understanding of the detailed hydraulic conditions leading to passive drift of stream invertebrates. In addition, they open new perspectives to improve models predicting the distribution of benthic invertebrates based on hydrodynamics by accounting for turbulence.
This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.
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