Role of vegetation in deposition and dispersion of air pollution in urban parks

A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river. Part one presents the statistical theory of turbulence; Part two addresses the coherent structures in open-channel flows and boundary layers.

Turbulence in Open-Channel Flows

Turbulence at water-vegetation interface in open channel flow: Experiments with natural-like plants

[1] The past decade witnessed rapid developments in remote sensing methods that now permit an unprecedented description of the spatial variations in water levels (Hw), canopy height (hc), and leaf area density distribution (a) at large spatial scales. These developments are now renewing interest in effective resistance formulations for water flow within and above vegetated surfaces so that they can be incorporated into simplified water routing models driven by such remote sensing products. The first generation of such water routing models linked the bulk velocity to gradients in Hw via a constant diffusion velocity that cannot be inferred from canopy properties (a and hc). The next generation of such hydrologic models must preserve the nonlinear relationship between the resistance value, canopy attributes (e.g., a and hc), and Hw without compromising model simplicity. Using a simplified scaling analysis on the depth-integrated mean momentum balance and a two-layer model for the bulk velocity, the Darcy-Weisbach friction factor (f) was shown to vary with three canonical length scales that can be either measured or possibly inferred from remote sensing products Hw, hc, and the adjustment length scale Lc = (Cda)−1, where Cd is the drag coefficient (of order unity). The scaling analysis proposed here reveals that these length scales can be combined in two dimensionless groups, Hw/hc and Lc/hc. The dependence of f on these two functional groups was then explored using a combination of first-order closure modeling and 130 experimental runs derived from a large number of flume experiments carried out for rigid and flexible vegetation. The results from the data and the model show a nonlinear decrease in f with increasing Hw/hc at a given Lc/hc and the nonlinear increase in f with decreasing Lc/hc. Furthermore, both model and data results did not exhibit any dependence on the bulk Reynolds number.

Hydraulic resistance of submerged rigid vegetation derived from first order closure models

Experimental analysis of wave attenuation and drag forces in a realistic fringe Rhizophora mangrove forest

While studies on vegetated channel flows have been developed in many research centers, studies on jets interacting with vegetation are still rare. This study presents and analyzes turbulent jets issued into an obstructed cross-flow, with emergent vegetation simulated with a regular array of cylinders. The paper presents estimates of the turbulence diffusion coefficients and the main turbulence variables of jets issued into a vegetated channel flow. The experimental results are compared with jets issued into unobstructed cross-flow. In the presence of the cylinder array, the turbulence length-scales in the streamwise and transverse directions were reduced, relative to the unobstructed crossflow. This contributed to a reduction in streamwise turbulent diffusion, relative to the unobstructed conditions. In contrast, the transverse turbulent diffusion was enhanced, despite the reduction in length-scale, due to enhanced turbulent intensity and the transverse deflection of flow around individual cylinders. Importantly, in the obstructed condition, the streamwise and transverse turbulent diffusion coefficients are of the same order of magnitude.

/pdf/how-vegetation-in-flows-modifies-the-turbulent-mixing-and-1u9131oxh7.pdf

How vegetation in flows modifies the turbulent mixing and spreading of jets

Experimental investigation on dispersion mechanisms in rigid and flexible vegetated beds

Despite the many studies on flow in partly obstructed open channels, this issue remains of fundamental importance in order to better understand the interaction between flow behavior and the canopy structure. In the first part of this study we suggest a new theoretical approach able to model the flow pattern within the shear layer in the unobstructed domain, adjacent to the canopy area. Differently from previous studies, the new analytical solution of flow momentum equations takes into account the transversal velocity component of the flow, which is modelled as a linear function of the streamwise velocity. The proposed theoretical model is validated by different experiments carried out on a physical model of a very large rectangular channel by the research group of the Department of Civil, Environmental, Building Engineering and Chemistry of the Technical University of Bari. An array of vertical, rigid, and circular steel cylinders was partially mounted on the bottom in the central part of the flume, leaving two lateral areas of free flow circulation near the walls. The three-dimensional flow velocity components were measured using a 3D Acoustic Doppler Velocimeter. A comparison of the measured and predicted data of the present study with those obtained in other previous studies, carried out with different canopy density, show a non-dependence of this analytical solution on the array density and the Reynolds number. In the second part of the paper, detailed observations of turbulent intensities and spanwise Reynolds stresses in the unobstructed flow are analyzed and discussed. Differently from some earlier studies, it was observed that the peak of the turbulence intensity and that of the spanwise Reynolds stress are significantly shifted toward the center of the shear layer.

Hydrodynamic behavior in the outer shear layer of partly obstructed open channels

Flow through rigid and emergent/submerged cylinder arrays are commonly found in several engineering application such as offshore structures, transmission lines, chimneys, array of silos and field array of trees. Various hydrodynamic phenomena may be occurring in the interaction between a flowing fluid and these structures. In this manuscript we focus on the study of flow structures in a channel partially obstructed by an array of equi-spaced, vertical, rigid, emergent, circular steel cylinders. Experimental results show that the presence of the cylinders array strongly affects the flow velocity distribution, forming a transversal sharp transition region at the interface between the obstructed and the unobstructed domains. At the interface, for the current and previous studies, it was observed that the flow distribution always resembles a boundary layer feature. This similarity in feature of the flow distribution as a boundary layer has led to adapting the universal law of the wall to describe the transversal profile of the mean flow velocity, considering, by analogy, the interface separating both domains as a virtual wall. The specific objectives addressed in this study is to propose and validate a new modified log-law predicting the representative transversal profile of the mean flow velocity at the interface between the obstructed and the unobstructed domains. The proposed analytical model is validated by a series of experiments carried out on a very large rectangular channel in the Department of Civil, Environmental, Building Engineering and Chemistry of the Technical University of Bari—Italy. The three-dimensional flow velocity components were measured using a 3D Acoustic Doppler Velocimeter ADV. As a result, it is observed that the measured and the predicted, applying the proposed modified log-law, mean flow velocity data have a perfect matching between them. Moreover, in the second part of the paper, detailed observations on the flow turbulence structure are analyzed and discussed.

Mouldi Ben Meftah

Papers

How vegetation in flows modifies the turbulent mixing and spreading of jets

Experimental investigation on dispersion mechanisms in rigid and flexible vegetated beds

Hydrodynamic behavior in the outer shear layer of partly obstructed open channels

A modified log-law of flow velocity distribution in partly obstructed open channels

Experimental study of a vertical jet in a vegetated crossflow.