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

Michele Mossa,Mouldi Ben Meftah,Heidi M Nepf,Francesca De Serio

doi:10.1038/s41598-017-05881-1

Michele Mossa, Mouldi Ben Meftah + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-05881-1

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Abstract

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.

Highlights

Aquatic vegetation provides a wide range of ecosystem services[1,2,3]
In the present paper the turbulent integral length scales, turbulent diffusion coefficients, and advective terms are analyzed in the streamwise and spanwise directions for a turbulent jet entering an obstructed flow and compared to the same jet
The first profile of run U4 is upstream of the jet exit and shows an undisturbed channel flow

Summary

Introduction

Aquatic vegetation provides a wide range of ecosystem services[1,2,3]. The uptake of nutrients and production of oxygen improve water quality[4]. Even if detrainment is usually associated with buoyancy-driven flows, such as plumes or density currents flowing in a stratified environment, Mossa and De Serio[31] proved theoretically that detrainment occurs when a momentum-driven jet is issued in a not-stratified obstructed current. This finding is relevant, because it can be extended to unconventional ideas of jets through porous obstructions, such as the case of outflows from different sources spreading among oyster farms, wind farms, solar plants as well as aerial pesticides sprayed onto orchards or river jets flowing at mouths through bar deposits[37] (see Appendix 1 for some examples). A spatial average[39, 40] is not applied, because it would erase important information on the spatial variation along and transverse to the jet

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