Secondary circulation in partially vegetated flows

Unigarro Villota, Stefania ORCID: 0000-0002-8453-7168 (2023). Secondary circulation in partially vegetated flows. University of Birmingham. Ph.D.

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Abstract

Partially vegetated flows like those over rivers with riparian vegetation are common in natural environments. In these flows, vegetation acts as a porous obstruction for the flow. Thus, the slow interstitial fluid inside the vegetation and the rapid free streams above and next to it, lead to large gradients of shear at the vegetation patch boundaries, where shear layers are created. These shear layers are characterised by large turbulent coherent structures that interact with secondary circulations developed due to the differential roughness between the vegetation and the adjacent unobstructed flow. This hydrodynamic interaction dictates the interfacial mass and momentum transport in aquatic environments. As vegetation
provides important ecosystem services, such as water quality improvement through nutrients and heavy metals retention, sediment trapping, and oxygen flux, the hydrodynamic transport occurring at the interface can significantly impact many physical, chemical and ecological processes. Therefore, flow characterisation through patchy vegetation is important for a system-level understanding of vegetated flows and the management of river and coastal systems. However, the characterisation of these flows is complex, owing to their three-dimensionality, which is not yet fully understood. This thesis presents the characterisation of the three-dimensional flow developed on a partially vegetated channel, focussing
upon the role of secondary circulation on mass and momentum transport. Firstly, an experimental approach was employed to determine the flow properties, taking 3D-point velocity measurements, using an Acoustic Doppler Velocimeter (ADV), in the cross-sectional plane on a partially vegetated flume. Experiments were configured to examine the influence of vegetation density, over a range of reported realistic field conditions, and shallowness on the mean and turbulent flow. Subsequently, an analytical model was used to predict depth-averaged streamwise
velocities, modelling the effects of secondary circulation on the momentum equation.

The results reveal that secondary circulation cells scale with the vertical shear layer thickness within the vegetation and the water depth beside the canopy patch. The rotational direction of the major pair of cells located at the lateral canopy-water interface inverts, either at high densities due to the growth of an emergent rotating pair of cells, or at reduced water depths. Secondary circulation contributions to vertical mass transport, for the higher submergence condition, are estimated to be of the same order of magnitude as that of turbulent diffusion. Advective dispersive stresses arising due to mean flow fluctuations are found to be up to 21% of the total vertical shear stress and the main contributors to lateral shear stress within the vegetation and in the main channel outside the mixing layer. Additionally, this work presents the first attempt at using the Shiono and Knight Method, an analytical model based on the depth-averaged Navier-Stokes equation, to model depth-averaged streamwise velocities in submerged partially vegetated channels. Appropriate advice on the values of calibration parameters and cross-section discretisation is given. The combined experimental and modelling approaches provide and insight into the 3D flow structure and highlights the importance of secondary circulation when evaluating mass and momentum transport in submerged partially vegetated flows.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
Sharifi, Soroosh	UNSPECIFIED	UNSPECIFIED
Jesson, Michael	UNSPECIFIED	UNSPECIFIED
Philip, Jimmy	UNSPECIFIED	orcid.org/0000-0002-1068-0340
Ghisalberti, Marco	UNSPECIFIED	orcid.org/0000-0002-6690-8922