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Turbulence structure in straight gravel-bed channels.

Kirkbride, Alistair David (1994) Turbulence structure in straight gravel-bed channels. PhD thesis, University of Sheffield.

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It is clear when looking at a gravel-bed river that the flow turbulence is not random: eddies and boils can be seen at the surface. Understanding the behaviour of gravel-bed rivers is limited until the flow structures are described and their origins explained. This work uses a combination of point monitoring of velocity and visualisation of flows over gravel beds in both the field and flume to describe and explain the structure of the boundary layer and the generation mechanisms of flow structures within it. The boundary layer over fluvial gravel beds can be split into two main flow zones. The inner zone consists of eddies developing and shedding off relatively large obstacles clasts and more confused eddying in the inter-obstacle wake zone. There is some evidence to suggest that flow directly above the inner zone is characterised by the domination of negative Reynolds stress associated with the upwards ejection of eddies at higher than average streamwise velocity. The visualisations show that the eddy shedding becomes more confused as the flow rate is increased, and is best described as intermittent. Above the obstacle crests is the outer flow zone in which there are larger flow structures. Visualisations of the vertical streamwise plane and simultaneous logging of velocity at three points suggest that the larger structures consist of inverted wedges of high-speed fluid which move at more than the mean streamwise velocity as "sweep-like" structures. Low-speed fluid is ejected away from the bed after a high-speed front has passed. These structures become increasingly dominant as the flow rate is increased. The high-speed zones appear to be associated with the passage of the front slopes of water surface waves, although this cannot explain some of those monitored in the field. The flow structures are observed to exhibit a crudely intermittent behaviour in the visualisations. Attempts to quantify the intermittency both from the vistlalisations and by time series analysis of both velocity and stress does not produce the clear quasi-periodic velocity signatures that some other workers have found. There is some agreement between the periods of dominant peaks in the spectra of Reynolds stress and calculated burst periods. This is interpreted as meaning that a similar bursting process occurs over mixed gravel beds as that reported over the smooth and uniformly rough surfaces. There is no evidence that eddy shedding produces a periodic signature in either the velocity or stress series. A mechanistic model is proposed which involves the coupling of eddy shedding with the outer flow wedges. In conditions of high relative roughness (or low flow). eddy shedding is predominantly responsible for the generation of turbulent structures in the boundary layer. whereas at low relative roughness (or high flow conditions). the outer zone wedges dominate. This allows the interpretation of flow structure in gravel bed rivers to be made with respect to that observed in sand bed rivers and in tidal flows over gravels. Observations of gravel entrainment in the visualisation experiments suggest that there are two modes of entrainment. At marginal transport conditions. fines are entrained from the wake zones of the larger obstacle clasts by vertical velocity fluctuations. At higher flow conditions, the high speed "sweep-like" structures are responsible for entraining the larger clasts, although the timescale of entrainment is greater that that of an individual flow structure.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Social Sciences (Sheffield) > Geography (Sheffield)
Identification Number/EthosID: uk.bl.ethos.629451
Depositing User: EThOS Import Sheffield
Date Deposited: 09 Jan 2017 12:05
Last Modified: 09 Jan 2017 12:05
URI: http://etheses.whiterose.ac.uk/id/eprint/15024

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