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Title: The influence of near bed turbulent flow structures on scour hole development at pipe culvert outlets.
Author: Liriano, Sarah Louise.
ISNI:       0000 0001 3610 8871
Awarding Body: University of Herfordshire
Current Institution: University of Hertfordshire
Date of Award: 1999
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This thesis presents the results of experimental measurements of scouring and turbulent velocity measurements in and around scour holes downstream of pipe culvert outlets. Centreline scour hole profiles have been measured at 4 flow rates and 4 tailwater depths resulting in up to 16 different experimental conditions. These results have enabled the maximum scour depth downstream of culvert outlets to be quantified in terms of flow rate and tailwater depth. An equation in terms of densimetric Froude number is presented to predict scour depth incorporating constants dependent on tailwater depth calculated from additional equations. The prediction of scour depth for different tailwater depths is of immediate benefit to design engineers. Additionally, measurementosf scourh ole profiles haveb een madeu sing four experimental facilities of different sizes. Froudian scaling protocols have been used to investigate the effect of model scale on scour hole development. Three experimental model facilities have been used at the University of Hertfordshire and a fourth prototype scale facility used at HR Wallingford. Model scale has been found to have an effect with small-scale models failing to accurately predict maximum scour depth. This is of particular significance as previous equations predicting scour depth downstream of outlets have been derived from studiesu sing small-scalem odel facilities. Velocity data was collected for three tailwater depths at one flow rate using a downward facing 3 component Acoustic Doppler Velocimeter. A method of bed fixing was used to enable velocity measurements to be made in scour holes at different stages of development without changes in bed form during the velocity data collection. From this data mean velocity vectors and contours have been plotted at different stages of scour hole development and turbulence intensities and Reynolds stresses have been determined for the centreline profiles. Further, using the quadrant analysis technique, the near-bed bursting events have been examined at different stages of development, which has shown that different events occur at different locations in the scour hole. The analysis of the turbulent flow structure in the scour hole has revealed that initial scour is a result of high velocities exceeding the critical velocity for sediment transport. As the scour hole develops the velocities close to the bed reduce and a gradual increase in scour depth takes place as a result of quadrant 4 events (sweeps) impacting on the bed in this region. Upstream of the dune the jet comes into contact with the bed and flow structures similar to those observed downstream of backward facing steps or dunes in open channel flow are noted. In particular it is suggested that hairpin vortices may be present in this region and lead to further scouring. The identification of flow structures in the scour hole may lead to the development of computer simulations of scouring downstream of pipe culvert outlets which in the long term could be used as a design tool.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fluid mechanics