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Title: Hydrodynamics of large-scale roughness in open channels
Author: Mulahasan, Saad
ISNI:       0000 0004 5993 4940
Awarding Body: Cardiff UNiversity
Current Institution: Cardiff University
Date of Award: 2016
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This thesis investigates the hydrodynamics of flow around/and or above an obstacle(s) placed in a fully turbulent developed flow such as flow around lateral bridge constriction, flow over bridge deck and flow over square ribs that are characterized with free surface flow. Also this thesis examines the flow around one-line circular cylinders placed at centre in a single open channel and floodplain edge in a compound, open channel. *Hydrodynamics studies of compound channels with vegetated floodplain have been carried out by a number studies of authors in the last three decades. To enrich our understanding of the flow resistance, comprehensive experiments are carried out with two vegetation configurations-wholly vegetated floodplain and one-line vegetation and then compared to smooth unvegetated compound channel. The main result of the flow characteristics in vegetated compound channels is that spanwise velocity profiles exhibit markedly different characters in the one-line and wholly-vegetated configurations. Moreover, flow resistance estimation results are in agreement with other experimental studies. *A complementary experimental study was carried out to investigate the water surface response in an open-channel flow through a lateral channel constriction and a bridge opening with overtopping. The flow through the bridge openings is characterized by very strong variation of the water surface including undular hydraulic jumps. The results of simulation that was carried by (Kara et al. 2014, 2015) showed a reasonable agreement between measured and computed water surface profiles for the constriction case and a fairly good was achieved for the overtopping case. *Evaluation of the shear layer dynamics in compound channel flows is carried out using infrared thermography technique with two vegetation configurations - wholly vegetated floodplain and one-line vegetation in comparison to non-vegetated floodplains. This technique also manifests some potential as a flow visualization technique, and leaves space for future studies and research. Results highlight that the mixing shear layer at the interface between the main channel and the floodplain is well captured and quantified by this novel approach. iii *Flume experiments of turbulent open channel flows over bed-mounted square bars at low and intermediate submergence are carried out for six cases. Two bar spacings, corresponding to transitional and k-type roughness, and three flow rates, are investigated. This experimental study focused on two of the most aspects of channel rough shallow flows: water surface profile and mean streamwise vertical velocity. Results show that the water surface was observed to be very complex and turbulent for the large spacing cases, and comprised a single hydraulic jump between the bars. The streamwise position of the jump varied between the cases, with the distance of the jump from the previous upstream bar increasing with flow rate. The free surface was observed to be less complex in the small spacing cases, particularly for the two higher flow rates, in which case the flow resembled a classic skimming flow. The Darcy-Weisbach friction factor was calculated for all six cases from a simple momentum balance, and it was shown that for a given flow rate the larger bar spacing produces higher resistance. The result of the simulation that was carried out by Chua et al. (2016) shows good agreement with the experiments in terms of mean free surface position and mean streamwise velocity. *Drag coefficient empirical equations are predicted by a number of authors for an array of vegetation. The research aims to assess the suitability of various empirical formulations to predict the drag coefficient of in-line vegetation. Drag coefficient results show that varying the diameter of the rigid emergent vegetation affects significantly flow resistance. Good agreement is generally observed with those empirical equations. Key Words: Flow Visualization; Infrared Thermography; Shallow Flows; Shear layer; Image processing; Experiment; Free surface; Bridge hydrodynamics; Bridge overtopping; Vegetation roughness, Emergent vegetation, Drag coefficient, blockage; Compound channel, Lateral velocity profiles; Hydraulic resistance; Hydraulic jump, Square bars.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General) ; TC Hydraulic engineering. Ocean engineering