Fluid Dynamics
Title
Hydrodynamics of grit moving round a curved channel
Project coordinator(s)
Dr. Laurence Gill
Email: gilll@tcd.ie
Tel: +353 1 896 1047
Research Student(s)
Mr. Titikish Patel
Email: patelt@tcd.ie
Description
When water passes round a bend in a channel, secondary currents are induced (due to centrifugal force) which means that any particles moving along the bed of the channel are pushed to the inside of the bend. The aim of this project is to study the hydrodynamics of particles as they pass around a bend in an open channel by using a combination of physical scale and Computational Fluid Dynamics (CFD) modelling. This phenomenon can be employed, for example, to remove grit from wastewater entering treatment works by placing a sump at the inside of the bend without any significant loss in head to the process flow or space considerations to the treatment plant.
A physical model of an open channel with a bend in the laboratory has been constructed and preliminary studies have shown the process to be very efficient at removing grit particles down to 0.3mm diameter but the efficiency drops off rapidly for smaller particles. A mathematical model was then built in CFD and successfully calibrated to the results from the physical model. The mathematical model has then been used to optimise the channel and study different characteristics such as angle of bend, position of sump, flow rate etc.
This project is not only of relevance to the grit removal process at wastewater treatment plants, but also the general understanding of sediment transport in open channel flow, for example meandering in rivers, whereby erosion takes place on outside and deposition on inside of the bend. It is also an interesting use of CFD to attempt to model the liquid phase and also the solid phase concurrently.
Cross Flow Velocity Vectors and Longitudinal Contours on bend
Title
Artificial Reefs for coastal protection and enhanced surfing conditions
Project coordinator(s)
Dr. Laurence Gill
Email: gilll@tcd.ie
Tel: +353 1 896 1047
Research Student(s)
Mr. Ronan Kane
Email: kanerc@tcd.ie
Description
In the past, extensive modelling has been carried out on the effects of emergent and submerged breakwaters in the near-shore zone with regards to sediment transport. Recent developments in the geotextile industry have afforded new possibilities for shoreline protection. One of these possibilities is the use of Artificial Reefs for coastal protection. Artificial reefs work by inducing natural wave breaking over the reef which is located offshore, thus reducing wave energy in the near-shore zone and reducing sediment transport. This study aims to investigate these alternatives to traditional coastal protection methods by testing the efficacy of artificial reefs for shoreline protection. Furthermore the deliberate manipulation of reef shape is being investigated for proficiency in creating a controlled wave environment suitable for surfing. As part of an existing study within the group, extensive mathematical modelling is being carried out to approximate wave breaking, hydrodynamics and shoreline response to the presence of artificial reefs using DHI’s MIKE21 software suite.
Garrettstown Strand in County Cork in south-west Ireland is being modelled as a case study since it has been suffering from severe erosion which has become particularly acute in recent years. As one of the potential mitigation schemes under consideration, submerged breakwaters are being examined as one of the potential solutions to the beach erosion which include the idea of incorporating a recreational value to the coastal protection scheme in the form of surfing and other beach-based activities. This has involved a bathymetry survey of the bay being carried out and the analysis of the long term off-shore wave climate of the area. Different submerged breakwater geometric designs have then been developed to promote optimal plunging waves at a peel angle of 450. The theoretical affect of the submerged structure on the adjusted near-shore wave climate and littoral response was then modelled numerically at various different positions within the bay. The optimum position was thus defined in order to minimise the existing beach erosion and also to promote sand accretion. The results were compared against the ongoing effects of the current situation and also against other possible erosion mitigation schemes such as groynes and submerged breakwaters.
MIKE21 simulation of pocket beach with Artificial Reef
Title
Residence Time Distribution modelling to characterise the hydraulic efficiency of reactor networks
Project coordinator(s)
Dr. Laurence Gill
Email: gilll@tcd.ie
Tel: +353 1 896 1047
Research Student(s)
Dr Ali Enbaia (project complete)
Description
The understanding of the hydraulic behaviour of any reactor is essential in order to optimise the efficiency of the process for which it has been designed. This research project is aiming to build a mathematical model to interpret the Residence Time Distributions (RTDs) which are generated by means of tracer studies carried out on reactor networks, particularly within the field of Wastewater Treatment plants. The project has progressed through a series of experiments on laboratory scale reactors with well understood hydraulic characteristics to produce RTD data sets which were then be used to validate and calibrate the mathematical model. The mathematical model has been formulated on the basis of tank-in-series theory. The laboratory tests have been carried out using Rhodamine WT as the tracer and the respective concentrations are analysed using the Self-Contained Underwater Fluorescence Apparatus (SCUFA). The initial experiments have been carried out using a single mixed tank reactor with the tracer injected in as a pulse input, step input and step-step input. The reactor was configured in a variety of ways to create known dead zones and short –circuits. The work then progressed to looking at various combinations of multiple tanks in series and in parallel to refine the mathematical model and develop it further. The model was finally tested at full scale Wastewater Treatment plants on activated sludge processes in an attempt to characterise the hydraulic performance of the reactors and in turn, possibly identify the source of any associated treatment problems with the processes. After some calibration experiments to determine how much of the Rhodamine was absorbed by the organic matter (bacteria) in the process, the model was successfully applied to the RTD results from the full-scale trials, identifying some flaws in the original tank designs that promoted sever short-circuiting and dead zones. It was also tested in a drill mixing process for the oil industry in Libya and was again successful as it could identify those tanks where mixing was inefficient and short-circuiting was prevailing.
RTD modeling of a 3 tank in series Activated Sludge treatment process