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Analysis of laboratory scale aerated basin using computational techniques Jenkinson, Robert Wayne


This study endeavored to apply computational fluid dynamics (CFD) techniques to aerated stabilization basins (ASBs) on a laboratory scale. A laboratory study was set up to assess the hydraulic impacts of a small-scale mechanical surface aerator in a still-water tank. A small surface mechanical aerator was built that would circulate from 0.5 to 4 L/sec. The aerator was employed in a tank and the velocity patterns imported by the aerator were measured using an acoustic Doppler velocimeter (ADV). The velocity profiles were converted into dispersion parameters for use in a CFD model. A local turbulent time scale method was used to determine an approximate dispersion value as a function of distance from the aerator. The analysis examined different aerator power settings under otherwise identical conditions. Three successful runs were completed. The chaotic patterns within the tank induced by the aerator, produced turbulent structures that were not easily resolved using a time averaged statistical turbulence approach. Tracer studies were performed in the same basin, modified to provide a flow-through environment. The studies were performed with and without aeration and at different flowthrough rates. Tracer study results were examined and some of the typical parameters used in the literature were extracted for comparative analysis. The trends were surprising as the dimensionless axial dispersion number was inversely related to the degree of mixing applied. A two-dimensional computational fluid dynamics (CFD) model was employed to model the fluid flow within the basin without aeration using a commercial package, FLUENT. The modeling results were compared with depth-averaged ADV velocity measurements taken within the basin. Many CFD modeling parameters were varied, including grid size and structure, turbulence modeling and boundary condition modeling. The model compared well with the ADV data. The results from the FLUENT modeling were applied to a second CFD model that solved the solute transport equations over a structured grid. The solute transport modeling only examined the transport equations with uniform dispersion over the basin and, although it was the original intent, no attempts were made to model aeration at this stage. The overall shape of the modeled tracer curves compared favourably to the tracer study data obtained in the lab.

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