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Title: Advanced turbulence modelling and near-wall treatments for rotating ducts and centrifugal compressor flows
Author: Boudjir, Amel
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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The main aim of the thesis is to report numerical investigations of developing flows through a centrifugal blade passage. The investigation focused on exploring alternative ways of predicting the near wall region and rotation using advanced stress transport models and wall functions within such geometries. In the initial stages of this study, the flow fields within stationary and rotating ducts have been reproduced through the use of various turbulence models and wall treatments. The aim of exploring such geometries was to gain insight into the flow physics within them as these flows share similar features found in the flow within centrifugal impellers. Initial refinements of the advanced wall treatment and the approaches used to couple the wall functions and the turbulence models were also performed on the ducts. This study confirmed that linear eddy-viscosity schemes fail to capture many features associated with such flows. The two stress transport schemes tested (a basic linear one, and more advanced non-linear form) performed similarly to each other, and gave reasonable agreement with data, although they both failed to faithfully represent the effects of secondary flows on rotation on the suction side. It was noted that the advanced wall function tested (the Analytical Wall Function, or AWF, based on solving a simplified form of the momentum equation across the near-wall cell) in its original form failed to reproduce the effects of rotation and that it was highly sensitive to the approach used to approximate convective terms within it. Reasonable results for the rotating ducts were obtained with the AWF using a refined approximation of these convective terms, although turbulence levels were generally underpredicted close to the wall. A stationary impeller blade passage of the NASA Low speed centrifugal compressor (LSCC) has then been simulated. The standard k-ε and k-ω models, and two stress transport schemes were used to model the core flow region, while the standard and analytical wall functions were used to predict the near-wall region. Although there is no corresponding available experimental data, the study was aimed at exploring the behaviour of the different turbulence models and wall treatments and gaining insight into the flow field in the absence of rotation. It was found that, in general, both stress transport models perform similarly in terms of reproducing the flow features within the blade passage and that in the first part of the passage, where there is substantial boundary layer growth and flow development, both the velocity and turbulence fields are mainly affected by the choice of the wall treatment, while in the second half, the predictions of the flow field are mainly dependent on the choice of the outer turbulence model. The turbulence levels predicted by the AWF were generally lower than those returned by the standard wall function. Finally the flow within the rotating NASA Low speed centrifugal impeller was modelled using the previously described turbulence models and wall treatments and comparisons were made with existing experimental data. It was noted that the various combinations of turbulence models and wall treatments produced generally similar results for the various velocity components, although the meridional velocity component was slightly underpredicted in locations. The spanwise velocity was not, in general, faithfully produced by the various turbulence models, especially in the second half of the impeller passage. Additionally, similar values for turbulence quantities were produced via the different turbulence models and wall functions, with the main difference being that the use of the AWF often resulted in lower levels of turbulent kinetic energy than those computed via the standard wall function.
Supervisor: Craft, Timothy ; Turan, Ali Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available