Application of optical flow for high-resolution velocity measurements in wall-bounded turbulence

Abstract

This thesis assesses the performance of an advanced wavelet-based optical flow (wOF) algorithm in extracting high accuracy and high-resolution velocity vector fields from tracer particle images in wall-bounded turbulent flows. wOF is first evaluated using synthetic particle images generated from a channel flow DNS of a turbulent boundary layer. The sensitivity of wOF to the regularization parameter (λ) is quantified and results are compared to state-of-the-art cross-correlation-based PIV. Results on synthetic particle images indicated different sensitivity to underregularization or over-regularization depending on which region of the boundary layer is being analysed. Nonetheless, tests on synthetic data revealed that wOF can modestly outperform PIV in vector accuracy across a broad λ range. wOF showed clear advantages over PIV in resolving the viscous sublayer and obtaining highly accurate estimates of the wall shear stress and thus normalising boundary layer variables.

Following the validation of the wOF algorithm using synthetic PIV images rendered from the DNS simulation, wOF is subsequently applied to an experimental data acquired in a well-established flow facility referred to as a side-wall quenching burner (SWQ) facility. This experimental facility is designed for reacting flows, but wOF is first applied to data acquired under non-reacting flow conditions, where a jet flow impinges onto a parallel wall, creating a developing turbulent boundary layer. Overall, wOF revealed good agreement with results from both PIV and a combined PIV + PTV (particle tracking velocimetry) method. However, wOF was able to successfully resolve the wall shear stress and correctly normalise the boundary layer streamwise velocity to wall units where PIV and PIV + PTV showed larger deviations. The enhanced vector resolution of wOF enabled improved estimation of instantaneous derivative quantities and intricate flow structure both closer to the wall and more accurately than the other velocimetry methods.

In the final investigation of the thesis, both wOF and PIV are applied to experimental PIV images from the same SWQ facility, now operated under reacting conditions to investigate the coupling between flame-wall interaction (FWI) and wall-turbulence. Characteristic vortex types were identified and a conditional analysis is performed to characterise the vortex dynamics and hydrodynamic influences affecting flame quenching. The action of a dominant vortex, known as a Type 3 vortex, is predominantly to push the flame deeper into the shear layer at the wall, leading to increased strains and heat loss which result in quenching. A flame vortex interaction mechanism, which has only been postulated previously in the literature, is revealed explicitly for the first time. The turbulent flow field facilitating the transport process due to this flame vortex interaction is analysed through a quadrant analysis of the turbulent fluctuations, explicitly revealing the role of the T3 vortex in entraining burnt gases into the reactants leading to unique thermochemical states.