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UBC Theses and Dissertations

Numerical investigation of transition and turbulence in planar and axisymmetric expansions Moallemi, Nima


This thesis documents a series of complementary numerical investigations aimed at understanding the flow instability, transition, and laminarization process through planar and axisymmetric expansions. Direct numerical simulation (DNS) is used to accurately resolve the spatial and temporal scales of the simulated flows. The first investigation addresses the mechanisms for the initial onset of flow instability in planar expansions at low Reynolds numbers. Various expansion ratios are studied using two-dimensional direct numerical simulations at flow Reynolds numbers up to 5000 and expansion ratios in the range of 1.33 to 4.00. Correlations are developed for the non-dimensional reattachment length and the maximum velocity magnitude in the reverse-flow region in terms of Reynolds numbers and expansion ratios. Bifurcation phenomena resulting in the loss of flow symmetry downstream of the sudden expansion is observed and critical Reynolds numbers for the onset of bifurcation for various expansion ratios are identified. The growth and decay mechanisms of the localized turbulence are analyzed through the transport budgets of instantaneous vorticity. The effect of transverse jets on the bifurcation and instability of the flow in planar expansions is examined. Transverse injections cause increased levels of mixing and fluctuations downstream of the flow by generating localized disturbances through the activation of inviscid instability modes in the transverse flow. Finally, the dynamics of turbulent flow in a gradual axisymmetric expansion is investigated. Turbulent inflow is generated in a precursor simulation of a periodic pipe with an annular rib roughness. Validation is accomplished in terms of published mean velocity profile, velocity defect, and fluctuation amplitudes as well as near-wall power spectra. Turbulence from the precursor study is passed to the gradual expansion to examine the downstream growth and decay of turbulence. This process is described in terms of the evolution of the Reynolds stress tensor in the turbulent and laminarizing regions, morphometrically in terms of the evolution of coherent vortical structures, and mechanistically through analysis of the budgets of the vorticity transport equation.

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