Numerical simulation of streamwise vorticity enhanced mixing

The goal of the present work is a detailed and comprehensive study to assess the accuracy of the numerical simulation of the mixing processes in a lobed mixer flow field via a Reynolds-averaged solution method. To meet this goal, the first objective of the current work was to establish the suitability of various meshing strategies that would allow the complex mixer geometries found in current gas-turbine engine designs to be captured, together with the associated convoluted shear layers. A second objective was targeted at providing further insight and understanding of the capability of eddy-viscosity-based turbulence models in capturing the convoluted shear layers. Simplified mixer configurations selected from the literature were studied under incompressible isothermal flow conditions. Two solution algorithms were employed to model the mixer flow fields. The first consisted of a pressure-based structured grid methodology developed for incompressible flows. A density-based mixed-unstructured grid algorithm for compressible flows was also used, with extensions to low Mach number flows made possible through a low Mach number preconditioner. The effects of turbulence were modelled using ak-e turbulence model. The absence of this model in the code made available for the unstructured algorithm necessitated its implementation as a first step in the current work. The effects of unstructured mesh type on the prediction of flows with internal mixing layers were first assessed for an incompressible planar mixing layer. This simplified case was used as a benchmark case to help understand the effects on the convoluted shear layers arising within the lobed mixer flows. To quantify the capability of a Reynolds-averaged approach in simulating the turbulent mixer flow field, two variants of the two equation k-e model were employed. The first constituted the standard linear high Reynolds number k-e model of Launder and Spalding [62]. The second model was a quadratic non-linear version developed by Speziale [99] for the prediction of secondary flows in non-circular ducts. The relative merits of these two models was assessed through detailed comparisons with experimental data taken from the literature. Of particular importance in the mixer flow was the formation and subsequent evolution of the vorticity field. Consequently, this motivated a detailed study of the evolving vorticity field. The investigations thus far were based on a simplified mixer configuration with no temperature differences between the two streams. Therefore, as a final step, a realistic scarfed mixer was modelled in an attempt to model the temperature mixing. The main contribution of the present work is the assessment of a grid-based Reynolds-averaged solution procedure for the prediction of lobed mixer flows. The study revealed that capturing the initial mixing region proved to be most difficult. Firstly, unstruc-tured meshes employing non-hexahedral elements were very inefficient at simulating the mixing layer in the early stages. Secondly, the initial mixing region presented significant difficulties for the Reynolds-averaged solution method in which neither turbulence model was capable of correctly reproducing the turbulence field. Despite this, global parameters such as momentum thickness and streamwise circulation were well captured in the predictions.