An incompressible fully-developed duct flow expanding into a diffuser whose upper and one side walls are appropriately deflected (two diffuser configurations differing with respect to the expansion angles were considered, Fig. 1), for which the experimentally obtained reference database was provided by Cherry et al. (2008, 2009), was studied computationally by using LES (Large Eddy Simulation), DES (Detached Eddy Simulation) and RANS-SMC (Reynolds-Averaged Navier-Stokes in conjunction with a high-Reynolds number Second-Moment Closure model) methods. In addition, a zonal Hybrid LES/RANS (HLR; RANS – Reynolds-Averaged Navier Stokes) method, proposed recently by Jakirlic et al. (2006, 2009) and Kniesner (2008), has been applied. The flow Reynolds number based on the height of the inlet channel is Reh=10000. The primary objective of the present investigation was the comparative assessment of the afore-mentioned computational models in this flow configuration characterized by a complex three-dimensional flow separation being the consequence of an adverse pressure gradient evoked by the duct expansion. The focus of the investigation was on the capability of the different modelling approaches to accurately capture the size and shape of the three-dimensional flow separation pattern and associated mean flow and turbulence features.

In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.

This thesis reports on a numerical study of a round, isothermal turbulent jet of incompressible fluid, impinging normally on a flat wall at a different temperature. The aim was to generate detailed information about the ime-dependent three-dimensional velocity and temperature field, and, based on this, to extract statistically averaged flow and turbulence properties, as well as to identify and analyze the dominant vortical structures, their evolution and thermal signature on the target wall. The main body of the thesis deals with LES studies using the dynamic subgrid-scale model, of flow and heat transfer in a round impinging jet at Re=20000 and orifice-to-plate distance h/D=2. The LES were performed using the in-house unstructured finite-volume computational code T-FlowS. Prior to the jet simulations, the computational code and its features (the numerical schemes and solver, boundary conditions, mesh generation and refinement, implementation of the dynamic sub-grid scale model into an unstructured code) as well as structure identification and interpretation, were tested in the simulation of a plane channel and a pipe flow, the latter with heat transfer. Because a round impinging jet contains several flow regions, each featured by different flow physics (free jet expansion, impingement, jet deflection with strong acceleration, radially spreading and decelerating wall jet), several mesh refinements (with up to 10 million mesh cells), and different conditions at the free inflow boundary of the computational domain were explored until satisfactory agreement with the available experimental results was achieved. The final results, believed to be credibly accurate, were processed to extract the mean flow and turbulence statistics, budgets of the transport equations for the Reynolds stresses and turbulent-heat-flux components, as well as to analyze the time dynamics of the vortical structure and its relation with the instantaneous and averaged wall heat transfer. The LES confirmed some of the experimentally detected features such as a dip and a second maximum in the Nusselt number and negative production of turbulence kinetic energy in the stagnation region, but also revealed some other interesting phenomena such as strong oscillation of the stagnation point and the unsteady flow separation at the onset of wall-jet formation. These events were identified as the main cause of the Nu-number nonuniformity, and were linked to the ring vortices generated in the jet shear layer, and their asymmetric break-up prior or after the impingement. The second focus of the thesis was the study of the feasibility of combining the LES and RANS approaches into a hybrid method. The goal was to provide the time resolved three-dimensional solutions of the velocity and temperature field (though associated with the larger turbulence scales only) while using the mesh size typical of that used in off-wall LES or in the RANS computation. Two directions were pursued in parallel: zonal approaches with predefined RANS and LES zones, and seamless methods with a single statistical model serving both as the RANS- and as the LES sub-grid scale model. Prior to hybrid simulations, several RANS models were tested in computation of several generic flows with heat transfer, aimed at examining their suitability to serve as the near-wall RANS model in hybridization with LES. In order to verify the hybrid approaches, simulations of the plane channel flow at a range of Reynolds numbers have been carried out with four different hybrid models. Two of the hybrid models tested were subsequently applied to simulate the round impinging jet having the same Re number and configuration as in the LES study, but using much coarser grid ($\approx $ 1.6 million). While the hybrid simulations yielded satisfactory results in plane channel flows, their performance in the impinging jet - though superior to the conventional LES when using the same (coarse) mesh, was not fully satisfactory. Several critical issues have been detected requiring further testing that was beyond the scope of this thesis, thus preventing the final conclusions to be drawn. Based on this research, some directions for further research both in hybrid LES/RANS and in conventional LES have been proposed.