Biglobal linear stability analysis for the flow in eccentric annular channels and a related geometry

Recently, it has been observed that simple geometry characterized by a low level of symmetry present interesting peculiarities in the process of transition from laminar Poiseuille flow to turbulent flow. Examples of this type of geometry are eccentric channels and, more generally, parallel channels containing a narrow gap. In the present work, a global linear stability analysis for the flow in this class of geometry has been performed. The problem is discretized through spectral collocation and the eigenvalue problem has been solved with the Arnoldi-method based algorithms and the QZ algorithm. Since no numerical studies of this type have yet been performed to address the issue of transition in this geometry, the codes have been validated toward results obtained in simplified geometries _e.g., concentric annular channel and square channel_. The eigenvalue spectra of the Poiseuille flow in eccentric channels and a U-shaped channel have then been computed and analyzed for a wide range of geometric parameters. After comparison with spectra typical of channel flow and pipe flow it is shown that an additional linear mechanism of instability is present, related to the spanwise variation of the laminar velocity profile

URANS SIMULATION OF CONFINED PARALLEL JETS MIXING

The simulation of parallel jets through steady state CFD has often proved to be problematic, in particular when identical jets are simulated. In the present work the simulation of parallel jet mixing by the Unsteady Reynolds Averaged Navier-Stokes (URANS) methodology has been carried out. Such methodology has the potential to improve the results of steady state simulations, along with hybrid RANS-LES modeling and DES at a limited computational cost. The experimental setup of Kunz et al., consisting of five parallel pipe jets mixing in a rectangular confinement, has been chosen as a benchmark test because of its similarity with the geometry of the IRIS reactor. The ensemble averaged time-dependent Navier –Stokes equations have been solved through the finite volume code STAR-CD 4.06. The specific algorithm is the PISO algorithm specifically designed for transient calculations. Several numerical models have been prepared: the minimal symmetry unity (1/4 of the single jet) with boundary conditions forcing the solution to be symmetric on the symmetric planes; one, two or four jets with periodic boundary conditions; and the whole domain with wall boundary conditions. The results confirm that steady state calculations tend to under-estimate the spreading (mixing) of the jets. In particular, the spreading is acceptable in the near inlet region, while a strong discrepancy is observed far from the inlet. The results of transient simulation carried out on domains including more than one jet indicate stable oscillatory behavior downstream from the jet inlets and the results are definitely in better agreement with the test data

Numerical Study of Parallel Jet Interaction

A numerical study of bounded adjacent jets to analyze the related mixing phenomenon is here proposed. Bounded jets are indeed present in several engineering applications, such as turbine blades, burners, electronic cooling systems, energy plant components and their interaction strongly influences the performance of these components. An analysis of Parallel Confined Jet (PCJ) mixing behaviour via Large Eddy Simulation (LES) approach is performed by using the commercial code ANSYS-FLUENT 6.3.26 and different configurations are investigated in order to validate periodic boundary conditions and to test the influence of wall function approach and grid size. The comparison between a single and a three-jet configurations with coarse (1.8 and 5.4 million of cells respectively) and fine grids (8.5 and 25.5 million of cells respectively) is reported. Mean velocity profiles at different axial stations, the spreading rate are reported and validated with experimental data from literature. Periodic boundaries seem to be appropriate for present case study. The influence of wall function approach and grid resolution on capability of prediction is tested and a finer grid in the core and near the wall seems to improve numerical results