José Miguel Pérez

Inception and evolution of coherent structures in under-expanded supersonic jets

The purpose of this paper is to examine the generation and nature of the large coherent structures observed experimentally in an under-expanded supersonic impinging jet. More specifically, the questions to answer are: What mechanisms govern the receptivity process at the nozzle lip?, how does the underlying flow field affect the evolution of the large-scale coherent structure generated from the initial instability? and what are the interactions between the large-scale (forced) coherent structures and the developing turbulence in the jet shear layer? In order to answer some of these questions both alternatives, that these structures come from global modal flow instabilities or from convective instabilities, the latter, are considered in this work. The stability analysis considered in the former case is performed in this work near the nozzle around the temporal average of the flow obtained by using an in-house LES (Large Eddy Simulation) code. The flow in this region is considered laminar, steady and without non-linear effects. The well known feedback loop in the impinging jet, according to which acoustic waves propagate upstream and excite the jet shear-layer (see Figure 2), advises against some of the hypothesis considered previously in the global stability analysis (ie. non-linear approximation). However the acoustic waves are orders of magnitude smaller than the hydrodynamic waves and should be smoothed out in the temporal average used in the calculation of the mean flow. The results show that both, axisymmetrical (m = 0) and azimuthal modes (m ≥ 1) are stable to global modal analysis and only convective instability could justify the instabilities observed in experiments in the shear layer. A study on the receptivity problem confirms that external disturbances may enter and excite the shear layer, being responsible of the instabilities observed in both experiments and direct numerical simulations.

Linear global instability of non-orthogonal incompressible swept attachment-line boundary layer flow

Instability of the orthogonal swept attachment line boundary layer has received attention by local1, 2 and global3–5 analysis methods over several decades, owing to the significance of this model to transition to turbulence on the surface of swept wings. However, substantially less attention has been paid to the problem of laminar flow instability in the non-orthogonal swept attachment-line boundary layer; only a local analysis framework has been employed to-date.6 The present contribution addresses this issue from a linear global (BiGlobal) instability analysis point of view in the incompressible regime. Direct numerical simulations have also been performed in order to verify the analysis results and unravel the limits of validity of the Dorrepaal basic flow7 model analyzed. Cross-validated results document the effect of the angle _ on the critical conditions identified by Hall et al.1 and show linear destabilization of the flow with decreasing AoA, up to a limit at which the assumptions of the Dorrepaal model become questionable. Finally, a simple extension of the extended G¨ortler-H¨ammerlin ODE-based polynomial model proposed by Theofilis et al.4 is presented for the non-orthogonal flow. In this model, the symmetries of the three-dimensional disturbances are broken by the non-orthogonal flow conditions. Temporal and spatial one-dimensional linear eigenvalue codes were developed, obtaining consistent results with BiGlobal stability analysis and DNS. Beyond the computational advantages presented by the ODE-based model, it allows us to understand the functional dependence of the three-dimensional disturbances in the non-orthogonal case as well as their connections with the disturbances of the orthogonal stability problem.

Laminar Separation Bubbles in Two-Dimensional Straight-Diverging-Straight Channel Flows

Geometries with sudden expansion have been a subject of study for decades now, owing to its engineering applications. While attention has been lavished on flow through symmetric channels with sudden expansion (SE) and backward-facing step (BFS), channels with other divergent angles are studied far less. Straight-diverging-straight (SDS) channels with finite angle of divergences have been studied here. Our focus is on the formation of the laminar separation bubble, typically in the diverging region, and its reattachment downstream. Computations have been carried out to estimate the effect of various parameters such as the angle of divergence \((\alpha )\), the outlet to intet height ratios \((D/d)\) and the Reynolds numbers \((Re)\) on the formation of the recirculation bubble. The extreme case with \(\alpha = 90^\circ \) can be compared to the flow through a symmetric sudden-expansion (SE) flow. The base flow obtained from the two open source codes is characterized for the formation of laminar separation bubble for very low Reynolds numbers \((Re)\) in the parametric space including the angle of divergence, \(\alpha \) and the expansion ratio, \(\kappa =D/d\) and \(Re\).