Numerical study of obstacle geometry effect on the vortex shedding suppression and aerodynamic characteristics

Purpose Two-dimensional incompressible fluid flows around a rectangular shape placed over a larger rectangular shape at low Reynolds numbers (Re) have been numerically analyzed in the present work. The vortex shedding is investigated at different arrangements of the two shapes allowing the investigation of three possible configurations. The calculations are carried out for several values of Re ranging from 1 to 200. The effect of the obstacle geometry on the vortex shedding is analyzed for crawling, steady and unsteady regimes. The analysis of the flow evolution shows that with increasing Re beyond a certain critical value, the flow becomes unstable and undergoes a bifurcation. This paper aims to observe that the transition of the unsteady regime is performed by a Hopf bifurcation. The critical Re beyond which the flow becomes unsteady is determined for each configuration. A special attention is paid to compute the drag and lift forces acting on the rectangular shapes, which allowed determining; the best configuration in terms of both drag and lift. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Re and the obstacle geometry. Hence, the values of vortex shedding frequencies are calculated in this work. Design/methodology/approach The dimensionless Navier–Stokes equations were numerically solved using the following numerical technique based on the finite volume method. The temporal discretization of the time derivative is performed by an Euler backward second-order implicit scheme. Non-linear terms are evaluated explicitly; while, viscous terms are treated implicitly. The strong velocity–pressure coupling present in the continuity and the momentum equations are handled by implementing the projection method. Findings The present paper aims to numerically study the effect of the obstacle geometry on the vortex shedding and on the drag and lift forces to analyze the flow structure around three configurations at crawling, steady and unsteady regimes. Originality/value A special attention is paid to compute the drag and lift forces acting on the rectangular shapes, which allowed determining; the best shapes configuration in terms of both drag and lift.

Experimental combustion chamber simulation at transient regimes

The transient regimes in a combustion chamber has to be as short as possible because flame front position and thickness can destroy the combustion chamber in couple seconds. The simulation of such a regime has to be performed unsteady. An experimental combustion chamber it is simulated at two unsteady regimes to see the flame front structure and comparison it is made with the experimental data to validate the results. For this analysis Ansys CFX was used and the turbulent model was DES while the combustion model was Eddy Dissipation. The two cases show different flame front structures while the boundary conditions for the two regimes are very similar.

Experimental and Numerical Analyses of Unsteady Flow Regimes and Mixing in a Micro T-Mixer

Despite the very simple geometry, T-shaped micro-mixers are characterized by different and complex laminar flow regimes. In the present work, experiments and direct numerical simulations are employed jointly to investigate unsteady periodic flow regimes, viz. the asymmetric and symmetric regimes. The first is characterized by a periodic dynamics of the three-dimensional structures and by a high degree of mixing, while in the second the flow always maintains a double mirror symmetry in the mixing channel, which causes a dramatic decrease of the mixing performance. A methodology, allowing us to quantitatively compare the numerical predictions with experimental flow visualizations, is used to investigate these unsteady regimes and to evaluate the relevant frequencies and the degree of mixing. In both regimes the characteristic non-dimensional frequency, based on the bulk velocity and hydraulic diameter of the mixing channel, increases with the Reynolds number, but a significant discontinuity is found at the transition from the first to the second regime.