RESEARCH INTERESTS

2001 - 2002

LES of the flow through a 3D transsonic nozzle with a WENO scheme and the mixed scale model

The simulation of the flow through the 3D channel with a third order WENO (Weighted Essentially Non-Oscillatory) scheme, has been performed on a new grid mesh. The WENO scheme is much less disspative than the second order TVD Harten Yee scheme which was used in the thesis (see document). It is important to control the numerical dissipation in the simulation as there is a competition between the subgrid scale modelling and the numerical dissipation. The computational domain has more points in the longitudinal (Ox) and the vertical (Oy) directions to better capture the unsteady phenomena. There are 6e6 grid points. Also, the average grid mesh size near the walls is more important than in the thesis simulation, which allows to integrate the equations over a larger time scale.

The large separation observed under the lambda shape like shock pattern at the initialization become unstable and some unsteady vortices are ejected. Some temporal spectra have been reconstructed near the lower wall and the right lateral wall, downstream the shock-wave/boundary layer interaction. The dimensionless characteristic frequency of the ejected vortices is St = 0.4. This Strouhal number has been calculated with the inlet velocity and the maximum heigh of the bump. This value is closed to the bluff body Strouhal number of 0.3 measured in the wake. Also some spatial correlation between two points have been obtained in this quasi homogeneous region. The spatial integral scale is then of order of Li = 4e-4 m. The grid mesh size value is lm = 1e-5 m at this location showing that we correctly resolved these coherent structures.

Time evolution of the longitudinal velocity component U obtained with the WENO scheme in a vertical plane where the boundary layer separates

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Statistical Simulation of the flow through a 2D axisymmetrical nozzle with secondary injection
Collaboration CEMIF-LIMSI-CNES

The study deals with the simulation of the over-expanded flow in the 2D axisymmetrical Ariane nozzle during takeoff. The geometrical configuration of the Ariane nozzle is optimized for the outflow conditions existing in the high atmosphere. The presence of a shock wave in the divergent at the shuttle launch leads to performance losses in the propulsion. Also, the shock wave/boundary layer interaction leads to recirculations that increase the asymmetrical lateral load.

The aim of the study is to simulate the flow in the over-expanded regime to analyse in detail the secondary flows observed in the nozzle in sight of control. The flow is calculated with a k-omega statistical model of turbulence through a nozzle for which experimental results are available (ONERA). First, the flow is simulated without secondary injection. Then after this reference flow will be obtained and analyzed, the principal jet will be coupled with a secondary injection of fluid. The injection of fluid in the separation zone, in the nozzle, should improve the performances of the propulsion jet as the back flow in the divergent should be less important.

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