Adrien GOMAR

Referred papers

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  Abstract

  A time-domain harmonic balance method for the analysis of almost-periodic (multi-harmonics) flows is presented. This method relies on Fourier analysis to derive an efficient alternative to classical time marching schemes for such flows. It has recently received significant attention, especially in the turbomachinery field where the flow spectrum is essentially a combination of the blade passing frequencies. Up to now, harmonic balance methods have used a uniform time sampling of the period of interest, but in the case of several frequencies, non-necessarily multiple of each other, harmonic balance methods can face stability issues due to a bad condition number of the Fourier operator. Two algorithms are derived to find a non-uniform time sampling in order to minimize this condition number. Their behavior is studied on a wide range of frequencies, and a model problem of a 1D flow with pulsating outlet pressure, which enables to prove their efficiency. Finally, the flow in a multi-stage axial compressor is analyzed with different frequency sets. It demonstrates the stability and robustness of the present non-uniform harmonic balance method regardless of the frequency set.

  Non-uniform time sampling for multiple-frequency harmonic balance computations

  Journal of Computational Physics, Volume 236, 1 March 2013, Pages 317–345, http://dx.doi.org/10.1016/j.jcp.2012.11.010
  Authors: T. Guédeney, A. Gomar, F. Gallard, F.Sicot, G. Dufour and G. Puigt
  
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  Abstract

  The present paper investigates a time-domain harmonic balance method as an alternative to classical time-marching schemes for stability studies of turbomachineries toward flutter. A weak coupling approach is applied, which requires to compute the fluid response to prescribed harmonic motions of the structure. The harmonic balance method is adapted to single-sector reduction and an arbitrary Lagrangian/Eulerian formulation. The 11th standard configuration for aeroelasticity is investigated to assess the accuracy of the approach. Then, an industrial test case is presented: a fan designed by Safran Snecma. The results show the good accuracy of the proposed harmonic balance method as well as significant reductions in computational time.

  A Time-Domain Harmonic Balance Method for Turbomachinery Aeroelasticity

  Revised in November 2012
  Authors: F. Sicot, A. Gomar, G. Dufour and A. Dugeai
  

Conference papers

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  Abstract

  The present work proposes to use LES in a 3D transonic axial compressor configuration, the NASA Rotor 37. Three meshes are investigated: a 10 million, a 25 million and a 100 million grid points mesh. The convergence criteria, the choice of the numerical schemes and the determination of the time-step are investigated to propose a methodology for LES calculations. The time needed to compute such simulations and comparison with the experimental results are given. Finally, an analysis of the mesh dependency for LES is assessed.

  pdf BibTeX

  High-Fidelity Simulation of the Turbulent Flow in a Transonic Axial Compressor

  In Proceedings of 9th European Turbomachinery Conference, Istanbul, Turkey, 2011
  Authors: A. Gomar, N. Gourdain and G. Dufour
  
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  Abstract

  This paper investigates the capability of numerical simulations to estimate unsteady flows in gas turbine components with both structured and unstructured flow solvers. Different numerical approaches are assessed, from steady-state methods based on the Reynolds Averaged Navier-Stokes (RANS) equations to more sophisticated methods such as the Large Eddy Simulation (LES) technique. Three test cases are investigated: a separated flow phenomenon in an internal turbine cooling channel, the wall heat transfer in a turbine guide vane and the aerodynamics of a high-pressure compressor. Steady flow simulations usually fail to predict the mean effects of unsteady flows (such as vortex shedding) and wall heat transfer, mainly because some phenomena are not well taken into account (laminar-to turbulent transition, shock/boundary layer interaction, etc.). Actually, only the LES (partially) succeeds to accurately estimate unsteady flows and wall heat fluxes in complex configurations. The results presented in this paper indicate that this method considerably improves the level of physical description (including boundary layer transition). However, the LES still requires developments and validations for such complex flows. This study also points out the dependency of results to parameters such as the freestream turbulence intensity. When feasible solutions obtained with both structured and unstructured flow solvers are compared to experimental data.

  pdf BibTeX

  Application of RANS and LES to the Prediction of Complex Flows in Gas Turbine Components

  In Proceedings of 46th Symposium of Applied Aerodynamics, AAAF, Orléans, France, 2011
  Authors: N. Gourdain, L.Y.M. Gicquel, A. Gomar, R. Fransen, E. Collado and T. Arts
  

Aeroelasticity of Counter Rotating Open Rotor

My Ph.D. research deals with Counter Rotating Open Rotor Aeroelasticity. To do so, a weak coupling approach is followed as the structure is assumed to have a sinusoidal motion. Unsteady simulations of the fluid are then run to get the fluid response to the vibration of the structure. Since the computations can be extremely costly, a spectral method is used, allowing to reduce the CPU time by a factor up to ten.

The method has been implemented into the elsA code by F. Sicot and extended by T. Guédeney. The aim of my Ph.D. is to assess such a method for CROR and to study the properties/limits of the approach.

Keywords: CROR, Harmonic Balance technique, Aeroelasticity

Large Eddy Simulations of turbomachineries

During my final year internship, I assesed Large Eddy Simulations (LES) for turbomachineries. LES is a high order method that solves the large turbulent structures, whereas the commonly used Reynolds Averaged Navier-Stokes approach (RANS) models turbulence. Therefore, LES shows to be much more accurate for the prediction of the flow than RANS do. However, as the Reynolds number is big in turbomachineries computations (up to ten million), such an approach requires extremely fine meshes that are not suitable for daily basis computations (up to 100 grid-points mesh for only one blade sector!). Thus, the work has been dedicated to the evaluation of the density of the mesh needed and the time required to run such computations. Moreover, convergence criterion have been investigated to state when the calculations were converged. Local results have been compared to experimental data and have shown that even if LES gives an insight of the real flow behavior, it is still not mature to accurately predict the global parameters. Below are shown two films highlighting the LES prediction of the NASA Rotor 37. All results and details of this work have been published for the European Turbomachinery Conference (ETC). The abstract and pdf are available above.

• Compressible flows (2^nd year students)
• Joukovsky theory (1^st year students)
• Flight dynamics of a commercial plane (2^nd year students)
• Laminar and turbulent boundary layer (2^nd year students)
• Supersonic airfoil (2^nd year students)