Research themes

    GPU Implementation of numerical simulation code for astrophysics : RAMSES-GPU

    In 2009-2010, with my colleagues F. Château et R. Teyssier, we have implemented several numerical schemes to solve Euler equations (gas dynamics) on GPU : 1. a finite volume code using a Godunov-type Riemann solver 2. a Riemann-free solver, the Turganov-Tadmor centered scheme. First results are reported in a proceeding presented at HPCTA 2010.
    This first attemp was further developped to give the highly efficient multi-GPU magnetohydrodynamics simulation code named RAMSES-GPU.

  • GPU computing applications in image processing

    1. Implementation of a DCT-based Inpainting algorithm for Nvidia GPU using CUDA. Joint work with Yassir Moudden and Antoine Pedron.
    2. Implementation of a CDF (Cohen-Daubechies-Feauveau) lifting scheme discrete wavelet transform algorithm
      See also the following slides : JTE GPU 04/12/2008 - slides
  • Wavelet-based multifractal analysis

    1. solar magnetograms and active regions (collaboration with the Trinity College of Dublin Astrophysics Group). See the slides of the talk given at conference ADA6 in May 2010: ADA6 2010
    2. study of the multifractale properties of simulations of the interstellar medium (collaboration with E. Audit from CEA/IRFU/Sap).
  • data acquisition system developpment for high-energy physics:

    design of data acquisition systems and digital signal processing systems for high-energy physics and astrophysics experiments.

    1. software and hardware developments for the UTS sub-system of Eclairs With S. Schanne from CEA/IRFU/SaP, we develop software code under RTEMS for the scientific trigger of Eclairs.
    2. FPGA digital signal processing study for BAO-radio project (dark energy survey through Baryon Acoustic Oscillations power spectrum measuments):
      • development of a VHDL 4096-points streaming FFT operator dealing  with a continuous 8bit, 500 MSPS data stream.
      • (in progress) development of a FFT-based correlator system (a 256-points FFT every 2 ns).
      • The BAO Radio acquisition system

PhD thesis manuscript (advisor: Alain Arnéodo):

Analyse multifractale 2D et 3D à l’aide de la transformation en ondelettes : application en mammographie et en turbulence développée

Abstract :

Since the end 80’s, wavelet transform has been recognized as a privileged tool to study fractal objects, providing a unified multifractal formalism for both functions and measures. In the first part, we use the 2D WTMM (Wavelet Transform Modulus Maxima) methodology to study mammography. We illustrate the usefulness of the methodology in the study of texture segmentation of rough surfaces and in the geometric characterization of clusters of microcalcifications, which are early signs of breast cancer. In a second methodologic part, we generalize the WTMM method to provide a multifractal description of both 3D scalar and vectorial data fields, introducing the tensorial wavelet transform. We show that a recursive filter technique allows to save between 25% and 60% of computing time, as compared with FFT based filtering techniques. Then we apply the 3D WTMM method to Direct Numerical Simulations (DNS) of the Navier-Stokes equations in turbulent regime with moderate Reynolds numbers. By mesuring a significantly non-zero cancellation exponent, we bring evidence that multifractal properties of both 3D dissipation and enstrophy fields are well accounted for non-conservative multiplicative cascading processes. Moreover, we observe that the cancellation exponent decreases as the Reynolds number increases. Finally, we present the first results of a fully vectorial multifractal analysis of both velocity and vorticity fields on the same numerical simulations showing that the value of the intermittence parameter C_2, as measured by the tensorial 3D WTMM method, is significantly larger than the one obtained by studying 1D longitudinal velocity increments.

Keywords

Multi-dimensionnal wavelet transform, rough surface, singularity, H”older exponent, multifractal, scale invariance, autosimilarity, WTMM methodology, singularity spectrum, fractional Brownian motion, stochastic process, random cascade, mammography, microcalcifications, fully developped turbulence.

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Pierre Kestener

Research Engineer at CEA/IRFU