Pietro Marco Congedo

A semi-intrusive deterministic approach to uncertainty quantification in non-linear fluid flow problems

This paper deals with the formulation of a semi-intrusive (SI) method allowing the computation of statistics of linear and non linear PDEs solutions. This method shows to be very efficient to deal with probability density function of whatsoever form, long-term integration and discontinuities in stochastic space.Given a stochastic PDE where randomness is defined on ?, starting from (i) a description of the solution in term of a space variables, (ii) a numerical scheme defined for any event ω ? ? and (iii) a (family) of random variables that may be correlated, the solution is numerically described by its conditional expectancies of point values or cell averages and its evaluation constructed from the deterministic scheme. One of the tools is a tessellation of the random space as in finite volume methods for the space variables. Then, using these conditional expectancies and the geometrical description of the tessellation, a piecewise polynomial approximation in the random variables is computed using a reconstruction method that is standard for high order finite volume space, except that the measure is no longer the standard Lebesgue measure but the probability measure. This reconstruction is then used to formulate a scheme on the numerical approximation of the solution from the deterministic scheme. This new approach is said semi-intrusive because it requires only a limited amount of modification in a deterministic solver to quantify uncertainty on the state when the solver includes uncertain variables.The effectiveness of this method is illustrated for a modified version of Kraichnan-Orszag three-mode problem where a discontinuous pdf is associated to the stochastic variable, and for a nozzle flow with shocks. The results have been analyzed in terms of accuracy and probability measure flexibility. Finally, the importance of the probabilistic reconstruction in the stochastic space is shown up on an example where the exact solution is computable, the viscous Burgers equation.

SURROGATE MODEL WITH CONSERVATIVE ERROR MEASURE FOR THE STAGNATION HEAT FLUX IN ATMOSPHERIC ENTRY FLOWS

This paper proposes the application of a conservative global error measure estimation to a kriging metamodel of the stagnation pressure and heat flux in the context of the atmospheric reentry of the EXPERT vehicle. In particular, a model based method and a generalized cross validation technique are compared to the actual root mean squared error in order to check whether the estimation is conservative. Furthermore, the quality of kriging metamodeling is compared to the one of classical polynomial chaos response surface by comparing their root mean squared errors.