Center for Turbulence Research Proceedings of the Summer Program 2010 27 Robust simulation of nonclassical gas-dynamics phenomena By P. M. Congedo†, P. Colonna‡, C. Corre†, J. Witteveen AND G. Iaccarino A computational study of the occurrence of rarefaction shock-wave (RSW) in a dense gas shock-tube is presented. The approach is based on the characterization of exper- imental variability and uncertainties in the thermodynamic properties of the fluid to investigate the realizability and reproducibility of a RSW in the FAST shock-tube at Delft University of Technology. The weakness of the RSW makes its occurrence highly sensitive to uncertainties on the initial conditions and on the equation of state. In this work, computational fluid dynamics (CFD) and uncertainty quantification tools are com- bined to predict the probability of the RSW to occur and also to define experimental error levels required to ensure the reproducibility of the measurements. 1. Introduction The experimental confirmation of nonclassical gas-dynamic effects in flows of dense vapors is one of the unanswered questions in fluid mechanics, since the initial theory was formulated by Bethe (1942) and later by Zel’dovich (1946) and Thompson (1971). For dense-vapor transonic flows of substances formed by complex organic molecules, phe- nomena such as rarefaction shock waves and compression fans are theoretically possible. Fluids that might exhibit nonclassical gas-dynamic phenomena are called BZT fluids from the name of the three scientists who first theorized their existence. These anomalies occur when the fundamental derivative of gas-dynamics Γ = 1 + ρ a  ∂a ∂ρ  s with ρ the fluid density, a the sound speed and s the entropy, becomes negative between the upper saturation curve and the Γ = 0 contour. Such a region is often referred to as the inversion zone and and the Γ = 0 contour is called the transition line. Nonclassical gas-dynamic effects could be exploited to design highly efficient turbine nozzles for small Organic Rankine Cycle (ORC) turbogenerators (Brown & Argrow 2000), whereby the formation of compression shocks in the turbine passages can be highly re- duced (or even suppressed) thus considerably increasing the isentropic efficiency of the expander (Monaco et al. 1997). ORC technology is among the best options for the con- version into electricity of renewable energy sources (solar radiation, biomass, geothermal heat, industrial waste heat). An attempt to experimentally prove for the first time the existence of nonclassical gas- dynamics is underway at the Delft University of Technology. A newly realized shock-tube will be used to generate a rarefaction shock-wave (RSW) as depicted in Fig. 1 (Colonna et al. 2008c). Within the TU Delft project, several advancements with regard to the fundamental theory, thermodynamic modeling of the fluids (Colonna et al. 2006, 2008a; Nannan et al. 2007), the maximization of the effects (Guardone et al. 2010), and the numerical simulation of the fluid flow (Colonna & Silva 2003; Colonna & Rebay 2004), † LEGI Lab, INPG (Institut National Polytechnique de Grenoble) ‡ Energy Technology Section, Delft University of Technology