Optimizing speed and accuracy of the Partially Averaged Navier-Stokes Method

The Partially Averaged Navier-Stokes (PANS) formulated by Girimaji et al. (2003) and Girimaji (2006) is being increasingly used to solve complex and high-Reynolds number industrial flows. This approach, which belongs to the bridging hybrid RANS-LES methods, is designed to resolve a part of the turbulence spectrum adjusting seamlessly from RANS to DNS (Direct Numerical Simulation).  Various researchers have contributed to the PANS method following two main paths: (a) applying the basic derivation rules on different RANS models e.g. k-w, k-e etc. thus providing the background PANS model and (b) developing calculation approaches for the main model resolution parameter, namely, the unresolved to total kinetic energy ratio (fk=ku/ktot). In this work, we analyse the performance of different ‘fk’ approaches for the external car aerodynamics on large computational meshes. Three different approaches for specifying fk are considered in this work: (1) using the constant value over the entire computational domain; (2) a dynamic update of fk per computational cell at every time step following the formula of Girimaji and Abdul-Hamid (2005); (3) using the most complex approach that solves additional equation for so called scale-supplying variable (Basara, Pavlovic, Girimaji 2018). Calculations are also performed by using a time activation ramp for different methods to balance speed and accuracy. Measurements, but also previous RANS calculations, are used as a reference point to the present calculations.