Performance enhancement of the Partial-Averaged Navier-Stokes method by using fractional step algorithm

An improved accuracy of hybrid RANS/LES models is evident in many industrial CFD applications, but the excessive computing time is still the main obstacle for more extensive use of such models. Therefore, the acceleration of hybrid RANS/LES simulations should be treated with the highest priority. In this work we deal with the Partially Averaged Navier-Stokes (PANS) formulated by Girimaji et al. (2003) and Girimaji (2006), which belongs to the bridging hybrid RANS-LES methods. This approach is being increasingly used to solve complex and high-Reynolds number industrial flows. It is designed to resolve a part of the turbulence spectrum adjusting seamlessly from RANS to DNS (Direct Numerical Simulation). In the previous reported work, we balanced speed and accuracy by using different models for the main PANS resolution parameter, namely, the unresolved to total kinetic energy ratio (fk=ku/ktot). Furthermore, an adaptive mesh refinement based on ‘fk’ criteria (refining cells above prescribed fk value) have been already reported. In this work, we employ the fractional step algorithm for coupling velocity and pressure fields and for solving the pressure correction equation. Preliminary calculations for the external car aerodynamics using meshes larger than one hundred million computational cells have shown significant speed up when compared to well-known SIMPLE like pressure algorithms. Measurements, but also previous RANS calculations, are used as a reference point to the present calculations.