Verification and validation of LES of a triple parallel jet flow in the context of a thermal striping investigation

Abstract

Large Eddy Simulation (LES) consists in explicitly resolving the largest turbulent scales while the small-scale motions are taken into account by means of a subgrid-scale (SGS) model. Although it remains computationally expensive, LES seems to constitute an increasingly employed tool for engineering applications in fluid mechanics. In a nuclear context e.g., LES is a relevant approach for the numerical study and prediction of the thermal striping, which can occur during the functioning of sodium-cooled nuclear reactors and may cause some damage to the structures. Numerous experiments where three differentially heated jets mix inside a cavity were designed and conducted by the Japan Atomic Energy Agency (JAEA) to study these phenomena. In order to evaluate the quality of LES using the CEA in-house TrioCFD code, a Verification and Validation study is proposed. Several LES of triple parallel jets are conducted with various grid sizes and SGS models. Following the idea of Celik et al. (2009), the effective SGS kinetic energy is written as a polynomial function of the grid size Δ:keff,sgs=amodΔpmod+anumΔpnum, where the two terms in the right-hand side are respectively the contributions of the numerical scheme and of the subgrid-scale model. In the Verification part, the orders of convergence of these two contributions are assessed for the Smagorinsky and WALE models. In the Validation part, an evaluation of the results is proposed by comparing computed velocity profiles to experimental measurements. The effects of grid resolution, SGS model and boundary conditions are discussed.