DOI: 10.1615/TSFP6
ANALYSIS OF GERMANO'S HYBRID FILTER FOR SIMULATION OF TURBULENT FLOWS
要約
Germano (2004) proposed a hybrid filter model, which additively combines an LES-like filter operator (F) and a RANS-like statistical operator (E) using a blending function k: H = kF +(1−k)E. Using turbulent channel flow as an example, we conducted a priori tests in order to gain further insight into the hybrid-filter approach. In this a priori test, two separate simulations of RANS (E) and LES (F) were performed using the same grid in order to construct a hybrid-filtered field (H). The resulting H-field solution which was used for the present analysis revealed several deficiencies of existing hybrid models. We showed that, contrary to the assumption made by most existing hybrid approaches, hybrid velocity fields are not divergence free. Based on the analysis of the instantaneous velocity fields and H-filtered turbulent kinetic energy, we have illustrated that while the artificial stochastic forcing term of Piomelli et al. (2003) was necessary in DES, the extra terms arising out of the hybrid-filtered Navier-Stokes (HFNS) equations derivation represents such forcing naturally, thus alleviating the need for any ad-hoc fix. Finally, our results also indicated that direct blending of two disparate RANS and LES models, as is commonly done in many hybrid approaches, is mathematically imprecise, lacking the ability to capture fundamental physics, even with a posteriori modifications.