fluent-v6.3-湍流.ppt

Modeling Turbulent Flows Introductory FLUENT Training What is Turbulence? Unsteady, irregular (aperiodic) motion in which transported quantities (mass, momentum, scalar species) fluctuate in time and space Identifiable swirling patterns characterize turbulent eddies. Enhanced mixing (matter, momentum, energy, etc.) results Fluid properties and velocity exhibit random variations Statistical averaging results in accountable, turbulence related transport mechanisms. This characteristic allows for turbulence modeling. Contains a wide range of turbulent eddy sizes (scales spectrum). The size/velocity of large eddies is on the order of mean flow. Large eddies derive energy from the mean flow Energy is transferred from larger eddies to smaller eddies In the smallest eddies, turbulent energy is converted to internal energy by viscous dissipation. Is the Flow Turbulent? Turbulent Flow Structures Overview of Computational Approaches Reynolds-Averaged Navier-Stokes (RANS) models Solve ensemble-averaged (or time-averaged) Navier-Stokes equations All turbulent length scales are modeled in RANS. The most widely used approach for calculating industrial flows. Large Eddy Simulation (LES) Solves the spatially averaged N-S equations. Large eddies are directly resolved, but eddies smaller than the mesh are modeled. Less expensive than DNS, but the amount of computational resources and efforts are still too large for most practical applications. Direct Numerical Simulation (DNS) Theoretically, all turbulent flows can be simulated by numerically solving the full Navier-Stokes equations. Resolves the whole spectrum of scales. No modeling is required. But the cost is too prohibitive! Not practical for industrial flows - DNS is not available in Fluent. There is not yet a single, practical turbulence model that can reliably predict all turbulent flows with sufficient accuracy. Turbulence Models Available in FLUENT RANS Modeling – Time Averaging Ensemble (time) averaging may be used to ext