∧NSY Modeling Turbulent Flows Introductory FLUENT Training e 2006 ANSYS Inc. All ANSYS, Inc. Proprietary
© 2006 ANSYS, Inc. All rights reserved. ANSYS, Inc. Proprietary Modeling Turbulent Flows Introductory FLUENT Training
Introductory FLUENT Notes Fluent User Services cente ANSYS LUENT v6. 3 December 2006 wwfluentusers. com FLUENT 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 a 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 C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-2 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 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
Introductory FLUENT Notes Fluent User Services cente ANSYS LUENT v6. 3 December 2006 wwfluentusers. com FLUENT Is the flow turbulent? External flows where Re PUL x≥5000 along a surface L L=x.d,、d,.etc Re>20000 around an obstacle Other factors such as free-stream turbulence. surface conditions and disturbances may cause transition Internal flows to turbulence at lower reynold Re,≥2,300 numbers Natural Convection Ra where Ra βgL△Tp2CD阝gD△7 is the rayleigh number P μ k a k is the Prandtl number C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-3 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 Is the Flow Turbulent? External Flows Internal Flows Natural Convection Rex 500,000 along a surface around an obstacle where where Other factors such as free-stream turbulence, surface conditions, and disturbances may cause transition to turbulence at lower Reynolds numbers Re 2,300 dh Re d 20,000 is the Rayleigh number = U L Re L , , ,etc. d dh L = x k g L T Cp g L T = = 3 2 3 Ra 9 10 Pr Ra k Cp = Pr = is the Prandtl number
Introductory FLUENT Notes Fluent User Services cente ANSYS LUENT v6. 3 December 2006 wwfluentusers. com FLUENT Turbulent flow structures Small L structures structures Iniection of energy Dissipation Large-scale Flux ofenergy Ls= / Re Energy Cascade Richardson(1922) C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-4 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 Turbulent Flow Structures Energy Cascade Richardson (1922) Small structures Large structures
Overview of Computational Approache to Introductory FLUENT Notes Fluent User Services cente LUENT v6. 3 December 2006 wwfluentusers. com UENT 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 C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-5 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 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