Introductory FLUENT Notes Fluent User Services cente ANSYS LUENT v6. 3 December 2006 wwfluentusers. com FLUENT Wall boundary conditions The k-e family and RSM models are not valid in the near-wall region, whereas Spalart-Allmaras and k-o models are valid all the way to the all (provided the mesh is sufficiently fine). To work around this, we can take one of two approaches Wall Function Approach Standard wall function method is to take advantage of the fact that(for equilibrium turbulent boundary layers ) a log-law correlation can supply the required wall boundary conditions(as illustrated in the previous slide) buf是 Non-equilibrium wall function method attempts to improve the results for sublayer flows with higher pressure gradients, separations, reattachment and Similar laws are also constructed for the energy and species equations Benefit: Wall functions allow the use of a relatively coarse mesh in the near-wall region outer layer Enhanced Wall Treatment Option Combines a blended law-of-the wall and a two-layer zonal model finher faye Suitable for low-Re flows or flows with complex near-wall phenomena Turbulence models are modified for the inner layer butter a Generally requires a fine near-wall mesh capable of resolving the viscous sublayer(at least 10 cells within the "inner layer) C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-16 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 Wall Boundary Conditions ◆ The k–ε family and RSM models are not valid in the near-wall region, whereas Spalart-Allmaras and k–ω models are valid all the way to the wall (provided the mesh is sufficiently fine). To work around this, we can take one of two approaches. ◆ Wall Function Approach ⚫ Standard wall function method is to take advantage of the fact that (for equilibrium turbulent boundary layers), a log-law correlation can supply the required wall boundary conditions (as illustrated in the previous slide). ⚫ Non-equilibrium wall function method attempts to improve the results for flows with higher pressure gradients, separations, reattachment and stagnation. ⚫ Similar laws are also constructed for the energy and species equations. ⚫ Benefit: Wall functions allow the use of a relatively coarse mesh in the near-wall region. ◆ Enhanced Wall Treatment Option ⚫ Combines a blended law-of-the wall and a two-layer zonal model. ⚫ Suitable for low-Re flows or flows with complex near-wall phenomena ⚫ Turbulence models are modified for the inner layer. ⚫ Generally requires a fine near-wall mesh capable of resolving the viscous sublayer (at least 10 cells within the “inner layer”) inner layer outer layer
Introductory FLUENT Notes Fluent User Services cente ANSYS LUENT v6. 3 December 2006 wwfluentusers. com FLUENT Placement of the first grid point For standard or non-equilibrium wall functions. each wall-adjacent cell centroid should be located within the log-law layer yn a 30-300 For enhanced wall treatment(eWt), each wall-adjacent cell centroid should be located within the viscous sublayer EWT can automatically accommodate cells placed in the log- law layer How to estimate the size of wall-adjacent cells before creating the grid The skin friction coefficient can be estimated from empirical correlations 0.037 f0.039 Flat plate 2 Rey Dh Use postprocessing tools (Xy plot or contour plot)toto double check the near wall grid placement after the flow pattern has been established C 2006 ANSYS. nc All ANSYS, Inc. Proprietar
© 2006 ANSYS, Inc. All rights reserved. 6-17 ANSYS, Inc. Proprietary Fluent User Services Center www.fluentusers.com Introductory FLUENT Notes FLUENT v6.3 December 2006 Placement of The First Grid Point ◆ For standard or non-equilibrium wall functions, each wall-adjacent cell centroid should be located within the log-law layer ◆ For enhanced wall treatment (EWT), each wall-adjacent cell centroid should be located within the viscous sublayer ⚫ EWT can automatically accommodate cells placed in the log-law layer ◆ How to estimate the size of wall-adjacent cells before creating the grid: ⚫ The skin friction coefficient can be estimated from empirical correlations: ◆ Use postprocessing tools (XY plot or contour plot) to to double check the nearwall grid placement after the flow pattern has been established. 2 f e w C U =U = 30−300 + p y 1 + p y + + = = u y y y u y p p p p 1 5 Re 0.037 2 L Cf 1 4 Re 0.039 2 Dh Cf Flat plate: Duct: