AFLR3 BL Normal Spacing
For BL grid regions the normal spacing grows geometrically from surfaces with grid BCs set for BL grid generation. The initial spacing is set explicitly or determined from either a laminar or turbulent flow approximation. Normal to the boundary surface the spacing then grows geometrically. Numerous options are available to control the normal spacing distribution. A description of the primary parameters for BL grids is presented here. See AFLR3 Option Flags and/or AFLR3 Parameter Options for specific details on all parameters.
The growth rate parameters are by default set internally using auto-BL-parameter-mode from the initial normal spacing and BL profile related parameters (-Re, -refx, -blf, -du, -y+, -du+, and/or -blds). The BL growth rate parameters (-bli, -blr, -bldr, -blrm, -blrend, and/or -bldel) can be adjusted from auto-BL-parameter-mode or explicitly set. In auto-BL-parameter-mode either a turbulent of laminar BL profile is assumed. With a turbulent BL profile the BL growth rate parameters are set so that the u+ velocity increment (velocity increment normalized with the friction velocity) is approximately constant throughout the BL. With a laminar BL profile the BL growth rate parameters are set so that the normalized velocity increment (velocity increment normalized with the velocity at the edge of the BL) is approximately constant throughout the BL.
The BL growth rate distribution is shown in Figure 1 for a turbulent BL profile. The same distribution profile is used (if plotted versus number of layers) for a laminar BL profile or if no specific type of profile is assumed. The growth rate is initially constant for a set number of layers (-bli). Then the growth rate increases using a constant geometric acceleration or growth in the growth rate (-bldr). The maximum growth rate is limited (-blrm). When the limit is reached the growth rate is held constant until transition to isotropic grid generation. If the BL thickness (-bldel) is specified then the growth rate transitions to the ending (-blrend) growth rate once the BL has advanced beyond the BL thickness. The ending growth rate (-blrend) increases using a constant geometric acceleration or growth in the growth rate (-bldrend). Also, the ending growth rate will be reset the maximum growth rate (-blrm) if the ending growth rate (-blrend) is set less than the maximum. If the BL has advanced beyond the BL thickness and the normal spacing is at the isotropic limit then BL grid generation will be terminated locally.
Figure 1. BL Growth Rate Distribution
The BL normal spacing and growth rate parameters are described briefly in the following.
-blds normal_spacing or dsdef=normal_spacing
Set initial normal spacing.
-Re reynolds_number or blre=reynolds_number
Set reference Reynolds number per unit grid dimension for determining the parameters that control spacing normal to BL surfaces.
-refx reference_x or refx=reference_x
Set reference length in grid units for calculating BL parameters. If reference_x is not specified or set to zero then the reference length is re-set to the maximum physical length of all BL objects. If reference_x is specified and greater than zero then the reference length is set to the value of reference_x
-blf fraction_of_laminar_bl or mbltype=1 blyp=fraction_of_laminar_bl
Specify laminar BL profile option and fraction of laminar BL thickness for determining the initial spacing normal to BL surfaces and growth rate parameters.
-du du_value or mbltype=1 bldup=du_value
Specify laminar BL profile option and normalized velocity increment (velocity increment normalized with the velocity at the edge of the BL) for a laminar BL profile to use in determining the initial spacing normal to BL surfaces and growth rate parameters.
-y+ y+_value or mbltype=2 blyp=y+_value
Specify turbulent BL profile option and initial y+ value for determining the initial spacing normal to BL surfaces and growth rate parameters.
-du+ du+_value or mbltype=2 bldup=du+_value
Specify turbulent BL profile option and u+ velocity increment (velocity increment normalized with the friction velocity) for a turbulent BL to use in determining the initial spacing normal to BL surfaces and growth rate parameters.
-bli number_of_layers or iblri=number_of_layers
Set number of constant growth BL normal spacing layers adjacent to the BL surface. The default value is automatically reset in auto-BL-parameter-mode.
-blr initial_growth_ratio or mblauto=0 cdfrbl=initial_growth_ratio
Set initial growth rate for BL normal spacing and turn off auto-BL-parameter-mode. The default value is automatically reset in auto-BL-parameter-mode.
-bldr growth_ratio_multiplier or mblauto=0 dcdfrbl=growth_ratio_multiplier
Set growth rate acceleration/multiplier for BL normal spacing and turn off auto-BL-parameter-mode. The default value is automatically reset in auto-BL-parameter-mode.
-blrm maximum_growth_ratio or cdfrblm=maximum_growth_ratio
Set maximum allowable growth rate for BL normal spacing. The default value is automatically initialized in auto-BL-parameter-mode.
-blrend end_growth_ratio or cdfrblend=end_growth_ratio
Set ending growth rate for BL normal spacing.
-bldrend growth_ratio_multiplier or dcdfrblend=growth_ratio_multiplier
Set ending growth rate acceleration/multiplier for BL normal spacing. The actual value used is limited to be greater than or equal to the maximum allowable growth rate.
-bldel thickness or deldef=thickness
Set BL thickness. The default value is automatically initialized in auto-BL-parameter-mode.