AFLR3 BL Normal Spacing

AFLR3 generates the mesh in BL regions using a normal spacing that grows geometrically from surfaces with grid BCs set for BL grid generation. The initial spacing is set explicitly. 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 Details for specific details on all parameters.

The growth rate parameters are by default set to default values of -bli 5 -blr 1 -bldr 1.05 -blrm 1.5. The default values correspond to those that match a turbulent BL profile with an initial normal spacing set at a y+ of 1. In addition, the u+ velocity increment (velocity increment normalized with the friction velocity) is approximately constant throughout the BL with the default parameters. The BL growth rate parameters (-bli, -blr, -blrm, -bldr, and/or -bldel) can be explicitly set to achieve a desired spacing distribution.

In the BL region these parameters are used to determine the normal spacing along a boundary normal curve for each boundary surface node. The resulting BL spacing along a normal curve smoothly varies and grows geometrically along a normal curve as shown below.

The spacing is calculated as follows

R_i = cdfrbl when i ≤ iblri
R_i = dcdfrbl • R_i-1 when i > iblri
R_i = max (R_i, cdfrblm)

∆S_i+1 = R_i • ∆S_i more
∆S_i+1 = max (∆S_i+1, cdfrblm • ∆X)

where i is the layer, R is the geometric growth rate, ∆S is the normal spacing, and ∆X is the tangential spacing on the boundary surface.

The BL growth rate distribution versus number of layers shown is typical for a turbulent BL profile. The same type of distribution profile is used 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 BL process attempts to advance until it reaches the BL thickness. The normal spacing grows until it reaches the limiting value on normal spacing. Once reached the normal spacing remains constant. The limit on normal spacing is determined by the parameter cblmnr (default value is 0.7). The local value for the normal spacing limit is cblmnr multiplied by the tangential surface spacing on the initial boundary surface. Typical normal spacing and normal distance distributions are shown below.

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.

-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 cdfrbl=initial_growth_ratio

Set initial growth rate for BL normal spacing.

-bldr growth_ratio_multiplier or dcdfrbl=growth_ratio_multiplier

Set growth rate acceleration/multiplier for BL normal spacing.

-blrm maximum_growth_ratio or cdfrblm=maximum_growth_ratio

Set maximum allowable growth rate for BL normal spacing.

-cblmnr normal_spacing_limit or cblmnr=normal_spacing_limit

Set coefficient for limiting maximum normal spacing.

-bldel thickness or deldef=thickness

Set BL thickness.

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