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 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 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. Alternatively, they can be set to match a turbulent BL profile if the option -blauto is used to activate auto-BL-parameter-mode and the turbulent BL profile related parameters (-Re, -refx, -blf, -y+, and/or -blds) are set. The default values correspond to those that match a turbulent BL profile with an initial normal spacing set at a y+ of 1. The BL growth rate parameters (-bli, -blr, -blrm, -bldr, 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.

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 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 0.5). 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.