4th AIAA CFD Drag Prediction Workshop

Gridding Guidelines

 

3 November 2008

 

 

Note:  The Configuration Geometry, Test Cases, and Gridding Guidelines are current as of 31 October 2008, but are subject to changes as developments require.   Please check the website periodically for updates, and/or register with dpw@cessna.textron.com to be notified directly.

1)    Grids to be Generated:

a)    Grid Convergence Cases – NASA Common Research Model:

i)      Wing-Body-Tail, iH=0¡:  ~3.5 Million, ~10 Million, ~35 Million, ~100 Million Cells/Nodes;
Where iH is the incidence of the horizontal tail

ii)     Coarse (3.5M), Medium (10M), and Fine (35M) grids are required;
The Extra-fine (100M) grid is optional

b)    Trim Drag Analysis - match Medium grid from Grid Convergence cases

i)      Wing-Body                                 ~8 Million Cells/Nodes

ii)     Wing-Body-Tail, iH=-2¡                ~10 Million Cells/Nodes

iii)    Wing-Body-Tail, iH=+2¡               ~10 Million Cells/Nodes

c)    RE Study, Wing-Body-Tail, iH=0¡:     Same Family as Medium, but different viscous spacing as required (See item 2-a-ii below)

2)    Grid Guidelines:

a)    Initial spacing normal to all viscous walls (RE Based on CREF=275.80Ó):

i)      coarse:      y+ ~ 1.0      dy = 0.001478 (RE= 5M)

ii)     medium:     y+ ~ 2/3      dy = 0.000985 (RE= 5M),  dy = 0.000273 (RE= 20M)

iii)    fine:           y+ ~ 4/9      dy = 0.000657 (RE= 5M)

iv)   extra-fine:                   y+ ~ 8/27    dy = 0.000438 (RE= 5M)

b)    Recommended: generate grids with 2 cell layers of constant spacing normal to viscous walls

c)    Total grid size to grow ~3X between each grid level for grid convergence cases

d)    For structured meshes, this growth is ~1.5X in each coordinate direction

e)    Grid convergence cases must maintain the same grid family between grid levels, i.e. maintain the same stretching factors, same topology, etc.

f)     Growth rate of cell sizes in the viscous layer should be < 1.25.

g)    Farfield located at ~100 CREFÕs for all grid levels.

h)    For the Medium Baseline Grids:

i)      Chordwise spacing for wing and tail leading edge (LE) and trailing edge (TE) ~0.1% local chord.

ii)     Wing and tail Spanwise spacing at root ~0.1% local semispan.

iii)    Wing and tail Spanwise spacing at tip ~0.1% local semispan.

iv)   Cell size near fuselage nose and after-body ~2.0% CREF.

i)      For the Coarse and Fine Baseline Grids, the above values should be scaled accordingly.

j)      Wing and Tail Trailing Edge Base

i)      Minimum of 8 cells across TE base for the coarse mesh

ii)     Minimum of 12 cells across TE base for the medium mesh

iii)    Minimum of 16 cells across TE base for the fine mesh

iv)   Minimum of 24 cells across TE base for the extra-fine mesh

k)     Be multi-grid friendly

l)      For unstructured grids designed for vertex based solvers, the spacings refer to inter-nodal spacings and the resulting grid sizes are expected to be similar to the structured grid sizes above. For unstructured grids for cell- centered solvers, the spacings refer to spacings between cell centers (or surface face centers), which corresponds approximately to a factor of 2 reduction in the overall number of surface points compared to the nodal solver case, for a triangular surface grid (this is based on triangle centroid separation distance of 2/3h). For tetrahedral cell-centered-solver meshes, the total number of grid points will be approximately 1/3 of the numbers listed above.

Feedback on Langley Products and Services
Accessibility