Results from the 3rd Drag Prediction Workshop using the
NSU3D Unstructured Mesh Solver
Dimitri J. MavriplisUniversity of Wyoming
Overview
• Description of Meshes • Description of NSU3D Solver
– Sample performance– Preliminary Sensitivity Evaluations
• WB and WBF Results• W1 and W2 Results
– Including runs performed at Cessna on 2nd family of grids
• Conclusions
General Gridding Guidelines
• Grid Convergence Cases:
– DLR F6 WBF• 3 grid levels required
– DLR F6 WB • Medium grid required, coarse/fine optional
– Wing1 and Wing2• Four grid levels required
General Gridding Guidelines• Grid Resolution Guidelines
– BL Region• Y+ < 1.0, 2/3, 4/9, 8/27 (Coarse,Med,Fine,VeryFine)• 2 cell layers constant spacing at wall• Growth rates < 1.25
– Far Field: 100 chords – Local Spacings (Medium grid)
• Chordwise: 0.1% chord at LE/TE• Spanwise spacing: 0.1% semispan at root/tip• Cell size on Fuselage nose, tail: 2.0% chord
– Trailing edge base:• 8,12,16,24 cells across TE Base (Coarse,Med,Fine,Veryfine)
General Gridding Guidelines
• Grid Convergence Sequence– Grid size to grow ~3X for each level refinement
• 1.5X in each coordinate direction (structured)– Maintain same family of grids in sequence
• Same relative resolution/topology/growth factors– Sample sizes (DLR F6 WBF):
• 2.7M, 8M, 24M pts (structured grids)• Unstructured grids should be similar
– Cell based vs. Node Based Unstructured solvers– 5 to 6 times more tetrahedra per nodes– 2 times more prisms than nodes
Available (Posted) Unstructured Grids
• VGRID (NASA Langley)– Node-Based grids NASA(W1,W2,WB,WBF)– Node-Based grids Cessna (W1,W2)– Cell Centered Grids Raytheon (WB,WBF)
• ANSYS Hybrid Meshes• Centaur (DLR, adapted) (Node Based)• AFLR3 (Boeing) (Cell Centered)• TAS (JAXA) (Node Based)• GridPro (Block-Structured/Unstructured)
VGRID NASA (Node Based)
• WB:– Coarse : 5.3M pts– Medium: 14.3M pts– Fine: 40.0M pts (> 200M cells)
• WBF:– Coarse: 5.6M pts– Medium: 14.6M pts– Fine: 41.1M pts ( > 200M cells)
VGRID Node Centered (NASA)
VGRID Node Centered (NASA)
VGRID Node Centered (NASA)
NSU3D Description• Unstructured Reynolds Averaged Navier-
Stokes solver– Vertex-based discertization– Mixed elements (prisms in boundary layer)– Edge data structure– Matrix artificial dissipation
• Option for upwind scheme with gradient reconstruction
– No cross derivative viscous terms• Thin layer in all 3 directions• Option for full Navier-Stokes terms
Solver Description (cont’d)• Spalart-Allmaras turbulence model
– (original published form)– Optional k-omega model
Solution Strategy• Jacobi/Line Preconditioning
– Line solves in boundary layer regions• Relieves aspect ratio stiffness
• Agglomeration multigrid– Fast grid independent convergence rates
• Parallel implementation– MPI/OpenMP hybrid model
• DPW runs: MPI on local cluster and on NASA Columbia Supercomputer
Grid Generation• Runs based on NASA Langley supplied
VGRIDns unstructured grids• Tetrahedra in Boundary Layer merged into
prismatic elements
• Grid sizes up to 41M pts, 240M elements
Sample Run Times• All runs performed on NASA Columbia Supercomputer
– SGI Altix 512cpu machines– Coarse/Medium (~15Mpts) grids used 96 cpus
• Using 500 to 800 multigrid cycles– 30 minutes for coarse grid– 1.5 hrs for medium grid
– Fine Grids (~40M pts) used 248 cpus• Using 500 to 800 multigrid cycles
– 1.5 to 2 hrs hrs for fine grid
– CL driver and constant incidence convergence similar– WB cases hard to converge (not entirely steady)
Scalability
• Near ideal speedup for 72M pt grid on 2008 cpus of NASA Columbia Machine
(~10 minutes for steady-state solution)
NSU3D Sensitivity Studies
• Sensitivity to Distance Function Calculation Method
• Effect of Multi-Dimensional Thin-Layer versus Full Navier-Stokes Terms
• Sensitivity to Levels of Artificial Dissipation
Sensitivity to Distance Function
• All DPW3 Calculations done with Eikonal equation distance function
Sensitivity to Navier-Stokes Terms
• All DPW3 Calculations done with Multidimensional Thin-Layer Formulation
Sensitivity to Dissipation Levels
• Drag is grid converging• Sensitivity to dissipation decreases as expected• All Calculations done with low dissipation level
WBF Convergence (fixed alpha)
• “Similar” convergence for all grids• Force coefficients well converged < 500 MG cycles
WBF Convergence
• Medium Grid (15M pts): Fixed alpha
WBF Convergence
• Medium Grid (15M pts): Fixed CL
WBF Convergence
• Similar convergence (Fixed CL or alpha)
WBF: Grid Convergence Study
• CP at wing break station (y/b=0.411)
WBF: Grid Convergence Study
• CP at wing break station (y/b=0.411)
WBF: Grid Convergence Study
• CP at wing break station (y/b=0.411)
WBF: Grid Convergence Study
• CF at wing break station (y/b=0.411)
WBF: Grid Convergence Study
• Good fairing design (coarse grid: 5M pts)
WBF: Grid Convergence Study
• Good fairing design (medium grid: 15M pts)
WBF: Grid Convergence Study
• Good fairing design (fine grid: 40M pts)
WBF: TE Separation
• Coarse grid: 5M pts
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CP at wing break station (y/b=0.411)
WBF: Drag Polar
• CFX at wing break station (y/b=0.411)
WBF: Drag Polar
• Full Polar run on all 3 grids (5, 15, 40M pts)
WBF: Drag Polar
• Full Polar run on all 3 grids (5, 15, 40M pts)
WBF: Moment
• Full Polar run on all 3 grids (5, 15, 40M pts)
WBF: Moment
• Full Polar run on all 3 grids (5, 15, 40M pts)
WB Convergence (fixed alpha)
• Separated Flow, unsteady shedding pattern• Smaller residual excursions with fewer MG levels• Moderate CL variations
WB Medium Grid
• Plot Min and Max unsteady CL values
WB Medium Grid
• Plot Min and Max unsteady CL values• Good overlap in polar– suitable drag values
WB Medium Grid
• Plot Min and Max unsteady CL values• Less overlap in CM
WB Medium Grid
• CP Values at Break Station (y/b=0.411)
WB Medium Grid
• CFX Values at Break Station (y/b=0.411)
WB Grid Convergence
• CP Values at Break Station (y/b=0.411)
WB Grid Convergence
• CFX Values at Break Station (y/b=0.411)
WB Grid Convergence
• Separation Pattern (Coarse grid : 5M pts)
WB Grid Convergence
• Separation Pattern (Medium grid : 5M pts)
WB Grid Convergence
• Separation Pattern (Fine grid : 40M pts)
WB TE Separation Pattern
• (Coarse grid : 5M pts)
Grid Convergence (WB+WBF)
• Grid convergence apparent (particularly for WBF)
Grid Convergence (WB+WBF)
• Some cancellation apparent: WBF less uniformly converging
Grid Convergence (WB+WBF)
• Grid Convergence Ranked 8th in Vassberg Fig. of Merit:– Best for unstructured solvers ….. Importance of uniform family of grids
Grid Convergence (WB+WBF)
• Grid convergence apparent (in this measure)
Grid Convergence (WB+WBF)
• Grid convergence apparent (in this measure)
WBF-WB Differences
• Medium grid comparisons
WBF-WB Differences
• Medium grid comparisons
WBF-WB Differences
• Medium grid comparisons
Grid Convergence of Drag Increment
• Consistent with one group of DPW3 Entries
Conclusions
• WBF appears to be grid converging• WB case is complex
– Previous results showed importance of grid topology
– New DPW3 grids are once again different• DPW1,2,3 pushing s.o.f of grid resolution
– DPW1: 1.6M pts– DPW2: 3M pts to 10M– DPW3: 5M to 40M pts
VGRID NASA (Node Based)• W1:
– Coarse : 1.8M pts– Medium: 4.5M pts– Fine: 11.5M pts– SuperFine: 36.9M pts
• W2:– Coarse: 1.9M pts– Medium: 4.7M pts– Fine: 11.9M pts– SuperFine: 38.5M pts
VGRID NASA (Cessna)• W1:
– Coarse : 0.98M pts– Medium: 2.4M pts– Fine: 6.1M pts– SuperFine: 12.7M pts
• W2:– Coarse: 0.95M pts– Medium: 2.3M pts– Fine: 5.9M pts– SuperFine: 12.4M pts
VGRID Node Centered (NASA)
W1 Convergence (fixed alpha=0.5)
• “Similar” convergence for coarse/med grids• Apparent unsteadiness in residual for finest grid• Force coefficients well converged < 500 MG cycles for all grids
W1 Grid Convergence Study
• CP at station 5:
W1 Grid Convergence Study
• CP at station 5:
W1 Grid Convergence Study
• CP at station 5:
W1 Grid Convergence Study
• CP at station 5:
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W1 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Convergence Study
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
W2 Grid Polar Sweep (Fine Grid)
• CP at station 5
Streamlines at 0.5 degrees (W1)
Streamlines at 0.5 degrees (W2)
W1-W2 Grid Polar Comparison(Fine Grid)
W1-W2 CL-Incidence Comparison(Fine Grid)
W1-W2 Moment Comparison (Fine Grid)
W1-W2 Grid Convergence Study
•Apparently uniform grid convergence
W1-W2 Grid Convergence Study
•Good grid convergence of individual drag component
W1-W2 Grid Convergence Study
•Ranked 1st by Vassberg Figure-of-Merit
W1-W2 Grid Convergence Study
W1-W2 Grid Convergence Study
W1-W2 Results
• Discrepancy between UW and Cessna Results
W1-W2 Results
• Despite uniform grid convergence: Results on 2 grid families not converging to same values
W1-W2 Results
• Removing effect of lift-induced drag : Results on both grid families converge consistently
65M pt mesh Results
• 10% drop in CL at AoA=0o: closer to experiment• Drop in CD: further from experiment• Same trends at Mach=0.3• Little sensitivity to dissipation
Summary
• W1-W2 appear to be in asymptotic grid convergence range– Cd difference ~ 1 count at 0.5 degrees
• Grids are getting finer …..40M pts ~1 hr on NASA Columbia Supercomputer
• Drag decomposition useful in providing better drag estimates on coarser grids