Drag Workshop Results Using CFL3D

Christopher RumseyRobert Biedron

NASALangley Research CenterHampton, VA

DLR-F4M=0.75Recref=3 million

Issues addressed

• Effect of grid (1-to-1 vs. overset)

• Comparison of 3 turbulence models

• Issue of transition for supposedly “fully turbulent” computations

• Effect of different versions of SA model

CFL3D V6.0

• Upwind, implicit 3-factor AF

• Finite volume, multigrid

• FDS (Roe)

• Globally 2nd order spatially accurate

• Multi-block capabilities, including 1-to-1, patched, and overset

• Parallel (MPI)

Grid convergence1-to-1 grid, M=0.75, CL=0.5

• If every-other-point grid is in asymptotic region for 2nd

order global spatial convergence (doubtful), then CD on fine grid is high by 30 counts!

• A finer-level grid of the same family is needed

N-2/3

CD

0 0.0002 0.0004 0.0006 0.00080.02

0.03

0.04

0.05

0.06

0.07

standard 1-to-1 grid(3,180,800 cells)

Effect of grid on surface pressuresα=0 deg, M=0.75, Re=3.e6

Drag Prediction Workshop standard grids

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.238

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.331

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.409

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.512

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.636

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.844

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

CFL3D, oversetCFL3D, 1-to-1exp

2y/B = 0.185

Detail

x/c

-Cp

0 0.2 0.4 0.6 0.8 1-0.8

-0.4

0

0.4

0.8

1.2

1.6

CFL3D, oversetCFL3D, 1-to-1exp

2y/B=0.331

Grid comparison near y=230 mm

Overset grid

X Y

Z

1-to-1 grid

3.2 million cells50c extent

3.7 million cells170c extent

Entire grid:

Effect of grid on forces & moments

α

CL

-4 -2 0 2 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA, oversetexp

CM

CL

-0.16 -0.12 -0.08 -0.04 00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA, oversetexp

Effect of grid on forces & moments

CD

CL

0 0.02 0.04 0.06 0.08 0.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA, oversetexp

CD, CDp, CDv

CL

0 0.01 0.02 0.03 0.04 0.050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA, oversetCDpCDp

CDvCDv

Effect of turbulence model on surface pressuresα=2 deg, M=0.75, Re=3.e6

Drag Prediction Workshop 1-to-1 standard grid

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.844

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.636

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.512

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.409

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.331

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

2y/B=0.238

x/c

-Cp

0 0.2 0.4 0.6 0.8 1

SASSTEASM

2y/B = 0.185

Detail

x/c

-Cp

0 0.2 0.4 0.6 0.8 1-0.8

-0.4

0

0.4

0.8

1.2

1.6SASSTEASM

2y/B=0.636

Streamlines at alpha=2 deg

SA SST

EASM

Effect of turbulence model on forces & moments

α

CL

-4 -2 0 2 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SST, 1-to-1EASM, 1-to-1exp

CM

CL

-0.16 -0.12 -0.08 -0.04 00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SST, 1-to-1EASM, 1-to-1exp

Effect of turbulence model on forces & moments

CD

CL

0 0.02 0.04 0.06 0.08 0.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SST, 1-to-1EASM, 1-to-1exp

CD, CDp, CDv

CL

0 0.01 0.02 0.03 0.04 0.050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SST, 1-to-1EASM, 1-to-1CDp

CDpCDp

CDvCDv

CDv

Actual “fully turbulent” transition locations for different turbulence models

alpha=0 deg

y, mm

x,m

m

100 200 300 400 500 600

300

400

500

600

700

wing outlineSASSTEASM

y, mm

x,m

m

350 400

500

510

520

530

540

550

560

570

580

590

600

wing outlineSASSTEASM

SA: 1.4%cSST: 2.8%cEASM: 5.7%c

At y=400 mm:

Effect of forcing SA transition to match “natural” transition of EASM

CD, CDp, CDv

CL

0 0.01 0.02 0.03 0.04 0.050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA (trans), 1-to-1CDpCDp

CDvCDv

Effect of SA version on transition• 2 versions of SA in wide use:

– SA (Ia): “official” version in Aerospatiale Journal– SA+fv3: “unofficial” version resulting from a Spalart

e-mail in early 90’s

• SA (Ia) transitions very near L.E.– Typically 1 – 2 % c for alpha=0 deg case

• SA+fv3 delays transition for low Re (1-10 million)– 7 – 8 % c or more for alpha=0 deg case (OVERFLOW

results similar)– Seems to show more sensitivity to grid & free stream

turbulence level chosen

Effect of SA version on forces

CD, CDp, CDv

CL

0 0.01 0.02 0.03 0.04 0.050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA + fv3, 1-to-1CDp

CDpCDv

CDv

α

CL

-4 -2 0 2 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

SA, 1-to-1SA + fv3, 1-to-1exp

Summary

• Grid issues– Official 1-to-1 grid too coarse to resolve

pressures (L.E. & shock under-resolved)– Nonetheless, global forces & moments similar

to those using better quality overset grid; at alpha=0:

• CD: ~8 count difference (2.4 %)

– Family of grids (2 or 3 for each type) needed for grid sensitivity study

Summary, cont’d

• Turbulence model comparison (1-to-1 grid)– SST & EASM give lower CD than SA by <20

counts (8.7 % difference at –3 deg, 3.3 % difference at +2 deg)

– Primarily due to lower friction drag

Summary, cont’d

• “Fully turbulent” is misnomer– All turbulence models “transition” on their own

– At low Re (order 1-10 million), transition is not at the leading edge! E.g., for alpha=0:

• SA: 1-2 %c typical

• SST: 2-5 %c typical

• EASM: 2-7 %c typical

– Effect is small: forcing SA to transition at EASM location changes CD by <1 count (0.1 %)

Summary, cont’d

• Two versions of SA are known to be present in today’s U.S. production codes– SA + fv3 (unofficial version) widely used, can

delay transition significantly for low Re (order 1-10 million) compared to official SA (Ia)

– Effect for alpha=0:• Delta CD=1.4 counts (0.4 %)

Summary of effects at alpha=0

grid turb model trans loc SA version

effe

ct,i

np

erce

nt

0

1

2

3

4

5

6

L

D

M

L

D

M L DM L

D

M

L=liftD=dragM=moment

Conclusions• Good quality grid a MUST

– It is possible to miss details in Cp yet do reasonably well on forces and moments

– Right answer for wrong reasons? – only a grid study using a family of grids will tell

• SA, SST, EASM turbulence models give verysimilar results for this case (but still ~20 count drag difference)

• CFD transition location should always be checked• Better version control and consistency checks are

needed for turbulence model coding

Comments on EASM

• Nonlinear explicit algebraic stress model, k-omega form (AIAA 2000-4323)

• More robust than earlier versions of EASM

• Roughly 40 % more expensive than SA

• As good as SA and SST for aerodynamic thin-shear flows, but better for flows where nonlinear and curvature effects are important

• Validation on-going

Recommendations for future workshops• Give out family of successively finer grids for a required

grid study– Grid study needed for CFD validation of this type– Some participants do not have 3D grid generation capability– “Official” grids ensure consistency– For wing body: 7 million, 3 million, 1.5 million cells? (structured)– Structured grids should be multigriddable

• Include surface Cps as part of required results– Integrated quantities hide things that could be helpful in evaluation

• More fixed-alpha cases and fewer fixed-CL cases– Fixed alpha cases are easier to run & better for comparing code-to-code

• To ensure transition location is not a cause of variability:– Force transition at specified locations (harder), or…– Include high Re (order 50 million) fully turbulent case (easier)