Aerodynamic Ground Effect: a case-study of the integration

of CFD and experiments

Tracie Barber & Stephen HallUniversity of New South Wales,

Sydney, Australia

Introduction

• Ground Effect

• Issues causing discrepancies in results

• Boundary Conditions– Deformable surface

• Viscous Effects

• Moving Ground design and analysis

• Continuing work

Ground effect - applications

WIG400

0

-400

-800

-1000

40

20

0

kPa m/s

Pressure Velocity

Previous Research

• Inviscid (ie panel methods) used extensively

• Analytical methods also used (many assumptions, limitations)

• Limited viscous CFD results• Inappropriate boundary conditions often

used• Experimental results scarce / unreliable

Current Work

• Reynolds-Averaged Navier Stokes Equations (CFX & Fluent)

• Higher order discretising schemes

• RNG or Realizable k- turbulence model, standard wall functions

Test-cases

Testcases: (2D)

• NACA 4412 wing

• Angles of attack:– 1.2O, 4O, 6.4O, 10O, 12O

• Reynolds Number:– 8,200,000

• Clearances (h/c):– 0.05, 0.10, 0.50, 1.00, free air

Boundary conditions

IMAGE SLIP

GROUND STATIONARY GROUND MOVING

u=Uu=U

Results - velocity vectorsIMAGE

GROUND MOVINGGROUND STATIONARY

SLIP

Results - velocity contours

Image Slip

Ground Stationary Ground Moving

Velocity Contours, H/c=0.025

0 37 14774 111

Image

Ground Moving

Slip

Ground Stationary

0 35 73 110 147

PIV Analysis

• Particle Image Velocimetry

• Pairs of images allow particle movement to be determined

• Nd-Yag laser, 532 nm, 100mJ/pulse

• Particles of spherical latex (5m)

• Initial investigation considered effect of moving ground

Moving Ground

Conveyor Belt

Wing

Drive System

Moving Ground

Camera

Conveyor Belt

Wing

Drive System

Moving Ground Stationary Ground

X X

Y Y

40

0 20 40 60 80 100 X

Y

60

40

20

0 0 20 40 60 80 100 X

Y

60

40

20

0

U 7.5 10 12

0 20 40 60 80 100 X 0 20 40 60 80 100 X

Y

60 40

20 0

Y

60 40

20 0

tke 0.25 0.50 1.0

Free Surface Effects

• Most WIG craft operate over water

• Lifting bodies produce a high pressure on their lower surface

• “Does this pressure change the surface shape, and does this affect the aerodynamic characteristics of the body?”

Free Surface Effects

• Froude number

• Reynolds number:

• CFD model (VOF) overcomes this issue

Uc

Re

gL

UFr

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Froude Number

Su

rface

De

form

ati

on

(m

)Max Surface Deformation Beneath

Airfoil vs Froude Number

Importance of Viscous Effects

• Panel methods (inviscid) frequently used for ground effect

• Real flow situations – will viscous effects greatly affect results?

Results: Three-Dimensional

• NACA 4412 wing, AR=6

• Angles of attack:– 1.2O, 4O, 6.4O, 10O

• Reynolds Number:– 8,200,000

• Clearances (h/c):– 0.05, 0.10, 0.50, 1.00, free air

Effect of Ground on Separation

Mid-Semispan Plane Mid-Semispan Plane

h/c=0.05 h/c=free air

Effect of Ground on Wake

h/c=1.00

h/c=0.05

Moving Ground - wing

PIV Results – Flow separation

70 80 90X(mm)

0

10

20

30

Y(mm)

k1.500.750.00

70 80 90X(mm)

20

30

40

50

Y(mm)

k1.500.750.00

360 370 380 390 400 410 420 430 440 450X(mm)

10

20

30

40

50

60

70

80

90

Y(mm)

K6.003.000.00

360 370 380 390 400 410 420 430 440 450X(mm)

10

20

30

40

50

60

70

80

90

Y(mm)

K6.003.000.00

0 10 20 30 40 50 60 70 80 90X(mm)

0

10

20

30

40

50

60

70

80

90

Y(mm)

k4.002.000.00

0 10 20 30 40 50 60 70 80 90X(mm)

0

10

20

30

40

50

60

70

80

90

Y(mm)

k4.002.000.00

3 6 0 3 7 0 3 8 0 3 9 0 4 0 0 4 1 0 4 2 0 4 3 0 4 4 0 4 5 0X ( m m )

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

Y(m

m)

K6 . 0 03 . 0 00 . 0 0

3 6 0 3 7 0 3 8 0 3 9 0 4 0 0 4 1 0 4 2 0 4 3 0 4 4 0 4 5 0X ( m m )

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

Y(m

m)

K6 . 0 03 . 0 00 . 0 0

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0X ( m m )

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

Y(m

m)

k4 . 0 02 . 0 00 . 0 0

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0X ( m m )

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

Y(m

m)

k4 . 0 02 . 0 00 . 0 0

7 0 8 0 9 0X ( m m )

0

1 0

2 0

3 0

Y(m

m)

k1 . 5 00 . 7 50 . 0 0

7 0 8 0 9 0X ( m m )

2 0

3 0

4 0

5 0

Y(m

m)

k1 . 5 00 . 7 50 . 0 0

PIV tke CFD tkeNACA 4412 wing at 12o, h/c=0.45,

trailing edge region

PIV tke CFD tkeNACA 4412 wing at 12o, h/c=0.05,

trailing edge region

Moving Ground

- Implementation of moving ground into 3ft x 4ft wind tunnel - Belt speed of 60m/s- Extensive CFD analysis to determine best

configuration- Uniform velocity profile- Uniform turbulence profile

Moving Ground - suspended

Velocity Contours

Moving Ground – suspended, leadup

Velocity Contours

Moving Ground, in line

Velocity Contours

Moving Ground, offset, leadup, suction

Velocity Contours

Moving Ground - turbulence100

50

0

Moving Ground – design & construction

Supersonic Ground Effect

Shock Wave Validation Cases

Stationary Ground Supersonic Ground Effect

(Experimental)

Stationary Ground Supersonic Ground Effect

(CFD)

Moving Ground Supersonic Ground Effect

(CFD)

Integration of CFD & Experiments

CFD

ExperimentReal World

CFD GLUE