experimental and cfd investigations into slamming of small, high speed craft dominic hudson, simon...

Experimental and CFD investigations into slamming of small, high speed craft

Dominic Hudson, Simon Lewis, Stephen Turnock

ONR Hull slamming workshop, Caltech

17-18th February 2009

Background• Work in support of

Design of High Performance Craft from a Human Factors Perspective

• This involves:

• Model and full scale testing• Measurements of muscle fatigue and

heart rate on passengers on board

• Prediction of motions of high speed craft

• Suspension seat design

Heart rate and Oxygen consumption

0%

10%

20%

30%

40%

50%

60%

70%

1 11 21 31 41 51 61 71 81 91 101 111

Time (minutes)

Pe

rce

nta

ge

ma

xim

al

He

art

ra

te

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

Pe

rce

nta

ge

VO 2m

ax

heart rate VO2

Outline

• Methods for prediction of planing craft motions

• Computational Fluid Dynamics (CFD) to predict vertical motion

• Improvements to CFD - boundary layer flow

• Wedge impact experiment

• Conclusions and future work

Prediction of motions

• Potential flow theory– Advantages:

• Simple• Computationally efficient

– Disadvantages:• Difficulties modelling more complex shapes

• Computational Fluid Dynamics– Advantages:

• Potential for accurate results– Disadvantages

• Complex setup • Computationally expensive

2D CFD - wedge impact• Computational fluid dynamics method using

– RANS equations (ANSYS CFX 11)

• Transient simulation

• Equations of motion solved at each timestep

• Initial investigations used published experimental data for validation

Results - wedge impact

CFD Improvements

• Boundary layer development on an impulsively started flat plate

– mesh size, domain size, turbulence model, and first cell distance from the wall

Bow section motion

• Experiments conducted at MARINTEK

• Test parameters

• Water entry velocity 2.44m/s• Mass: 261kg

• Measured pressures, accelerations and forces

CFD simulation

Inflow boundary

Symmetry planeOutflow boundary condition

Smooth wall, no slip condition

0.8m

0.4m

CFD Parameters

• Using Ansys CFX v11.0

• Finest mesh: 30000 cells

• First element situated 2*10-5m from the wall

• Turbulence model used is k-omega

• Y+ value at the wall is 0.6

• Inhomogeneous multiphase model

• Motions are calculated through user defined functions in Matlab for each timestep

Results - visualisation

• Images of flow

Results – pressure (1)

Predicted and experimental pressure (transducers P1 and P2)

-5000

0

5000

10000

15000

20000

25000

30000

35000

-0.02 0 0.02 0.04 0.06 0.08 0.1

Time (s)

Pre

ssur

e (P

a)

P1 pressure prediction

P1 experiment

P2 Pressure prediction

P2 experiment

Results – pressure (2)

Predicted and experimental pressure (transducers P3 and P4)

0

5000

10000

15000

20000

25000

30000

35000

-0.02 0 0.02 0.04 0.06 0.08 0.1

Time (s)

Pre

ssu

re (

Pa)

P3 Pressure prediction

P3 experiment

P4 Pressure prediction

P4 experiment

Experimental testing

• Rig designed to investigate free-falling wedge

– Provide detailed validation data – Include uncertainty analysis– Improve understanding

• Synchronised high speed video, pressure and acceleration data

• Pressure, acceleration sampled at 10kHz

• Mass and drop height varied

Comparison of sample rates

Drop test rig

Results – experimental (1)

Pressure N/m2

8.8ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

15ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

21.6ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

30.9ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

42.8ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

57.1ms after impact

-5000

0

5000

10000

15000

20000

25000

30000

0 50 100 150 200 250 300

Horizontal distance from wedge apex (mm)

P6 P5 P4 P3 P2 P1

Results – experimental (2)

Results - uncertainty

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

0

0.5

1

1.5

2

x 104

Time (s)

Pre

ss

ure

(P

a)

Comparison of different methods of calculating error

Statistical

Systematic

Results - repeatability

Outcomes of experiment

• Synchronisation of measurements enhances understanding of impact.

• Images allow comparison between CFD and experiment.

Determining point of impact

- Accelerometer responds to impact at 2.5 msafter apex enters water- Video indicates distance travelled approx. 1cm

- Position sensor agrees with video

Future work - motions

Potential Flow solver

using strip theory

Computational Fluid Dynamics

Hybrid model

3D CFD mesh (Azcueta,2002)

• The hybrid approach is used to improve the accuracy of the numerical predictions.

Future work - general

• Use ‘flexible’ wedge – measure structural responses

– Strain gauges, thermo-elastic stress analysis?, digital image correlation?

• Effect of hull features on flow – deadrise, spray rails, hull shape, RIB collars

• Inclined wedge entry – heeled conditions

• Use high-speed video to investigate spray characteristics

• Modify rig for forced wedge entry/exit

Conclusions• Experimental study provides good data for

validation of wedge impact.

• Improvements to CFD predictions for highly non-linear flows such as water impact.

• Hybrid approach can be used to improve the accuracy of high speed craft motions prediction.

1 2 3 4 5 6 7 8

x 10-3

-4000

-2000

0

2000

4000

6000

8000

10000

12000

Time (s)

Pre

ssur

e (P

a)

0.005s

P1

0.005667s0.00533s0.006s0.006333s0.006667s0.007s0.007333s0.007667s0.008s

Questions

?

Thank you for your attention.