experimental and cfd investigations into slamming of small, high speed craft
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Experimental and CFD investigations into slamming of small, high speed craft. Dominic Hudson , Simon Lewis, Stephen Turnock ONR Hull slamming workshop, Caltech 17-18 th February 2009. Background. Work in support of Design of High Performance Craft from a Human Factors Perspective - PowerPoint PPT PresentationTRANSCRIPT
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Experimental and CFD investigations into slamming of small, high speed craft
Dominic Hudson, Simon Lewis, Stephen Turnock
ONR Hull slamming workshop, Caltech17-18th February 2009
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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
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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
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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
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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
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Results - wedge impact
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CFD Improvements• Boundary layer development on an
impulsively started flat plate– mesh size, domain size, turbulence
model, and first cell distance from the wall
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Bow section motion• Experiments conducted
at MARINTEK• Test parameters
• Water entry velocity 2.44m/s• Mass: 261kg
• Measured pressures, accelerations and forces
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CFD simulation
Inflow boundary
Symmetry planeOutflow boundary condition
Smooth wall, no slip condition
0.8m
0.4m
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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
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Results - visualisation• Images of flow
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Results – pressure (1)
Predicted and experimental pressure (transducers P1 and P2)
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Results – pressure (2)
Predicted and experimental pressure (transducers P3 and P4)
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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
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Comparison of sample rates
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Drop test rig
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Results – experimental (1)
Pressure N/m2
8.8ms after impact
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21.6ms after impact
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30.9ms after impact
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42.8ms after impact
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57.1ms after impact
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Horizontal distance from wedge apex (mm)
P6 P5 P4 P3 P2 P1
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Results – experimental (2)
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Results - uncertainty
0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
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Time (s)
Pre
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a)Comparison of different methods of calculating error
StatisticalSystematic
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Results - repeatability
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Outcomes of experiment
• Synchronisation of measurements enhances understanding of impact.
• Images allow comparison between CFD and experiment.
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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
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Future work - motionsPotential Flow
solverusing 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.
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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
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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.
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Questions
?Thank you for your attention.