FROTH: Fundamentals and Reliability of Offshore Structure ... · FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics Conclusions • Multi-stage impact loadings

FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics


EPSRC 2012 - 2015


Drop Tests: experiments and numerical modelling

T. Mai, D. Greaves & A. Raby

School of Marine Science and Engineering

Plymouth University

Z.H. Ma, L. Qian, D. Causon, C. Mingham & P. Martínez Ferrer

School of Computing, Mathematics and Digital Technology

Manchester Metropolitan University


Motivation

• To carry out experiments (WP1) and numerical computations (WP2) to measure/calculate the impact loadings on a flat plate.

• To improve the understandings of the hydrodynamic characteristics of violent water entry of a flat plate.

• To investigate the fluid compressibility and aeration effects on the impact loadings.

10/11/2016 FROTH Workshop 18 November 2015 3


Experiment/Computation setup

• Free fall of block onto (calm) water surface:– Pure water (sound speed: cs = 1484 m/s)

– Aerated water: with void fraction up to 10% will give cs = 33.3 m/s according to Wood’s law (1940)

– Impact velocity: 2 m/s ~ 8 m/s

– Block masses: 32kg ~ 52 kg

– Geometry of the impact plate:

• Square plate: W x L x H = 0.25 m x 0.25 m x 0.012 m

417/06/2013 T. Mai, D. Greaves & A. Raby


Experiment/Computation setup


• Pressure measurement: P1 – P9

• Accelerometer: A1

The falling block and guide rails. The impact plate.


Instruments on the impact plate

• 5 XPM10 pressure transducers: range of 100 bar (10 000 kPa)

• 1 accelerometer (model 4610): range of 200g (g = 9.81 m/s2 )

• Sampling rate: 50 kHz

17/06/2013 T. Mai, D. Greaves & A. Raby 6

XPM10Accelerometer -

Model 4610


Experiment setup

• The test rig is mounted on the gantry in the ocean tank at PU

717/06/2013 T. Mai, D. Greaves & A. Raby

The falling block. The bubble generator.


Numerical Method

• AMAZON-CW: mathematical equations



Numerical Method

• AMAZON-CW: features– Compressible air and water

– Hull cavitations

– One pressure, one velocity

– Volume of fluid method

– Approximate Riemann solver

– Programming languages: C++

– Parallelisation: OpenMP + CUDA


– Validation cases:

• Liquid piston

• Freefall of a water column

• Water-air shock tubes

• Dam break

• Incipient cavitations

• Underwater explosions

• Slamming problems


The process: pure water entry (video)



Impact loadings: pure water entry, v=5.5 m/s


Total impact force. Pressure at P1.




Pressures on the plate (block 1). Pressures on the plate (block 2).




T=-0.035 ms T=2.365 ms

Pressure contours on the impact plate.




T=-0.035 ms T=2.365 ms

Pressures along the horizontal central section.


Impact loadings: pure water entry, v=7 m/s


Total impact force. Pressure at P1.




Pressures on the plate (block 1). Pressures on the plate (block 2).




T=-0.013ms T=2.487 ms

Pressure contours on the impact plate.




T=-0.013 ms T=2.487 ms

Pressures along the horizontal central section.


The process: aerated water entry (video)




Impact loadings: aerated water entry, v=5.5 m/s

Block 1 Block 2 Numerical

Impact pressures at P1 and P2




Aeration effects on the peak impact loadings.

Pressure at P1 Total force on the plate




Aeration effects on the impulse of shock loadings.

Pressure impulse at P1 Total force impulse on the plate



Impact loadings: aerated water entry, v=7 m/s

Block 1 Block 2 Numerical

Impact pressures at P1 and P2




Aeration effects on the peak impact loadings.

Pressure at P1 Total force on the plate




Aeration effects on the impulse of shock loadings.

Pressure impulse at P1 Total force impulse on the plate


Conclusions

• Multi-stage impact loadings– Shock load: the highest pressure peak, 2 ms duration

– Low pressure load: water in tension, 4 ms duration

– Secondary re-load: much smaller than the shock load

• Aeration effects– Local pressures and total force can be effectively reduced.

– Peak loadings can be halved by 1.6% aeration.

– The duration of shock load is prolonged by aeration.

– The variation of shock load impulse is less sensitive to the change of aeration than the peak loading.



References:

• MMU and Plymouth. “Pure and aerated water entry of a flat plate”.Revision submitted to Physics of Fluids.

• Z.H. Ma, D.M. Causon and L. Qian et al. “A compressible multiphase flowmodel for violent aerated wave impact problems”. Proc. R. Soc. A 470:20140542

• Z.H. Ma, D.M. Causon and L. Qian et al. “A GPU based compressiblemultiphase hydrocode for modelling violent hydrodynamic impactproblems”. Computers and Fluids 120 (2015): 1-23

• Z.H. Ma, D.M. Causon and L. Qian et al. “The role of fluid compressibility inpredicting slamming loads during water entry of flat plates”. ISOPE 2015,pp. 642--646.

• T. Mai, D. Greaves and A. Raby. “Aeration effect on impact: Drop test of aflat plate”. ISOPE 2014, pp 703—709.

• F. Gao, Z.H. Ma and J. Zang et al. “Simulation of breaking wave impact on avertical wall with a compressible two-phase flow model”. ISOPE 2015, pp679—683.


Documents

FROTH: Fundamentals and Reliability of Offshore Structure ... · FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics Conclusions • Multi-stage impact loadings