FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics EPSRC

Transcription

1 FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics EPSRC

2 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

3 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

4 Experiment/Computation setup Free fall of block onto (calm) water surface: Pure water (sound speed: c s = 1484 m/s) Aerated water: with void fraction up to 10% will give c s = 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 m 17/06/2013 T. Mai, D. Greaves & A. Raby 4

5 Experiment/Computation setup Pressure measurement: P1 P9 Accelerometer: A1 The falling block and guide rails. The impact plate. 10/11/2016 FROTH Workshop 18 November

6 Instruments on the impact plate 5 XPM10 pressure transducers: range of 100 bar ( kpa) 1 accelerometer (model 4610): range of 200g (g = 9.81 m/s 2 ) Sampling rate: 50 khz XPM10 Accelerometer - Model /06/2013 T. Mai, D. Greaves & A. Raby 6

7 Experiment setup The test rig is mounted on the gantry in the ocean tank at PU The falling block. The bubble generator. 17/06/2013 T. Mai, D. Greaves & A. Raby 7

8 Numerical Method AMAZON-CW: mathematical equations 10/11/2016 FROTH Workshop 18 November

9 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 10/11/2016 FROTH Workshop 18 November

10 The process: pure water entry (video) 10/11/2016 FROTH Workshop 18 November

11 Impact loadings: pure water entry, v=5.5 m/s Total impact force. Pressure at P1. 10/11/2016 FROTH Workshop 18 November

12 Impact loadings: pure water entry, v=5.5 m/s Pressures on the plate (block 1). Pressures on the plate (block 2). 10/11/2016 FROTH Workshop 18 November

13 Impact loadings: pure water entry, v=5.5 m/s T= ms T=2.365 ms Pressure contours on the impact plate. 10/11/2016 FROTH Workshop 18 November

14 Impact loadings: pure water entry, v=5.5 m/s T= ms T=2.365 ms Pressures along the horizontal central section. 10/11/2016 FROTH Workshop 18 November

15 Impact loadings: pure water entry, v=7 m/s Total impact force. Pressure at P1. 10/11/2016 FROTH Workshop 18 November

16 Impact loadings: pure water entry, v=7 m/s Pressures on the plate (block 1). Pressures on the plate (block 2). 10/11/2016 FROTH Workshop 18 November

17 Impact loadings: pure water entry, v=7 m/s T=-0.013ms T=2.487 ms Pressure contours on the impact plate. 10/11/2016 FROTH Workshop 18 November

18 Impact loadings: pure water entry, v=7 m/s T= ms T=2.487 ms Pressures along the horizontal central section. 10/11/2016 FROTH Workshop 18 November

19 The process: aerated water entry (video) 10/11/2016 FROTH Workshop 18 November

20 Impact loadings: aerated water entry, v=5.5 m/s Block 1 Block 2 Numerical Impact pressures at P1 and P2 10/11/2016 FROTH Workshop 18 November

21 Impact loadings: aerated water entry, v=5.5 m/s Pressure at P1 Total force on the plate Aeration effects on the peak impact loadings. 10/11/2016 FROTH Workshop 18 November

22 Impact loadings: aerated water entry, v=5.5 m/s Pressure impulse at P1 Total force impulse on the plate Aeration effects on the impulse of shock loadings. 10/11/2016 FROTH Workshop 18 November

23 Impact loadings: aerated water entry, v=7 m/s Block 1 Block 2 Numerical Impact pressures at P1 and P2 10/11/2016 FROTH Workshop 18 November

24 Impact loadings: aerated water entry, v=7 m/s Pressure at P1 Total force on the plate Aeration effects on the peak impact loadings. 10/11/2016 FROTH Workshop 18 November

25 Impact loadings: aerated water entry, v=7 m/s Pressure impulse at P1 Total force impulse on the plate Aeration effects on the impulse of shock loadings. 10/11/2016 FROTH Workshop 18 November

26 Multi-stage impact loadings Conclusions 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. 10/11/2016 FROTH Workshop 18 November

27 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 flow model for violent aerated wave impact problems. Proc. R. Soc. A 470: Z.H. Ma, D.M. Causon and L. Qian et al. A GPU based compressible multiphase hydrocode for modelling violent hydrodynamic impact problems. Computers and Fluids 120 (2015): 1-23 Z.H. Ma, D.M. Causon and L. Qian et al. The role of fluid compressibility in predicting slamming loads during water entry of flat plates. ISOPE 2015, pp T. Mai, D. Greaves and A. Raby. Aeration effect on impact: Drop test of a flat plate. ISOPE 2014, pp F. Gao, Z.H. Ma and J. Zang et al. Simulation of breaking wave impact on a vertical wall with a compressible two-phase flow model. ISOPE 2015, pp /11/2016 FROTH Workshop 18 November