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: 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 0.012 m 17/06/2013 T. Mai, D. Greaves & A. Raby 4
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 2015 5
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/s 2 ) Sampling rate: 50 khz XPM10 Accelerometer - Model 4610 17/06/2013 T. Mai, D. Greaves & A. Raby 6
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
Numerical Method AMAZON-CW: mathematical equations 10/11/2016 FROTH Workshop 18 November 2015 8
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 2015 9
The process: pure water entry (video) 10/11/2016 FROTH Workshop 18 November 2015 10
Impact loadings: pure water entry, v=5.5 m/s Total impact force. Pressure at P1. 10/11/2016 FROTH Workshop 18 November 2015 11
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 2015 12
Impact loadings: pure water entry, v=5.5 m/s T=-0.035 ms T=2.365 ms Pressure contours on the impact plate. 10/11/2016 FROTH Workshop 18 November 2015 13
Impact loadings: pure water entry, v=5.5 m/s T=-0.035 ms T=2.365 ms Pressures along the horizontal central section. 10/11/2016 FROTH Workshop 18 November 2015 14
Impact loadings: pure water entry, v=7 m/s Total impact force. Pressure at P1. 10/11/2016 FROTH Workshop 18 November 2015 15
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 2015 16
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 2015 17
Impact loadings: pure water entry, v=7 m/s T=-0.013 ms T=2.487 ms Pressures along the horizontal central section. 10/11/2016 FROTH Workshop 18 November 2015 18
The process: aerated water entry (video) 10/11/2016 FROTH Workshop 18 November 2015 19
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 2015 20
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 2015 21
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 2015 22
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 2015 23
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 2015 24
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 2015 25
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 2015 26
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: 20140542 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. 642--646. T. Mai, D. Greaves and A. Raby. Aeration effect on impact: Drop test of a flat plate. ISOPE 2014, pp 703 709. 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 679 683. 10/11/2016 FROTH Workshop 18 November 2015 27