Towards Multiscale Green Sea Loads Simulations in Irregular Waves - - PowerPoint PPT Presentation

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Jan 12, 2023 146 likes •368 views

Towards Multiscale Green Sea Loads Simulations in Irregular Waves with the Naval Hydro Pack Inno Gatin, Vuko Vuk cevi c, Hrvoje Jasak Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia 25/July/2017 FSB 1/20

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SLIDE 1

Towards Multiscale Green Sea Loads Simulations in Irregular Waves with the Naval Hydro Pack

Inno Gatin, Vuko Vukˇ cevi´ c, Hrvoje Jasak

Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia

25/July/2017

FSB

Motivation Procedure and methods Conclusion 25/July/2017 1/20

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SLIDE 2

Different procedures and methods for different scales are presented.

Motivation, Procedure and methods, Preliminary results.

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Motivation Procedure and methods Conclusion 25/July/2017 2/20

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SLIDE 3

The objective is a complete numerical framework for green sea load calculation.

  • A multiscale framework comprising CFD and a large scale method,
  • The large scale method takes into account the statistical

nature of wave loads,

  • CFD uses the results from the large scale method to compute

highly nonlinear wave loads due to green sea.

FSB

Motivation Procedure and methods Conclusion 25/July/2017 3/20

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SLIDE 4

Three–scale procedure is proposed in this work.

  • 1. Linear seakeeping → exploration of multiple sea states and

heading angles → selection of the most adverse condition,

  • 2. Coarse CFD → conducting a three hour seakeeping simulation for

the selected sea state → detecting green sea events,

  • 3. Fine CFD → conducting a detailed green sea simulation on

the critical part of the deck structure → loads on deck structures.

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Motivation Procedure and methods Conclusion 25/July/2017 4/20

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SLIDE 5

Procedure and methods

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Motivation Procedure and methods Conclusion 25/July/2017 5/20

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SLIDE 6

Step 1: Hydrodynamic coeffs obtained with linearised free surface solver.

  • Single–phase simulations with linearised free surface model,
  • Efficient wave diffraction and radiation simulations.

50 100 150

Time, s

1.524e+09 1.526e+09 1.528e+09 1.53e+09 1.532e+09 1.534e+09

FZ, N

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Motivation Procedure and methods Conclusion 25/July/2017 6/20

1

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SLIDE 7

Linearised free surface solver agrees well with potential flow methods.

0.2 0.3 0.4 0.5 0.6 ω, rad/s 2e+07 4e+07 6e+07 8e+07 1e+08 1.2e+08 1.4e+08 FZ, N/m Naval Hydro HydroSTAR 0.2 0.3 0.4 0.5 0.6 ω, rad/s 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 φFz, rad Naval Hydro HydroSTAR 0.2 0.3 0.4 0.5 0.6 ω, rad/s 1e+09 2e+09 3e+09 4e+09 5e+09 6e+09 MY, Nm/m Naval Hydro HydroSTAR 0.2 0.3 0.4 0.5 0.6 ω, rad/s 1 2 3 4 5 6 φMy, rad Naval Hydro HydroSTAR

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Motivation Procedure and methods Conclusion 25/July/2017 7/20

1

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SLIDE 8

Step 2: Use linear seakeeping methods to assess green water probability.

  • Using the calculated hydrodynamic coefficients, calculate the

green water probability for a large number of sea states,

  • Select the most adverse sea state, which will serve as a starting

point for the next step.

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Motivation Procedure and methods Conclusion 25/July/2017 8/20

2

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SLIDE 9

Step 3: Calibrate the input spectrum using Higher Order Spectrum method.

Higher Order Spectrum (HOS) method:

  • Pseudo-spectral method for solving nonlinear boundary

conditions for free surface waves,

  • Takes into account nonlinear wave–wave interaction and

modulation,

  • Appropriate for efficient nonlinear irregular sea state propagation,
  • Applicable for coupling with CFD,
  • Low CPU expense.

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Motivation Procedure and methods Conclusion 25/July/2017 9/20

3

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SLIDE 10

Directional wave spectrum is efficiently propagated using HOS.

3 hours of real time simulated in 5 minutes of CPU time.

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Motivation Procedure and methods Conclusion 25/July/2017 10/20

3

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SLIDE 11

Low CPU expense of HOS enables fast calibration of input spectrum.

  • The selected wave energy spectrum is calibrated using HOS in
  • rder to produce the target spectrum,
  • Up to 100 three–hour realisations using HOS needed for the

calibration → a bit difficult with CFD. . .

0.3 0.4 0.5 0.6 0.7 ω, rad/s 50 100 150 200 Sζζ, m

2 s

Target: JONSWAP HOS result after calibration HOS result before calibration

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Motivation Procedure and methods Conclusion 25/July/2017 11/20

3

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SLIDE 12
  • Comparing HOS and CFD wave spectrum reveals that minimal

wave damping occurs in CFD.

0.2 0.3 0.4 0.5 0.6 0.7 ω, rad/s 50 100 150 200 Sζζ, m

2 s

HOS CFD

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Motivation Procedure and methods Conclusion 25/July/2017 12/20

3

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SLIDE 13

Step 4: Perform a three hour CFD seakeeping simulation.

  • SWENSE method is used to couple potential flow and CFD,
  • Fast and robust simulations with coarse temporal (200

time–steps/period) and spatial (600 000 cells) resolution,

  • Small number of nonlinear iterations per time–step is enabled

using enhanced 6–DOF–fluid flow coupling.

1000 2000 3000 4000 5000 6000 7000 Time, s

  • 3e+06
  • 2e+06
  • 1e+06

1e+06 2e+06 3e+06 FX, N 3650 3700 3750 3800 3850 3900-3e+06

  • 2e+06
  • 1e+06

1e+06 2e+06 3e+06

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Motivation Procedure and methods Conclusion 25/July/2017 13/20

4

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SLIDE 14

Enhanced coupling reduce CPU time by a factor of 4.

4 8 12 N 5 5.2 5.4 5.6 5.8 6 RT0, N 4 8 12 N 20.5 21 21.5 22 22.5 23 RT1, N 4 8 12 N

  • 0.32
  • 0.31
  • 0.3
  • 0.29
  • 0.28

γRT1, rad Strongy coupled Enhanced

  • Ref. solution

4 8 12 N

  • 0.185
  • 0.18
  • 0.175
  • 0.17
  • 0.165
  • 0.16

z0, m 4 8 12 N 0.84 0.87 0.9 0.93 z1, m 4 8 12 N

  • 2
  • 1.9
  • 1.8
  • 1.7
  • 1.6

γz1, rad 4 8 12 N

  • 0.085
  • 0.08
  • 0.075
  • 0.07
  • 0.065

φ0, rad 4 8 12 N 0.985 0.99 0.995 1 φ1, rad 4 8 12 N

  • 0.57
  • 0.56
  • 0.55
  • 0.54

γφ1, rad

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Motivation Procedure and methods Conclusion 25/July/2017 14/20

4

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SLIDE 15

Step 5: Detect green water events and perform detailed CFD simulation.

  • Customised post–processing tools detect the situations where

water on deck occurred in the three hour simulation,

  • Select the green water incident which is considered the most

dangerous,

  • Conduct a detailed CFD simulation with fine spatial and temporal

resolution, including complex geometries.

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Motivation Procedure and methods Conclusion 25/July/2017 15/20

5

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SLIDE 16

Detailed V&V of green sea loads has been performed.

Experiments performed for a static FPSO model at SNU:

Lee, H.H., Lim, H.J. and Rhee, S.H.: Experimental investigation of green water on deck for a CFD validation database (2012).

  • Green water pressure is compared at ten locations on deck,
  • Nine incident waves are considered,
  • isoAdvector geometric VOF method is used for interface tracking,
  • Grid, temporal and periodic uncertainty is assessed.

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Motivation Procedure and methods Conclusion 25/July/2017 16/20

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SLIDE 17

isoAdvector preserves a sharp interface for green water simulations.

Author: Dr. Johan Roenby, DHI.

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Motivation Procedure and methods Conclusion 25/July/2017 17/20

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SLIDE 18

Pressure peaks and pressure impulses are compared to experiments.

A detailed verification study has been performed for nine waves:

  • Numerical uncertainties = periodic + discretisation

uncertainties,

  • Experimental uncertainties = periodic + measuring

uncertainties,

  • 20 wave periods simulated to achieve periodic convergence,
  • Four grid levels used: from ≈ 200 000 to ≈ 4 000 000 cells.

14 16 18 20 22 24 26 Time, s 2000 4000 6000 8000 p, Pa

Location near the breakwater.

6 8 10 12 14 16 Time, s 100 200 300 400 500 600 700 p, Pa

Location further from the breakwater.

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Motivation Procedure and methods Conclusion 25/July/2017 18/20

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SLIDE 19

Results agree well with experiments.

H = 13.5 cm, λ = 2.25 m

1 2 3 4 5 6 7 8 9 10 Pressure gauge label 100 200 300 400 500 600 pmax, Pa CFD EFD

Pressure peaks,

1 2 3 4 5 6 7 8 9 10 Pressure gauge label 50 100 150 200 250 300 P, Pa s CFD EFD

Pressure impulses (time integrals), H = 15.0 cm, λ = 3.0 m

1 2 3 4 5 6 7 8 9 10 Pressure gauge label 100 200 300 400 500 600 700 800 pmax, Pa CFD EFD

Pressure peaks,

1 2 3 4 5 6 7 8 9 10 Pressure gauge label 50 100 150 200 250 300 P, Pa s CFD EFD

Pressure impulses (time integrals).

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Motivation Procedure and methods Conclusion 25/July/2017 19/20

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SLIDE 20

All the components of the procedure have been thoroughly validated.

A comprehensive procedure for green water load assessment includes:

  • Stochastic nature of ocean waves,
  • Complicated geometries.
  • Validated numerical methods:

Higher Order Spectrum method, Linearised free surface solver, SWENSE method with enhanced 6–DOF–fluid flow coupling algorithm, isoAdvector and Ghost Fluid Method for highly resolved green water simulations.

Future work:

  • Conduct the complete procedure for an example vessel.

FSB

Motivation Procedure and methods Conclusion 25/July/2017 20/20

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SLIDE 21

All the components of the procedure have been thoroughly validated.

A comprehensive procedure for green water load assessment includes:

  • Stochastic nature of ocean waves,
  • Complicated geometries.
  • Validated numerical methods:

Higher Order Spectrum method, Linearised free surface solver, SWENSE method with enhanced 6–DOF–fluid flow coupling algorithm, isoAdvector and Ghost Fluid Method for highly resolved green water simulations.

Future work:

  • Conduct the complete procedure for an example vessel.

Questions?

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Motivation Procedure and methods Conclusion 25/July/2017 20/20