Validation of STAR-CCM+ for 2Dirregular waves

Luca Oggiano(Reseach Scientist)

Email:luca.oggiano@ife.no

Linkedin: www.linkedin.com/in/luca-

oggiano

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• Background and motivation• Wave Loads project• Simulations• Results and comparison• Discussion

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Overview

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Motivation• Increase of the offshore wind energy market• Increase in wind turbine size• Increase in monopile size• Models are crude and somehow primitive• CFD can help

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Wave Loads Project• Henrik Bredmose• Signe Schløer• Robert Mikkelsen• Stig Øye• Torben Juul Larsen• Taeseong Kim• Anders Melchior Hansen

• Jesper Mariegaard• Flemming Schlütter• Jacob Tornfeldt Sørensen• Ole Svendstrup Petersen• Hans Fabricius Hansen• Anders Wedel Nielsen• Bjarne Jensen• Iris Pernille Lohman• Xerxes Mandviwalla• Bo Terp Paulsen• Harry Bingham

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vTask A:Boundary conditions forphase resolving wavemodels

Task C:Aero-elastic responseto fully nonlinear waveforcing

Task B:CFD methods for steepand breaking waveimpactsDTU, (DHI)

Task D:Physical model tests

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Experimental Facility

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Solver

• RANS (Reynolds averaged Navier Stokes) solver based onconservation of mass and momentum with VOF model to capture theinterface between the fluids [1]

The density ρ, and the viscosity, µ, are specified in terms of the water volume fraction, α

[1] Hirt, C. W. and B. D. Nichols (1981). Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries. Journal of Computational Physics 39, 201–225.

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• Where the water volume fraction α, once the velocity field is known, is Advanced in time by the transport equation:

• And the force is calculated by pressure integration on the wettedsurface area

𝐹𝐹 𝑡𝑡 = �𝑊𝑊𝑆𝑆𝑝𝑝 𝑡𝑡 𝑑𝑑𝑛𝑛

[1] Hirt, C. W. and B. D. Nichols (1981). Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries. Journal of Computational Physics 39, 201–225.

Solver

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• A second order central scheme was used to solve the NS eqations• Symmetrical boundary conditions were used at the side walls and top

wall while a no-slip walls conditions were used at the bottom in order to reproduce the evolution of the waves on the slope.

• A prescribed velocity profile was previously precalculated and prescribed at each time step

Solver

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Linear reconstruction of incident waves

Assuming the paddle to be unknown, a linear reflection analysis was used with the assumption that the free surface elevation at any location in the domain can be described by a Fourier series [2]

[2] Bredmose, H., a. Hunt-Raby, R. Jayaratne, and G. N. Bullock (2009, November). The ideal flip-through impact: experimental and numerical investigation. Journal of Engineering Mathematics 67(1-2), 115–136.

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Inlet Boundary conditions for CFD

Inlet

H,Usurf

MWLU

• The velocities were transformed from the linear domain into the physical domain by the second-order kinematics recommended in the DNV standard (which is a Taylor expansion (extrapolation) of the linear velocity profile). The model, for the first order components reduces to:

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Grid Structure

1.2 Hmax

1.2 Hmin

Free Surface refinementBased on previous findings, a mesh refinement was used only in the free surface areawhile the rest of the domain had a coarser mesh allowing for a lower number of cellsand thus a reduced computational time

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Domain• Both 2D and 3D simulations were carried out and a simplified model• The full wave tank, slope included, was modeled• The inlet was placed where the sensor 1 is placed (at the beginning

of the slope)• The outlet was extended to increase numerical damping, and avoid

wave refelctions1

Damping area

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The two time series were incident waves where 2D irregular waves given by a JONSWAP spectrum.

At full scale the significant wave height and the peak period was Hs =11 m and Tp =14 s respectively, which at model scale corresponds to Hs = 0.1375 m and Tp = 1.5652 s.

KCpeak = 0.2, which indicates that most the waves will be in the long wave regime as required by the Morison equation.

The equivalent KC number is ca. 6 meaning that drag component can be considered much smaller than the inertia

Positioning the waves on the Le Mahute plot shows that thewaves are developing into an aera where breaking waves mightoccurr in both cases

Experiments

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As suggested in [3], the time series were analyzed with in the two-dimensional time/frequency domain by a wavelet Morlettransform. The type wavelet transform used was implentedfrom [4].

where g is the Morlet waveletThe color indicates the magnitude of the force for a given time and frequency and has the unit of [ms1/2] [Ns1/2]

[3] Paulsen Et. Al. Steep Wave Loads From Irregular Waves On An Offshore Wind Turbine Foundation: Computation And Experiment –OMAE2013[4] Christopher Torrence and Gilbert P. Compo- A Practical Guide to Wavelet Analysis Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado

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v2D Grid Sensitivity StudyBase Grid

Model Fullscale Model Fullscale Model Fullscale[m] [m] [m] [m] [s] [s] Type

98 0.26 20.8 0.10375 8.3 1.408723 12.6 2D irregular99 0.26 20.8 0.1375 11 1.565248 14 2D irregular

96 0.26 20.8 0.1375 11 1.565248 14 2D regular

Hs Tphmodel

Model Fullscale Model Fullscale Model Fullscale[m] [m] [m] [m] [s] [s] Type

98 0.26 20.8 0.10375 8.3 1.408723 12.6 2D irregular99 0.26 20.8 0.1375 11 1.565248 14 2D irregular

Hs Tphmodel

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[5] - Oggiano et. Al. - Comparison of experiments, CFD simulations and a finite element code on a stiff monopile in shallow water under shoaling regular waves Proceedings of ISOPE 2016

The base grid was based on previous grid dependency study on regular waves with H=Hs and T=Tp

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2D simulations on a selected part of thetime series

Single cell in the y direction

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• 80s time serie extracted from the full experimental time series.

• Ch1 used as inlet for CFD simulations

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Wavelet Transform (Ch2 Close to inlet)

[ms1/2]

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Wavelet Transform (Ch18 Indistrubed waves)

[ms1/2]

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FFT Transform

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The cylinder was excited at its natural frequency after the passage of the first steep wave. As we were mainly interested in the pure hydrodynamic loading and since the structural vibrations are not included in the numerical model, a low-pass FFT filter, with cut-off frequency of 6 Hz was applied

Extreme Event 3D simulationInline forceWave elevation

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vExtreme EventInline forceWave elevation

WaveElevation

Ch17

InlineForce

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vExtreme Event (free surface)

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Conclusions• A method to simulate irregular waves and extreme events was

developed• The computations show good agreements with the experiements

• In the time domain (extreme values)• In the frequency domain

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Publications and Funding• Submitted to ISOPE 2017

• Projects• NOWITECH

• Norwegian Reseach Centre for Offshore wind Technology• https://www.sintef.no/projectweb/nowitech/

• DIMSELO• Dimensioning Sea Loads• http://www.dimselo.no/

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Papers to refer to• Nygaard, T. A., De Vaal, J., Pierella, F., Oggiano, L. and Stenbro, R.

(2016). Development, Verification and Validation of 3DFloat; Aero-Servo-Hydro-Elastic Computations of Offshore Structures. Energy Procedia Volume 94, September 2016, Pages 425–433

• Oggiano, L., Pierella, F., Nygaard, T. A., De Vaal, J.and Arens, E. (2016). Reproduction of steep long crested irregular waves with CFD using the VOF method. DEEPWIND conference 2017

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Luca Oggiano

Email:luca.oggiano@ife.no

Linkedin: www.linkedin.com/in/luca-oggiano