Recent Advances in Simulationfor Marine Applications

Milovan Perić

CD­adapco, Nürnberg Office

www.cd­adapco.comMilovan.Peric@de.cd­adapco.com

Introduction

Numerical towing tank has slowly become a reality...

The major advances in simulation technology which led to the acceptance of simulation as a design and optimisation tool are:

− The ability to handle complex geometry with all relevant details;

− An adequate modelling of turbulence;

− An adequate modelling of free surface effects;

− An adequate modelling of cavitation;

− The ability to handle non-linear waves;

− Coupled simulation of flow and flow-induced motion of bodies;

− Coupled simulation of flow and structural deformation.

Handling of Complex Geometry, I

During the past decade, the ability to handle complex geometry with all relevant details has been greatly improved:

− Tools for automatic and manual repair of CAD-models (which are often imperfect) have been developed;

− A surface-wrapping tool has been introduced, which creates a closed surface around assemblies of solid parts;

− Tools for automatic generation of polyhedral, trimmed hexahedral or extruded meshes have been developed;

− Automatic and manual definition of local mesh refinement requirements have been created, based on:

local curvature,

proximity of other walls,

pre-defined volume shapes, etc.

Handling of Complex Geometry, II

Surface-wrapping of an oil rig

Handling of Complex Geometry, III

Re-meshed surface of an oil rig

Handling of Complex Geometry, IV

Simulation of air flow around an oil platform

Handling of Complex Geometry, V

Volume shapes used to enforce local mesh refinementin a study of flow around a KVLCC-hull

Turbulence Modelling, I

For many types of analysis, the standard k-ε or k-ω turbulence models are adequate...

In order to predict secondary flows better, more sophisticated models are needed, e.g. Reynolds-stress model...

A special turbulence model is needed to predict transition from laminar to turbulent flow, e.g. when predicting resistance of a racing yacht...

For predicting noise sources, pressure fluctuation etc., large-eddy-simulation (LES or DES) type of analysis with special subgrid-scale turbulence models is needed.

The analyst needs to select the most appropriate model for his analysis...

Turbulence Modelling, II

Predicted iso-lines of constant velocity, showing boundary layer growth

KVLCC half model,comparison withwind tunnel test...

Turbulence Modelling, III

Comparison of computed and measured data...

Experiment

CFD

Reynolds-stressturbulence modelused in simulation

Turbulence Modelling, IV

y/L

0.00 0.02 0.04 0.06 0.08 0.10

u/U

, v/

U,

w/U

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2u/U (exp.)v/U (exp.)w/U (exp.)u/U (CFD)v/U (CFD)w/U (CFD)

Velocity cut in propeller plane at z/L=-0.05

Comparison of measured and predicted velocity profiles

Turbulence Modelling, V

Comparison of computed and measured resistance...

Modelling of Free-Surface Effects, I

Interface-capturing methods have been developed, which allow for the simulation of both gas and liquid flow...

Where the two fluids are not expected to mix, a sharp interface (within one control volume) is produced, with minimized numerical mixing.

Trapped gas bubbles of liquid droplets are adequately accounted for (gravity and surface tension effects).

Arbitrary free surface deformation and fragmentation can be accounted for.

Phase change models (cavitation, boiling) are integrated into this method to allow more complex phenomena to be modelled.

Modelling of Free-Surface Effects, II

Distribution of liquid volume fraction after 101 periods of roll-oscillation in an LNG-tank...

No numerical mixing: even unresolved drops and bubbles can be tracked until they reach free surface...

Roll-motion ofa full-size tank...

Free-surfaceresolution withinone cell...

Modelling of Cavitation, I

If the aim is to avoid cavitation, one only needs to predict its onset (pressure below saturation level)...

If cavitation cannot be avoided, its effect on performance needs to be assessed, so one has to model cavitation...

Models based on bubble dynamics (Rayleigh-Plesset equation) have proven robust and sufficiently accurate...

One additional equation for volume fraction of vapour is solved, with two parameters:

− seed density (number of seeds per m3 of liquid);

− initial radius of seed bubble.

These parameters are related to “liquid quality” and depend on region (for sea waters) or treatment (like de-gassing or filtering in a laboratory)...

Modelling of Cavitation, II

Cavitation in experiment: flow around NACA0015 foil at 10.3° angle of attack (chord length 0.2 m, water speed 6 m/s, channel 0.57 x 0.57 m, cavitation number 1.7, absolute pressure 32000 Pa); HSVA in 1999.

t0 t0 + 10,4 ms t0 + 20,7 ms

t0 + 31 ms t0 + 41,4 ms t0 + 51,7 ms

Modelling of Cavitation, III

Simulation of cavitation: flow around NACA0015 foil at 10.3° angle of attack (chord length 0.2 m, water speed 6 m/s, channel 0.57 x 0.57 m, cavitation number 1.7, absolute pressure 32000 Pa): CD-adapco, 2009

t0

0 10 ms

20 ms 30 ms

40 ms 50 ms

Modelling of Cavitation, V

The original design of Voith Water Jet led to substantial cavitation in the upper range of speeds... Experiments (performed after simulation) confirmed this...

Simulation Experiment

Courtesy of Voith Turbo Schneider Propulsion GmbH & Co. KG

Modelling of Cavitation, V

Courtesy of Voith Turbo Schneider Propulsion GmbH & Co. KG

With the optimized design, cavitation starts at a much higher speed and is less intensive – a substantial increase of efficiency has been achieved...

Waves, I

Simulation of wave impact and added resistance requires realistic waves in the solution domain...

Creation of incoming waves at a boundary requires long computing times...

The solution can be initialized using wave theory (e.g. Stokes 5th-order theory)...

This can substantially reduce solution domain size and the duration of simulation...

Coupling of solution methods based on RANSE and potential theory is useful when wave propagation over long distance is of interest:

− solving RANSE in regions where potential flow theory is not applicable (boundary layer, wake, breaking waves etc.);

− using potential theory elsewhere...

Waves, II

Waves break when they come into shallow water...

The Stokes 5th-order wave with19.9 m wave height in 33.5 m water depth breaks after about 1 period...

Wave shape shortly after initialization (top), after one period (middle) and after 1.5 periods (bottom)

Waves, III

Water velocity in the crest region increases with time as the wave tends to break: from 9.9 m/s after initialization to about 22.9 m/s during overturning (wave propagation: 17.6 m/s)...

max. 9.9 m/s max. 22.9 m/s

max. 17.7 m/s max. 21.1 m/s

Waves, IV

Impact of the first wave(nearly Stokes-wave)

Waves, V

Impact of the second wave(steeper, nearly breaking)

Waves, VI

Horizontal force on the platform during two wave encounters

Impact with the first wave(nearly Stokes wave)

Impact with the second wave(steeper, not yet breaking) More than

twice higherload!

Waves, VII

Courtesy of Germanischer Lloyd AG

Waves, VIII

Courtesy of Germanischer Lloyd AG

Floating Bodies, I

Implicit simulation of flow and flow-induced motion of floating (or flying) body:

− has no restriction on time-step size for stability reasons (time step can be chosen according to accuracy requirements);

− allows extreme motions of bodies, like lifeboat launching;

− handles well even very light bodies...

Many extensions are under way:− to allow for multiple bodies moving relative to each other;

− to allow for body connections (elastic spring, rigid body, flexible with constraints, etc.);

− to allow for external forces (thrust, mooring, towing etc.);

− to allow for relative motion of parts (hull, propeller, blade...).

Floating Bodies, II

Flow chart of coupled solution of equations for fluid flow and body motion...

Floating Bodies, III

Free fall of a lifeboat into flat water: Experiment by NORSAFE

Floating Bodies, IV

Comparison of computed and measured accelerations (from Mørch et al, 2008):

Prediction of effects of changes in lifeboat geometry has also been validated...

Floating Bodies, V

Initial wave position varied by 20 m (drop from 32 m height).

Following wave (180°)

Wavelength ca. 220 m, wave height 13.5 m, water depth 33.5 m

The questions to be answered:

− When is the load on the structure the highest?

− When are accelerations the highest?

Floating Bodies, VI

Vertical acceleration at center of gravity for different hit points, following wave (180° incidence).

Floating Bodies, VII

Pressure at one monitoring point for different hit points, 180°

Fluid-Structure-Interaction, I

Coupled simulation of flow and flow-induced deformation of solid structures is of high interest...

Coupling of FV-codes simulating fluid flow and FE-codes computing solid deformation is nowadays explicit – thus not applicable or inefficient in many practical situations...

An implicit coupling – as for the special case of floating body – is required for robustness and computational efficiency...

In many cases, structural model can be simplified – e.g. ship may be represented as a beam which can sustain bending and torsion...

Such simplified structural models have been included into CFD-simulation in an implicit way – with success...

Future Trends

More powerful and affordable computers = higher demands from simulation:

− More transient simulations (URANS, DES and LES);

− Prediction of pressure fluctuation and noise sources (turbulence, cavitation);

− More fluid-structure-interaction applications (slamming, sloshing);

− Simulation of manoeuvring tests (circle, zig-zag etc.) and other experiments – to be done in the design phase...

− More automatic optimization studies...

Simulation Experiments

Voith uses a ship simulator to train captains...

Hydrodynamic coefficients used to come from experi-ments – now they come from CFD analyses...

Simulator

Experiment SimulationCourtesy of Voith Turbo Schneider Propulsion GmbH & Co. KG

Optimization

An automatic calculation strategy is developed that includes the following tasks:

- parametrization of geometry - automatic generation of geometry - automatic mesh generation - automatic computation and post-processing of results - embedding the procedure in an optimization loop

automatic optimization loop

Azimuth-Thruster Optimisation

Courtesy of Voith Turbo Schneider Propulsion GmbH & Co. KG

Smart Engineers, I

No matter what the software can do, it remains just a tool...

How quickly and well problems get solved depends on the craftsman using the tool – the engineer remains indispensable!

Engineers need to be educated how to best use the tools at hand:

− theoretical analysis,

− numerical simulation (different kinds, different efforts),

− experiment (model, full scale).

The solution should be:− reliable (one needs to estimate numerical and modelling

errors);

− obtained with minimum cost (time and money)...

Smart Engineers, II

An example:

A mega-yacht with 4propellers suffered from vibrationsat a cruising speed of about 16 - 18 kn.German Lloyd and CD-adapco solvedthe problem with the help of 2 CFD-simulations, one FE-analysis and two field experiments (on real object)...

Old design

New design

Vortex shedding was the cause...