CFD for Aerodynamics of Fast Ships

Volker Bertram

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Scenario - what, why, etc.

Aerodynamic flow around a ship superstructure is important in many ways:

• Exhaust dispersal• Ventilation of occupied spaces• Wind forces, especially for maneuvering• Special operational conditions - helicopter landing, etc.

Used to make design & operational decisions

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Wind tunnel proven tool

This information now predominantly from wind tunnel tests

Wind tunnel proven tool to provide useful information about the airflow.

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Aero CFD: An alternative now!

Aerodynamics CFD effective in other engineering fields:• aerospace• automotive• civil engineering

Advantages: • All information available at any time• More precise control over what is viewed• More details are possible • Full scale (but still idealized...)• Non-intrusive

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CFD for ship aerodynamics now a topic

Problem difficult due to various factors:

• Grid generation very difficult• Large grid cell count • Complex physics

Recent progress addresses these issues:

• Unstructured, more robust solvers• Improved, automatic grid generation tools• Advanced numerical modeling techniques• Affordable parallel computing

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Several applications in last years

DMI

• DMI• Sirehna• Marintek• NRL• JJMA• Stanford• KRISO• ... (?)

Sirehna

JJMA

NRL

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Several applications also at HSVA

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Tools and Methods Used

Typical geometry imported from IGES format

Unstructured, tetrahedral grids generated using ICEM-CFD, cell count of up to 5 million cells

Calculations with Comet on a parallel PC cluster

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Geometric modeling of all superstructures impossible

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Baffle Elements Tested

model global effect of replacing filigree structures by semi-permeable cell boundaries

22 000 cells 6 000 cells

cell count for a 2D case

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Baffle elements disappointed

geometric model

Assorted baffle parameters

k and |v| for mast

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Simple block does the trick

geometric model Simple block

k and |v| for mast

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Application to fast ferry

Superfast VI, HDW, 29 kn

IGES file from yard too detailed: several weeks work to downstrip

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Grid: 680,000 cells per symmetry half

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Model tests performed at IFS wind tunnel

Physical model (1:150) in wind tunnel

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Local refined grid reduces discretization errors

Exhaust concentration

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CFD

Similar agreement for wind from abaft

Experiment

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They cannot be fulfilled all at the same time!

Some similarity laws always violated

ratio of velocities

geometric similarity

ratio of mass flux

Reynolds number of the inflow

Reynolds number of the jet

Froude number of the jet

ratio of momentum flux

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Parameter studies

exhaust gas temperature 300°C

inviscous computation

model test parameters

full scale Rn

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stream lines (0°) pressure distribution (30°)

turbulent kinetic energy k (0°)

Visualisation of different quantities

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Forces OK for small-to-medium angles

Drag

Side force

Roll moment

Differences for large oblique angles attributed to flow separation

insufficiently captured by turbulence model

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Application to fast SES

AGNES 200, French SES, 40 kn

First step:Create IGES description

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Grid topology allows easy re-gridding

Inner cylinder in outer blockMatching every 5°2.9 million cells

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Pressures change with angle of attack

180° 170° 150°

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Vortex formation behind superstructure

streamlines =180°

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Strongly 3-d flow

streamlines =180°

5 cm higher

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Features similar for 170°

streamlines =170°

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Less complex “foil” flow for for 150°

streamlines =150°

Streamlines return to original direction further downstream

Flow followslow-pressure side of SES

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Flow strongly 3-dimensional

Virtual Reality may help understanding the flow

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Virtual Reality comes in many shapes

Sources: VRL, Univ. Of Michigan; VRCA RWTH Aachen

cave head-gear PC

Poor man’s VR suffices!

VRML

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What is VRML?

• 3D file interchange format

• 3D analogue to HTML

• ISO standard

VRML = Virtual Reality Modeling Language

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VRL, Univ. of Michigan

INSEAN INSEAN+TUHH TUHH

Steps creating a CFD VRML model

• Study experience of others

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Direct export:43000 polygons2810 KByte

Steps creating a CFD VRML model

• Study experience of others

• Export geometry data from RANSE solver

• Downsize geometry

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Direct export:43000 polygons2810 KByte

After merging:900 polygons130 KByte

Steps creating a CFD VRML model

• Study experience of others

• Export geometry data from RANSE solver

• Downsize geometry

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Steps creating a CFD VRML model

• …

• Build VRML geometry model

• Process and downsize flow data

• Add flow data to VRML model

• Add interaction to VRML model

Interactivehigh-lighting

Interactiveselection

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Pressures use colour VRML interpolation

Work continues:

• Refine algorithm to downsize model

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Conclusions

• CFD offers more insight than wind tunnel• Further work required for validation• Wind tunnel may be too optimistic for smoke tracing• VRML suitable for post-processing