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CFD Applications for Marine Foil Configurations Volker Bertram, Ould M. El Moctar

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CFD Applications for Marine Foil ConfigurationsVolker Bertram, Ould M. El Moctar

- RANSE solver:
- Conservation ofmass1
- momentum3
- volume concentration1
- In addition:k- RNG turbulence model2
- In addition:cavitation model (optional)1
- HRIC scheme for free-surface flow
- Finite Volume Method:
- arbitrary polyhedral volumes, here hexahedral volumes
- unstructured grids possible, here block-structured grids
- non-matching boundaries possible, here matching boundaries

Surface-piercing strut

Rudder at extreme angle

Cavitation foil

Wing profile bad choice in this case

Flow highly unsteady: port+starboard modelled

1.7 million cells, most clustered near CWL

8 L

4 L

10 L to each side

10 L

10 L

Starboard half of grid (schematic)

Circular section strut, Fn=2.03, Rn=3.35·106

thickness

almost

doubled

Thickness “60”Thickness “100”

circular section strut, Fn=2.03, Re=3.35·106

parabolic strut cylinder

Fn=2.03, Re=3.35·106

Transverse

plate

attached

Parabolic strut, Fn=2.03, Re=3.35·106

Parabolic strut, Fn=2.03, Rn=3.35·106

Transverse

plate

attached

Transverse

plate (ring)

attached

cylinder, Fn=2.03, Re=3.35·106

Large initial time steps

overshooting leading-edge wave for usual number of outer iterations

convergence destroyed

Use more outer iterations initially

leading-edge wave reduced

convergence good

- High Froude numbers require unsteady computations
- Comet capable of capturing free-surface details
- Realistic results for high Froude numbers
- Qualitative agreement with observed flows good
- Response time sufficient for commercial applications
- Some “tricks” needed in applying code

Surface-piercing strut

Rudder at extreme angle

Cavitation foil

Rudder profiles employed

in practice

- Concave profiles:higher lift gradients and max lift than NACA profiles of same maximum thickness
- IfS-profiles:highest lift gradients and maximum lift due to the max thickness close to leading edge and thick trailing edge
- NACA-profiles feature the lowest drag

Stall Conditions

Superfast XII

Increase maximum rudder angle to 45º

RANSE grid with 1.8 million cells, details

- 10 c ahead
- 10 c abaft
- 10 c aside
- 6 h below

Radial Force Distribution

l

Root

Tip

Source Terms

Rudder angle 25°

Superfast XII, rudder forces in forward speed

lift

drag

shaft

moment

Velocity distributionat 2.6m above rudder base

25º 35º 45º

Velocity distributionat top for 35°

forward reverse

no separation massive separation

- RANSE solver useful for rudder design
- higher angles than standard useful

Surface-piercing strut

Rudder at extreme angle

Cavitation foil

different seed types &

spectral seed distribution

„micro-bubble“ &

homogenous seed distribution

average seed radius R0

average number of seeds n0

V

„micro-bubble“ R0

liquidVl

vapor bubble R

Vapor volume fraction:

Cavitation model: Effective fluid

The mixture of liquid and vapor is treated as an effective fluid:

Density:

Viscosity:

Cavitation model: Convection of vapor bubbles

Lagrangian observation

of a cloud of bubbles

&

Equation describing the transport of the vapor fraction Cv:

convective transport bubble growth or collapse

Task: model the rate of the bubble growth

Conventional bubble dynamic

=

observation of a single bubble in infinite stagnant liquid

„Extended Rayleigh-Plasset equation“:

Inertia controlled growth model by Rayleigh:

Application to typical hydrofoil

Stabilizing finrudder

First test: 2-D NACA 0015

Vapor volume fraction Cv for one period

First test: 2-D NACA 0015

Comparison of vapor volume fraction Cv for two periods

Periodic cavitation patterns

on 3-D foil

Vapor volume fraction Cv

for one period

Pressure coefficient Cp

for one period

Comparison of

vapor volume fraction Cv

with

pressure coefficient Cp

for one time step

Experiment by Ukon (1986)

Cv= 0.05

pressure distribution Cpand vapor volume fraction Cv

Cv= 0.005

Cv= 0.5

Correlation between

visual type of cavitation

and

vapor volume fraction Cv ?

Pressure distribution

withand without

calculation of cavitation

Exp.

Minimal and maximal

cavitation extent with

vapor volume fraction Cv=0.05

- cavitation model reproduces essential characteristics
- of real cavitation
- reasonable good agreement with experiments
- threshold technology