Cfd applications for marine foil configurations volker bertram ould m el moctar
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CFD Applications for Marine Foil Configurations Volker Bertram, Ould M. El Moctar. COMET employed to perform computations. RANSE solver: Conservation of mass 1 momentum 3 volume concentration 1 In addition: k-  RNG turbulence model2

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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 configurations volker bertram ould m el moctar

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


Comet employed to perform computations

COMET employed to perform computations

  • 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


Diverse applications to hydrofoils

Diverse Applications to Hydrofoils

Surface-piercing strut

Rudder at extreme angle

Cavitation foil


Motivation struts for towed aircraft ill designed

Motivation: Struts for towed aircraft ill-designed

Wing profile bad choice in this case


Similar flow conditions for submarine masts

Similar flow conditions for submarine masts


Similar flow conditions for hydrofoil boats

Similar flow conditions for hydrofoil boats


Grid designed for problem

Grid designed for problem

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)


Cells clustered near free surface

Cells clustered near free surface


Flow at strut highly unsteady

Flow at strut highly unsteady

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


Wave height increases with thickness of profile

Wave height increases with thickness of profile

thickness

almost

doubled

Thickness “60”Thickness “100”

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


Wave characteristic changed from strut to cylinder

Wave characteristic changed from strut to cylinder

parabolic strut cylinder

Fn=2.03, Re=3.35·106


Transverse plate reduces waves

Transverse plate reduces waves

Transverse

plate

attached

Parabolic strut, Fn=2.03, Re=3.35·106


Transverse plate reduces waves1

Transverse plate reduces waves

Parabolic strut, Fn=2.03, Rn=3.35·106

Transverse

plate

attached


Transverse plate less effective for cylinder

Transverse plate less effective for cylinder

Transverse

plate (ring)

attached

cylinder, Fn=2.03, Re=3.35·106


Problems in convergence solved

Problems in convergence solved

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


Remember

Remember:

  • 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


Diverse applications to hydrofoils1

Diverse Applications to Hydrofoils

Surface-piercing strut

Rudder at extreme angle

Cavitation foil


Concave profiles offer alternatives

Concave profiles offer alternatives

Rudder profiles employed

in practice


Cfd applications for marine foil configurations volker bertram ould m el moctar

  • 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


Validation case whicker and fehlner dtmb

Validation Case (Whicker and Fehlner DTMB)

Stall Conditions


Superfast xii ferry used hsva profiles

Superfast XII Ferry used HSVA profiles

Superfast XII

Increase maximum rudder angle to 45º


Fine ranse grid used

Fine RANSE grid used

RANSE grid with 1.8 million cells, details

  • 10 c ahead

  • 10 c abaft

  • 10 c aside

  • 6 h below


Grid generation allows easy rotation of rudder

Grid generation allows easy rotation of rudder


Body forces model propeller action

Radial Force Distribution

l

Root

Tip

Source Terms

Body forces model propeller action


Pressure distribution tip vortex

Pressure distribution / Tip vortex

Rudder angle 25°


Maximum before 35

Maximum before 35º

Superfast XII, rudder forces in forward speed

lift

drag

shaft

moment


Separation increases with angle

Separation increases with angle

Velocity distributionat 2.6m above rudder base

25º 35º 45º


Reverse flow also simulated

Reverse flow also simulated

Velocity distributionat top for 35°

forward reverse

no separation massive separation


Stall appears earlier in reverse flow

Stall appears earlier in reverse flow


Remember1

Remember:

  • RANSE solver useful for rudder design

  • higher angles than standard useful


Diverse applications to hydrofoils2

Diverse Applications to Hydrofoils

Surface-piercing strut

Rudder at extreme angle

Cavitation foil


Cavitation model seed distribution

Cavitation model: Seed distribution

different seed types &

spectral seed distribution

„micro-bubble“ &

homogenous seed distribution

average seed radius R0

average number of seeds n0


Cavitation model vapor volume fraction

Cavitation model: Vapor volume fraction

V

„micro-bubble“ R0

liquidVl

vapor bubble R

Vapor volume fraction:


Cfd applications for marine foil configurations volker bertram ould m el moctar

Cavitation model: Effective fluid

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

Density:

Viscosity:


Cfd applications for marine foil configurations volker bertram ould m el moctar

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


Cavitation model vapor bubble growth

Cavitation model: Vapor bubble growth

Conventional bubble dynamic

=

observation of a single bubble in infinite stagnant liquid

„Extended Rayleigh-Plasset equation“:

Inertia controlled growth model by Rayleigh:


Cfd applications for marine foil configurations volker bertram ould m el moctar

Application to typical hydrofoil

Stabilizing finrudder


Cfd applications for marine foil configurations volker bertram ould m el moctar

First test: 2-D NACA 0015

Vapor volume fraction Cv for one period


Cfd applications for marine foil configurations volker bertram ould m el moctar

First test: 2-D NACA 0015

Comparison of vapor volume fraction Cv for two periods


3 d naca 0015

3-D NACA 0015

Periodic cavitation patterns

on 3-D foil


2 d naca 16 206

2-D NACA 16-206

Vapor volume fraction Cv

for one period


2 d naca 16 2061

2-D NACA 16-206

Pressure coefficient Cp

for one period


2 d naca 16 2062

2-D NACA 16-206

Comparison of

vapor volume fraction Cv

with

pressure coefficient Cp

for one time step


3 d naca 16 206 validation with experiment

3-D NACA 16-206: Validation with Experiment

Experiment by Ukon (1986)

Cv= 0.05


3 d naca 16 206

3-D NACA 16-206

pressure distribution Cpand vapor volume fraction Cv


3 d naca 16 2061

3-D NACA 16-206

Cv= 0.005

Cv= 0.5

Correlation between

visual type of cavitation

and

vapor volume fraction Cv ?


3 d naca 16 2062

3-D NACA 16-206

Pressure distribution

withand without

calculation of cavitation


3 d naca 16 2063

3-D NACA 16-206

Exp.

Minimal and maximal

cavitation extent with

vapor volume fraction Cv=0.05


3 d naca 16 206 vrml model

3-D NACA 16-206: VRML model


Remember2

Remember

  • cavitation model reproduces essential characteristics

  • of real cavitation

  • reasonable good agreement with experiments

  • threshold technology


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