cfd applications for marine foil configurations volker bertram ould m el moctar n.
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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 model 2

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Presentation Transcript
comet employed to perform computations
COMET employed to perform computations
  • RANSE solver:
  • Conservation of mass 1
  • momentum 3
  • volume concentration 1
  • In addition: k- RNG turbulence model 2
  • 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

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)

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

slide19

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
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
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

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:

slide34

Cavitation model: Effective fluid

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

Density:

Viscosity:

slide35

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:

slide37

Application to typical hydrofoil

Stabilizing fin rudder

slide38

First test: 2-D NACA 0015

Vapor volume fraction Cv for one period

slide39

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

with and 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

remember2
Remember
  • cavitation model reproduces essential characteristics
  • of real cavitation
  • reasonable good agreement with experiments
  • threshold technology