cfd applications for marine foil configurations volker bertram, ould m. el moctar
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
CFD Applications for Marine Foil Configurations
Volker Bertram, Ould M. El Moctar
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COMET employed to perform computations
RANSE solver:
Conservation of mass 1momentum 3volume concentration 1
In addition: k- RNG turbulence model 2In 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
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Diverse Applications to Hydrofoils
Surface-piercing strut
Rudder at extreme angle
Cavitation foil
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Motivation: Struts for towed aircraft ill-designed
Wing profile bad choice in this case
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Similar flow conditions for submarine masts
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Similar flow conditions for hydrofoil boats
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Grid designed for problem
Flow highly unsteady: port+starboard modelled1.7 million cells, most clustered near CWL
10 L to each side
8 L
4 L
10 L 10 LStarboard half of grid (schematic)
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Cells clustered near free surface
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Flow at strut highly unsteady
Circular section strut, Fn=2.03, Rn=3.35·106
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Wave height increases with thickness of profile
thickness almost
doubled
circular section strut, Fn=2.03, Re=3.35·106
Thickness “60” Thickness “100”
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Wave characteristic changed from strut to cylinder
parabolic strut cylinderFn=2.03, Re=3.35·106
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Transverse plate reduces waves
Transverseplate
attached
Parabolic strut, Fn=2.03, Re=3.35·106
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Transverse plate reduces waves
Transverseplate
attached
Parabolic strut, Fn=2.03, Rn=3.35·106
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Transverse plate less effective for cylinder
Transverseplate (ring)attached
cylinder, Fn=2.03, Re=3.35·106
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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
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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
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Diverse Applications to Hydrofoils
Surface-piercing strut
Rudder at extreme angle
Cavitation foil
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Concave profiles offer alternatives
Rudder profiles employed in practice
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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
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Validation Case (Whicker and Fehlner DTMB)
Stall Conditions
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Superfast XII Ferry used HSVA profiles
Superfast XII
Increase maximum rudder angle to 45º
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Fine RANSE grid used
RANSE grid with 1.8 million cells, details
• 10 c ahead• 10 c abaft• 10 c aside• 6 h below
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Grid generation allows easy rotation of rudder
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Body forces model propeller action
Radial Force Distribution
RootTip
Source Terms
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Pressure distribution / Tip vortex
Rudder angle 25°
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Maximum before 35º
Superfast XII, rudder forces in forward speed
lift
shaft moment
drag
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Separation increases with angle
Velocity distribution at 2.6m above rudder base
25º 35º 45º
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Reverse flow also simulated
Velocity distribution at top for 35°
forward reverse no separation massive separation
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Stall appears earlier in reverse flow
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Remember:
• RANSE solver useful for rudder design• higher angles than standard useful
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Diverse Applications to Hydrofoils
Surface-piercing strut
Rudder at extreme angle
Cavitation foil
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Cavitation model: Seed distribution
average seed radius R0average number of seeds n0
different seed types &spectral seed distribution
„micro-bubble“ &homogenous seed distribution
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Cavitation model: Vapor volume fractionV
liquid Vl
„micro-bubble“ R0
vapor bubble R
Vapor volume fraction:
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Cavitation model: Effective fluid
The mixture of liquid and vapor is treated as an effective fluid:
Density:
Viscosity:
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Cavitation model: Convection of vapor bubbles
Task: model the rate of the bubble growth
convective transport bubble growth or collapse
Lagrangian observation of a cloud of bubbles
Equation describing the transport of the vapor fraction Cv:
&
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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:
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Application to typical hydrofoil
Stabilizing fin rudder
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First test: 2-D NACA 0015
Vapor volume fraction Cv for one period
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First test: 2-D NACA 0015
Comparison of vapor volume fraction Cv for two periods
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3-D NACA 0015
Periodic cavitation patternson 3-D foil
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2-D NACA 16-206
Vapor volume fraction Cvfor one period
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2-D NACA 16-206
Pressure coefficient Cp for one period
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2-D NACA 16-206
Comparison ofvapor volume fraction Cv
with
pressure coefficient Cp for one time step
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3-D NACA 16-206: Validation with Experiment
Experiment by Ukon (1986) Cv= 0.05
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3-D NACA 16-206
pressure distribution Cp and vapor volume fraction Cv
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3-D NACA 16-206
Cv= 0.005 Cv= 0.5
Correlation between visual type of cavitation
andvapor volume fraction Cv ?
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3-D NACA 16-206Pressure distribution
with and without calculation of cavitation
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3-D NACA 16-206
Minimal and maximalcavitation extent with
vapor volume fraction Cv= 0.05
Exp.
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3-D NACA 16-206: VRML model
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Remember
• cavitation model reproduces essential characteristics
of real cavitation• reasonable good agreement with experiments • threshold technology
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