Seakeeping Analysis of Sailing Yacht Hulls and Centerboard ...
Transcript of Seakeeping Analysis of Sailing Yacht Hulls and Centerboard ...
Seakeeping Analysis of Sailing Yacht Hulls and Centerboard Effect: Comparison between
Different Computational Methods
Author: Eng. Giovanni Bailardi
Supervisor: Prof. Dario Boote (UNIGE)
External Reviewer: Prof. Pierre Ferrant (Ecole Centrale de Nantes)
Contents Introduction
Numerical Methods Overview
B.E.M. - HydroStar
R.A.N.S. - Star CCM+
Comparisons
Conclusion
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Seakeeping Analysis
The seakeeping study is normally used to evaluate the ship response to a generic sea state.
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Seakeeping Analysis
Computational methods can allow the evaluation of the yacht motion, velocity and acceleration in all the 6 DoFs
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B.E.M. Frequency Domain
R.A.N.S. Time Domain
R. A. O. Response Amplitude Operator
Sea
State
Yacht
Response
R. A. N. S.
B. E. M.
Thesis Workflow
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Inertia Estimation (Excel Sheet)
Input geometry (Matlab)
Meshing + Seakeeping (HydroStar)
3D Model (Rhinoceros,
FreeShip, MaxSurf)
Meshing + Seakeeping + Postprocessing (Star CCM+)
Postprocessing (Matlab)
Comparison (Matlab)
CoG
CoG keel
CoG bulb
CoG hull
zobj
Inertia Estimation
In order to solve the yacht dynamic system, mass inertias must be evaluated and then trasported to the relative CoG:
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𝑀 + 𝐴(𝑡) 𝑋 + 𝐵 𝑡 + 𝐵ν 𝑋 + 𝐾 𝑋 = 𝐹(𝑡)
𝐼𝑥𝑥_𝑜𝑏𝑗 = 𝐼𝑜𝑏𝑗 + 𝑧𝑜𝑏𝑗2 ∙ 𝑚𝑜𝑏𝑗 𝐼𝑥𝑥_𝐶𝑜𝐺 = 𝐼𝑥𝑥_𝑜𝑏𝑗
Boundary Element Method
The most used method for the seakeeping evaluation comes directly from the Potential Theory applied to a Panel Method:
Non viscous (ν = 0)
Irrotational (𝛻 × 𝑉 = 0);
Incompressible (𝜌 = 𝑐𝑜𝑛𝑠𝑡)
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+ Fast computation + Spectral analysis possible + LTI system + 2nd order effects
- Only volume under waterline - No lifting effect - No viscous damping effect - Optimum for simple geometry
body
HydroStar
Computational Domain
The yacht is modeled by 3D panels, where Green functions are applied, up to the waterline
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The Wave is computed as Monochromatic perturbation along the mean Free Surface with constant Amplitude
NO TOPSIDE VOLUME
RAO: Heading Effect (V = 0kn)
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Heave has a wider spectrum with beam seas
Pitch motion present resonance around 0.3Hz hence waves of about 3s period
Speed Effect: Heave
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Beam sea (β=90°) Head sea (β=180°)
Very Slight modification
The increase of velocity generates a peak of resonance
The higher is the velocity the higher is the peak
Trim effect: Pitch & Roll
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Head sea (β=180°) Beam sea (β=90°)
Pitch motion peak enhanced by negative trim (bow up)
Reduced by positive trim (bow
down)
The trim angle changes the underwater volume and thus the RAO spectrum, the resonance frequency and its peak.
Cricitcal Viscous Damping
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Roll response amplitude strongly influeced by the vicous damping coefficient Need of exact coefficient for yacht roll performance estimation, seasickness and
motion
R.A.N.S. CFD
Numerical solution of the Reynold Averaged Navier-Stokes equations in a Finite Volume domain
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+ Viscous effects + Lifting+Vorticity effects + Dynamic unsteady simulation + Complete body volume
considered
- Very long computation - Strong Mesh influence - Spectral waves input under
development
Star CCM+
Computational Domain
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Eulerian Volume with the fluids interface described by the Volume Of Fraction model (no multiphase)
Dynamic Fluid-Body Interaction
Mesh refinement «ad-hoc»
Overset Mesh
Regular Waves Peculiarities
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Waves affect the wetted wolume Each Wave requires special refinement in order to avoid smearing
1st Order Wave (T=2.5s)
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1st Order Wave (T=4s)
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1st Order Wave (T=8s)
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RANS: Speed Effect
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Different response period because of different encounter frequency No differences in Heave motion Amplitude Different mean Dynamic Trim but not visible amplitude difference
Upwind Sailing: Heeled Condition
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Roll Motion: mesh
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The domain loses symmetry: nr. cells at least doubled
The informations don’t spread in a favourite direction
Hull Aft Shape
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NIKKA: Soft Chines BELLA DONNA: Round Stern
Hull Aft Shape: Speed Influence
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0 kn
8 kn
Centerboard up/down: RAO Comparison
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NIKKA
BELLA DONNA
Centerboard up/down: Focus on Roll
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NIKKA: HydroStar vs Star CCM+
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The general trend between the two methods is similar Heave resonance behaviour seemed halved with viscous simulations At very long periods/low frequencies a lower ratio of Heave amplitude
is shown for Star CCM+
Head sea (β=180°)
BELLA DONNA: HydroStar vs Star CCM+
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The general trend between the two methods is similar Heave resonance behaviour seemed halved with viscous simulations At very long periods/low frequencies a lower ratio of Heave amplitude
is shown for Star CCM+
Head sea (β=180°)
HydroStar vs Star CCM+
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Heave motion shows a slightly reduced spectrum for RANS solver Concerning sailing yachts, Roll motion cannot be correctly predict by
non-viscous solver
Beam sea (β=90°)
Conclusions
For the seakeeping of complex dynamic systems like the sailing yachts, RANS
solvers are needed.
Forward velocity resonance effects that need to be experimentally validated;
Soft Chines damp mostly the Roll response but its influence decreases
increasing the velocity;
The Lifting Centerboard don’t affect the longitudinal motion in heeled
condition with head sea (roll fixed) while strongly modifies the rolling
behaviour.
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Future Developments
Experimental validation at forward speed
Modern trend of Hard Chined hulls (Volvo, Vendeé Globe, Mini Transat 6.5, Class 40);
Influence of Bulb shape on roll damping;
Heading Angle + 6 DoFs: the future of VPP;
Aerodynamic + Hydrodynamic RANS simulation (America’s CUP - Oracle USA)
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