CFD PROJECTS – Heave Response

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2013 CAE NAVAL & OFFSHORE
Windsor Guanabara, Rio de Janeiro/RJ – Brasil
13 de Junho de 2013
SIMULATION OF FLOW AROUND FLOATING
STRUCTURES: SHIPS AND PLATFORMS
Alexandre T. P. Alho
Laboratório de Sistemas de Propulsão
DENO/POLI, UFRJ
Engenharia Naval e Oceânica
COPPE/UFRJ & EP/UFRJ
INTRODUCTION

Preliminary Considerations
▪
Growing demand for high efficiency systems
▪
Demand for accurate predictions in less time and at low costs.
▪

Accurate CFD models: designers can rely on as an effective
design tool.

CFD model must be developed based on a good compromise
between the quality of the numerical result and the computational
effort.
Performance prediction of ships and offshore platforms

Experimental methods are well-established, but are usually
expensive and time-consuming.

Optimization process is virtually impossible based on experimental
methods: very high costs.
2013 CAE NAVAL & OFFSHORE – Windsor Guanabara, Rio de Janeiro/RJ – Brasil
INTRODUCTION


Examples of CFD Projects
▪
CFD Predictions of the Hull Resistance and the Wave System of a
Catamaran.
▪
Investigate the performance of passive damping foils on heave
response of a catamaran.
▪
Develop a CFD model to study the effectiveness of passive damping
devices on heave motions of mono-column platforms.
Methodology
▪
The flow around vessel/platform hulls was simulated by means of
commercial CFD code (ANSYS CFX).
▪
Results are validated against experimental data (if available).
CFD PROJECTS – Resistance & Wave Cut

Motivation
▪
Growing demand for high speed multihull vessels.


Objective
▪

Catamaran/SWATH concept has been received special attention
 good performance in terms of speed and transversal stability.
Validate a CFD model in terms of its performance on estimating hull
resistance and calculating the wave cuts generated by the hull.
Main Particulars
▪
Length (BP):
27.6 m
▪
Beam (each hull):
2.97 m
▪
Draft (design load): 1.5 m
▪
Block coefficient:
0.653

Main Particulars
▪
Length (BP):
27.6 m
▪
Beam (each hull):
2.97 m
▪
▪
Block coefficient:
0.653
IF. Sep 22
IF. Sep.42
IF. Sep.62
0,55

0,45
Demihull separation
▪
2.75 m (22), 5.25 m (42)
and 7.75 m (62):
  0.9..2.6 B.
IF
0,35
0,25
0,15
0,05
-0,05
Significant interference effects
-0,15
0,1
0,2
0,3
0,4
0,5
Fn
0,6

Hull Resistance
▪
In most cases, numerical errors are lower than 5.0% (max. 7.2%).
9000
8000
Resistance (gf)
7000
Exp.
Hump & hollow
behavior well
described.
CFD
6000
5000
4000
3000
2000
Unable to resolve
wave-breaking.
1000
0
0,25
0,3
0,35
0,4
0,45
Fn

Free surface elevations
FN = 0.332
0,03
Exp.
0,02
CFD
Wave Elevation
0,01
0
FN = 0.389
0,03
-0,01
Exp.
CFD
0,02
-0,02
0,01
-1
-0,5
0
0,5
1
1,5
x-position
Wave Elevation
-0,03
2 0
2,5
3
3,5
-0,01
0,04
-0,02
0,03
FN = 0.430
Exp.
CFD
0,02
-0,03
-1
-0,5
0
0,5
1
1,5
x-position
Wave Elevation
0,01
-0,04
2 0
2,5
3
3,5
-0,01
-0,02
Good correlation upstream
and along the hull.
-0,03
-0,04
-1
-0,5
0
0,5
1
1,5
2
2,5
x-position
3
3,5

Objective
▪

Investigate the performance of passive damping foils on heave
response of a catamaran  viscous damping coefficient.
Main Particulars
▪
Length (BP):
27.6 m
▪
Beam (each hull):
2.97 m
▪
▪
Block coefficient:
0.653
Passive damping foil.

Heave Response
Without Damping Foil
With Damping Foil

Heave Response
Without Damping Foil
With Damping Foil

Objective
▪

Develop a CFD model to study the effectiveness of passive damping
devices on heave motions of mono-column platforms.
Vertical Circular Cylinder

External dia.:
 Moonpool dia.:

110 m
50 m
Central Moonpool

Devised to improve response
in waves.
External skirt: damping device

Free Decay Simulation: Original Skirt
Validation: Original Skirt
Decay period: good correlation!
Vertical displacement [Norm.]

Over-estimated amplitude: numerical
simulation did not include the damping
effect of mooring lines, risers, etc.
Numerical (CFD)
Experimental
Time [s]

Free Decay Simulation: Alternative Skirt Geometry
Alternative B
CFD PROJECTS – Seft-propulsion Test

Objective
▪

Focus
▪

Develop a CFD model dedicated to estimate the propulsion factors and
to simulate the self-propulsion test of a hull.
Design applications.
Main Particulars:
▪
Length (Loa):
73.4 m
▪
Length (Lpp):
70.6 m
▪
Breath (B):
14.8 m
▪
Design draught (T):
2.6 m
▪
Service Speed (VS):
9.5 knt

Hull Performance
▪ Test speed (VS):
9.5 knt
▪ Total resistance (RT):
50.6 kN
▪ Wake coefficient (w):
0.153

Test Results
▪ Propeller revolutions (N):
433 rpm
▪ Propeller thrust (Treq):
65.3 kN
N = 420 rpm

Results Evaluation
▪ Comparison against statistical estimation.
▪ Wake fraction, thrust deduction fraction and relative-rotative efficiency
predictions based on Holtrop & Mennen (1984).
Statistical
Numerical
Dif.
Propeller Revolutions
456
433
-5.2%
rpm
Propeller Thrust
70.4
65.3
-7.8%
kN
Wake Fraction
0.181
0.153
-16.7%
---
Thrust Deduction Fraction
Relative-rotative Efficiency
0.243
1.028
0.184
1.024
-32.3%
-0.4%
-----
FINAL REMARKS

The overall performance achieved suggests that the CFD
numerical models were able to resolve the physics of the
flow around vessel/platform hulls.

The comparison against experimental results showed that
the numerical models were able to provide reasonable
performance predictions, suggesting that designers can rely
on CFD models as an effective design tool.

CFD PROJECTS – Heave Response

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