Windsor Guanabara, Rio de Janeiro/RJ Brasil 13 de Junho de ... · predictions based on Holtrop & Mennen (1984). CFD PROJECTS

Engenharia Naval e Oceânica

COPPE/UFRJ & EP/UFRJ

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


COPPE/UFRJ & EP/UFRJ 2013 CAE NAVAL & OFFSHORE – Windsor Guanabara, Rio de Janeiro/RJ – Brasil

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.



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.

Catamaran/SWATH concept has been received special attention

good performance in terms of speed and transversal stability.

Objective

▪ 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





Demihull separation

▪ 2.75 m (22), 5.25 m (42)

and 7.75 m (62):

0.9..2.6 B.

Significant interference effects

-0,15

-0,05

0,05

0,15

0,25

0,35

0,45

0,55

0,1 0,2 0,3 0,4 0,5 0,6

IF

Fn

IF. Sep 22

IF. Sep.42

IF. Sep.62




Hull Resistance

▪ In most cases, numerical errors are lower than 5.0% (max. 7.2%).

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0,25 0,3 0,35 0,4 0,45

Re

sis

tan

ce

(g

f)

Fn

Exp.

CFDHump & hollow

behavior well

described.

Unable to resolve

wave-breaking.




Free surface elevations

Good correlation upstream

and along the hull.

-0,03

-0,02

-0,01

0

0,01

0,02

0,03

-1 -0,5 0 0,5 1 1,5 2 2,5 3 3,5

Wave E

levati

on

x-position

Exp.

CFD

-0,04

-0,03

-0,02

-0,01

0

0,01

0,02

0,03

-1 -0,5 0 0,5 1 1,5 2 2,5 3 3,5

Wave E

levati

on

x-position

Exp.

CFD

-0,04

-0,03

-0,02

-0,01

0

0,01

0,02

0,03

0,04

-1 -0,5 0 0,5 1 1,5 2 2,5 3 3,5

Wave E

levati

on

x-position

Exp.

CFD

FN = 0.389

FN = 0.430

FN = 0.332



CFD PROJECTS – Heave Response

Objective

▪ Investigate the performance of passive damping foils on heave

response of a catamaran viscous damping coefficient.

Main Particulars





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.: 110 m

Moonpool dia.: 50 m

Central Moonpool

Devised to improve response

in waves.

External skirt: damping device




Free Decay Simulation: Original Skirt




Validation: Original Skirt

Time [s]

Ve

rtic

al d

isp

lace

me

nt

[Norm

.]

Decay period: good correlation!

Over-estimated amplitude: numerical

simulation did not include the damping

effect of mooring lines, risers, etc.

Numerical (CFD)

Experimental



Free Decay Simulation: Alternative Skirt Geometry

Alternative B




Objective

▪ Develop a CFD model dedicated to estimate the propulsion factors and

to simulate the self-propulsion test of a hull.

Focus

▪ 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

CFD PROJECTS – Seft-propulsion Test



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 0.243 0.184 -32.3% ---

Relative-rotative Efficiency 1.028 1.024 -0.4% ---



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.

FINAL REMARKS

Documents

Windsor Guanabara, Rio de Janeiro/RJ Brasil 13 de Junho de ... · predictions based on Holtrop & Mennen (1984). CFD PROJECTS