SNAME-2008-p

Hydrodynamics Analysis of Ships Side by Side in Waves using AQWA and Resistance and Diffraction Simulation over a Ship Hull using ANSYS-CFD

Hydrodynamics Analysis of Ships Side by Side in Waves using AQWA and Resistance and Diffraction Simulation over a Ship Hull using ANSYS-CFD

Franz Zdravistch, Ph.D.Technical Account ManagerFranz Zdravistch, Ph.D.Technical Account Manager

2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Technical Account ManagerANSYS Inc.Technical Account ManagerANSYS Inc.

OutlineOutlineOutlineOutline

Hydrodynamic analysis of ships side by side in Hydrodynamic analysis of ships side by side in Hydrodynamic analysis of ships side by side in Hydrodynamic analysis of ships side by side in

wavewavewavewaves

Introduction to modeling ships side by side Theoretical background of potential flow Numerical examples and discussion


Resistance and Diffraction Simulation over a Ship Resistance and Diffraction Simulation over a Ship Resistance and Diffraction Simulation over a Ship Resistance and Diffraction Simulation over a Ship

Hull using ANSYSHull using ANSYSHull using ANSYSHull using ANSYS----CFDCFDCFDCFD

RANS CFD Solver: ANSYS-FLUENT DTMB 5415 geometry description Resistance Test case Steady Resistance Test case

ConclusionsConclusionsConclusionsConclusions

Introduction (1)

Motivation


Offshore LNG offloading system(M. Naciri, OMAE 2007)

Replenishment-at-sea

Operational condition personnel and structural safety

AnalysisRelative motions,mooring forces, etc

under wave, wind, current (forward speed)

Introduction (2)

Difficulty: Standing waves between the gapIncident wave(a = 1.0m, = -450 )

Causes: Resonant fluid motion

in restricted region, Unrealistically enlarged


Diffraction wavea(max)=2.2m

by ideal fluid theory.

Consequences: Inaccurate RAO results,Divergent in time domain

Introduction (3)

Methods for suppression of standing waves

Potential theory, boundary integration approach,Fictitious lid elements on free-surface between gap

Rigid lid (Huijsmans et al, 2001)


Rigid lid (Huijsmans et al, 2001)

Flexible lid with defined modal shapes(Newman, 2004)

Free surface damper lid(Chen, 2004)

used in this case

Lid elements

Theoretical background (1)

AssumptionIdeal fluid, irrotational and incompressibleSmall wave elevation

Governing equationsLaplace equation in fluid regionBody boundary conditionFar field radiation condition,


Far field radiation condition,Seabed conditionFree surface condition

Boundary integration approach

with pulsating source Greens function,S: wetted hull surface only

dszyxGzyxs

),,;,,(41),,( pi

=

Wetted surface under water(in blue colour)

Theoretical background (2)

Free surface damper lidConventional linear free surface condition

Absorbing beach in non-linear time domain

02

=

gze


Absorbing beach in non-linear time domain

Damped free surface condition on lid

),()(21

),(

e

e

gDtDDtD

+=

= xxx

Damping factorWetted hull surface with lid elements(in blue colour)

0)( 22

=+ i

gz

Numerical calculation and

Discussions (1) Kodan Model

3.1 Kodan ModelModel test: Conventional ship with a rectangular barge (Kodan,1984)


Ship: Lpp =2.085m, dR =0.131m; Barge: Lpp =3.125m, dR =0.113m; PL=1.2m

Motions and forces were measured (Fn=0.0)

Principal dimensions only were known, estimated body plans used for numerical calculation


Discussions (2) Kodan model

Damping factor effects on resonant response (standing wave) (=0.72rad/s, =-450)


Damping lid suppresses waves Proper damping factor needed

Amplitude of diffraction wavewithout suppression,scales to 2.5m, for 1m incident wave

=0.01

=0.1



Damping factor effects on diffraction waves (=0. 45rad/s, =-450)


=0.01

=0.1

Amplitude of diffraction wavewithout suppression,Scale=1.2m, for 1m incident wave

Damping lid suppresses waves, Wave pattern keeps unchanged, Amplitude changes, but not big

as at standing wave frequency



Damping factor effects on wave exciting forces

0.2

0.3

0.4

A

W

R

hydro-int non-inter vlid=0.01vlid=0.02 vlid=0.1 test(Kodan, 1984)

0.6

0.9

1.2

g

A

W

R



Hydrodynamic interaction is evident standing wave is due to this interaction =0.01 gives closer results =0.1 over-damped the wave exciting forces at standing wave frequency

0.0

0.1

0.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

( **2/g)dR

F

2

/

g

0.0

0.3

0.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(**2/g)dRF

3

/

g



Damping factor effects on ship motions

0.4

0.6

0.8

1

S

w

a

y

/

hydro-int non-inter plid=0.01plid=0.02 plid=0.1 test(Kodan, 1984)

0.6

0.9

1.2

H

e

a

v

e

/



Hydrodynamic interaction is evident increases, RAOs at standing wave frequency decrease Hull viscous damping not included => =0.1 is closer because force over-damped

0

0.2

0.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(**2/g)dR

S

w

a

y

/

0

0.3

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(**2/g)dRH

e

a

v

e

/

Resistance and Diffraction Simulation over

a Ship Hull: Mathematical Description

Governing equations:( ) 0v

t=+

r

( ) ( ) ( ) +=+ pvvvt

rrr

vr

( ) + Ivvv T rrr 2: velocity vector in the Cartesian coordinate system

The stress tensor is given by

Mass conservation:

Momentum conservation:

p: static pressurewhere is molecular viscosity


( ) + Ivvv 3The stress tensor is given by where is molecular viscosity After Reynolds averaging the above equations can be written as

( ) 0uxt ii

=

+

( ) ( )jij

i uux

ut

+

+

+

=

l

lij

i

j

j

i

ji x

u

x

u

x

u

xx

p 32 ( )ji

juu

x

+

the Reynolds stresses iji

it

i

j

j

itji

x

uk

x

u

x

uuu

+

+

=

32

''

Interface tracking between the phases is achieved by solving a continuity equation for the volume fraction of each one of the phases (VOF method)

RANS CFD solver: ANSYS-FLUENT

Works based on cell centered finite volume discretization schemes

Works with structured and unstructured (tetrahedral, prism, polyhedral) and hybrid mesh topologies


prism, polyhedral) and hybrid mesh topologies

General purpose CFD solver with many physical models and turbulence models

DTMB 5415

DTMB 5415 : Geometry description Conceived as a preliminary design for a Navy Surface combatant The hull geometry includes a sonar dome and transom stern There is a large EFD database for Model 5415 due to a current

international collaborative study on EFD/CFD and uncertainty


assessment

Reference http://www.nmri.go.jp/cfd/cfdws05/index.html

Resistance: Computational Grid

Outlet

Inlet

Hexahedral mesh with 1.8 Million cells

Half domain modeled to exploit


symmetry

The ship is fixed i.e. all the 6 degrees of freedom are off

Average wall Y+ is 36.5

Resistance: Problem description

Ship Length, Lpp = 5.72 m

Ship speed = 2.1 m/s (Froude Number = 0.28) Fixed attitude


Ship moving in calm water

Resistance: Simulation setup

Turbulence models Realizable k-e SST k-omega

Open channel flow

Boundary Conditions


Boundary Conditions Inlet boundary: Pressure-inlet outlet boundary: pressure-outlet Side, center, top and bottom: symmetry

Discretization schemes Modified HRIC for VOF Second order upwind for momentum and turbulence SIMPLE pressure-velocity coupling in FLUENT

Resistance: Wave Elevation Contours


Kelvin wave pattern predicted by ANSYS-FLUENT simulation (filled contours)

Resistance: Wave Elevation Contours


Kelvin wave pattern predicted by FLUENT simulation (contour lines)

Resistance: Wave Profile and Forces

-0.005

0

0.005

0.01

Z

/

L

p

p

EXP SST RKE

-0.005

0

0.005

0.01

0.015

0.02

Z

/

L

p

p

EXP SST RKE


-0.01-0.5 0.0 0.5 1.0 1.5

X / Lpp

-0.01-0.5 -0.25 0 0.25 0.5

X / Lpp

Expt. SST RKE

[N] [N] % diff. [N] % diff.

Total Drag 45.08 43.90 2.6 42.45 5.8

Viscous Drag 30.69 30.99 0.9 29.90 2.5

Wave profile along y/Lpp = 0.172 plane Wave profile along the hull

Diffraction: Computational Grid

Hexahedral mesh with 3 Million cells

Half domain modeled to exploit symmetry

Damping zone to apply numerical beach condition

OutletDamping zone

Inlet


beach condition

Constant mesh size in the flow direction from inlet to the bow, to preserve the incoming wave form

The ship is fixed all the 6 degrees of freedom are off

Inlet

Diffraction: Problem description

Ship Length, Lpp = 3.048 m

Ship speed = 1.53 m/s (Froude Number = 0.28) Fixed attitude, moving into incoming head sea waves


Wave length = 4.572 m

Wave height = 0.018 m

Resulting encounter period, Te = 1.088 sec

Resulting encounter velocity, Ve = 4.2 m/s

Diffraction: Boundary Conditions

( )[ ]( ) ( )nnnynxn n

nnn tykxkhk

hzkAv

u

+

+=

=

cossincos

coshcosh

1

( )[ ]( ) ( )nnnynxnnn tykxkhk

hzkAw ++=

sincosh

sinh

Incoming wave boundary condition


( ) nnnynxnnn n hk= cosh1coskkx = sinkk y =where the wave numbers in x-y directions are:

h: calm water tank depthA: wave amplitude : wave heading: wave frequency

Reference: Kim, M.H., Niedzwecki, J.M., Roesset, J.M., Park, J.C., Hong, S.Y., and Tavassoli, A., Fully Nonlinear Multidirectional Waves by a 3-D Viscous Numerical Wave Tank, ASME J. Offshore Mecahnics and Arctic Eng., Vol. 123, August 2001

Diffraction: Simulation Setup

SST k-omega turbulence model Open channel flow Boundary Conditions

Inlet boundary: Pressure-inlet outlet boundary: pressure-outlet Side, center, top and bottom: symmetry


Side, center, top and bottom: symmetry Wave bc: through user defined function (udf) Numerical beach condition at the outlet: through udf

Discretization schemes Modified HRIC for VOF Second order upwind for momentum and turbulence First order time accuracy SIMPLE pressure-velocity coupling in FLUENT

Diffraction: Wave Elevation Contours

Incoming waves Waves dampened due to numerical beach conditionShip hull


Wave elevation contours coloured by wave height, seen from top view

Diffraction: Wave elevation contours


Wave elevation contours coloured by wave height, diffracted waves



Wave pattern along the ship hull, with transparent free-surface



Experiment ANSYS-FLUENT

Diffraction: Forces & moment

0

0.002

0.004

0.006

0.008

0.01

0.012

C

d

EXP CFD

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

C

h

EXP CFD


-0.0020 0.5 1 1.5 2 2.5 3

t / Te

Drag Force coefficient (Cd) Heave Force coefficient (Ch)

Moment coefficient (Cm)

-0.10 0.5 1 1.5 2 2.5 3

t / Te

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0 0.5 1 1.5 2 2.5 3

t / Te

C

m

EXP CFD

Conclusions (1)

Side-by-side ships floating in waves Standing wave (resonant response of fluid in restrict region)

exists; Its amplitude needs to be damped if using potential theory Free surface damping lid method is an applicable/reliable


Free surface damping lid method is an applicable/reliable approach;

Damping factor on lid is about 0.01, but more experimental data needed.

Conclusions (2)

The RANS CFD solver ANSYS-FLUENT is used to validate resistance and diffraction tests

The resistance simulation was performed using SST k-w and Realizable k-e turbulence models and the SST model found to give better results

The resistance drag predictions were of the order of 0.9% to 5.8% error


The diffraction simulation results show good qualitative comparison in terms of the wave elevation contours

The diffraction force predictions show phase difference and error in the peak force predictions, one of the reasons for the discrepancy could be first order time accuracy

Overall results show good comparison with the experimental data for a real life application

Conclusions (3)

Both AQWA and ANSYS-CFD provide useful and complementary design information AQWA simulations much faster than CFD. Allows for preliminary

evaluation of larger number of design options CFD simulations provide more detailed physics, including viscous

effects


Currently working on integrating AQWA-Suite and ANSYS-CFD: Couple potential flow and viscous effects (where needed) for increased

accuracy and efficiency Use a unified environment (Workbench) for case set up, execution and

post-processing

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