hydrodynamics analysis of ships side by-cfd.pdf

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 gap

Incident wave(a = 1.0m,β = -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 Green’s function,S: wetted hull surface only

dszyxGzyxs

),,;,,(41

),,( ζηξσπ

ϕ ∫∫=

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

=−∂∂ φωφ

gze


Absorbing beach in non -linear time domain

Damped free surface condition on lid

),()(21

),(

e

e

gDt

DDt

D

φφνφφηφ

νφ

−−∇⋅∇+−=

−−∇= xxx

Damping factor

Wetted hull surface with lid elements

(in blue colour)

0)( 22

=−+∂∂ φαωφ

igz

Numerical calculation and

Discussions (1) Kodan Model

3.1 Kodan ModelModel test: Conventional ship with a rectangular barge (Kodan,1 984)


Ship: Lpp =2.085m, d R =0.131m; Barge: Lpp =3.125m, d R =0.113m; P L=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 (standi ng wave) (ω=0.72rad/s, β=-450)


► Damping lid suppresses waves► Proper damping factor needed

Amplitude of diffraction wave

without suppression,

scales to 2.5m, for 1m incident wave

α=0.01

α=0.1



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


α=0.01

α=0.1

Amplitude of diffraction wave

without 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

ζζ ζζAW

R

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

0.6

0.9

1.2

gζζ ζζA

WR

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


► Hydrodynamic interaction is evident <=> standing wave is due to this interaction► α=0.01 gives closer results► α=0.1 over-damped the wave exciting forces at sta nding wave frequency

0.0

0.1

0.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(ωωωω **2/g)dR

F2

/ ρρ ρρg ζζ ζζ

0.0

0.3

0.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(ωωωω**2/g)dR

F3

/ ρρ ρρg



• Damping factor effects on ship motions

0.4

0.6

0.8

1

Sw

ay /

ζζ ζζ

hydro-int non-inter plid=0.01

plid=0.02 plid=0.1 test(Kodan, 1984)

0.6

0.9

1.2

Hea

ve /

ζζ ζζ

hydro-int non-inter vlid=0.01

vlid=0.02 vlid=0.1 test(Kodan, 1984)


► Hydrodynamic interaction is evident► α increases, RAOs at standing wave frequency decrease► Hull viscous damping not included => α=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

(ωωωω**2/g)dR

Sw

ay /

0

0.3

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(ωωωω**2/g)dRH

eave

/

Resistance and Diffraction Simulation over

a Ship Hull: Mathematical Description

• Governing equations:

( ) 0vt

=⋅∇+∂∂ rρρ

( ) ( ) ( )τρρ ⋅∇+−∇=⋅∇+∂∂

pvvvt

rrr

vr

( )

⋅∇−∇+∇≡ Ivvv T rrr 2µτ

: velocity vector in the Cartesian coordinate system

The stress tensor is given by

Mass conservation:

Momentum conservation:

p: static pressure

where µµµµ is molecular viscosity


( ) ⋅∇−∇+∇≡ Ivvv

3µτThe stress tensor is given by where µµµµ is molecular viscosity

• After Reynolds averaging the above equations can be written as

( ) 0uxt i

i

=∂∂+

∂∂ ρρ

( ) ( )jij

i uux

ut

ρρ∂∂+

∂∂

∂∂

−∂∂

+∂∂

∂∂+

∂∂−=

l

lij

i

j

j

i

ji x

u

x

u

x

u

xx

p δµ3

2 ( )jij

uux

′′−∂∂+ ρ

the Reynolds stresses iji

it

i

j

j

itji x

uk

x

u

x

uuu δµρµρ

∂∂

+−

∂∂

+∂∂

=−3

2''

• 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 /

Lpp

EXP SST RKE

-0.005

0

0.005

0.01

0.015

0.02

Z /

Lpp

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

( )[ ]( ) ( )nnnynx

n n

nnn tykxk

hk

hzkA

v

uεω

θθ

ω −−+×

+=

∑

∞

=

cossin

cos

cosh

cosh

1

( )[ ]( ) ( )nnnynx

nnn tykxk

hk

hzkAw εωω −−++=∑

∞

sincosh

sinh

• Incoming wave boundary condition


( ) nnnynxn

nn

n hk∑= cosh1

θcoskkx = θsinkk y =where the wave numbers in x-y directions are:

h: calm water tank depthA: wave amplitudeθ : wave headingω: 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

Cd

EXP CFD

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

Ch

EXP CFD


-0.002

0 0.5 1 1.5 2 2.5 3

t / Te

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

Moment coefficient (Cm)

-0.1

0 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

Cm

EXP CFD

Conclusions (1)

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

exists;

• Its amplitude needs to be damped if using potential theory

• Free surface damping lid method is an applicable/re liable


• Free surface damping lid method is an applicable/re liable approach;

• Damping factor on lid is about 0.01, but more exper imental data needed.

Conclusions (2)

• The RANS CFD solver ANSYS-FLUENT is used to validat e 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 b etter results

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


• The diffraction simulation results show good qualit ative comparison in terms of the wave elevation contours

• The diffraction force predictions show phase differ ence 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 exper imental 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 p reliminary

evaluation of larger number of design options– CFD simulations provide more detailed physics, incl uding viscous

effects


• Currently working on integrating AQWA-Suite and ANSYS-CFD:

– Couple potential flow and viscous effects (where ne eded) for increased accuracy and efficiency

– Use a unified environment (Workbench) for case set up, execution and post-processing

hydrodynamics analysis of ships side by-cfd.pdf

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