SNAME-2008-p
description
Transcript of 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
2008 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary
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)
2008 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary
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,
2008 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary
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)
2008 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary
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
-
Numerical calculation and
Discussions (2) Kodan model
Damping factor effects on resonant response (standing wave) (=0.72rad/s, =-450)
2008 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary
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
-
Numerical calculation and
Discussions (3) Kodan model
Damping factor effects on diffraction waves (=0. 45rad/s, =-450)
2008 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary
=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
-
Numerical calculation and
Discussions (4) Kodan model
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
hydro-int non-inter vlid=0.01vlid=0.02 vlid=0.1 test(Kodan, 1984)
2008 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary
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
-
Numerical calculation and
Discussions (5) Kodan model
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
/
hydro-int non-inter vlid=0.01vlid=0.02 vlid=0.1 test(Kodan, 1984)
2008 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary
( ) + 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
2008 ANSYS, Inc. All rights reserved. 14 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary
Ship moving in calm water
-
Resistance: Simulation setup
Turbulence models Realizable k-e SST k-omega
Open channel flow
Boundary Conditions
2008 ANSYS, Inc. All rights reserved. 18 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 19 ANSYS, Inc. Proprietary
Kelvin wave pattern predicted by ANSYS-FLUENT simulation (filled contours)
-
Resistance: Wave Elevation Contours
2008 ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 21 ANSYS, Inc. Proprietary
-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
2008 ANSYS, Inc. All rights reserved. 22 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 23 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 24 ANSYS, Inc. Proprietary
( ) 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
2008 ANSYS, Inc. All rights reserved. 25 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 26 ANSYS, Inc. Proprietary
Wave elevation contours coloured by wave height, seen from top view
-
Diffraction: Wave elevation contours
2008 ANSYS, Inc. All rights reserved. 27 ANSYS, Inc. Proprietary
Wave elevation contours coloured by wave height, diffracted waves
-
Diffraction: Wave elevation contours
2008 ANSYS, Inc. All rights reserved. 28 ANSYS, Inc. Proprietary
Wave pattern along the ship hull, with transparent free-surface
-
Diffraction: Wave elevation contours
2008 ANSYS, Inc. All rights reserved. 29 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 30 ANSYS, Inc. Proprietary
-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
2008 ANSYS, Inc. All rights reserved. 31 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 32 ANSYS, Inc. Proprietary
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
2008 ANSYS, Inc. All rights reserved. 33 ANSYS, Inc. Proprietary
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