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© 2011 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2011 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
Phil Stopford
ANSYS UK
Engineering Simulation
Software for the Offshore,
Marine and Wave/Tidal
Renewable Energy
Industries
Viscous CFD Applications
© 2011 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
• Introduction to viscous CFD
• CFD capabilities
• Offshore and marine applications
– Hydrodynamic characterisation/loading
– Motion response
– Vortex-induced vibration
– Added mass and damping
– Two-way fluid structure interaction
• Wind/Tidal renewable energy applications
– Oscillating water columns
– Tidal turbines
– Wind farm layouts
• Summary
Agenda
© 2011 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary
Fluid dynamics
• Complex and sometimes non-intuitive
• Depends on the interaction of multiple
features
1m/s 1m/s
Which situation will see the
highest velocity?
A B
© 2011 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary
Results
Velocity (m/s) Pressure Field
Adverse
pressure
gradient
~60% higher
© 2011 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary
What is CFD?
• Computational Fluid Dynamics (CFD)…
• Flow simulation allows a prototype to be modelled on
the PC workstation
– Complementing physical testing
• CFD can be used on
– Any geometry at any scale
– Most flow physics including free surfaces and motion
…is the science of predicting fluid flow, heat transfer,
mass transfer and related phenomena by solving the
mathematical equations which govern these physical
processes, using a numerical approach (i.e. on a
computer) including viscous effects
© 2011 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary
Introduction to CFD applications
• Many applications for fluid flow analysis with viscous
CFD
– Hydrodynamic characterisation and loading of floating and
submerged hull forms, structures and devices
• Viscous drag, form drag
• Wave-making, sea-keeping
– Motion response
• Vortex induced vibration
– Added mass and damping analysis
– Tidal turbine hydraulic performance
– Tidal/wind turbine farm layout and wake effects
– Providing fluid loading results for fluid-structure-interaction
assessment
© 2011 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary
• CFD capabilities for offshore, marine and wave/tidal– Flow visualisation
– Quantitative information• Pressures, velocities, ...
• Viscous/pressure forces, drag, lift, ...
– Free surface models• Simple wave generation
• Wave/body interactions
– Dynamic response• Rigid body 6-DOF solutions
• Added mass and damping calculations
– Tidal/Wind turbine-specific tools• Rotating and stationary components
• Performance and power extraction
• Cavitation modelling
CFD capabilities
© 2011 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary© 2011 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary
Hydrodynamic
characterisation
© 2011 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary
Hydrodynamic Characterisation
and Loading
• Viscous CFD provides a way to
characterise the overall forces
on a floating or submerged
body
• Viscous and form drag
• 5415 Destroyer test case
• At 4.03 knots
• Drag
• CFD 43.9 +/- 2 N
• Experiment 44.3 N
© 2011 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary
Free-surface flows
• Wigley Hull test-case
– Validation of ANSYS CFD capability
to calculate wave structure for an
analytical hull shape
– Excellent agreement with experiment
© 2011 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary
Racing Yacht CFD
• Racing yacht geometry at model scale• Fully appended with rudders, keel and bulb
• Different speeds give different hull orientation
© 2011 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary
Drag vs Side Force
20 Heel & 14 knots
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 10000 20000 30000 40000 50000 60000 70000 80000
Side Force [N]
To
tal
Dra
g [
N]
CFX
Experiment
Validation: Racing Yacht
Side Force vs. Yaw Angle
20 Heel & 14 knots
0
10000
20000
30000
40000
50000
60000
70000
80000
-3 -2 -1 0 1 2 3 4 5
Yaw [deg]
Sid
e F
orc
e [
N]
CFX
Tank
• Forces at 20 heel• Constant speed
• Variation of yaw angle
• Good agreement
© 2011 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary
Transient wave-loading with CFD
• CFD can be used to
look at transient
loadings on structures
– Extreme wave events
– Peak load transfer to
ANSYS Mechanical model
– Automated 1-way transfer
of load from ANSYS CFD
to Mechanical within
Workbench
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Motion response
© 2011 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary
• It is also possible to use viscous CFD to understand– The effect of geometry motion on
fluid flow (a prescribed motion)
– Geometry motion due to fluid flow and resulting loads (a flow-driven motion)
• All this can be done in ANSYS CFD software if moving solids are:– Rigid bodies
– Have deformations that are simple to describe in the CFD software
CFD simulations with moving bodies
Prescribed motion
Flow-driven motion
© 2011 ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary
• Dynamic Sink and Trim• Speedboat example
• Six degree of freedom
• Free surface flow
Rigid Body CFD Solution
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• Dynamic Sink and Trim• Mono-chromatic waves
generated at inlet by simplelinear theory
• Pitch and heave from6-DOF solution
Rigid Body CFD Solution
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Mooring example
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Mooring Model
• The addition of mooring lines as part of a CFD
calculation is now possible
– Complementing AQWA capability
• Simple Spring-Damper model for tethers
• Includes capability to have multiple mooring points
• Implemented to allow for 3D cases
k x
c x. F = k x + c x
.
Moving
Body
Force Applied By Mooring
© 2011 ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary
Model Setup
For each mooring:
• Specify an arbitrary mooring point, (x,y,z)
• Provide an initial location for the attachment point on the
moving body, (x,y,z)
• Input values for stiffness and damping coefficients, k and c
• Set the length of the tether, L
Mooring
Point
Attachment
Point
L, k, c
© 2011 ANSYS, Inc. All rights reserved. 21 ANSYS, Inc. Proprietary
Mooring example
• Two moorings defined
Open Channel
Flow Direction
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Vortex Induced Vibration
© 2011 ANSYS, Inc. All rights reserved. 23 ANSYS, Inc. Proprietary
Vortex induced vibrations
• An important topic for the
offshore industry
– Offshore platforms need to be placed in
more and more hostile environments
• A challenging fluid-structure interaction
(FSI) application
– Complex response of riser, etc to ocean waves
and currents
– Length to diameter ratios of order 103
– Reynolds numbers of order 104
• Several simulation approaches of varying
complexity
– CFD with embedded rigid-body mechanics
– CFD with coupling to flexible structural mechanics
An offshore platform
near Sakhalin
(Russia)
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Simple 2-D VIV
• Use 2-D CFD with 2 degrees of freedom and numerical tethers to understand riser motion
– Computationally inexpensive
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More advanced methods for VIV
• Strip theory
– Fluid flow fields are computed in
multiple two-dimensional planes
positioned along the riser
– Computationally cheaper than full
3-D CFD
– Doesn’t take into account three-
dimensional flow features
– Only resolves flow forces at specific
locations
– Potentially useful methodology for
coupling ANSYS CFD to beam (riser
specific) structural simulation software
© 2011 ANSYS, Inc. All rights reserved. 26 ANSYS, Inc. Proprietary
More advanced methods for VIV
• Full 3-D CFD simulations dynamically to structural
simulations (ANSYS Mechanical)
• Two-way fluid-structure-interaction– Fluid flow field is computed with a full 3D CFD model
– CFD results passed to ANSYS Mechanical as loads
– ANSYS Mechanical calculates deformation and passes
geometry displacement back to ANSYS CFD
– Computationally expensive but shows potential
© 2011 ANSYS, Inc. All rights reserved. 27 ANSYS, Inc. Proprietary
Experimental Set-up
• Delta Flume in Holland
• Inlet velocity: 0.16m/s
• Top tension: 405N
• Bending stiffness: 29.9NM2
• Axial stiffness: 5.88MN
• Structural dumping: 0.33%
• Mass ratio relative to the surrounding water: 3
• Riser diameter: 28mm
• Length to diameter ratio ~470
• Submerged part: 42.5% of the riser length
• Re ~5000
Water surface
inside the
vacuum tank
Water surface
in the flume
Vacuum tank
13.12m
Riser
Cabin
Incident
velocity
profile at the riser
1195
from Chaplin et al. (2004)
© 2011 ANSYS, Inc. All rights reserved. 28 ANSYS, Inc. Proprietary
ANSYS CFD Set-up
top surface
(free slip)
tank walls
(no slip)
water surface
(free slip)
outlet
Floor
(free slip)
Computational domain and
boundary conditions
Fluid/solid interface
• The case was run as laminar
• Time discretization scheme: Second Order Backwards Euler
• Spatial discretization scheme: Second Order Upwind
• Convergence criterion:
10-5 for RMS Residual
• Maximum number of coefficient loops: 10
inlet
© 2011 ANSYS, Inc. All rights reserved. 29 ANSYS, Inc. Proprietary
F=405N (applied to the
central node)No movement in xz
No constraint in y
No movement in xz
No constraint in y
No movement in xyz
- All nodes but
central
- central node
Boundary conditions
Fluid/solid interface
ANSYS Structural Set-up
• The nonlinear transient solver (i.e. the large displacement transient option) was used
• Riser was modeled as a solid cylinder with Solid185 elements (3D 8-node structural solid)
• Young's modulus of 9.55 109Pa was chosen to match the axial stiffness
• Ramped loading was switched off
© 2011 ANSYS, Inc. All rights reserved. 30 ANSYS, Inc. Proprietary
View of Flow Structure
A view of vorticity field at different heights. Red and blue colours
represent positive and negative vorticity respectively
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Results – Riser Motion
Moving riser started from rest…
© 2011 ANSYS, Inc. All rights reserved. 33 ANSYS, Inc. Proprietary© 2011 ANSYS, Inc. All rights reserved. 33 ANSYS, Inc. Proprietary
Added-Mass and
Damping Calculation
© 2011 ANSYS, Inc. All rights reserved. 34 ANSYS, Inc. Proprietary
Added Mass and Damping
• When simulating floating bodies, or mooring systems,
some 3D-panel method codes and multi-body
dynamics codes require additional coefficients in
order to get an accurate response.
• The effect of these coefficients is implicitly included in
full CFD analyses
• Sometimes coefficients can be estimated for simple
geometries
• For complex geometry we can calculate them quickly
using CFD
© 2011 ANSYS, Inc. All rights reserved. 35 ANSYS, Inc. Proprietary
Added Mass and Damping
• Perform transient simulation with prescribed
sinusoidal motion, e.g. heave, sway, and look at
variation with amplitude and frequency
• Examine reaction force response of the structure and
the phase change (compared to displacement)
• Coefficients obtained by extracting Fourier
coefficients of the fundamental frequency over a time
period
• Higher order components of coefficients could also
have been extracted using similar techniques based
on Fourier analysis
© 2011 ANSYS, Inc. All rights reserved. 36 ANSYS, Inc. Proprietary
Damping Example
• Lowering of structure to seabed
– Need added mass and damping for
accurate dynamics simulation
• Perform transient CFD calculation
on one mudmat
– Separate horizontal and vertical motion
prescribed
– Sinusoidal moving mesh
– Simulation duration of 3-5 cycles only
• Information courtesy of Saipem
(UK) Ltd
EniG R O U P
© 2011 ANSYS, Inc. All rights reserved. 37 ANSYS, Inc. Proprietary
Damping Example
• Same geometry and mesh can be
used for heave and sway
calculations
Hydrodynamic Forces in Heave
Direction (Inverted Can)
-5.E+05
-4.E+05
-3.E+05
-2.E+05
-1.E+05
0.E+00
1.E+05
2.E+05
3.E+05
0 20 40 60 80 100
Time (Seconds)
Fo
rce
(k
N)
© 2011 ANSYS, Inc. All rights reserved. 38 ANSYS, Inc. Proprietary
Damping Example
• Results analysed in CFD-Post
– Coefficients extracted from amplitude
and phase of reaction force plot
– Also examined:
• Effects of holes in geometry
• Effect of proximity to sea bed
Sway Motion
Heave MotionPeriod
(s)
Amplitude
(cm)
Calculated Value
(no holes)
Model Tests
(4 holes)
Added Mass in Heave
6.0
0.375
76.33
88.0
6.0
0.5
77.17
90.0
6.0
0.75
81.38
92.0
7.0
0.25
76.4
-
Period
(s)
Amplitude
(cm)
Calculated Value
(no holes)
Model Tests
(4 holes)
6.0
0.375
17.91
12.0
6.0
0.5
19.55
13.5
6.0
0.75
18.86
18.0
Damping in Heave
© 2011 ANSYS, Inc. All rights reserved. 41 ANSYS, Inc. Proprietary
Response Amplitude Operators (RAO)
• Comparison of CFD to Free Floating
Calm Buoy measurement
– Prescribed heave motion
– RAO calculated from Added Mass,
Damping and Restoring coefficients
Surge RAO
0
0.2
0.4
0.6
0.8
1
4 5 6 7 8 9 10
Full scale period (s)
RA
O (
m/m
)
Experimental CFD
Heave RAO
0
0.2
0.4
0.6
0.8
1
1.2
1.4
4 6 8 10 12
Full scale period (s)
RA
O (
m/m
)
Experimental CFD
Buoy model
Anchored
with three
mooring lines
LINE #1LINE #3
OPTICAL
SYSTEM
FOR
MEASUREMENT
OF BUOY
MOTION
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Transient two-way
fluid-structure-
interaction
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Transient Dynamics: Two-way FSI
• 2-Way Coupled Fluid Structure Interaction
– Motion of vessel calculated not prescribed
– Structural FEA code used to solve for vessel displacement
– Loads exchanged in both directions• Between CFD and FEA code
– More ‘coupled’ solution than 1-way• Introduce concept of ‘coupling convergence’
– Transient or steady-state• Single exchange per timestep – explicit
• Multiple exchange per timestep – implicit
– Important for strongly coupled problems
© 2011 ANSYS, Inc. All rights reserved. 45 ANSYS, Inc. Proprietary
Transient Dynamics: Two-way FSI
• Basic sea-keeping– Two-way FSI
– Fluid flow simulation in
ANSYS CFD• Waves generated as boundary
condition again
– Structural mechanics in
ANSYS FEA
– Examine slamming for
example, and stress
response
© 2011 ANSYS, Inc. All rights reserved. 46 ANSYS, Inc. Proprietary
Guest presentation
• BMT presentation
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Wave Energy:
Oscillating Water Column
Simulation
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OWC principle
• Waves in sea generate
oscillation in vertical duct
• Resonance occurs if duct
diameter and length are
carefully chosen
• Resonance can increase the
wave height significantly
• Cylinder can be on sea-bed,
or at surface
Reference: Lighthill, J., 1979, ‘Two-dimensional analyses related to wave-energy
extraction by submerged resonant ducts’, J. Fluid Mech, 91, part 2, 253-317.
© 2011 ANSYS, Inc. All rights reserved. 49 ANSYS, Inc. Proprietary
OWC example application
• Compression
chamber above
OWC
• Energy can be
harnessed (e.g. via
Wells turbine)
Source:http://news.bbc.co.uk/1/hi/sci/tech/1032148.stm
© 2011 ANSYS, Inc. All rights reserved. 51 ANSYS, Inc. Proprietary
Vertical cylinder on sea-bed
• Pressure distribution at
resonance
– Amplitude elevation in
cylinder
© 2011 ANSYS, Inc. All rights reserved. 52 ANSYS, Inc. Proprietary
Vertical cylinder
• Pressure time trace at monitoring points on seabed
– Point 1 inside cylinder, Point 2 outside cylinder
upstream
More resonant
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Wave-piercing design
• Air Pressure variation
inside cylinder
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Air movement through the hole at the
top of the OWC
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Tidal turbine
simulation using CFD
and one-way FSI
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Tidal turbine CFD
• CFD provides the ability to perform detailed hydraulic assessment of tidal turbine devices
• Quantitative results
– Blade loading
– Torque
– Axial thrust
– Power and efficiency
• Flow visualisation
– Streamlines
– Pressure, temperature, velocity plots
• It can show why a machine design is good or bad
– Where are the losses due to separation and swirl at certain operating conditions?
• CFD also provides pressure loads for structural mechanics
© 2011 ANSYS, Inc. All rights reserved. 57 ANSYS, Inc. Proprietary
Quantitive Analysis
Forces and Torque on Blades
• Resultant Force in X direction
– -3092.84 [N]
• Resultant Force in Y direction
– 60.0122 [N]
• Resultant Force in Z direction
– 911208 [N]
• Resultant Torque
– 725450 [Nm]
© 2011 ANSYS, Inc. All rights reserved. 58 ANSYS, Inc. Proprietary
Imported CFD pressure into
structural calculation
• Imported
Hydraulic
Forces from
CFD
calculation
applied to
structural
calculation
– One-way
FSI
© 2011 ANSYS, Inc. All rights reserved. 59 ANSYS, Inc. Proprietary
Structural mechanics
• Displacement
due to
centrifugal
and hydraulic
loading
– Calculated
in ANSYS
Mechanical
© 2011 ANSYS, Inc. All rights reserved. 60 ANSYS, Inc. Proprietary
Structural mechanics
• Stress
contours due
to centrifugal
and hydraulic
loading
© 2011 ANSYS, Inc. All rights reserved. 61 ANSYS, Inc. Proprietary
East River: Verdant Generation 5
Kinetic Hydropower Systems
• Effect of turbines in East River
• ‘Non-rotating units create small wake regions, especially behind the
pylon, pile, blades and tail cone. Very little flow acceleration is visible;
generally well above the river bottom
• The turbulent wake lead to regions of increased mixing and flow
disturbance, however, these regions are generally well above the river
bottom. The impact of the pile wake, which is near the river bottom, is
reduced by the lower water velocities in the fully developed turbulent
boundary layer.‘• Jonathan A Colby, Hydrodynamic Analysis of Kinetic Hydropower arrays,
http://www.theriteproject.com/uploads/VP_HydrodynamicAnalysisKHPS.pdf
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Wind and tidal turbine
farm layouts
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Site Specific Issues
• Trickle down – wind -> tidal
• WindModeller, vertical application based on ANSYS CFD
– Currently being extended to tidal flows
• Effect of geometry;
– Orography / Bathymetry
• Flow physics;
– Atmospheric
– Marine, free surface....
– Turbulence
– Inflow / ambient conditions
• Turbine interactions and the environment
– Resolved Turbines / Actuator Disk models
– Wakes, towers
sciencenw.com/uploads/horns_rev.jpg
© 2011 ANSYS, Inc. All rights reserved. 64 ANSYS, Inc. Proprietary
Site Specific Studies
• Geometry available in various
formats
• Point values: x,y,z csv
• Various GIS formats (.map,
NTF, Seazone)
• Point values of depth,
referred to LAT
• Digitised contours of
coastline
• Convert to STL
• Morph template mesh to
terrain for automation
• Or ANSYS AMP / ICEM CFD
© 2011 ANSYS, Inc. All rights reserved. 65 ANSYS, Inc. Proprietary
Offshore wind turbine wakes
Horizontal velocity Turbulence intensity
Uref = 10 m/s at 70m, z0 = 0.0001m, upstream TI = 6%
Wind direction: sector 285
© 2011 ANSYS, Inc. All rights reserved. 66 ANSYS, Inc. Proprietary
Tidal Turbine: Fall of Warness
Peak mean spring current = 3.6 m/s
Typical water depth = 34 m
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Processing of Bathymetric Data
• Seazone Data
• The reference level of the depth data
is Lowest Astronomical Tide (LAT)
• About 6 m resolution, in places as
low as 1 m
• Data contains approximately 11
million points
• Gridded data triangulated and
converted to STL
• Basis for meshing
• Important to have methods that cope
well with anisotropic meshes
• Number of meshing approaches tried
• Black Box, morph template hex mesh
© 2011 ANSYS, Inc. All rights reserved. 69 ANSYS, Inc. Proprietary
Calculations
• 18 mins elapsed time, 30 iterations
• About 30-45 mins to complete converged
run
• 4 processors
• 894894 nodes
• Resolution about 25 m square
• Input profiles
– Constant and logarithmic (ABL profile)
and 1/7th profile
• Use tidal diamonds from naval charts for
initial studies
© 2011 ANSYS, Inc. All rights reserved. 71 ANSYS, Inc. Proprietary
Sample results vs Data
Figure 10: Comparison between CFD results, Tidal
Diamond Information and ADCP Measurements. 1 hour
after High Water.
Figure 11: Orientation of the velocity at the ADCP locations, 1
hour after High Water
© 2011 ANSYS, Inc. All rights reserved. 72 ANSYS, Inc. Proprietary
Results with Turbines
Depth and landmass
Overall Flow Speed
Turbine wakes,
colours zoomed to
illustrate wakes
Turbine wakes
zoomed – local speed
© 2011 ANSYS, Inc. All rights reserved. 73 ANSYS, Inc. Proprietary
Conclusions
• ANSYS viscous CFD provides a useful and
complementary simulation technology
• Offshore and marine applications
– Hydrodynamic characterisation/loading
– Motion response
– Vortex-induced vibration
– Added mass and damping
– Two-way fluid structure interaction
• Wind/Tidal renewable energy applications
– Oscillating water columns
– Tidal turbines
– Tidal farm layouts