Engineering Simulation Software for the Offshore, Marine ... UK/staticassets/04... · Engineering...

© 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


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


Results

Velocity (m/s) Pressure Field

Adverse

pressure

gradient

~60% higher


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


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


• 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


Hydrodynamic

characterisation


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


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


Racing Yacht CFD

• Racing yacht geometry at model scale• Fully appended with rudders, keel and bulb

• Different speeds give different hull orientation


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


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


Motion response


• 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


• Dynamic Sink and Trim• Speedboat example

• Six degree of freedom

• Free surface flow

Rigid Body CFD Solution


• Dynamic Sink and Trim• Mono-chromatic waves

generated at inlet by simplelinear theory

• Pitch and heave from6-DOF solution

Rigid Body CFD Solution


Mooring example


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


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


Mooring example

• Two moorings defined

Open Channel

Flow Direction


Vortex Induced Vibration


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)


Simple 2-D VIV

• Use 2-D CFD with 2 degrees of freedom and numerical tethers to understand riser motion

– Computationally inexpensive


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


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


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)


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


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


View of Flow Structure

A view of vorticity field at different heights. Red and blue colours

represent positive and negative vorticity respectively


Results – Riser Motion

Moving riser started from rest…


Added-Mass and

Damping Calculation


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


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


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


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)


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


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


Transient two-way

fluid-structure-

interaction


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


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


Guest presentation

• BMT presentation

ANSYS_BMT - Marine Seminar - June 2011.pptx


Wave Energy:

Oscillating Water Column

Simulation


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.


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


Vertical cylinder on sea-bed

• Pressure distribution at

resonance

– Amplitude elevation in

cylinder


Vertical cylinder

• Pressure time trace at monitoring points on seabed

– Point 1 inside cylinder, Point 2 outside cylinder

upstream

More resonant


Wave-piercing design

• Air Pressure variation

inside cylinder


Air movement through the hole at the

top of the OWC


Tidal turbine

simulation using CFD

and one-way FSI


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


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]


Imported CFD pressure into

structural calculation

• Imported

Hydraulic

Forces from

CFD

calculation

applied to

structural

calculation

– One-way

FSI


Structural mechanics

• Displacement

due to

centrifugal

and hydraulic

loading

– Calculated

in ANSYS

Mechanical


Structural mechanics

• Stress

contours due

to centrifugal

and hydraulic

loading


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


Wind and tidal turbine

farm layouts


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


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


Offshore wind turbine wakes

Horizontal velocity Turbulence intensity

Uref = 10 m/s at 70m, z0 = 0.0001m, upstream TI = 6%

Wind direction: sector 285


Tidal Turbine: Fall of Warness

Peak mean spring current = 3.6 m/s

Typical water depth = 34 m


SeaZone Bathymetry Data


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


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


Results without Turbines


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


Results with Turbines

Depth and landmass

Overall Flow Speed

Turbine wakes,

colours zoomed to

illustrate wakes

Turbine wakes

zoomed – local speed


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

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