Viscous CFD applications - Ansys · •Introduction to viscous CFD ... accurate dynamics simulation •Perform transient CFD calculation –Separate horizontal and vertical motion

© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Justin Penrose

ANSYS UK Ltd

Seminar

30th November 2010

Engineering Simulation

Software for the Offshore,

Marine and Wave/Tidal

Renewable Energy Industries

Viscous CFD

applications

© 2010 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

• Introduction to viscous CFD

• CFD capabilities

• Introduction to CFD applications

• ANSYS CFD applications (Part 1)– Drag and motion simulation

• Guest presentation– BMT Fluid Mechanics

• Hydrodynamic loading on a subsea pipeline

• Ballast tank sloshing in hurricanes

• Submarine hydrodynamics

• ANSYS CFD applications (Part 2)• OWCs, tidal turbines

• Guest presentation– Cardiff University

• Tidal stream energy

• ANSYS CFD applications (Part 3)– Farm layouts

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 turbine farm layout and wake-effect investigation

– 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 turbine-specific tools• Rotating and stationary components

• Performance and power extraction

• Cavitation modelling

CFD capabilities


Hydrodynamic

characterisation


Hydrodynamic

characterisation/loading

• Viscous CFD provides a way to

characterise the overall forces

on a floating or submerged

body

• Viscous and form drag

• 5415 Destroyer testcase

• 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


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


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

– Having deformations that are simple to describe in the CFD software

CFD simulations with moving bodies

A prescribed motion

Flow driven motion


• Dynamic Sink and Trim• Monochromatic waves

generated at inlet by simplelinear theory

• Pitch and heave from6dof 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 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 2D VIV

• Use 2D 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 3D 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 3D 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

• 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 (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 9.55109Pa 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 colors

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 forced sinusoidal motion simulation

– Eg. 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 T

• 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

– 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)

• Compare to Free Floating Calm Buoy

– 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

– 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 Fluid Mechanics– Hydrodynamic loading on a subsea

pipeline

– Ballast tank sloshing in hurricanes

– Submarine hydrodynamics

– …


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 principle

• 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


Wave-piercing design

• Air Pressure variation

inside cylinder


Air movement trough 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


Tidal turbine CFD

• Pressure

contours on

blade and

support

surfaces


Tidal turbine CFD

• Pressure

contours on

blade and

support

surfaces

• Velocity

Vectors


Tidal turbine CFD

• Pressure

contours on

blade and

support

surfaces

• Velocity

Vectors

• Surface

Streamlines


Tidal turbine CFD

• Pressure

contours on

blade and

support

surfaces

• Streamlines of

velocity


Quantitative 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


Tidal turbine CFD

• Hydrogen Kinetics– Unshrouded double turbine

– CFD used to analyse initial design• Results provided useful flow visualisation

and quantitative information

– Geometrical modifications to blade profiling guided by CFD results

– Second design analysed in CFDCourtesy of Hydrogen Kinetics

Design 1 Design 2


• Model two draft tube

designs for comparison

• Determine individual

component losses

• Benefits from CFD:

– Improve matching

between various parts of

machine

– Model ‘what if’ scenarios

Tidal Barrage Turbine CFD



• Original Design:

– Draft Tube area expansion

angle = 7 deg

– Machine Power: 354.894

[kW]

– Machine efficiency: 48.65 %

• It’s just an example!

– Maximum loss was found to

be in the draft tube.

Individual Component losses

ComponentTotal Pressure

InTotal Pressure

OutTotal Pressure

drop % Drop

Bulb/Intake 5.08E+05 5.06E+05 2.11E+03 0.81

Distributor 5.06E+05 4.80E+05 2.59E+04 9.92

Runner 4.80E+05 3.13E+05 1.67E+05 63.99

Draft Tube 3.13E+05 2.46E+05 6.61E+04 25.29

Machine 5.08E+05 2.46E+05 2.61E+05



• Design modifications:

– Reduce draft tube

expansion angle to 4 deg

– Extend the hub cone in the

draft tube

– Machine Power: 378.326

[kW]

– Machine Efficiency: 55.4 %

– Lesser flow area compared

to 7 deg angle and better

attached flow on the longer

cone

Individual Component losses

ComponentTotal Pressure

InTotal Pressure

OutTotal Pressure

loss % loss

Bulb/Intake 5.08E+05 5.06E+05 2.08E+03 0.85

Distributor 5.06E+05 4.80E+05 2.60E+04 10.67

Runner 4.80E+05 3.04E+05 1.76E+05 72.09

Draft Tube 3.04E+05 2.64E+05 3.99E+04 16.37

Machine 5.08E+05 2.64E+05 2.44E+05


Guest presentation

• Cardiff University– Tidal Stream Energy


Tidal turbine farm

layouts

Ian Jones


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


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


Tools for Automated Solution

• Objective

– From Map to Mesh to CFD

to Report

– Data Extraction and

Automation of Analysis

• Wind and Tidal Flows

CFD solution + automated post-processing


Modelling Large Arrays:

Horns Rev

•8x10 WT

•Diameter of 80m

•Hub height of 70m

•Wind turbine spacing: 7 diameters

•Domain size:

•10 km radius

•1.0 km height

•Wind turbine thrust curve: Vestas

V80 WT

•ABL boundary layer profiles at

inlet

geou

z

zln

uu ),

~(min

0

*

z

u~

3

*

0,max~ zzzz ground

22

3inletrefTIuk

22

*

k

uC


Mesh characteristics

Hexahedral mesh

Non-uniform in horizontal and vertical planes

Background mesh before adaption ~690k nodes


Mesh characteristics

Hexahedral mesh

Non-uniform in horizontal and vertical planes

Mesh after adaption ~1.4M nodes


Results at hub height – sector 280

Horizontal velocity Turbulence intensity

Uref = 8 m/s at 70m, z0 = 0.0002m

Wind direction: sector 280


Array Efficiency Sector 270-285,

Compared with Measurements

Array efficiency at 8 m/s

compared to measurements 1

Good agreement between

model and measurements

Relatively large scatter

(and asymmetry) in

measurements

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

270 275 280 285 290

Eff

icie

nc

y

direction

Array efficiency, sector +- 2 degrees

Case C

RHS, Barthelmie et al

LHS, Barthelmie et al

Sector +- 2 degrees

firstrow

windfarm

PN

P

1. Barthelmie et al , Modelling the impact of wakes on power output at Nysted and Horns Rev,

EWEC 2009, Marseille.


East River: Verdant Generation 5

KHPS

• 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

Documents

Viscous CFD applications - Ansys · •Introduction to viscous CFD ... accurate dynamics simulation •Perform transient CFD calculation –Separate horizontal and vertical motion