Engineering Simulation Software for the Offshore, Marine and Wave/Tidal Renewable Energy Industries. Viscous CFD Applications. Phil Stopford ANSYS UK

Size: px

Start display at page:

Download "Engineering Simulation Software for the Offshore, Marine and Wave/Tidal Renewable Energy Industries. Viscous CFD Applications. Phil Stopford ANSYS UK"

Kellie Hamilton
6 years ago
Views:

1 Engineering Simulation Software for the Offshore, Marine and Wave/Tidal Renewable Energy Industries Viscous CFD Applications Phil Stopford ANSYS UK 2011 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2 Agenda 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 2011 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

3 Fluid dynamics Complex and sometimes non-intuitive Depends on the interaction of multiple features Which situation will see the highest velocity? A B 1m/s 1m/s 2011 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary

5 What is CFD? Computational Fluid Dynamics (CFD) 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 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 2011 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary

6 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. 6 ANSYS, Inc. Proprietary

.. Free surface models Simple wave generation Wave/body interactions Dynamic response Rigid body 6-DOF solutions Added mass and

7 CFD capabilities 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 2011 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary

Hydrodynamic characterisation 2011 ANSYS, Inc.

and form drag 5415 Destroyer test case At 4.03 knots Drag CFD 43.

9 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 /- 2 N Experiment 44.3 N 2011 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary

analytical hull shape Excellent agreement with experiment

10 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. 10 ANSYS, Inc. Proprietary

Different speeds give different hull orientation

11 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. 11 ANSYS, Inc. Proprietary

12 Side Force [N] Total Drag [N] Validation: Racing Yacht Forces at 20 heel Constant speed Variation of yaw angle Good agreement Drag vs Side Force 20 Heel & 14 knots Side Force [N] CFX Experiment Side Force vs. Yaw Angle 20 Heel & 14 knots Yaw [deg] CFX Tank 2011 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary

Mechanical model Automated 1-way transfer of load from ANSYS CFD to

13 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 2011 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary

CFD simulations with moving bodies 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

15 CFD simulations with moving bodies 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 Prescribed motion Flow-driven motion 2011 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary

Rigid Body CFD Solution Dynamic Sink and

freedom Free surface flow 2011 ANSYS, Inc.

linear theory Pitch and heave from 6-DOF solution

17 Rigid Body CFD Solution Dynamic Sink and Trim Mono-chromatic waves generated at inlet by simple linear theory Pitch and heave from 6-DOF solution 2011 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary

19 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 Moving Body Force Applied By Mooring c ẋ F = k x + c ẋ k x 2011 ANSYS, Inc. All rights reserved. 19 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

20 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 L, k, c Attachment Point Mooring Point 2011 ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary

Mooring example Two moorings defined Open Channel Flow Direction

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)

23 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 10 3 Reynolds numbers of order 10 4 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) 2011 ANSYS, Inc. All rights reserved. 23 ANSYS, Inc. Proprietary

Simple 2-D VIV Use 2-D CFD with 2 degrees of freedom and numerical tethers to understand riser

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

25 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 threedimensional 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. 25 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

26 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. 26 ANSYS, Inc. Proprietary

27 Experimental Set-up Delta Flume in Holland Inlet velocity: 0.16m/s Top tension: 405N Bending stiffness: 29.9NM 2 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 Cabin Water surface inside the vacuum tank Water surface in the flume 13.12m Riser Vacuum tank Incident velocity profile at the riser from Chaplin et al. (2004) 2011 ANSYS, Inc. All rights reserved. 27 ANSYS, Inc. Proprietary

ANSYS CFD Set-up Computational domain and boundary conditions inlet top surface (free slip) tank walls (no slip) water surface (free slip) outlet Floor (free slip) Fluid/solid interface The case was

28 ANSYS CFD Set-up Computational domain and boundary conditions inlet top surface (free slip) tank walls (no slip) water surface (free slip) outlet Floor (free slip) 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: ANSYS, Inc. All rights reserved. 28 ANSYS, Inc. Proprietary

29 ANSYS Structural Set-up Boundary conditions 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 Fluid/solid interface - All nodes but central - central node 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 Pa was chosen to match the axial stiffness Ramped loading was switched off 2011 ANSYS, Inc. All rights reserved. 29 ANSYS, Inc. Proprietary

30 View of Flow Structure A view of vorticity field at different heights. Red and blue colours represent positive and negative vorticity respectively 2011 ANSYS, Inc. All rights reserved. 30 ANSYS, Inc. Proprietary

33 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. 34 ANSYS, Inc. Proprietary

34 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. 35 ANSYS, Inc. Proprietary

Damping Example Eni G R O U P Lowering of structure to seabed Need added mass and damping for accurate dynamics simulation Perform transient CFD calculation on one mudmat Separate horizontal

35 Damping Example Eni G R O U P 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 2011 ANSYS, Inc. All rights reserved. 36 ANSYS, Inc. Proprietary

E+05 0 20 40 60 80 100 Time (Seconds) Hydrodynamic Forces in Heave Direction

36 Force (kn) Damping Example Same geometry and mesh can be used for heave and sway calculations 3.E+05 2.E+05 1.E+05 0.E+00-1.E+05-2.E+05-3.E+05-4.E+05-5.E Time (Seconds) Hydrodynamic Forces in Heave Direction (Inverted Can) 2011 ANSYS, Inc. All rights reserved. 37 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

37 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 Added Mass in Heave Period (s) Heave Motion Amplitude (cm) Calculated Value (no holes) Model Tests (4 holes) Damping in Heave Period (s) Amplitude (cm) Calculated Value (no holes) Model Tests (4 holes) ANSYS, Inc. All rights reserved. 38 ANSYS, Inc. Proprietary

RAO (m/m) RAO (m/m) Response Amplitude Operators (RAO) Comparison of CFD to Free Floating Calm Buoy measurement 1 0.8 Surge RAO Prescribed heave motion 0.

38 RAO (m/m) RAO (m/m) Response Amplitude Operators (RAO) Comparison of CFD to Free Floating Calm Buoy measurement Surge RAO Prescribed heave motion 0.6 RAO calculated from Added Mass, Damping and Restoring coefficients Experimental CFD Buoy model Anchored with three mooring lines OPTICAL SYSTEM FOR MEASUREMENT OF BUOY MOTION Full scale period (s) Heave RAO 1.4 LINE #3 LINE # Experimental CFD ANSYS, Inc. All rights reserved. 41 Full scale period (s) ANSYS, Inc. Proprietary

40 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. 44 ANSYS, Inc. Proprietary

Structural mechanics in ANSYS FEA Examine slamming for example, and

41 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. 45 ANSYS, Inc. Proprietary

44 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, ANSYS, Inc. All rights reserved. 48 ANSYS, Inc. Proprietary

OWC example application Compression chamber above OWC Energy can be harnessed (e.g. via Wells turbine) Source:http://news.

47 Vertical cylinder Pressure time trace at monitoring points on seabed Point 1 inside cylinder, Point 2 outside cylinder upstream More resonant 2011 ANSYS, Inc. All rights reserved. 52 ANSYS, Inc. Proprietary

Wave-piercing design Air Pressure variation inside cylinder

Air movement through the hole at the top of the OWC 2011

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

51 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. 56 ANSYS, Inc. Proprietary

52 Quantitive Analysis Forces and Torque on Blades Resultant Force in X direction [N] Resultant Force in Y direction [N] Resultant Force in Z direction [N] Resultant Torque [Nm] 2011 ANSYS, Inc. All rights reserved. 57 ANSYS, Inc. Proprietary

53 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. 58 ANSYS, Inc. Proprietary

Structural mechanics Stress contours due to centrifugal and hydraulic

East River: Verdant Generation 5 Kinetic Hydropower Systems Effect of turbines in East River Non-rotating units create small

Very little flow acceleration is visible; generally well above the river bottom The turbulent wake lead to regions of increased

The impact of the pile wake, which is near the river bottom, is reduced by the lower water velocities in the fully developed

56 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, ANSYS, Inc. All rights reserved. 61 ANSYS, Inc. Proprietary

Wind and tidal turbine farm layouts 2011 ANSYS,

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;

58 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 sciencenw.com/uploads/horns_rev.jpg Turbine interactions and the environment Resolved Turbines / Actuator Disk models Wakes, towers 2011 ANSYS, Inc. All rights reserved. 63 ANSYS, Inc. Proprietary

map, NTF, Seazone) Point values of depth, referred to LAT Digitised contours of

59 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. 64 ANSYS, Inc. Proprietary

60 Offshore wind turbine wakes U ref = 10 m/s at 70m, z 0 = m, upstream TI = 6% Wind direction: sector 285 Horizontal velocity Turbulence intensity 2011 ANSYS, Inc. All rights reserved. 65 ANSYS, Inc. Proprietary

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

63 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. 68 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

64 Calculations 18 mins elapsed time, 30 iterations About mins to complete converged run 4 processors nodes Resolution about 25 m square Input profiles Constant and logarithmic (ABL profile) and 1/7 th profile Use tidal diamonds from naval charts for initial studies 2011 ANSYS, Inc. All rights reserved. 69 ANSYS, Inc. Proprietary

66 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. 71 ANSYS, Inc. Proprietary

67 Results with Turbines Depth and landmass Turbine wakes zoomed local speed Turbine wakes, colours zoomed to illustrate wakes Overall Flow Speed 2011 ANSYS, Inc. All rights reserved. 72 ANSYS, Inc. Proprietary

68 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 2011 ANSYS, Inc. All rights reserved. 73 ANSYS, Inc. Proprietary

Engineering Simulation Software for the Offshore, Marine and Wave/Tidal Renewable Energy Industries. Introduction. Nigel Atkinson ANSYS UK Ltd

Engineering Simulation Software for the Offshore, Marine and Wave/Tidal Renewable Energy Industries Introduction Nigel Atkinson ANSYS UK Ltd 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary