GOSUD/SAMOS Workshop 2-4 May 2006

Airflow modeling for research vessels

B. I. Moat1, M. J. Yelland1, R. W. Pascal1, S. R. Turnock2, S. Popinet3

1) National Oceanography Centre, UK2) University of Southampton, UK.

3) National Institute of Water and Atmospheric Research, New Zealand.

GOSUD/SAMOS Workshop 2-4 May 2006

Outline1) Problem2) Background3) Introduction to Computational Fluid Dynamics

(CFD)4) CFD in ship design for atmospheric research5) Examine various CFD codes6) Summary

Problem

• Finding a measure of the acceleration/deceleration at any given instrument location

• Determining the effective measurement height (vertical displacement of air)

• Developing an effective tool that can be used during the ship design phase to best site atmospheric instrumentation

• Order 50 ships? - balance between time/finanical cost etc


Background

• The effects of flow distortion:– are not that sensitive to variations in wind speeds between 6

to 20 ms-1 (<0.5%)– are very sensitive to relative wind direction and anemometer

location (20 %)– are sensitive to the ship design (~5%)


(VOSClim brochure)

The 4 stage CFD modelling process

• Digitize plans (autocad)

• Femgen (Femsys, UK)

• 4 WEEKS PER SHIP

• Or, obtain model from naval architects

• Significant learning curve

• 1) Geometry creation



High resolution mesh

• Historically the most difficult part of CFD modelling

• Variable mesh sizes and types• Different mesh for each wind

direction

• 1) Geometry creation• 2) Mesh generation



10 m/s 15 m/s

• Run times are machine dependant (1 - 10 days)

• 64 bit workstation• 1- 3 Gbytes memory

• 1) Geometry creation• 2) Mesh generation• 3) Compute solution




10 m/s 15 m/s

• 1) Geometry creation• 2) Mesh generation• 3) Compute solution• 4) Post-processing

• Estimate wind speed bias and vertical displacement


Research ship design

• RRS Charles Darwin• 5% wind speed error due

to ship structure

• RRS Discovery• Streamlined• Wind speed errors (bow-

on 1%)



RRS RRS James CookJames Cook to be delivered in to be delivered in September 2006September 2006



RRS Charles Darwin

RRS James Cook

RRS Discovery

Win

d s

pe

ed

bia

s (%

)W

ind

sp

ee

d b

ias

(%)

Relative wind directionRelative wind direction

Request a streamlined superstructureForemast as tall and far forward as possibleUncluttered foremast platform

CFD CODES available


• VECTIS - www.ricardo.com

– Cost: $11k academic license ($5.4k per additional processor)– Accuracy: 2% for low flow distortion sites (up to 10% bias)– Best validated – 11 ships modeled - (Yelland et al. 2002)– Fast mesh generation process (staff time = 3 hours/run)– Computation times of 3-5 days per relative wind direction

CFD CODES available


• VECTIS - www.ricardo.com

– Cost: $11k academic license ($5.4k per additional processor)– Accuracy: 2% for low flow distortion sites (up to 10% bias)– Best validated – 11 ships modeled - (Yelland et al. 2002)– Fast mesh generation process (staff time = 3 hours/run)– Computation times of 3-5 days per relative wind direction

• FLUENT - www.fluent.com

– Cost: $?k depends upon application (equivalent to VECTIS )– Accuracy: ? % – 1 ships modeled - Dupuis et al. (2003)– Mesh generation process

• Initial Staff time = 2 weeks• Less for other wind directions

– Computation times of 1 day

CFD CODES available


• GERRIS - gfs.sourceforge.net

– Cost: FREE (GNU software license)– Accuracy: 4% (1 ships modeled - Popinet et al. 2004)

• University of Tokyo are currently modeling 2 ships (part of a PhD. Project)

– Mesh generation fully automatic– Mesh adapts to the flow where specified– Computation times vary 3 to 5 days (depends on

geometry detail required)


New ‘parametric method’ proposed by Ship Science

• Assumption: hulls are similar and geometries don’t need to be accurate everywhere

• Script to transform ship shape into representations of other ships (each deck xyz +position rel. to bow)

• Investigate the level of detail required


New ‘parametric method’ proposed by Ship Science

• Automatic generation of the large scale geometry• Detailed foremast model added for each ship• Automatic mesh generation for each wind direction• Flow then simulated in FLUENT or CFX

Possible parametric method projects?


– Project based at Ship Science, Southampton University, UK with NOC advising

– Initial feasibility study to determine accuracy of method - needs $27k funds

– Full scale project aimed at modelling a significant fraction of the 50 ships - needs at least $36k/year funds

Comparison of approaches


• Present detailed geometry approach– GERRIS/VECTIS/FLUENT for 1 ship at 10 wind

direction:• 8 weeks staff plus analysis time• 5 to 10 weeks computation

– OK for a low number of ships – Of known accuracy– Time scale: could start now (given staff time and

software)– NOC would make avaialble existing geometries of

the RV Ron Brown, RV Knorr and FS Polarstern


• Parametric geometry and mesh creation– Once developed, for 1 ship at 10 wind directions:

• about 2 days staff plus analysis time• 1 day computation (parallel processing)

– Feasible for studying the airflow over 50 ships– Accuracy not known yet – Validate by comparison to existing detailed models– Lead time: active research area which requires

funding

Comparison of approaches

SAMOS aims


• Accuracy required?

• What relative wind directions to study?

• Number of ships to study?

• When ?

• Funding ?

CONTACTS


• VECTIS - [email protected]/JRD/MET/cfd_shipflow.php

• GERRIS - [email protected]

http://gfs.sourceforge.net

• NEW PARAMETRIC METHOD - [email protected]

Documents

GOSUD/SAMOS Workshop 2-4 May 2006