VIS
ION
SSC
IENCETECH
NO
LO
GY
RESEARCHHIGHL
IGH
TS
22
VT
T S
CIE
NC
E 2
2
RA
NS
an
alyse
s of c
avita
ting
pro
pelle
r flo
ws
ISBN 978-951-38-7946-4 (soft back ed.) ISBN 978-951-38-7947-1 (URL: http://www.vtt.fi/publications/index.jsp)ISSN 2242-119X (soft back ed.) ISSN 2242-1203 (URL: http://www.vtt.fi/publications/index.jsp)
RANS analyses of cavitating propeller flows
Cavitation is an important and complex phenomenon in ship propeller flows. Cavitation induces vibrations to the ship structures, and noise to the interior of the ship and to the environment encumbering people on board and underwater fauna. Cavitation may also cause erosion to propellers, rudders, and other ship structures limiting the service time of vessels.
This publication discusses numerical modelling of cavitation incepting in propeller flows. The cavitation model implemented in FINFLO, a general purpose CFD code, has been validated for quasi-steady and time-dependent propeller flow problems. The validation of the numeri-cal model has been performed against model test results performed in cavitation tunnels. The results of the implemented numerical model and the tests are found to have good correlation.
CFD-based methods give a way to understand the physics inside the cavities more deeply. Numerical investigations of cavitating propeller flows in the early propulsor and ship design stage help to decrease the noise and vibration levels of vessels.
RANS analyses of cavitating propeller flows Tuomas Sipil
VTT SCIENCE 22
RANS analyses of cavitating propeller flows
Tuomas Sipil
Licentiates Thesis submitted in partial fulfilment of the requirements for the de-gree of Licentiate of Science in Technology at the Aalto University School of En-gineering, on the 10th of September, 2012.
ISBN 978-951-38-7946-4 (soft back ed.) ISSN 2242-119X (soft back ed.)
ISBN 978-951-38-7947-1 (URL: http://www.vtt.fi/publications/index.jsp) ISSN 2242-1203 (URL: http://www.vtt.fi/publications/index.jsp)
Copyright VTT 2012
JULKAISIJA UTGIVARE PUBLISHER
VTT PL 1000 (Tekniikantie 4 A, Espoo) 02044 VTT Puh. 020 722 111, faksi 020 722 7001
VTT PB 1000 (Teknikvgen 4 A, Esbo) FI-2044 VTT Tfn. +358 20 722 111, telefax +358 20 722 7001
VTT Technical Research Centre of Finland P.O. Box 1000 (Tekniikantie 4 A, Espoo) FI-02044 VTT, Finland Tel. +358 20 722 111, fax +358 20 722 7001
Technical editing Anni Repo
Kopijyv Oy, Kuopio 2012
http://www.vtt.fi/publications/index.jsphttp://www.vtt.fi/publications/index.jsp
3
RANS analyses of cavitating propeller flows
Kavitoivien potkurivirtausten RANS-analyysi. Tuomas Sipil. Espoo 2012. VTT Science 22. 136 p.
Abstract This publication presents validation studies for the cavitation model implemented in the Reynolds-averaged Navier-Stokes equation solver FINFLO. The validation studies relate to ship propellers in uniform and non-uniform inflow conditions.
The main physical phenomena involved in cavitation are first introduced. Then, the cavitation phenomena related to marine applications are presented, and the physics behind sheet and vortex cavitation are explained. As cavitating flows are strongly related to turbu-lence, the physics behind turbulence and its simulation methods are also introduced.
The benefits and uncertainties related to cavitation tests are described. It is important to understand the drawbacks of experimental methods when comparing the simulation results with the test observations. A brief description of the existing cavitation models is also given, and the utilized cavitation model and its numerical implementation are described in detail.
The validation cases are introduced and the simulation results are compared to the out-come of the cavitation tests. The simulation results generally showed good correlation with the experiments. Sheet cavitation was observed in the tests on both the suction and pres-sure sides of the blades in the validation cases, which was also found in the simulations. The cavitating tip vortices were also found to be similar in the experiments and simula-tions. The propeller slipstream must be discretized with a high resolution grid in order to predict the cavitating tip vortices and the wakes of the blades with reasonable accuracy.
A verification and validation analysis was performed for the global propeller perfor-mance characteristics according to the methodology recommended by the ITTC. The influence of the empirical constants in the utilized mass transfer model on the cavitating tip vortices is studied.
Finally, explanations for the similarities and differences between the results of the ex-periments and the simulations are discussed. The main differences are found to be caused by laminar flow separation at the leading edge of the blades in the tests, and the limitations of the turbulence and cavitation models utilized in the present simulations.
Keywords Cavitation, CFD, hydrodynamics, propeller, RANS, simulation, tip vortex, turbulence
4
Kavitoivien potkurivirtausten RANS-analyysi
RANS analyses of cavitating propeller flows. Tuomas Sipil. Espoo 2012. VTT Science 22. 136 s.
Tiivistelm Tyss on esitetty validointilaskentaa Reynolds-keskiarvoistettujen Navier-Stokes-yht-liden ratkaisijaan FINFLOhun implementoituun kavitaatiomalliin. Validointilaskennat liit-tyvt laivapotkurisovelluksiin sek tasaisessa ett eptasaisessa sisntulovirtauksessa.
Kavitaatioon liittyvt trkeimmt fysikaaliset ilmit on selitetty opinnytteen alussa. Tmn jlkeen on esitetty laivasovelluksissa esiin tulevat kavitaatioilmit. Levy- ja krki-pyrrekavitaation fysiikka on selitetty tarkasti. Koska kavitaatio liittyy lheisesti virtauk-sen turbulenttisuuteen, on mys turbulenssin fysiikka ja sen simulointi- ja mallinnusmene-telmt kuvattu.
Kavitaation kokeelliseen tutkimukseen liittyvt edut ja epvarmuudet ovat mys esitet-ty. On trke ymmrt kokeelliseen toimintaan liittyvt epvarmuudet, kun verrataan kavitaation simulointituloksia koetuloksiin. Olemassa olevia erilaisia kavitaatiomalleja on lyhyesti kuvattu. Tyss kytetty kavitaatiomalli ja sen numeerinen implementointi on selitetty yksityiskohtaisesti.
Validointitapaukset on esitetty ja simulointituloksia on verrattu kavitaatiokokeiden tu-loksiin. Simulointitulokset ovat yleisesti ottaen lhell kokeellisia tuloksia. Kokeissa ja laskennassa levykavitaatiota havaittiin sek imu- ett painepuolilla lapaa eri validointita-pauksissa. Kavitoiva krkipyrre oli samankaltainen kokeissa ja simulointituloksissa. Jttvirtauksen diskretointi on tehtv huolellisesti laskennoissa, jotta kavitoiva krkipyr-re sek lavan vanavesi tulevat mallinnettua tarkoituksenmukaisella tarkkuudella.
Lasketuille potkurin toimintaa kuvaaville globaaleille suureille on tehty verifiointi- ja validointitarkastelu ITTC:n suosittelemalla tavalla. Kytetyss massansiirtomallissa olevi-en empiiristen kertoimien vaikutusta kavitoivan krkipyrteen laskentatuloksiin on mys tutkittu.
Tyn lopussa on selitetty syit kokeellisten ja laskennallisten tulosten vastaavuuksiin ja eroihin. Trkeimmt syyt tulosten eroihin ovat levykavitaation synnyn viivstyminen kokeissa lavan johtoreunalla ilmenevn laminaarisen virtauksen johdosta sek simuloin-nissa kytettyjen turbulenssi- ja kavitaatiomallien rajoitukset.
Avainsanat Cavitation, CFD, hydrodynamics, propeller, RANS, simulation, tip vortex, turbulence
5
Preface This thesis was prepared for Aalto Universitys Marine Technology research group, and the work described was carried out in the Ship Hydrodynamics team at the Vehicle Engi-neering knowledge centre of VTT Technical Research Centre of Finland. I wish to extend my gratitude to Professor Jerzy Matusiak, thesis supervisor and examiner, for his valuable input and comments. My sincere thanks also to the second examiner of my thesis, D.Sc. (Tech.) Patrik Rautaheimo, for the valuable comments that helped to improve this thesis.
I also wish to thank Professor Timo Siikonen, instructor of my thesis, for his support and guidance and for our fruitful co-operation over the last decade in the field of computa-tional fluid dynamics.
I am deeply grateful to my VTT colleagues D.Sc. (Tech.) Jaakko Pylkknen (retired) and D.Sc. (Tech.) Antonio Snchez-Caja for introducing me to propeller hydrodynamics, for their enthusiasm for the subject and for their unwavering patience in guiding me through the world of ship propulsors.
My special thanks to Jussi Martio our many discussions on and off topic helped me to persevere during the hard times. A huge thanks to all members of the Ship Hydrodynamics team for creating such a good working atmosphere.
I would also like to acknowledge my sincere gratitude to Esa Salminen and Juho Ilkko of FINFLO Ltd. for their valuable guidance in using FINFLO and its auxiliary programs and for solving the bugs in the codes. My deep thanks also to Tiina Jrvilehto from Seman-tix Finland Ltd. for proofreading this thesis.
The financial support provided for the work of this thesis is gratefully acknowledged. The majority of the work was carried out under the EUs 6th Framework Programme project VIRTUE and the Tekes the Finnish Funding Agency for Technology and Innova-tion project VIRKOOT. These projects created fruitful environments and networks in which to study cavitation using numerical methods. I would like to thank Ilkka Saisto for his expert handling of the red tape related to these projects. The final part of this work was conducted under VTTs self-funded project CFDShip and
