Vessel efficiency competition case study - Simon Lewis computational flow dynamics

Propulsion efficiency improvement through CFD

Vessel Efficiency

Simon Lewis Tuesday 27th November2012

Overview of presentation

CJR Propulsion and initial propeller design tools

Analysis of a hull using CFD

Optimisation of propeller design

P-bracket analysis and design using CFD – A case study

Spray analysis

Rudder design

Recent successes

Future work

CJR Propulsion Ltd. – Company background

CJR propulsion is a leading propeller design and manufacture company

Also manufacture other underwater hull appendages such as rudders, P-brackets and propeller shafts.

World leader in advanced manufacturing methods – One of the only companies in Europe with the capability to machine propellers with a 5-axis CNC milling machine

Initial Propeller performance calculations

Historically, propeller design was based on propeller series data and experience

A lifting surface model and ship resistance prediction program was introduced to the design procedure in 2007. This allows:

• Accurate hull resistance prediction

• Accurate propeller performance prediction

• Cavitation on the propeller blades to be predicted (subject to accurate inflow data)

• Pressure field around the hull as well as pressure pulses on the hull are calculated – these are responsible for propeller noise and vibration (subject to accurate inflow data)

Limitations of this method

No method for determining the realistic inflow to the propeller plane – a uniform flow is assumed, although this is not the case as the flow has to travel passed the shaft and shaft bracket before reaching the propeller.

Cavitation and propeller pulses cannot be accurately predicted if the inflow conditions are not known.

In order to improve the rudder design, the flow entering the rudder region must be known.

No method of determining the effect of the shaft, shaft bracket and other appendages on the propeller performance

Knowledge transfer partnership

In order to overcome these limitations, CJR decided to seek assistance from the University of Southampton.

The two organisations won funding for a 2 year knowledge transfer partnership (KTP) funded by the TSB

The aim of the KTP was to improve sterngear design through the use of advanced computational fluid dynamics (CFD).

CFD analysis of a planing hull

CFD mesh of the hull and appendages.

Free surface showing hull wake.

Analysing the flow into the propeller

CFD mesh of the hull and appendages.

CFD analysis of a planing hullStreamlines of the flow

under the hull

Cross flow velocities in the propeller plane

CFD Study - Results

Case Study: P-bracket design

The aim is to demonstrate how the P-bracket design alters the flow into the propeller. This is achieved by

• Simulating the flow around a hull using CFD to gain a better understanding of the flow into the propeller.

• Altering the P-bracket design and analysing the effects:

-8º 15º 26º

Preliminary CFD Study – Propeller plane

Velocity in the x direction (forward velocity)

-8º P-bracket 15º P-bracket 26º P-bracket

Propeller Design

Four propellers are analysed once the wake predictions are completed

Propellers are analysed in the following flow regimes:

• Uniform wake• CFD predicted wake with -8º P-bracket• Trials data with -8º P-bracket• CFD predicted wake with 15º P-bracket• CFD predicted wake with 26º P-bracket

Propellers are analysed using in-house code and a vortex lattice method

Propeller Design – Thrust predictions

Propeller Design – Torque predictions

Uniform-8

-8 trials15

26

11.5

12

12.5

13

13.5

14

14.5

15

5x42.5x49

5x42.5x50.5

5x42.5x50.5 MOD

5x42.5x50.5 REV

5x42.5x49

5x42.5x50.5

5x42.5x50.5 MOD

5x42.5x50.5 REV

Torq

ue (k

Nm

)

Propeller Design –prediction of pressure pulses on hull

Uniform-8

-8 trials15

26

0

5

10

15

20

25

30

5x42.5x49

5x42.5x50.5

5x42.5x50.5 MOD

5x42.5x50.5 REV

5x42.5x49

5x42.5x50.5

5x42.5x50.5 MOD

5x42.5x50.5 REV

Pres

sure

pul

ses (

kPa)

Propeller Design –prediction of cavitation

Cavitation erosion

Case study conclusions

A design procedure for improving stern gear has been presented

The initial results suggest that there are significant savings to be made in terms of stern gear drag and propeller noise and vibration.

P-bracket design affects the propeller performance and optimisation of this component provides• A cleaner flow into the propeller.• Significant reduction in the predicted pressure pulses on the hull.• Increase in propeller thrust and torque.

Cavitation predictions are comparable with reality when the CFD wake is used

Trim and resistance analysis

• Prediction of drag and running trim.• Calculation of optimum position of the centre of gravity.• Sensitivity studies can be undertaken to evaluate the effect of changing

the hull parameters including displacement.

Variation of drag with displacement for three different

trim angles.

Propeller race

CFD simulation of the propeller and entire hull

Axial velocity

Vertical velocity

Rudder design

Two rudder designs are analysed

Rudder A is a wedge rudder with a blended stock, and toed in by 2.5 °Rudder B is a wedge rudder without a blended stock and has no toe in angle

Rudder design

Pressure on rudder surfaces at 0 degrees pitch.

Rudders at 35 degrees pitch, with streamlines.

CFD Spray analysis

CFD mesh of the hull is refined at the free surface.

CFD Spray analysis

• Evaluate the spray of a planing craft in calm water.• Effect of changes in the hull design (such as spray rail dimensions) on

the spray.

Analysis of the free surface flow.

Recent success storiesThe propeller design of the following yachts has used all or some of the method presented:

Manufacturer Yacht Required speed (kts)

Achieved speed (kts)

Alnmaritec 16m Pilot boat 25 27.6

Alnmaritec 19m patrol boat 36 37.6

Holyhead Marine 16m Pilot boat 25 28

Mustang Marine Humber pilot boat 25.7 (previous props) >27

Seaward Marine Tenerife pilot boat 21 23.5

Seaward Marine Guernsey ambulance boat 25 26.7

Conclusions

The collaborative research and development project has been a huge benefit to both the University of Southampton and CJR Propulsion:

CJR made extensive use of the resources at the university such as the Iridis 3 computer cluster and the in house CFD expertise gained from years of research.

The university has improved links to industry, and gained insight and knowledge from the research carried out during the project.

The improved lines of communication between the two has allowed further collaboration in the area of composite propellers which included a student at the university working on a summer placement at CJR.

CJR now offer a CFD consultancy service

CFD consultancy work by CJR

Picture courtesy of ICAP leopard

CFD consultancy work by CJR

Future work

Improve the propeller momentum source in the CFD to include variations in the propeller thrust and torque as each blade sweeps around the disk

Further sea trials are planned with a variable rudder toe in angle in order to fully quantify the effect of this against speed and turning performance

Including hull motion in the CFD to allow the hull to find its own heave and trim, and eliminate the need to carry out a matrix of nine simulations

Include the full propeller model in the CFD simulation to further enhance predictions.

Future work

Questions

?Simon Lewis

[email protected]