DANSIS meeting

Lyngby, 13 May 2009

Application of CFD in connection with ship design

• Background • Method• Examples• Summary

Claus Daniel Simonsen

FORCE Technologywww.force.dk

• When a ship, which consists of hull, propeller and rudder, is designed it is important to obtain an efficient solution for allcomponents working together:

The hull form should be optimized to minimize the resistance, which willreduce the engine power required to drive the ship through the waterand hereby also the fuel consumption

The propulsion system should be optimized to work efficiently in the wake behindthe ship, which also will influence the required power and the fuel consumption

The rudder should be optimized in order to maneuver the ship safely, butalso to improve the efficiency of the propeller

• The resistance is highly influenced by the hull design, but the hull designer has to keep in mind that the wake field also should be optimized seen from the propeller designers point of view due tothe strong interaction between the hull and propeller flows. Further, the rudder operates in the propeller slipstream

Background

• Today the most commonly method used for checking combined hull-propeller flows is towing tank/cavitation tunnel testing. However,due to the cost of model testing, the combined hull and propellerdesigns are checked relatively late in the design process

• The idea is to investigate if CFD can be used for evaluation of hulland propeller geometries in conditions where the propeller isworking behind the ship.

No requirement to physical model means a tool that can be used toimprove the propeller-hull designs in the early design

Calculation of forces and moments will give a picture of the performance of thesystem in the early design stage

Calculation of the flow field will give insight in the physics of the of thesystem in the early design stage

Background

• Some of the basic model tests that are used today in connectionwith design of ship hulls and propellers cover:

Resistance test and wake surveys (Hull alone)

Open-water propeller measurements (Propeller alone)

Cavitation tunnel testing (Hull+propeller)

Self-propulsion test (Hull+propeller)

• Question is if CFD can used to replicate these test with enough accuracy to be useful in the design process

Background

Example

• As an example, results from calculations of the flow around the geometry of a 70m naval inspection vessel including rudder, ice fins and propeller are shown

• Work based on a joint CFD project between MAN-Diesel A/S and FORCE Technology in Denmark

• The project was run under the Danish Centre for Maritime Technology (DCMT), which goal is to promote Danish know-how within maritime technology through increased research and development in the maritime industry

• All simulations are done with the RANS solverSTAR-CCM+ in model scale

• MeshingSurface wrappingTrimmed/polyhedral mesh approachPrism layer meshing in boundary layerZonal refinements

• Physics modeling2 phase VOF model for free surface modelingSteady and transient calculations depending on modelMRF and RBM used in connection with the propellermodelSliding interfaces used for rotating propellerk-ω SST turbulence model, all Y+ treatment

Method

Example: Open-water propeller

• Open-water propeller model

Modeled with steady state MRF and 2 million cellsPressure distribution illustrates loading on the blades

Example: Open-water propeller

• Open-water propeller model

• Thrust and torque coefficients Kt and Kq

• Open water data is measured in FORCE’s towing tank

• Design condition, J~0.8

• Kt and Kq predicted within 3.5% and 0.5% of measured values

Example: Resistance test

• Model of ship without propeller

Steady state solution with 3 million cellsFree surface prediction is essential for resistance calculations elevations Wake field prediction is important for propellerResistance predicted within 3% of measurement

Example: Caviation tunnel setup

• Model of ship with propeller

Transient simulation and 4.5 million cellsRotating propeller, RBM and sliding interfaces between propeller and hull domainNo free surface, no cavitationResults used to study flow featuresTime varying results: movie

Example: Caviation tunnel setup

• Model of ship with propeller at atmospheric pressure (no cavitation)

• Thrust and torque coefficients Kt and Kq

• Resistance not considered in cavitation tunnel

• Design condition, J~0.8

• Kt and Kq predicted within 3% of measured values

Example: Caviation tunnel setup

• Model of ship with propeller including cavitation

Transient simulation and 13 million cellsRotating propeller, RBM and sliding interfaces between propeller and hull domainRayleigh-Plasset model Qualitative agreement with model test results on propeller bladesRANS is too dissipative to maintain tip vortex cavitation

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Transient simulation and 6 million cellsRotating propeller, RBM and sliding interfaces between propeller and hull domainVisualization of free surface features Breaking bow wave capturedUnsteady and asymmetric stern wave

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Qualitative comparison with measured wave profile Overall wave pattern capturedSpray in bow region reduced in CFD

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Visualization of flow in stern regionInstantaneous pictureFlow around ice fins

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Visualization of flow on rudderInstantaneous picture“Typical” behavior of streamlines

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Visualization of flow in propeller slip stream in cross section at AP Instantaneous pictureAcceleration of axial flowTip and hub vortices

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Visualization of tip vortices along the rudderInstantaneous pictureNot exactly same propeller positionImportant that vortex passes below rudder

Example: Ship at self-propulsion

• Model of ship with propeller in towing tank

Comparison between measured and calculated data for propeller and hull quantitiesFairly good agreement with measurement. Propeller quantities and resistance arewithin 3% of measured data

Summary

• Summary

Advanced CFD appears to be a strong tool in the design phase of the ship,because detailed evaluation of design alternatives can be done early in the designprocess

Concerning hydrodynamic loads on the ship (incl. propeller) CFD is fairly accurate,and as such a good supplement to the model test. But, experiments are stillneeded to make the final check of the final design

When is comes to insight in the flow details and understanding of the physics of theflow CFD is very strong, because the information can be used to improve thedesign and to make sure that the flow behaves well before the physical model isbuild.

Experience shows that level of validation can vary between applicationsi.e. for different propeller and hull geometries. However, even so the CFD method is perceived as being a strong tool in the ship design process

Questions?

Thank you for your attention!

Questions?