Application of CFD in connection with ship design

Transcription

1 DANSIS meeting Lyngby, 13 May 2009 Application of CFD in connection with ship design Background Method Examples Summary Claus Daniel Simonsen FORCE Technology

2 Background When a ship, which consists of hull, propeller and rudder, is designed it is important to obtain an efficient solution for all components working together: The hull form should be optimized to minimize the resistance, which will reduce the engine power required to drive the ship through the water and hereby also the fuel consumption The propulsion system should be optimized to work efficiently in the wake behind the ship, which also will influence the required power and the fuel consumption The rudder should be optimized in order to maneuver the ship safely, but also 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 to the strong interaction between the hull and propeller flows. Further, the rudder operates in the propeller slipstream

3 Background Today the most commonly method used for checking combined hullpropeller flows is towing tank/cavitation tunnel testing. However, due to the cost of model testing, the combined hull and propeller designs are checked relatively late in the design process The idea is to investigate if CFD can be used for evaluation of hull and propeller geometries in conditions where the propeller is working behind the ship. No requirement to physical model means a tool that can be used to improve the propeller-hull designs in the early design Calculation of forces and moments will give a picture of the performance of the system in the early design stage Calculation of the flow field will give insight in the physics of the of the system in the early design stage

4 Background Some of the basic model tests that are used today in connection with 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

5 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

6 Method All simulations are done with the RANS solver STAR-CCM+ in model scale Meshing Surface wrapping Trimmed/polyhedral mesh approach Prism layer meshing in boundary layer Zonal refinements Physics modeling 2 phase VOF model for free surface modeling Steady and transient calculations depending on model MRF and RBM used in connection with the propeller model Sliding interfaces used for rotating propeller k-ω SST turbulence model, all Y+ treatment

7 Example: Open-water propeller Open-water propeller model Modeled with steady state MRF and 2 million cells Pressure distribution illustrates loading on the blades

8 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

9 Example: Resistance test Model of ship without propeller Steady state solution with 3 million cells Free surface prediction is essential for resistance calculations elevations Wake field prediction is important for propeller Resistance predicted within 3% of measurement

10 Example: Caviation tunnel setup Model of ship with propeller Transient simulation and 4.5 million cells Rotating propeller, RBM and sliding interfaces between propeller and hull domain No free surface, no cavitation Results used to study flow features Time varying results: movie

11 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

12 Example: Caviation tunnel setup Model of ship with propeller including cavitation Transient simulation and 13 million cells Rotating propeller, RBM and sliding interfaces between propeller and hull domain Rayleigh-Plasset model Qualitative agreement with model test results on propeller blades RANS is too dissipative to maintain tip vortex cavitation

13 Example: Ship at self-propulsion Model of ship with propeller in towing tank Transient simulation and 6 million cells Rotating propeller, RBM and sliding interfaces between propeller and hull domain Visualization of free surface features Breaking bow wave captured Unsteady and asymmetric stern wave

14 Example: Ship at self-propulsion Model of ship with propeller in towing tank Qualitative comparison with measured wave profile Overall wave pattern captured Spray in bow region reduced in CFD

15 Example: Ship at self-propulsion Model of ship with propeller in towing tank Visualization of flow in stern region Instantaneous picture Flow around ice fins

16 Example: Ship at self-propulsion Model of ship with propeller in towing tank Visualization of flow on rudder Instantaneous picture Typical behavior of streamlines

17 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 picture Acceleration of axial flow Tip and hub vortices

18 Example: Ship at self-propulsion Model of ship with propeller in towing tank Visualization of tip vortices along the rudder Instantaneous picture Not exactly same propeller position Important that vortex passes below rudder

19 Example: Ship at self-propulsion Model of ship with propeller in towing tank Comparison between measured and calculated data for propeller and hull quantities Fairly good agreement with measurement. Propeller quantities and resistance are within 3% of measured data

20 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 design process 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 still needed to make the final check of the final design When is comes to insight in the flow details and understanding of the physics of the flow CFD is very strong, because the information can be used to improve the design and to make sure that the flow behaves well before the physical model is build. Experience shows that level of validation can vary between applications i.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

21 Questions? Thank you for your attention! Questions?