Innovative CFD tools for hydrodynamic design of IACC boats

1. Innovative CFD tools for hydrodynamic design of IACC boats J. García-Espinosa, COMPASS IS, julio@compassis.com A. Souto, ETSIN, asouto@etsin.upm.es

2. CFDComputational Fluid Dynamics • Techniques to solve problems in fluid mechanics on computers • Algorithms to simulate real behavior of fluids • Tools used to predict/understand real behavior of a system in a fluid flow

3. Numerical simulation + Validation Model testing + Extrapolation Numerical Simulation Model Testing

4. (Available) CFD Applications • Hull and appendages hydrodynamic resistance calculation / Wave pattern analysis • Lift forces calculation • 2D profiles optimisation • Local phenomena analysis • Sails simulation • Mast profile optimisation • Aerodynamic resistance • Seakeeping analysis • Other (dynamic loads estimation, velocity prediction, ...)

5. (Current) CFD Applications • Hull and appendages hydrodynamic resistance calculation / Wave pattern analysis. • Widely used: Linear/Nonlinear FS potential flow solver • Frequently used: Fully nonlinear FS RANSE solver • Lift forces calculation • Widely used: Linear/Nonlinear FS potential flow solver • Frequently used: Fully nonlinear FS RANSE solver • 2D profiles optimisation • Widely used: RANSE solver • Local phenomena analysis • Widely used: Reynolds-averaged Navier Stokes (RANSE) solver. • Comming: LES turbulence models, direct simulations • Sails simulation • Widely used: Potential flow solver coupled to boundary layer solver • Coming: ALE RANSE solvers • Mast profile optimisation • Widely used: RANSE solver • Aerodynamic resistance • Widely used: RANSE solver • Seakeeping analysis • Widely used: Linear (frequency domain) solvers. • Comming: Non linear FS (time domain) solvers. • Other (dynamic loads estimation, velocity prediction, ...) • Frequently used: RANSE VOF – Level Set methods • Comming: Lagrangean - Meshless methods

6. Tdyn: Main Characteristics • Tdyn includes a fully integrated pre/postprocessor based on GiD system, incorporating advanced CAD tools (NURBS importation, creation and edition) • Data insertion (control volume generation, physical properties, boundary conditions, etc) is guided by the use of specific wizard tools for naval applications • Mesh can be automatically generated from the CAD information within the system. It also allows elements size assignment and quality check of the resulting mesh • System also includes a great variety of postprocessing options and tools for fast report generation • System includes a tool (procserver) which allows to perform Grid-Computing calculations • The same environment includes tools for performing a variety of analysis (structural, heat transfer, transport of species, user defined problems, …)

7. TdynFEM Fluid Dynamics RANSE-NAVAL module

8. RANSE solver. Characteristics. • Implicit and accurate scheme: In most of the cases of interest for the naval architecture the time step imposed by the stability criteria (smallest elements) may be orders of magnitude smaller than the time step required to obtain time-accurate results (physical time step), rendering explicit schemes impractical. -> Developed a new Predictor-Corrector implicit-second-order-time-accurate method • Pressure-Velocity segregation: Pressure-Velocity coupled schemes are very expensive from a computational point of view (velocity and pressure discrete equations are coupled). However the use of standard Pressure-Velocity segregation schemes impose strong limitations to the stability of the algorithm. -> Developed a new implicit-second-order-time-accurate method based on a pressure-velocity segregation scheme • Stable algorithm: Stability of the numerical algorithm is still today an open issue. There is a need for an accurate, pressure and advection stable algorithm, based on the physics of the problem. -> Developed a new stabilization methodology: Finite Increment Calculus based on the discrete balance of the quantities • Coupled free surface solver: Free surface condition must be coupled to the fluid flow solver in a stable and accurate way. -> Use of the transpiration technique.

9. Numerical and Implementation AspectsRANSE-NAVAL module • RANSE and FS equations are integrated by standard Finite Element Method (linear/quadratic tetrahedra, hexahedra, prisms, …) • RANSE and FS solvers have been optimized for working with unstructured meshed of linear tetrahedra/triangles • Implicit 2nd order time accurate fractional step algorithm is used to go faster to steady state • Boundary conditions may be defined by analytical functions (i.e. allowing to run different drifts angles with the same geometry/mesh) • RANSE-FS solver has been fully integrated within the multi-physics environment Tdyn

10. RANSE solver. Real Free Surface deformation and Sink and Trim treatment. • Free surface deformation as well as dynamic sinkage and trim angle are calculated during the program execution being boundary conditions and mesh automatically updated. • Finally a new calculation is performed with the real geometry. This is done in four steps: • New free surface NURBS definition, taking the resulting deformation into account, is generated: • NURBS Cartesian support grid of MxN points is created. • Z coordinate of the points, representing the wave elevation, is interpolated into the grid. • Finally, the NURBS surface based on the support grid is generated. • Geometry of the vessel if moved according to calculated sinkage and trim angle. • New control volume and mesh are automatically generated. New mesh is generated with a quality criterion based on using an error estimator based on FIC technique. • New calculation is carried out with fixed mesh

12. Numerical and Implementation AspectsLINEAR module • FS - Potential flow equations are integrated by panel method • The algorithm uses structured meshes of quadrilateral panels for free surface calculations • Boundary conditions are easily defined and automatically transferred to the mesh • Potential solver has been fully integrated within the multi-physics environment Tdyn

13. Application: IACC example 9 kn

14. Application: IACC example 9 kn

15. Application: Rioja de España • We present as example, the Spanish America’s Cup boat Rioja de España, participant in the races of 1995 • POTENTIAL Flow simulations have been carried out with a symmetric full geometry • RANSE simulations have been carried out at full scale, using a two layer k-e turbulence model, in combination with an extended law of the wall • Results are compared to experimental data extrapolations performed at El Pardo towing tank with a model at scale 1/3.5

16. Application: Rioja de España Acknowledgements: Authors thank National Institute of Aerospace Technique (INTA) for permitting the publication of the towing tank tests of Rioja de España, and the model basin El Pardo (CEHIPAR) for sending the full documentation of the tests. Special gratefulness to IZAR shipbuilders for allowing the publication of the Rioja de España hull forms.

17. Application: Rioja de EspañaExperimental data Every case was towed at equivalent velocities of 10, 9, 8.5, 8.0, 7.5 and 7.0 knots

18. Application: Rioja de España 9 kn

20. Application: Rioja de España All grids used have been generated with the same quality criteria (in terms of size transition and minimum angle) and using element sizes from 5mm to 2000 mm.

21. Application: Rioja de España E15D2 Initial mesh Final meshE0D0 All meshes have been generated with the same quality criteria (in terms of size transition and minimum angle) and using element sizes from 5mm to 2000 mm. E15D2 keel-bulb union detail E25D2 Final Mesh

22. Application: Rioja de España E25D2 Streamlines and velocity modulus contours (V = 9 Kn)

23. Experimental Simulation Application: Rioja de España E25D2 Wave profile 9Kn

24. Application: Rioja de España E25D2 Wave profiles and pressure contours 9Kn

25. Application: Rioja de España E0D0 Pressure contours 10Kn

26. Application: Rioja de España E25D2 Pressure contours and streamlines 9Kn

27. Application: Rioja de España E0D0 E15D2 E25D2 E15D4

28. Comments • RANSE solver. Numerical results obtained in the analysis of America’s Cup Rioja de España boat indicate that Tdyn can be an useful tool for practical design purposes. • Evaluation of total resistance gives less that 5% difference with towing tank extrapolations in most part of the analysis range. • Evaluation of induced (lift) forces in non-symmetric cases gives even less differences. • Obtained wave profiles are also very close to those measured in towing tank. • Qualitative results including: wave maps, streamlines, pressure and velocity contours and turbulence distribution, show also reasonable patterns • Numerical experience indicates that the RANSE formulation is very efficient for free surfaces flows, when the critical time step of the problem is some orders of magnitude smaller than the time step required to obtain time-accurate results - physical time step - (i.e. 4 CPU h / 1.5Mtetras standard PC single processor, including s&t final calculation – use of a integrated Grid Computing System “ProcServer”). • Potential flow solver. Numerical results obtained in the analysis of America’s Cup Rioja de España boat indicate that the proposed method can be a useful tool for practical design purposes. • Evaluation of wave resistance gives less that 5% difference with towing tank estimation in most part of the analysis range. • Evaluation of induced (lift) forces in non-symmetric cases gives even less differences. • Obtained wave profiles are also very close to those measured in towing tank. • Qualitative results including: wave maps, pressure and velocity contours show also reasonable patterns • Potential flow solver is a very efficient tool for free surfaces flows. It can be used instead of RANSE solver when local effect hasn’t interest for design (first stages of the design). Standard PC-CPU time required for one calculation is about 10 min. • Tdyn system has been used in the design of ALIGNHI and GBR America’s Cup boats

29. (Future) CFD Applications • Highly accurate results • Automatic mesh generation. • Boundary layer simulation. • Accurate turbulence models. • Error estimation. • Mesh refinement. • Automatic optimization of design • Complex criteria (stability) • Parametric geometry definition • Automatic geometry/mesh update • Coupled fluid-structure calculation (hydro-elasticity) • VPP-CFD tool (coupled) • Hydrodynamic flow. Free surface. • Aerodynamic flow. • Structural (membrane) analysis. • Automatic mesh regeneration. • VPP-CFD tool in waves • Hydrodynamic flow. Free surface. • Aerodynamic flow. • Structural (membrane) analysis. • Small CPU time requirements

30. further information at ... http://www.compassis.com http://www.gidhome.com email:info@compassis.com