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CFD for Aerodynamics of Fast Ships Volker Bertram

CFD for Aerodynamics of Fast Ships Volker Bertram. Scenario - what, why, etc. Aerodynamic flow around a ship superstructure is important in many ways: Exhaust dispersal Ventilation of occupied spaces Wind forces, especially for maneuvering

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CFD for Aerodynamics of Fast Ships Volker Bertram

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  1. CFD for Aerodynamics of Fast Ships Volker Bertram

  2. Scenario - what, why, etc. • Aerodynamic flow around a ship superstructure is important in many ways: • Exhaust dispersal • Ventilation of occupied spaces • Wind forces, especially for maneuvering • Special operational conditions - helicopter landing, etc. • Used to make design & operational decisions

  3. Wind tunnel proven tool This information now predominantly from wind tunnel tests Wind tunnel proven tool to provide useful information about the airflow.

  4. Aero CFD: An alternative now! • Aerodynamics CFD effective in other engineering fields: • aerospace • automotive • civil engineering • Advantages: • All information available at any time • More precise control over what is viewed • More details are possible • Full scale (but still idealized...) • Non-intrusive

  5. CFD for ship aerodynamics now a topic • Problem difficult due to various factors: • Grid generation very difficult • Large grid cell count • Complex physics • Recent progress addresses these issues: • Unstructured, more robust solvers • Improved, automatic grid generation tools • Advanced numerical modeling techniques • Affordable parallel computing

  6. Several applications in last years • DMI • Sirehna • Marintek • NRL • JJMA • Stanford • KRISO • ... (?) DMI JJMA NRL Sirehna

  7. Several applications also at HSVA

  8. Tools and Methods Used Typical geometry imported from IGES format Unstructured, tetrahedral grids generated using ICEM-CFD, cell count of up to 5 million cells Calculations with Comet on a parallel PC cluster

  9. Geometric modeling of all superstructures impossible

  10. Baffle Elements Tested model global effect of replacing filigree structures by semi-permeable cell boundaries cell count for a 2D case  22 000 cells 6 000 cells

  11. Baffle elements disappointed k and |v| for mast geometric model Assorted baffle parameters

  12. Simple block does the trick k and |v| for mast geometric model Simple block

  13. Application to fast ferry Superfast VI, HDW, 29 kn IGES file from yard too detailed: several weeks work to downstrip

  14. Grid: 680,000 cells per symmetry half

  15. Model tests performed at IFS wind tunnel Physical model (1:150) in wind tunnel

  16. Local refined grid reduces discretization errors Exhaust concentration

  17. Similar agreement for wind from abaft Experiment CFD

  18. Some similarity laws always violated ratio of velocities ratio of mass flux ratio of momentum flux Reynolds number of the inflow Reynolds number of the jet Froude number ofthe jet geometric similarity They cannot be fulfilled all at the same time!

  19. Parameter studies exhaust gas temperature 300°C model test parameters full scale Rn inviscous computation

  20. Visualisation of different quantities pressure distribution (30°) stream lines (0°) turbulent kinetic energy k (0°)

  21. Forces OK for small-to-medium angles Drag Roll moment Side force Differences for large oblique angles attributed to flow separation insufficiently captured by turbulence model

  22. Application to fast SES AGNES 200, French SES, 40 kn First step: Create IGES description

  23. Grid topology allows easy re-gridding Inner cylinder in outer block Matching every 5° 2.9 million cells

  24. Pressures change with angle of attack 180° 170° 150°

  25. Vortex formation behind superstructure streamlines =180°

  26. Strongly 3-d flow streamlines =180° 5 cm higher

  27. Features similar for 170° streamlines =170°

  28. Less complex “foil” flow for for 150° Flow follows low-pressure side of SES streamlines =150° Streamlines return to original direction further downstream

  29. Flow strongly 3-dimensional Virtual Reality may help understanding the flow

  30. Virtual Reality comes in many shapes Poor man’s VR suffices! Sources: VRL, Univ. Of Michigan; VRCA RWTH Aachen cave head-gear PC VRML

  31. What is VRML? VRML = Virtual Reality Modeling Language • 3D file interchange format • 3D analogue to HTML • ISO standard

  32. Steps creating a CFD VRML model • Study experience of others VRL, Univ. of Michigan INSEAN INSEAN+TUHH TUHH

  33. Steps creating a CFD VRML model • Study experience of others • Export geometry data from RANSE solver • Downsize geometry Direct export: 43000 polygons 2810 KByte

  34. Steps creating a CFD VRML model • Study experience of others • Export geometry data from RANSE solver • Downsize geometry Direct export: 43000 polygons 2810 KByte After merging: 900 polygons 130 KByte

  35. Steps creating a CFD VRML model • … • Build VRML geometry model • Process and downsize flow data • Add flow data to VRML model • Add interaction to VRML model Interactive selection Interactive high-lighting

  36. Pressures use colour VRML interpolation Work continues: • Refine algorithm to downsize model

  37. Conclusions • CFD offers more insight than wind tunnel • Further work required for validation • Wind tunnel may be too optimistic for smoke tracing • VRML suitable for post-processing

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