Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec
Possible solutions to this problem can be divided into those which keep up the strategy of single moving grid and those who do not.
Solution Options with Single-Grid Moving Grid 1. Better alignment of the grid lines with the expected heel angles will certainly give a better representation of the free surface, but produces complicated grid topologies.
2. Refinement of the grid in the far field will probably give the expected. However, this is not very
desirable because it computational increases the effort. Nonetheless refinement
might still be an option if other grid topologies can be used which allow a more distinguished distribution of grid cells, for example trimmed Cartesian, polyhedral or tetra grids.
Solution Options without Single-Grid Moving Grid 1. Mesh deformation is one possible grid strategy. Here the grid around the boat is deformed to allow for the motion of the boat instead of moving the whole grid. This method has been used before by the authors and was abandoned since it was neither very flexible nor reliable since it tended to produced negative Jacobian of the Control Volumes.
2. Overlapping Grids are another strategy which might be useful. Here two grids are used: An orthogonal grid is employed for the far field and a box around the yacht which could be resolved with a fine polyhedral grid. In the authors opinion this would be the most
flexible and
elegant solution, alas it is not available in the current Version STAR-CCM+. This leads to the conclusion that until overlapping grid technique is available the other mentioned solution approaches have to be tried out.
8. CONCLUSION
The paper presented here shows a new method to predict the performance of sailing vessels. Instead of resembling towing tank procedures to produce coefficients
from
RANSE simulations for a subsequent VPP analysis, it proposes to directly include the prediction of velocity into the
applying the sail forces via a dynamic sail force vector. It thus
simulation of the boat hydrodynamics by takes advantage of the possibilities of modern
viscous CFD codes, allowing to include a solution for the body motion equation of solid flow bodies under the impact of internal (RANSE-calculated) flow forces as well as external forces.
First results indicate that the method is generally feasible and the expected reduction of the computational costs for performance prediction
of sailing yachts has been
achieved. As an example, a GP26 boat design was investigated with RVPP. The results gained have been compared with data from a conventional VPP method.
3. 2.
The advantages of the method include a lesser number of simulations necessary to calculate a velocity polar plot and therefore a smaller computational effort. It can calculate boat performance at some discrete points of the polar plot as combinations of TWS and TWA, thus allowing for
conventional VPP methods it should come with an small increase in
accuracy in
rapid prototyping. Compared with the consideration of the
hydrodynamics, however this has to be validated in the future.
A disadvantage is that the calculated performance prediction is valid only for a specific sail set. If the performance of others sail sets shall be investigated, the calculation procedure has to be redone.
Since the method is still in its development phase it is naturally not flawless. The major problem in the current approach is vested in the need to accurately resolve the free surface in heel angles of +/- 30° around the boat’s DWL. Possible solutions to this include grid refinement, grid morphing and overlapping grids.
Further objectives include the implementation of yaw balance, which has to be based around an improved formulation to predict the LCE of the sails. Another desirable feature is to include the dynamic pitch and roll moment applied by movement of the crew or other types of ballast.
As next and most important step, the method has to be validated and its accuracy has to be assessed. It is planned to do so against conventional VPP results with two set of databases, one gained from processing of high quality towing tank data and the other from numerical towing tank test employing the same computational grid.
9. 1.
REFERENCES
GRAF, K. and BÖHM, C.: A New Velocity Prediction Method for
Post-Processing of
Towing Tank Test Results, Proc. 17th Chesapeake Sailing Yacht Symposium, Annapolis, Maryland, March 2005
F
ERZIGER, J.H. and PERIC, M.: Computational Methods for Fluid Dynamics, Springer, New York 2002
BÖHM, C. and GRAF, K.: Validation of RANSE simulations ACCV5 design
of a fully
Proceedings of the INNOV'Sail08, France, April 2008
4. using towing tank
appended data
Lorient,
CLAUGHTON, A., WELLICOME, AND SHENOI: Sailing Yacht Design / Theory, Addison Wesley Longman Limited, Essex, GB, 1998
©2011: The Royal Institution of Naval Architects
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