Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec


taking into account a reduction of sail luff length as the name implies, with corresponding impact on lift, drag and effective span. Whilst theoretically available, the factor reef is usually omitted in the current version of RVPP. This mainly owed to the fact that the development of RVPP aims towards racing yacht for which reefing most often is not an option. For details of the IMS method see Claughton [4] or the IMS documentation from the Offshore Racing Congress [8].


By using the aggregate sail coefficients as calculated in (19) and (20) one can calculate the sail force FS as shown in (21):


FS


1 2


 


 AAWS c


Air Sails LTotal


 





cLTotal cos AWA cDTotal sin AWA 0


sin AWA cDTotal cos AWA 


 Experience gained from seakeeping investigations


showed that the added air mass of sails contributes significantly to the moment of inertia of a sailing yacht, especially around the longitudinal axis. For details see Graf [5]. Therefore an additional added mass force FSA due to the movement of the sails in the surrounding air is taken into account:


FSB


A cA Aero 4


 M Air Sailsω zce  (22)


This leads to the final definition of the sail force vector as shown below which includes added mass and damping effects into the aerodynamics. Whilst the authors are aware, that


aerodynamic forces,


this approach won’t give exact unsteady the approximation is


deemed


applicable since the primary goal of the simulation is to get a steady state solution. To achieve this goal, aerodynamic added mass and damping are added to help the simulation to converge more quickly. Doing better here would have made determination of sail forces much more complicated and was therefore


neglected as


unnecessary for the current purpose. If the need arises to investigate dynamic yacht behaviour, the aerodynamic force model may be replaced by a more sophisticated solution for unsteady forces in the future.


Using similar auxiliary means for the hydrodynamics is unnecessary,


since a viscous FFS SAF flow code inherently


accounts for added mass and damping. Aero 


(23)


After transformation from the coordinate system planar to the water surface to a boat-fixed coordinate system, the sail force vector FAero is applied to the boat at the position of aerodynamic longitudinal and vertical centre of efficiency. At the current state of the program the


2 (21)


The optimizer applied here is a custom modification of Brent’s Method, combining a bracket search with a parabolic search algorithm.


Optimizing boat speed is based on the principle of “depowering” the sail; flat reduces total lift coefficient and total drag coefficient, as to eq. (19) and (20). Linear reduction of lift generates quadratic reduction of drag, giving better lift to drag ratio In addition, the heeling moment and consequently the heel angle is reduced with the consequence of a more favourable AW as to eq. (15).


4. IMPLEMENTATION


The theoretical method as described in the previous chapter is implemented in the commercial RANSE code Star-CCM+5.02,


and a cell centred flow which has been used for the


investigation presented here. Star-CCM+ solves mass- and momentum transport equations using a finite volume approach with polyhedral grid cells, Cartesian velocity components arrangement.


Turbulence is modelled using the Shear Stress Transport Model. For wall boundary conditions the SST turbulence model uses a wall treatment scheme, where low Reynolds number modelling of turbulence is used near a wall if the local dimensionless wall distance y+ falls below the limits of the logarithmic wall functions.


Motions of the rigid body are accounted for using the DFBI-Solver which calculates 6-DOF Motion of a rigid body. Translations and rotations of the yacht are resolved by moving the whole computational grid. The equations of motion are solved for in 5-DOF with the yaw rotation being kept fixed. This simplification is taken because compensating for yaw moment would require implementing a rotating rudder or at least some kind of virtual


rudder force model along with an automatic


control which balances aerodynamic and hydrodynamic yaw moments. While this is certainly a goal for future development, it hasn’t been implemented yet.


Star-CCM+ is based on a client-server architecture with a lightweight client based on a JAVA-Interface and a


©2011: The Royal Institution of Naval Architects B-87 variable


longitudinal part of the centre of effort CEAero is approximated to be at the geometrical centre of the sails. Since this approximation does not coincide with the physical position of CEAero, a better approximation is currently under development.


3.4 OPTIMIZER


To reflect the efforts of the sailors in order to trim the sails for optimum boat speed an optimizer has to be applied on the trim parameter flat.


fla 0


  u


t (24)


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