also included in the RANS simulations but added as additional drag components.
By way of illustration, in the above table we have a partial extract from an analysis of interference drag; we have the force, moment and centre of efforts of our CFD solution for one of the points simulated. The global force summary listed in the ‘All’ row at the top of the table, and then each component we specified in the simulation’s set-up is broken down individually in the rows below (such as jib, main, mast, aft beam and so on). Each row lists force summaries for each component. With this method you can start to see which piece of the global package is carrying load and then determine its efficiency. A yacht designer would then use this information to adjust the design to suit the intended need/use and/or highlight where they should be spending resources to better understand the flow and/or loadings. The bottom line is that the drag of items located in the near flow field of another body is now better understood and the data has increased in accuracy. Using this method the number of boat components not included in the windage simulation has been greatly reduced.
The North VPP
In our simulation the performance effect of the platform was predicted using North’s 6 DOF VPP. This in-house VPP functions as a constrained optimiser, which runs atop a collection of user- defined modules of tabulated data. In this case the platform and aero forces were considered a unit, which incorporated heel, trim and AWA (yaw) into the data module.
As an example: a data module can be a table of dagger-foil force and moment components listed for various foil-setting parameters – like foil extension, rake, yaw and pitch, plus in-flow conditions such as boat speed, leeway, sink and trim. They are the same variables for a data module for the platform/sail aero package as they are for a particular rig/sail set.
With the collection of data modules, each data set is first run through a pre-processor. The pre-processor converts the tabulated data into continuous multi-dimensional surfaces of force and moment components, all as functions of the previously user-defined parameters.
The VPP processor then takes into account all such user-defined surface modules. The optimiser aims to find the combination of user-defined parameters for which the combined force and moment components are in equilibrium, and for which the user-defined performance – say boat speed or VMG – is maximised. The force and moment equilibriums, aka degrees of freedom (DOF), are treated as constraints in the optimisation. Hence the user can specify which DOFs to be enabled or disabled. Typically all six DOFs are enabled for a complex foiling multihull. Additional constraints, such as lower/upper bounds, can be placed on the user-defined parameters. Likewise, certain relationships can be enforced on individually selected sets of parameters. Next month… Into practice with the Superfoiler
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