OrcaFlex Manual - Orcina

System Modelling: Data and Results, 6D Buoys
The angles define the orientation of the local wing axes relative to the buoy axes as follows:
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w
� Start with the wing axes Wxyz aligned with the buoy axes Bxyz and then rotate Wxyz about Bz by the azimuth
angle. This leaves Wz aligned with Bz but Wx now points in the direction towards which the declination is to be
made.
� Now rotate by the declination angle about the new direction of Wy. This declines Wz down into its final
direction, i.e. Wz now points along the direction whose azimuth and declination angles are as specified.
� Finally rotate by the gamma angle about this final Wz direction. This is a rotation about the principal wing axis,
so it allows you to adjust the pitch of the wing.
For each of these rotations, positive angles mean clockwise rotation and negative angles mean anti-clockwise
rotation, when looked at along the axis of rotation.
When setting these orientation angles, it is easiest to first set the azimuth and declination values to give the desired
Wz-direction. This is the direction of the axis about which the wing pitch is set. Then set gamma to give the correct
pitch of the wing. This process is best done with the Draw Local Axes option set on (see the View menu or the
Tools | Preferences menu) since the wing axes are then visible on the 3D view and you can check that the resulting
orientation is correct.
Wing Type
Determines the properties of the wing. You can define a number of wing types – click the "Wing Types" button to
access the wing types data form.
6.9.5 Wing Type Data
6D buoys can have a number of wings attached, each having its own data and type.
Name
Used to refer to the wing type.
Wing Type Properties
The properties of each wing type are specified by giving a table of lift, drag and moment coefficients as a function of
the incidence angle of the flow relative to the wing.
A 'Graph' button is provided, which displays a graph of the 3 coefficients so that you can visually check your data.
Incidence Angle
The incidence angle is the angle, α, that the relative flow vector makes to the wing surface. This equals 90° minus the
angle between Wy and the relative flow vector.
The incidence angle is always in the range -90° to +90°, where positive values mean that the flow is towards the
positive side of the wing (i.e. hitting the negative side) and negative values mean that the flow is towards the
negative side of the wing (i.e. hitting the positive side).
The incidence angles in the table must be given in strictly increasing order and the table must cover the full range of
incidence angles, so the first and last angle in the table are set to -90° and +90° and cannot be changed. Linear
interpolation is used to obtain coefficients over the continuous range of angles.
Note: The wing lift, drag and moment are assumed to depend only on the incidence angle, not on the
angle of attack in the wing plane. OrcaFlex will therefore use the same lift, drag and moment
coefficients for flow (with the same incidence angle) onto the front, the side or the back of the wing,
even though your data may only apply over a limited range of in-plane attack angles. You can
check that the angle of attack in the wing plane stays within the range of your data by examining
the Beta angle result variable.
Lift, Drag and Moment Coefficients
These define the hydrodynamic and aerodynamic loads applied to the wing. Aerodynamic loads are only applied if
the Include wind loads on 6D Buoy Wings option is enabled in the Environment data.
The lift coefficient Cl(α) defines the lift force applied to the wing, as a function of incidence angle α. The lift
coefficients can be positive or negative and the lift force is given by:
Lift Force = P ½.Cl(α).ρ.A.V 2