OrcaFlex Manual - Orcina

System Modelling: Data and Results, Lines
User Specified model
316
w
This option allows you to define a model (of both wake drag reduction and wake lift effect) by specifying drag and
lift coefficient factors as a function of the position of the downstream object relative to the wake of the upstream
object.
The wake effects are specified by giving a table of wake drag and lift coefficient factors for the downstream
cylinder, as a function of the downstream cylinder position relative to the upstream cylinder wake, as follows. See
above for notation.
� The Position columns of the table define, in non-dimensional form, a number of downstream cylinder centre
positions relative to the upstream cylinder wake frame of reference. This is done by specifying nondimensionalised
distances L/Du (downstream) and T/Du (transverse) from the upstream cylinder centre to the
downstream cylinder centre, where Du is the normal drag diameter of the upstream cylinder.
� The Coefficient Factor columns of the table define the wake effects at the given (L/Du, T/Du) positions, by
specifying drag and lift coefficient factors. Note that these data are scaling factors, not the drag and lift
coefficients themselves. The drag factors are scaling factors that are applied to the reference drag coefficient
CDd0 of the downstream cylinder, as specified on the line type data form. The lift coefficient factors are signed
scaling factors that are applied to the Reference Wake Lift Coefficient CLd0, which is specified on the wake
models data form. A +ve lift coefficient factor means a lift force in the +ve wake frame y-direction, so the lift
coefficient factor at a given T/Du position will normally have the opposite sign to the T/Du value, since wake lift
effects are normally towards the centre line of the wake.
OrcaFlex uses linear triangular interpolation to obtain the drag and lift coefficient factors to use for wake frame
positions between those specified in the table.
Note: The drag coefficient factors can be negative, in which case they represent flow reversal at that
position in the wake. This can happen, and indeed the Blevins model can give flow reversal just
behind the upstream cylinder. However the drag factors must not be greater than 1, so flow
enhancement cannot be modelled.
Wake drag effects are normally symmetric, and wake lift effects anti-symmetric, either side of the wake centre line.
So to avoid the need to specify in the table both +ve and -ve values of T/Du you can tell OrcaFlex to Reflect Data. In
this case you must only specify table rows for one half of the wake plane, i.e. either for T/Du ≥ 0 only, or for T/Du ≤ 0
only. OrcaFlex will then automatically reflect all your data points that are not on the wake centre line, by internally
duplicating them and negating T/Du and the lift coefficient, and will then interpolate over that new
specified+reflected data set, which now covers both sides of the wake centre line.
Results
Log Results
Note: Data reflection will not in general give perfectly symmetric response characteristics. This is
because the interpolation involves triangulation of the data, and that triangulation might not be
symmetric either side of the wake centre line. However any lack of symmetry will be proportional
to the spacing of the points you specify in the data - more closely spaced data points will give closer
to perfect symmetry.
This option is checked by default and this means that simulation results at all points on the Line are available. If this
option is unchecked then no simulation results are available for this Line.
OrcaFlex stores simulation results in an efficient way, only logging a minimal set of variables to the simulation file.
Other results variables which have not been logged are then derived when the results are requested. Usually this
means that simulation files are a reasonable size and we recommend that this value is checked.
Should you need to reduce the size of simulation files then this option can be unchecked for those lines for which
you do not need results.
Arc length axis, Arc length axis inverted, Value axis inverted
These data items allow you customise the way range graphs are displayed. The Arc length axis setting allows you to
control whether the arc length axis is horizontal or vertical. The latter option would typically be used for vertical
risers.
Normally the axes on an OrcaFlex graph display increasing values to the right (for a horizontal axis) or upwards (for
a vertical axis). The axis inverted options allow you to reverse the axes. Again this would typically be used for