OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
Wake Equation of Motion
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The wake equation of motion is typically a nearly linear, second order, ordinary differential equation. It is not
usually derived from physical laws, but is chosen to be one whose qualitative characteristics are known to be similar
to VIV. For example there are differential equations that are known to have solutions that are oscillatory, self
generating and self limiting.
The wake equation of motion involves parameters whose values are calibrated to match empirical results. This sort
of modelling ethos is commonly known as an inverse method. This is where one attempts to reproduce empirical
data without recourse to the fundamental physics of the system. Rather, one simply writes down a system of
equations that have the right sort of characteristics and then adjusts parameters in the equations to tune them to
best match the empirical data.
Almost universally, wake oscillator models only give the lift force and say nothing about the effect of VIV on the drag
force. The main aim behind the wake oscillator paradigm is to model the oscillating lift force.
Using a Wake Oscillator Model
Wake oscillator models are time-domain models and so can only be used in the dynamic analysis. To use a wake
oscillator model set the Dynamics VIV to that model.
When the simulation is run OrcaFlex creates and attaches a wake oscillator, of the chosen model, to each node in the
line. Each such oscillator then obeys the equations of the chosen model. There is no linkage between the wake
oscillators except through the structure. It is therefore effectively being assumed that the interaction between VIV at
different levels occurs predominantly through the structure, not through the fluid.
Lift Direction
The wake oscillator models are single degree of freedom models, i.e. they only model the transverse direction. Note
that this direction can change during the simulation, either because the line orientation changes or because of wave
motion changing the fluid velocity direction. When this happens the wake oscillator model is effectively being
rotated and there is an implicit assumption that this rotation does not significantly affect the wake.
Node Steady Motion Included
The wake oscillator models require the flow velocity as input. In OrcaFlex this input flow velocity is taken to be the
fluid velocity minus the filtered node velocity. This allows non-VIV motion, e.g. in a towed case, to contribute to VIV,
providing its period is significantly longer than the filter period. The filtering is necessary to prevent the VIV motion
itself feeding back into the input to the wake oscillator.
Current and Wave Motion Are Both Included
The input velocity to the wake oscillator models include the fluid velocity due to both current and any waves
specified. The models can therefore in principle be used to model the effect of waves on VIV. However please note
that the models were developed and calibrated for steady state conditions, so unsteady flow is outside their
intended area of application.
Inline drag amplification
The effect of inline drag amplification can be modelled by means of a table relating amplification factor to transverse
A/D.
Data Common to Wake Oscillator Models
Model Parameters
Both the Milan and the Iwan and Blevins models have various parameters that determine their properties. You can
choose to either use the Default set of values for these parameters or else choose to use your own Specified values.
Except for the Initial Value (see below), the Default values are the parameters given in the original published
papers, i.e. in the Milan and Iwan and Blevins papers. If the Specified option is chosen then you have complete
control over the model parameters.
Warning: The Specified option has been provided principally to allow users the option of calibrating the
model against other experimental results. If you are not doing this then we strongly recommend
that you use the Default parameters.
The following two model parameters are common to both wake oscillator models.