Milan wake oscillator model

The Milan model is an implementation of the model developed by a group in Italy and documented in the paper by Falco, Fossati and Resta. It is a wake oscillator model; see the wake oscillator models topic for information that applies to all such models.

Data

There are no data specific only to the Milan model. See instead

Results

The vortex force is available as a line force result which reports the total lift and drag force. This is the sum of the force generated by the wake oscillator model (which is in the transverse direction but doesn't include the drag force in that direction) and the standard Morison drag force in the inline, transverse and axial directions.

Transverse VIV offset is also available as a line position result.

Milan model implementation in OrcaFlex

In implementing the Milan model in OrcaFlex we came across the following issues.

Lift force

In the Milan model the standard Morison drag force in the transverse direction must be added to the force generated by the wake oscillator. OrcaFlex therefore calculates the drag forces as usual and then adds in the force generated by the Milan wake oscillator. The line motion therefore depends significantly on the drag coefficient specified for the transverse direction.

Notes: The Milan model was calibrated by assuming a transverse drag coefficient of 1.2 and a transverse added mass coefficient of 1.0, so other values of these coefficients take the model outside its domain of calibration.
The transverse force factor is only applied to the force generated by the Milan model, not to the transverse component of the standard drag force.

Node mean position

The Milan model needs to know the node offset in the lift direction, relative to its mean position, i.e. relative to the position about which VIV is occurring. For this mean position OrcaFlex uses the filtered position of the node. This enables OrcaFlex to handle cases such as towed lines, where the VIV excitation is due to motion of the line rather than (or as well as) that of the fluid. It is important that the filter period is suitably set.

Our experience of the Milan model