OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
Our experience of the Milan Model
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� If a line end node doesn't move at all (e.g. because it is fixed) then the wake oscillation does not develop and so
the Vortex Force decays to zero. The Milan model is therefore not suitable for predicting the vortex force on
fixed line end nodes.
� We have run the Milan model for the case of a simple spring-mounted cylinder, and compared the results with
the empirically based response curve published by Skop and Balasubramanian. For details contact Orcina.
� We have also run the Milan model for flexible riser cases and compared it against experimental results. The
results so far suggest that the Milan model is reasonable for cases where the flow velocity is uniform along the
riser, but the model is less satisfactory when the flow velocity varies a lot along the riser.
Iwan and Blevins Wake Oscillator Model
This wake oscillator model is as published by Iwan and Blevins. In their paper the model is developed from
theoretical considerations of momentum, and the hidden wake degree of freedom is such that its rate of change is a
measure of the fluid momentum in the transverse direction.
The resulting model makes the wake degree of freedom obey a Van der Pol equation. This is a type of equation that
has been used in other wake oscillator models, and it is known to have VIV-type characteristics such as frequency
lock-in.
The authors calibrated the model against experimental results for fixed and forced cylinders, and then compared the
model's predictions against experimental results for spring-mounted cylinders.
In addition note that the force generated by the Iwan and Blevins model (unlike the Milan wake oscillator model)
includes the standard Morison drag force in the transverse direction. When this model is used OrcaFlex therefore
suppresses the transverse component of the usual drag force, so the drag coefficient specified for the transverse
direction is not used.
Results
The Vortex Force is available as a line force results variable. This reports the total lift and drag force. Note that 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, plus the standard Morison drag force in the inline, transverse and axial directions.
Transverse VIV Offset is also available as a line position results variable.
9.2.2 Vortex Tracking Models
Overview
Two vortex tracking models are available in OrcaFlex, which we refer to by number: Vortex Tracking (1) and Vortex
Tracking (2). Both are based on the underlying physical equations of boundary layer theory and the Navier-Stokes
equation. As a result they introduce physical realism that is absent from the wake oscillator models.
Vorticity is a measure of a fluid's rotation and it is often advantageous to analyse fluid dynamics in terms of
vorticity. The reason for this is that the vorticity is often confined to narrow sheet-like regions and the important
fluid forces on a body in the flow are intimately related to the vorticity. One can focus on the sheet-like regions and
this is far more efficient from a computational point of view. Vortex methods are prevalent throughout
computational fluid dynamics.
The vortex tracking models are much more computationally demanding than the wake oscillator models. In fact they
are a type of computational fluid dynamic model, but they are much less computationally demanding than 'full'
Computational Fluid Dynamics (CFD). Work to date shows considerable promise, and we hope they will offer a
practical analysis technique which gives much of the realism of full CFD without the associated extremely long run
times.
The vortex tracking models model the full fluid flow field. In OrcaFlex the vortex tracking models are used to give
the force acting on the line, but they can also give other results, such as the fluid velocity and pressure at any point.
For example we have experimentally used them to calculate the pressure variations on the line surface due to VIV. If
other results such as these interest you then please contact Orcina for further details.
The vortex tracking models are based on the relative velocity of the flow past the line. They can therefore be used
both for cases where the excitation is due to current or waves, and also where the excitation is due to the line
moving, for example towed cases.