OrcaFlex Manual - Orcina

System Modelling: Data and Results, General Data
Outer time step (fraction of Wake Oscillator Strouhal period)
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w
This data item is only available if you are using a Wake Oscillator VIV model. The recommended outer time step will
be no greater than the minimum Wake Oscillator Strouhal Period divided by this value.
6.4.5 Implicit Integration
Implicit Integration Parameters
Use variable time step
For the implicit solver OrcaFlex offers both constant and variable time step algorithms.
The default is to use a constant time step and in general this is to be preferred. Variable time step schemes can
introduce high frequency noise into a system which in turn can lead to inaccurate results, for example noisy time
histories, non-physical spikes in results etc. Note that this is a feature of all variable time step algorithms. For the
majority of systems no problems arise when using a variable time step. However, if you are using variable time steps
then we do recommend that you check the quality of your results.
The variable time step algorithm chooses the time step based on the number of iterations used for previous time
steps. If a large number of iterations were required for previous time steps then the time step is reduced.
Conversely, if a small number of iterations were required then the time step is increased. The results variables
Implicit solver iteration count and Implicit solver time step can be used to track the performance of the variable
time step algorithm.
Time step / Maximum time step
If you are using a constant time step then this data item determines that time step. OrcaFlex has a default value of
0.1s. However, different systems will require shorter time steps and indeed some systems can give perfectly
accurate answers with longer time steps. If you wish to optimise run times then you may need to experiment with
different time step values.
If you are using a variable time step then this data item limits the time step used by OrcaFlex and it will never
exceed this value.
Warning: Implicit solvers can produce inaccurate results, even for stable simulations, if the time step is
chosen to be too large. Because of this we strongly recommend that you carry out sensitivity
studies on your time step to ensure accuracy. A very useful additional technique is to compare
results from the implicit solver and the explicit solver.
Maximum number of iterations
The implicit integration scheme uses an iterative method to solve the dynamic equilibrium equations. The
calculation is abandoned if convergence has not been achieved after this number of iterations.
If you are using a variable time step then this results in the time step being reduced rather than the simulation being
aborted. Accordingly we recommend that a relatively small value is used, we recommend using the default value of
20. This allows OrcaFlex to abandon the current iteration quite early and try again with a shorter time step.
Conversely if you are using a constant time step then the simulation is aborted if convergence cannot be achieved
inside this number of iterations. Because of this we recommend using a larger value than for variable time steps.
The default value of 100 is usually a good choice.
Tolerance
A non-dimensional value which controls the accuracy of the solution. Increasing this value can result in reduced
computation time since fewer iterations are needed to solve the dynamic equilibrium equations. However, doing so
may also result in inaccurate results.
Notes: We recommend that you use the default tolerance value. Should you choose to increase it then we
strongly recommend that you perform sensitivity studies to confirm the accuracy of your results.
For systems where the only responses are extremely slowly varying (e.g. drift motions, Spar VIM)
you might need to use a smaller tolerance than the default value to obtain accurate results.