Analysis Parameters

The analysis parameters control which VIV loads that are applied, how the response frequencies are combined and how the response iteration is carried out.

1. VIV analysis parameters

1.1. VIV load type

Option for the type of VIV loads to be applied.

  • Cross-flow : Cross-flow VIV loads are applied

  • In-line : In-line VIV loads are applied

  • Combined cross-flow and in-line : Both cross-flow and in-line VIV loads are applied

1.2. Response frequency option

Option for combining the response frequencies.

  • Concurrent : The response frequencies act concurrently, i.e. space sharing

  • Consecutive : The response frequencies act consecutively, i.e. time sharing

The response process observed in VIV experiments is often somewhere between concurrent and consecutive. The response frequency at a location shifts between different consecutive values, but other frequencies often dominate at other locations.

1.3. Damping

  • Relative structural damping : Relative structural damping applied to the total stiffness matrix. A value of 0.1 gives 10% relative damping.

  • Additional Structural Damping Specification : If ☑, additional material and slip damping may be specified and read from file.

Table 1. The damping input options in VIVANA. Relative damping, RELDAM, is applied to the total stiffness matrix. The additional damping from file is limited to the bending deformation only.
Stiffness matrix

Damping input

Material and geometric

Material

Deformation

RELDAM

X

axial, bending and torsion

additional damping from file

X

bending

X

bending

2. Advanced analysis parameters

2.1. Response calculation

Iterations are needed as the VIV loads are a function of the VIV response.

The VIV response is calculated separately for each identified response frequency in turn.

With Fixed point iterations the response is re-calculated with updated VIV loads until the change in calculated response is sufficiently small.

With Newton-Raphson iteration the previous response is modified by an increment calculated from the unbalanced loads. This is repeated until the increment is sufficiently small.

  • Iteration method : Fixed point or Newton-Raphson response iteration

  • Retry : If ☑ then if the response iteration does not converge, a second attempt will be made with the other response iteration method.

  • Convergence criterion :

    • AMPNOR : Norm of amplitude change for the translations. The squared sum of the amplitude changes for translations, normalised with the product of the number of nodes and the squared average diameter

    • DIFMAX : The maximum absolute change in amplitude, i.e. phase change ignored.

    • NONE : No convergence test, the maximum number of iterations will be performed

  • Convergence limit : Convergence limit for the iteration. Recommended values are 0.001 - 0.01 for the amplitude change norm (AMPNOR).

  • Max iterations : Maximum number of iterations

  • Initial response estimate : Scaling factor for the initial response estimate.

    • If zero, the corresponding mode shape is scaled by the average A/D for zero excitation in the excitation zone, weighted by the mode shape.

    • If non-zero, the initial response estimate will have a maximum translation of the scaling factor multiplied by the average diameter.

2.2. Response frequency selection

Possible response frequencies will be ranked according to an excitation parameter calculated for each frequency. The response frequency with the highest value is called the dominating frequency. The ranking is used to assign excitation zones in the case of concurrent response frequencies (space sharing) or relative duration in the case of consecutive response frequencies (time sharing).

  • Amplitude limit : Amplitude limit for including frequencies in the calculated response, normalised by the minimum diameter

  • Cut-off ratio below dominating frequency : Cut-off excitation parameter ratio for frequencies below the identified dominating frequency

  • Cut-off ratio above dominating frequency : Cut-off excitation parameter ratio for frequencies above the identified dominating frequency

For cylinders with constant diameter in linearly sheared flow, cut-off ratios of 0.2 below and 1.0 above the dominating frequency agree well with experiments.

2.3. Force calculation option and print switch

  • Force switch : Method used to calculate the forces

    • Use Stiffness : Forces are calculated using the stiffness matrix

    • Use Curvature : Forces are calculated using the curvature and the axial strain

  • Print switch : Amount of results printed to the MatrixPlot file

    • Final results only : Only the final results are printed

    • Final results and final iteration : The final results, the results from the final iteration, and axial force and bending moments are printed

    • Final results and all iterations : The final results, detailed results from all iterations, and axial force and bending moments are printed

Calculating the forces from the curvature and the axial strain requires that the axial and bending stiffness is linear for all elements. If nonlinear stiffness is found, forces, stress and fatigue will not be calculated. Curvature time series can still be printed, see Fatigue Analysis.

If the forces are calculated using curvature and axial strain, shear stiffness cannot be modelled and must be set equal to zero for all cross sections.

For details of all parameters, see VIV response analysis - input reference and Control information - input reference.