1. VIV response analysis

1.1. Data group identifier, one input line

RESPonse ANALysis PARAmeters

1.2. Response parameters, one input line

RELDAM IOPFRC IPRINT

  • RELDAM: real, default: 0: Relative structural damping

    • RELDAM = 0.1 gives 10% relative damping.

  • IOPFRC: integer, default: 1: Force switch

    • IOPFRC = 0: Forces calculated using stiffness matrix

    • IOPFRC = 1: Forces calculated using curvature and axial strain. This option 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 VIV fatigue analysis.

  • IPRINT: integer, default: 1: Print switch

    • IPRINT = 1: Final results are printed to Matrix Plot file

    • IPRINT = 2: Final results, results from the final iteration and axial force and bending moments are printed to Matrix Plot file

    • IPRINT = 5: Final results, detailed results from all iterations and axial force and bending moments are printed to Matrix Plot file.

If the forces are calculated using curvature and axial strain (IOPFRC = 1), shear stiffness cannot be modelled and must be set equal to zero for all cross sections.

1.3. Response parameters, one input line

IFRIT MAX_ITER CHILIM CONLIM SCAINI IOPTRY

  • IFRIT: integer, default: 2: Response iteration method

    • IFRIT = 1: Fixed point

    • IFRIT = 2: Newton - Raphson

  • MAX_ITER: integer, default: 30: Maximum number of iterations

  • CHILIM: character(6), default: AMPNOR: Convergence criterion

    • CHILIM = AMPNOR: Norm of amplitude change for translations. The norm is equal to the squared sum of the amplitude changes normlized with the product of the number of nodes and squared the average diameter.

    • CHILIM = DIFMAX: Maximum difference. This is the maximum absolute change in amplitude; i.e. phase change ignored.

    • CHILIM = NONE: No convergency test. MAX_ITER iterations will be performed.

    • CHILIM = 5: As AMPNOR, Norm of amplitude change for translations

    • CHILIM = 3: As DIFMAX, Maximum difference

    • CHILIM = 0: As NONE, No convergency test

    • CHILIM = 1: Quadratic norm. Not recommended.

    • CHILIM = 2: Relative maximum difference. Not recommended.

    • CHILIM = 4: Unbalanced force. Not recommended.

  • CONLIM: real, default: 0.0001: Convergence limit for the iteration. Recommended values 0.001 - 0.01 for CHILIM = AMPNOR.

    • CHILIM = NONE or 0: Dummy

    • CHILIM = DIFMAX or 3: Dimension \(\mathrm {[L]}\)

    • All other values of CHILIM: Nondinemsional

  • SCAINI: real, default: 0.5: Scaling factor for the initial response estimate

    • SCAINI = 0: Scale the corresponding mode shape by the average \(\mathrm {\frac{A}{D}*D}\) for zero excitation in the excitation zone, weighted by the mode shape. The IL part of combined CF and Il loading, IRSTYP = 3, is scaled with half the value found for CF.

    • SCAINI > 0: Scale initial response estimate so that the maximum amplitude is \(\mathrm {SCAINI*D_{avg}}\) for CF loading, IRSTYP = 1, IL loading, IRSTYP = 2 and the CF part of combined CF and Il loading, IRSTYP = 3. The IL part of combined CF and Il loading, IRSTYP = 3, is scaled with half this value.

  • IOPTRY: integer, default: 0: Option to re-try response iterations if the response iteration does not converge. Under development.

    • IOPTRY = 0: the analysis will continue and the non-converged results used

    • IOPTRY = 1: a second attempt will be made with the other response iteration method. May not be combined with data group Structural damping specification.

1.4. Response parameters, one input line

IOPTSH NUDDF ADLIM REXPAL REXPAH

  • IOPTSH: integer, default: 0: Option for combining response frequencies

    • IOPTSH = 0: Response frequencies act concurrently, i.e. space sharing

    • IOPTSH = 1: Response frequencies act consecutively, i.e. time sharing

  • NUDDF: integer, default: 0: Number of dominating frequencies given in user defined frequency ranking. Not used for IOPTSH = 1.

  • ADLIM: real, default: 0.01: Amplitude limit for including frequencies in the calculated response, normalized by the minimum diameter \(\mathrm {[1]}\).

  • REXPAL: real, default: 0: Cut-off excitation parameter ration for frequencies below the identified dominating frequency \(\mathrm {[1]}\).

  • REXPAH: real, default: 0: Cut-off excitation parameter ration for frequencies above the identified dominating frequency \(\mathrm {[1]}\).

Analysis of frequency content in time series from VIV experiments with high response frequencies shows that the response process is often somewhere between concurrent and consecutive. The response frequency at a location shifts between different consecutive values. However, other frequencies often dominate at other locations. Further work is needed before firm recommendations may be given.

For cylinders with constant diameter in linearly sheared flow the values REXPAL = 0.2 and REXPAH = 1.0 agree well with experiments.

1.5. User defined frequency ranking, NUDDF input lines

IDOMFRQ

  • IDOMFRQ: integer: Frequency number

The most dominating frequency is to be specified first, then the second most dominating frequency is specified etc.

If NUDDF = 0 the program will give the possible response frequencies a ranking according to an excitation parameter for each frequency.

\(\mathrm {E_i=\sum\limits_{j=1}^{N_{Ex}}l_j\,D_j^2\,u_j^2(\frac{A}{D})_{C_e=0,\,j}}\)

where

  • \(\mathrm {E_i}\) is the excitation parameter for frequency i.

  • \(\mathrm {l_j}\) is the length of element j within the excitation zone.

  • \(\mathrm {D_j}\) is the diameter of element j.

  • \(\mathrm {u_j}\) is the flow velocity at the midpoint of element j.

  • \(\mathrm {N_{Ex}}\) is the number of elements in the excitation zone.

  • \(\mathrm {(\frac{A}{D})_{C_e=0,\,j}}\) is the normalised response amplitude that give \(\mathrm {C_e=0}\), given as a function of the local value of \(\mathrm {\hat{f}}\) for frequency i.