Hydrodynamic Properties

The hydrodynamic properties define the excitation and damping model used in the VIV analysis. Property sets of each type are defined once and then referred to from the cross section property table, so that the same set may be reused for several segments.

If a property type is not referred to for a segment, the program uses its default model for that segment. Default excitation zone limits, added mass curves, excitation coefficients, damping and Strouhal number are described in Specification of section properties - input reference.

1. Hydrodynamic cross sections properties

One row must be given for each segment in the model. Each row refers to the property sets to be used for that segment:

  • Global segment : Global segment number

For each global segment number, specify the name of the property set. Default cross-section properties will be used, when no property set is referenced for a segment, see Default cross-section properties.

The global segment numbering follows the order the lines are defined in. The first segment of the first defined line is global segment 1 and the last segment of the last defined line has global number equal to the total number of segments in the system.

  • Excitation zone properties: Cross section property specification for excitation zone limits.

  • Added mass properties : Added mass for cross-flow or in-line added mass as a function of non-dimensional frequency. The interpretation of the curve depends on the VIV load type specified.

    • If VIV load type = Cross flow or Combined cross-flow and in-line: cross flow data

    • If VIV load type = In-line : in-line flow data

  • Excitation coefficients: Excitation coefficient for cross-flow or in-line excitation coefficient as a function of non-dimensional frequency. The interpretation of the curve depends on the VIV load type specified.

    • if VIV load type = Cross flow or Combined cross-flow and in-line: cross flow data

    • if VIV load type = In-line : in-line flow data

  • Hydrodynamic damping factors: Cross section property specification for hydrodynamic damping.

  • Strouhal number properties: Cross section property specification for Strouhal number

If combined cross-flow and in-line are specified, see VIV load type , the cross section property specification for in-line must specified separately:

  • In-line added mass properties : In-line added mass as a function of non-dimensional frequency.

  • In-line excitation coefficients: In-line excitation coefficient as a function of non-dimensional frequency.

Hydrodynamic cross sections properties must be specified for all segments in the Slender system.

2. Hydrodynamic coefficients

  • Water temperature : Used to calculate the kinematic viscosity of the water, and thereby the Reynolds number

2.1. Excitation zone properties

The excitation zone is the part of the structure where the vortex shedding excites the response. It is defined by a range of the non-dimensional frequency:

  • Name : Name identifying the excitation zone property set

  • Min : Minimum non-dimensional frequency in the excitation zone

  • Max : Maximum non-dimensional frequency in the excitation zone

Elements outside the excitation zone add damping to the system.

It is not allowed to specify a broader excitation range for a cross-section than the range covered by the associated excitation coefficient curves.

2.2. Added mass transition

Constant added mass is recommended for high modes, e.g. above mode 15. A smooth transition between the frequency dependent added mass and the constant still water added mass can be selected.

  • Added Mass Frequency Dependence : How the added mass varies with the mode number

    • Use frequency-dependent added mass for all modes : The frequency dependent added mass curve is used for all modes

    • Use constant still-water added mass for all modes : The constant still water added mass is used for all modes

    • Transition from frequency-dependent to still-water added mass : A transition zone is applied between the two, given by the mode numbers:

      • Last mode number for full frequency-dependent added mass : Last mode number in the active VIV direction for which the full frequency dependent added mass curve is used

      • First mode number for constant still-water added mass : First mode number in the active VIV direction for which the constant still water added mass is used

The specified transition is also applied to the default added mass model.

2.3. Added mass properties

The added mass coefficient is given as a function of the non-dimensional frequency:

  • Name : Name identifying the added mass property set

  • Non dimensional frequency

  • Added mass coefficient

A constant added mass is obtained by giving a single point.

2.4. Excitation coefficient properties

The excitation coefficient is used in the response analysis to calculate the excitation force on the cylinder. The excitation coefficient is given as a function of non-dimensional frequency and transverse response amplitude normalized by the diameter. It may be given in two ways:

  • Name : Name identifying the excitation coefficient property set

  • Type : Curve or Table

2.4.1. Curve

The excitation coefficient curve is described by four parameters for each non-dimensional frequency:

  • Non-dimensional frequency

  • A/D ratio for zero excitation, CE=0

  • A/D ratio for maximum excitation, CE=CEmax

  • Maximum excitation coefficient

  • Excitation coefficient for A/D=0

2.4.2. Table

For each non-dimensional frequency a curve of excitation coefficient versus amplitude to diameter ratio is given:

  • Amplitude to diameter ratio

  • Excitation coefficient

Both the amplitude to diameter ratio and the non-dimensional frequency must be given in either increasing or decreasing order.
The data must cover the complete A/D ratio range. The minimum A/D ratio should be zero and the maximum value approximately 2. All extrapolation should be done outside of VIVANA.

If the excitation curves are used to model sections covered with VIV suppression devices, note that the theoretical model can cover cases with up to approximately 75% coverage. For larger coverages the straked riser takes control of the VIV behaviour, and the responding frequencies and modes are generally lower.

2.5. Hydrodynamic damping properties

The hydrodynamic damping outside the excitation zone may be scaled relative to the Venugopal damping model:

  • Name : Name identifying the damping property set

  • Type : Venugopal damping factors, or damping calculated from the excitation curves

For the Venugopal type:

  • Still water damping contribution : Factor on the Venugopal still water damping contribution

  • Low velocity region : Factor on the Venugopal low velocity region

  • High velocity region : Factor on the Venugopal high velocity region

Alternatively, the damping may be calculated from the excitation curves defined for the section. Then, only a still water damping factor is given.

If the damping is calculated from the excitation curves, the section must have excitation coefficients given on table format.

2.6. Strouhal number properties

The Strouhal number relates the vortex shedding frequency to the flow velocity and the diameter.

  • Name : Name identifying the Strouhal number property set

  • Type :

    • Default : The program calculates the local Strouhal number for each element as a function of the Reynolds number using the default curve

    • Constant : A fixed Strouhal number is used for all elements in the segment

    • User defined : The program interpolates in a user defined curve of Strouhal number versus Reynolds number

For the User defined type, the following is given for each point:

  • Reynolds number

  • Strouhal number

The Reynolds numbers must be given in increasing order.