# Modal

The [Modal] section of the aNySIM configuration file contains parameters to control aNySTA runs. (MARIN inhouse use only. Not for aNySIMpro users)

# Frequency range

In order to process jobs efficiently, aNySTA only scans for solutions (natural frequencies) inside a limited range of angular frequencies. The parameters below define that range.

keyword Unit Description Options Version
FrequencyLow rad/s Lower bound of the frequency range. aNySTA_V8.1
FrequencyHigh rad/s Upper bound of the frequency range. aNySTA_V8.1
FrequencyStep rad/s Resolution inside the frequency range. aNySTA_V8.1

Notes:

• If you specify a narrow frequency range, you run the risk of missing solutions. The range should include all frequencies of importance to your application.
• aNySTA cannot distinguish between solutions that are less than one frequency step apart. If the step size is set too coarse, you risk missing different solutions that happen to share almost the same period.
• The following inequalities must hold, otherwise aNySTA will report an error:
     0.0 < FrequencyLow < FrequencyHigh < 100.0
(FrequencyHigh - FrequencyLow) / FrequencyStep ≥ 10.0


A typical frequency range is shown below.

     [Modal]
FrequencyLow = 0.025
FrequencyHigh = 1.5
FrequencyStep = 0.001


With these parameters, aNySTA will look for solutions with a period between roughly 4 seconds and 4 minutes (using $T=2\pi/\omega$).

# Output files

aNySTA is able to generate a plethora of information from each job. However, not all of it may be of interest to you. The following parameters select which output files are written. Here jobname is the name of your configuration file (jobname.ini).

Keyword Unit Description Options Version
IsInr Inertia matrix (jobname.inr) yes, no aNySTA_V8.1
IsPer Natural oscillation periods and eigenvectors (jobname.per) yes, no aNySTA_V8.1
IsRep Job report file (jobname.rep) yes, no aNySTA_V8.1
IsRes Time sequence results (jobname.res) yes, no aNySTA_V8.1
IsStf Stiffness matrix (jobname.stf) yes, no aNySTA_V8.1

All of these parameters are optional. The defaults are as follows:

    [Modal]
IsInr = no
IsPer = yes
IsRep = yes
IsRes = no
IsStf = no


# Tolerance

aNySTA finds a natural frequency when the determinant of a certain matrix is zero. To allow for numerical errors, this is implemented as close to zero ( $|\det \mathbf{A}|<\epsilon$) within the tolerance parameter defined below.

keyword unit description options
EpsDeterminant Tolerance parameter

The tolerance parameter should be set to a small positive value (0.0 < EpsDeterminant << 1.0). A typical value is:

    [Modal]
EpsDeterminant = 1e-6


# [BODY#]

aNySTA determines the stiffness of the whole system before it looks for eigen modes of this mass-spring system. To do so, an offset is applied independantly to each body in every motion mode (surge, sway, heave, roll, pitch and yaw). Therefore the offset needs to be provided for every body in order to do the modal analysis.

Keyword Unit Description Options Version
OffsetPos [m, m, m, deg,deg,deg] Position offset given in the local coordinate system {SFt0} {X, Y, Z, Roll, Pitch, Yaw} aNySIM_v9.0

This keyword is compulsory.

    [BODY1]
PosOffset = { 0.1, 0.1, 0.1, 1.0, 1.0, 1.0 }