Time Variables in Static Analysis |
 
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Time Variables in Static Analysis |
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Naturally, all time domain analyses require the specification of time variables. Since a static analysis is one which considers time invariant loading and structure response, time variables are effectively meaningless in this context. On that basis, it may seem curious that you are also required to specify time variables for a static analysis in Flexcom. This is purely a consequence of the historical evolution of the software. Although Flexcom now incorporates frequency domain analysis capabilities, the program has traditionally been primarily a time domain analysis tool. Given the importance of static analysis in terms of providing a platform for subsequent dynamic analysis, versions of Flexcom right from the beginning have provided a static analysis capability in addition to the (effectively core) time domain dynamic analysis capability. In an effort to standardise the solution inputs across all analysis types, time variables became mandatory for static analyses also, even if the specified values are somewhat notional. This has remained the case ever since, for various reasons including the following:
(i)Maintaining keyword compatibility with previous versions
(ii)Consistency of restart information storage and retrieval for static and time domain analysis
(iii)Consistency of database output storage and retrieval for static and time domain postprocessing
While the situation regarding time variables in static analysis is somewhat artificial and essentially a legacy stemming from early versions of Flexcom, the time variables do nonetheless serve a useful function in providing control over the rate of build-up of the applied static loading
As the time variables are largely notional in a static analysis, an initial static analysis is typically run from t=0 to t=1 second, while a static restart analysis (for example, to introduce a vessel offset) is typically run from t=1 to t=2 seconds. Any further static restarts (for example, to introduce current loading etc.) would normally be run from t=2 to t=3 seconds, from t=3 to t=4 seconds, and so on.
In many cases static loads and displacements can be applied in a single step, and if so, you are not required to specify any value of Time Step – this defaults to the difference between the analysis start and end times. If a time step is specified, then the static loads and displacements are built up to their full values at the end of the analysis over the specified number of steps. So for example in a static analysis running from t=1 to t=2 seconds with a fixed time step of 0.1 seconds, the loads increase linearly to their full value over 10 steps.
Note also that the Ramp Time entry is intended specifically for dynamic analysis, to allow control over the build-up of dynamic loads and displacements in an analysis with waves and/or vessel motions. For example, wave loads in a regular wave analysis are typically ramped on over 1 wave period, with the solution proceeding for a further 3-4 wave periods to achieve a steady state solution. Any Ramp Time specified in a static analysis is simply ignored, as the static loads and displacements are built up to their full values over the entire simulation time.
Two options are provided in terms of Ramp Type, both Linear and Nonlinear. With the former, the loads increase linearly to their full values between the analysis start to end times. When a Nonlinear ramp is specified (and this is the default), a half cosine ramp function is used (as opposed to a linear function).
Flexcom has two time stepping algorithms, fixed and variable. When you specify a fixed time step, the simulation proceeds from the start time to the end time using the fixed time step specified. In a variable step analysis, the choice of time step magnitude is made by the program based on a number of criteria. The time step is continuously monitored and varied as appropriate by the program within user-specified limits, to ensure a stable and convergent solution.
The vast majority of static analyses use a fixed time step, although a variable time step may be necessary in some extreme cases. The relevant inputs in this context are a Suggested Time Step, a Minimum Time Step, a Maximum Time Step, and optionally a Step Length parameter. The first three parameters are self-explanatory. The Step Length parameter pertains specifically to dynamic analyses, but is mentioned briefly here for completeness. If you are using a variable time step, the program chooses an optimum time step based on two main criteria, namely (i) the number of iterations required for the last three convergent solutions, and (ii) the instantaneous current response period, which is a measure of the dominant period in the response at any particular instant. The optimum time step, when this is based on the dominant response period, is obtained by multiplying the instantaneous current period by the Step Length parameter. As the notion of a dominant response period is meaningless for static analysis, any value you specify for Step Length is simply ignored. The variable step algorithm is most typically employed in dynamic analyses, but is also available also in static runs. For static analyses, the size of the step is selected only on the basis of the number of iterations required for convergence. The larger the number of iterations, the smaller the size of the time-step, and vice versa.
•*TIME is used to define time parameters for an analysis.
If you would like to see an example of how this keyword is used in practice, refer to any of the standard Flexcom Examples.