01-Oct-2014: Hydrodynamic Forces Added
It’s confirmed. Testing is completed. Hydrodynamic forces are now incorporated into oFreq. This includes crossbody forces; reactive forces (stiffness, added damping, and added mass); and active forces. These are all the forces that can arise from hydrodynamic interactions.
Let me take a moment to explain one of the most impressive features of the hydrodynamic forces. Classic wave theory assumes that hydrodynamic forces scale linearly with wave amplitude. Double the wave amplitude, and you double all your forces. Most of the existing seakeeping software goes with this assumption. Only problem: experimental evidence clearly shows that this is not true. Forces vary non-linearly. But back when much of the seakeeping software was developed, the computer were not powerful enough to consider the added complexity from this.
No skip to today. What can oFreq do for you:
That’s right. oFreq can do both. The best part: You don’t have to do anything to select between the models. You just supply oFreq with all the directories for the different data sets you have. This data can come from anywhere: model tests; other seakeeping programs; CFD RANS calculations; anywhere at all. The data gets converted into the format that oFreq expects. (We still need to develop some handy converter programs.) Your data does not need to match. Each data set can have different wave directions and different wave frequencies. oFreq can work with it all.
When you do a run of oFreq, you specify the wave directions and frequencies that you want outputs for. For each set of conditions, oFreq looks at the hydrodynamic data sets available to it. It does require that the hydrodynamic data can not have a wave direction more than 45 deg away from the current wave direction. Within that requirement, it will search through all the data available and find which ones apply to this situation. If oFreq can find more than one data set for the conditions you specify, it will use non-linear wave scaling, based on a power function. (oFreq assumes that all hydrodynamic forces are 0 when you have zero wave amplitude.) That is the preferred method. oFreq always takes the closest wave amplitudes.
If oFreq can only find one data set, it will default to use linear wave scaling.
You do not need to do anything to specify this. oFreq does it all automatically.