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-   -   interDymFoam_ possible rolling without decay? (http://www.cfd-online.com/Forums/openfoam-solving/77547-interdymfoam_-possible-rolling-without-decay.html)

Saśl Balsa June 26, 2010 11:34

interDymFoam_ possible rolling without decay?
 
Hello Foamers,

I consider myself a newbie in all the stuff related to OpenFOAM since I am still starting to use it for my PhD work. Nowadays my main interest concerns with Passive Stabilization tanks used on ships ( to dampen roll movements ). For this I need to make use of interDymFoam, in fact, adapting the Sloshing2D tank tutorial. I already did some simulations just to get familiar with the software but now I am involved in some comparisons of the results obtained with some experimental data, that's why I need to be sure of the values I need to introduce on the dynamicMeshdict file.

My question is related to Q ( damping coefficient ) and some other stuff. I have been digging a bit on the code and I happened to find the next piece of code involving this parameter (SDA.C)

scalar rollA = max(rollAmax_*exp(-sqr(Tpi - Tpn_)/(2*Q_)), rollAmin_);
scalar Tpi = Tp_ + dTp_*(time/dTi_); // Current roll period [sec]

The variables showed above mean,

Tpi: Current roll period. Which varies according to ...
Tp:Time period for liquid. This a value I know on my problem ( calculated according to Sloshing theory ).
dTp:Incr. in Tp/unit dTi
dTi: reference time step.

Q: Damping coefficient.
Tpn= Ship's roll period ( thus, tanks roll period )

Mi intention is to set up the problem in a manner I could get a tank rolling in 2D between the values of max. roll and min roll, thats all. However, it seems that roll is decaying exponentially as stated above. I guess I could get my tank rolling between these two max/min roll values just by setting Tpi equal to Tpn, meaning this resonant conditions, but my experimental case has different periods, namely ( Tp=1.32 and Tpn=1.19 ) and this has to remain as such.

So, having to get tight to these conditions How could I get my tank rolling between the maximum values of roll without having a decay on the roll??? is that possible ?

On the other hand, I have been trying to find out an expression to calculate Q but this damping ratio seems to have different possibilities of being calculated depending on the problem. Does anyone of you have any idea about an analytical expression for smooth sloshing cases ( without any kind of free surface breaking )?

I would appreciate any guidance on this since I am stuck at this point of my work.

Many many helps in advance to anyone who wish to help !

Greetings,
Saśl


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