[vince60270]

hello everybody,

I am trying to set a moving Center of mass for a DFBI, corresponding to 3 phases (stop, acceleration, constant Speed) depending on time. Here is the function i tried to set but it doesn't work and i am not so confident with programming :/. If someone can help it would be great!

($Time<3)?[1530, 0,-100]$Time<6)?[1030, 0.0, -100][1930,0,-100])

Thanks!

[me3840]

This appears to compile just fine. What problems are you having with it?

This function appears to just make the position 'jump'. Is that what you really want?

[vince60270]

Hello,

the Problem is that when i compile it it s fine but when i click enter in the Center of mass line for DFBI i have an error message telling me there is a Problem with unit like [x, y, z] x unit, y unit, z unit....

[me3840]

Are you trying to make the initial center of mass dynamic? That doesn't really make sense - the initial value is only for time t=0. After that DFBI calculates the center of mass depending on the body position.

[vince60270]

OhHHH do you meen that the Center of mass you set here won't stay the same during the Simulation ? If this is right how can i set a Center of gravity that will match my function ?

[vince60270]

In fact i am simulating really small and light boat but really powerfull. And during the three different Phases of the run ( stop/acceleration/stable Speed) the Pilot is moving in order to make the Center of mass move if it 's more clear.

[me3840]

This is just the initial center of mass, so you can't make it dependent on time.

Changing the CM is outside of my DFBI knowledge, but my guess is you could get clever with a Catenary coupling somehow, or perhaps coulping with another body (say for example a moving arm?)

But it's for certain that making the initial values a function of time will not work, so you should look at other means.

[vince60270]

hello,
i am not sure if i am able to do this Kind of stuffs on star ccm+... My second guess is that it would be possible for me to simulate this Change in the CM Location by writing a field function to force a specific angle of attack of the DFBI ( effects of a moving CM) do you know if it s possible and how i can manage it ?
thx

[ping]

i am pretty sure that the c.o.m. is locked once you initialise the sim

so what about setting it to a very low value and then replacing it with a vertical external force since its value can be changed via a field function but not its position so this could be done by adding an external moment

or the position or coordinate systems of the force can be changed in a macro

[vince60270]

Would it be possible then to do as the CM was moving to set 3 Forces at 3 different places corresponding to the Locations i wanted and to set with a field function which strenght and when is it active ? If yes could you explain to me a Little bit more about Setting external Forces or give me some tutorials or similar examples ?

[ping]

yes it is easier to just define 3 external forces at the required locations and have field functions to control their z component as a function of time so that one is on and the others are off at any time and with the correct magnitude

cant think of a tutorial on this but it is easy other than the field functions but you almost have that under control in your first post

i would do the boat in waves tutorial and then add an external force or two to that case to try the method out

[vince60270]

hello, i have tried to set the Forces but it appears that Body Forces are not available with the eulerian multiphase model but with the lagrangian model . Is it right ? and if yes do you know why ?