Hi all,

In a dyanmic mesh problem, is it possible to assign a rigid body motion without the use of UDF/Profile fns? i.e, can the motion of the rigid body be determined by the flow itself? For example, a high pressure gas expanding behind a moving piston, in which the forward motion of the piston is due only to the gas pressure behind it.

thanks for reading

Rajesh

UDF! you habe to decribe the motion of your piston due to the pressure force. That's mean the UDF should inttegrate twice the pressure force on your body to determine the motion at each time step. Take a look at MDM tutorial. The UDF described, is exactly what you need. Hope it helps.

Dear Max,

Thanx for your reply.

Does that mean fluent is unable to calculate the motion of a piston by the expanding gas behind it without any user input?

For example, suppose, I have a high pressure zone and a low pressure zone initially separated by a wall, and the piston is kept in the low pressure zone. If I remove the wall, then the gas expansion will move the piston forward. The motion of the piston is decided by the pressure difference across it and the mass of the piston.

Am I not able to model the phenomina without the help of a UDF? It seems stange to me(...may be due to my inexperience!), as while writng a udf, we have to give the pressure force acting on the piston at each instant, as you told, and this force is decided implicitely by the veolcity of the piston at that instant, while the latter is decided by the pressure force. Thus the velocity and pressure force are coupled at any inastant. In such a case we cannot calculate the pressure force unless we analytically solve the equation of motion of the piston( I think, I have made it clear!)

BW: What's MDM?

thanx for reading

regards, Rajesh

MDM is for Moving Deforming Mesh. As long as your piston will move, you must have a deforming Mesh (Layering, Remeshing or Smooting). In The MDM Panel, you have to define moving walls, deforming walls and stationnary walls. For the Moving wall, eg: Rigid Body, the motion has to be decribed, either with a Profile (that means that the motion is already known, can be also deescribed with a UDF) or with a UDF (that means that the motion will be calculated, in your cas with the integration of the pressure force: at each time step: F=ma -> a=dv/dt=F/m -> dv=F/m.dt -> dx/dt= dv -> dx = dv.dt)

Thank you very much mAx

I will have a look at the UDF manual and try to figure out my problem.

Thanx for reading

regards,

Rajesh

I work with this UDF in my applications: valve-spool wich moves under pressure force and spring effects.

Hi mAx,

That's nice to hear. This is the first time I trying to do a problem with the use of UDF. So please don't mind if I bother you with my silly doubts!

Thank you very much,

regards,

Rajesh

no problem. this forum is exactly for that.

Hi Max, I'm working about compressible(density=idealgas) incylinder3d problem. 3000rpm,staring angle=180 finish angle=720 anglestep=0.5. when preview mesh no problem. but when ı give pressure inlet-pressure outlet and starting iteration continuity gets bad results and crash 250 degre. I change anglestep,iteration per time step,mesh size but every time it crash neary 250-255.I try all k-e k-o reynolds all models but it only change 240 to 245. whats the problem? please give me some advise...