Two bodies in same fluid but different velocities
 

I wish to simulate a flow in which two solid bodies move at different but constant speeds (v1 and v2) through the same fluid. I'm using overset and dynamic mesh but I'm struggling with setting up the case correctly.


For example, if there was only one body (b1) and I wanted to move it through the fluid at v1 velocity, I would just give a farfield boundary condition to my domain with velocity v1 and that'll simulate a flow past the body at v1 velocity. Using standard sign conventions it'll mean a flow going from left to right over the body.


Now that i have two bodies moving at different velocities, i can't give any farfield velocity. Hence I have to move the two component zones containing the bodies, and the bodies themselves with velocity -v1 and -v2 respectively. that is the bodies will be moving from right to left now in an almost stationary background domain (given a velocity about ~1m/s just to initialize).


The problem is that no matter how I try to define the motion of the bodies, whether with UDF or with profile files, since they are moved with a negative speed, it creates a negative velocity region in front of the body. that is effectively creating a huge bluff body instead of a smooth aerodynamic body that I'm actually simulating for.


If it helps, what I'm actually trying to simulate is the effect of the wake of a slow moving but large body on the motion of the faster but smaller body behind it.


If somebody who has worked on something similar, or knows how to set up a case with relative motions of bodies can just give some pointers here, it'll be a great help to me.