Water Jet
 

I am having trouble simulating a water jet impacting different impact plates in 3Dimensions in Star CCM+. I realise that one has to set up the control volume including the nozzle and impact plate. I am having trouble assigning atmospheric air to the control volume area between the nozzle outlet and impact plate. As well as water flowing into the control volume.
Is this done in the initial conditions and if so, how can one do this? Using Field Functions
I also understand that you need to use a multiphase simulation.
Can anyone please assist me.

it is unclear what you are really trying to do. So I will assume you have box with a water nozzle (= circular window surface imprinted on that wall) on one side and on the other side of the box is a wall which will be the impact plate. All other surfaces can be walls or pressure outlets. You choose VOF, unsteady physics with phase 1 as water, phase 2 as air. Need a reasonably fine mesh along expected jet path. Then for initial conditions set volume fractions to 0.0,1.0 (ie all air). Then your jet boundary will be an velocity inlet or mass flow inlet with volume fractions 1.0,0.0 (ie all water). If you have pressure boundaries set them to 0.0,1.0 so if reverse flow occurs they suck in air. Timestep needs setting - try .01s first - might need to go much lower depending on velocities and mesh size. Do the VOF tutorial in Help if this does not make sense.

That is what I want and I think I may have managed to get the simulation working, I have attached a picture of the scalar scene with volume fraction of water. Now i need to measure the impact force of the water jet on the impact surface. I think i may have found it using reports however I am unsure. Please Help

I guess this is a problem of mesh dependency. I had a same issue with water-jet impingement project. Since VOF method resolves free-surface, it is must required to have fine (say extra fine) mesh. Otherwise, doesn't resolve the phenomena correctly.