aircore in hydrocyclone
 

Hi there
I simulate flow within a hydrocyclone and I stuck in a bad problem. When I simulate my case in cfx, unfortunately the aircore has no continuious form(please see pics capture and capture1) and I've examined my simulation with some of turbulent models like e-base and w-base models. I think I have a problem in BC.
Inlet: bulk mass flow rate, volume fraction of air=0, volume fraction of water=1
Vortex Finder(Overflow): Opening(because water exit and air enter within the hydrocyclone), Mass and Momentum= Opening pres. and Din, Volume fraction of air=zero gradient(because I have no idea what percentage of volume fraction enter to body), Volume fraction of water= zero gradient( like air I don't have any idea what percentage of volume fraction of overflow outlet exit from body)
Apex(underflow): Opening(because water exit and air enter within the hydrocyclone), Mass and Momentum= Opening pres. and Din, Volume fraction of air=zero gradient(because I have no idea what percentage of volume fraction enter to body), Volume fraction of water= zero gradient( like air I don't have any idea what percentage of volume fraction of underflow outlet exit from body)
Other problem I stuck in it is after running about 600 iteration I have air just near outlets and I don't have continous air core within body(see picture capture 2 and capture 3).
I really don't know what do I do.
Please help me ASAP.
Thank you in advance