Fluent and friction etc

Furness

Hi there (glad I found this board),

I am modeling fluid flow over air craft cavities but I have found some problems. The flow is viscous, yet increasing the length of the floor before the cavity has no effect on the pressure distribution or velocities. Can anyone explain?

Also I read that Pressure outlet is sometimes better than Outflow - under what circumstances? If I do not declare an edge as any particular type of boundary, what will Fluent default to?

Finally how good is a 2d model in predicting flow like this? should I scrap my 2d flow and focus on 3d?