Air flow accuracy based on computational domain size

Hi all,

I'm currently running  a simulation past an airfoil at a high Reynold number of 6*10^6.

I have messed with two diffferent settings, whereby I used the default comp. domain and then another with a much larger domain ( so I dont get flow interaction with the boundaries) .

The resultant forces are considerably different.

Now I do want more accuracy but with a 1.8 GHz processor and 3 GB of RAM I'm pretty limited, a typical large domain calculation took me around 30 hours the other day.

Also, Im running it for a physical time of 1 sec, would increasing that time lead to a better accuracy ?

Thanks for your time, any input would be appreciated !

SolidworksFlow Simulation