problem with static pressure

Nina

Hi, I've a problem with the static pressure.My geometry consist in a pipe, with a diameter of 2,5mm and a lenght of 29mm, inside to another pipe, with a diameter of 4mm, that after 20mm to the end of the first pipe presents a bifurcation.I have imposed as a BC, at the input of the pipe, a velocity equal to 0.8m/s and at the end of the two branches of bifurcation a gauge pressure of 0 Pa.I have used k-epsilo model. I have obtained that the static pressure at the end of first pipe is negative. So I wonder: -How can be the static pressure negative and then become zero? -Why the pressure drop is different from what I expect from theoretical calculations? Where I have wrong? Please help me!!! Thanks a lot

Nina