problem on DPM coupled with CFD
 

in recent days, I am developing code on dpm coupled with cfd. now i meet with such problem, when i apply my code to gas-solid flow in 2d duct, i find the value of pressure at some points of computational domains is negative sometimes. so is it normal? i simulate multi-flow in fluidization bed, no such problem occurs. this condition only appear when the inlet velocity in the duct increases over a special value(4m/s). by the way, i want to ask some guys that, what do the pressure in NS equations belong to? static pressure or dynamic pressure?