Pressure boundary outlet

Hi Dear all,

I am using D3Q19 model for simulating a 3D indoor air flow. Both the inet and outlet are on the top of the domain. I want to set a inlet velocity bounary condition and pressure at outlet .

I choose bc.addVelocityBoundary1P( 1,nx-2,ny-1,ny-1, 1,nz-2, omega); // top boundary
Unfortunately the pressure (the density) at outlet is ignored according to the user guide, Could I set half the top pressure boundary and the other half velocity boundary?

Anyone has the experience to share with me, i would be much appreciate. Many thanks!