# boundarycondition in inlet and outlet

Hi,there!
Now,I am in trouble of the use of pressureboundarycondition.
I am modeling a flow with const velocity.In a box, there is one inlet, one outlet and four sidewall. About the boundarycondition, I set like this:

``````  OnLatticeBoundaryCondition3D<T,DESCRIPTOR>* boundaryCondition = createLocalBoundaryCondition3D<T,DESCRIPTOR>();
Box3D inlet  = Box3D(0   ,nx-1,0   , 0   , 0   , nz-1);
Box3D outlet = Box3D(0   ,nx-1,ny-1, ny-1, 0   , nz-1);
Box3D front  = Box3D(nx-1,nx-1,1   , ny-2, 0   , nz-1);
Box3D back   = Box3D(0   ,0   ,1   , ny-2, 0   , nz-1);
Box3D top    = Box3D(1   ,nx-2,1   , ny-2, nz-1, nz-1);
Box3D bottom = Box3D(1   ,nx-2,1   , ny-2,    0, 0   );
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, inlet, boundary::dirichlet );
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, outlet, boundary::outflow );
T vy = getInletVelocity(param.inletVelocityLB, 0, param.startIter);
setBoundaryVelocity(*lattice, inlet, Array<T,3>( (T) 0,vy, (T) 0));
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, bottom, boundary::freeslip );
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, front, boundary::freeslip );
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, back, boundary::freeslip );
boundaryCondition->setVelocityConditionOnBlockBoundaries ( *lattice, top, boundary::freeslip );
initializeAtEquilibrium( *lattice, lattice->getBoundingBox(), param.rhoLoLB, Array<T,3>( (T) 0,vx, (T) 0));
lattice->initialize();
lattice->toggleInternalStatistics(false);

The speed at the boundary advances from the entrance to the exit. After reaching the exit, the speed increases and in turn advances toward the entrance. After several cycles, the overall speed becomes larger until the calculation is wrong.
is there anyone can give me any advice?``````