I have been simulating a simple 2D problem in which a fluid flows between two stationary parallel plates with a given inlet velocity. I took 1600x80 lattice domain, inlet lattice velocity as 0.1 and lattice viscosity as 0.01. The boundary condition for top and bottom walls is bounce-back. I chose Zou/He BC for inlet and first order zero-gradient boundary condition for outlet. I used a single distribution function based BGK D2Q9 Model.

After the simulation is complete and I plot horizontal velocity along horizontal centerline of the lattice domain. It correctly increases gradually, then becomes stagnant at value which is 1.5 times than the inlet velocity (the maximum velocity in a channel flow should be 1.5 times the average velocity in the developed region). But the problem comes at the last part of the domain. Between lattice cell number 1550 to 1600, the velocity graph suddenly drops too much. What could be the problem ? Kindly help me.