Channel flow acceleration
 

I simulated a 2D planar channel flow with a velocity-inlet on one side, and an outflow BC on the other, with no-slip walls on both sides. The channel is 10m x 1m and the velocity is 0.01 m/s (Re ~ 600 using air), so it should fully develop long before reaching the outflow BC. The problem is that when I simulate it, the flow speeds up rather than slow down. Boundary layers do develop at the walls, and the static pressure does drop, but velocity is accelerating rather than decelerating. The problem does converge rapidly. Would anyone have any idea what the trouble could be?