calculate the flow velocity

This might be simple but I can't remember how to do it.

problem: we have 2 tanks. One filled with atmospheric air at 1 bar and the other is vacuum. The two tanks are connected by a rigid pipe which has a valve. I open the valve and air flows to the vacuum tank. How do I calculate the air velocity?

P.s: I don't think Bernoulli is valid here because the flow isn't steady I guess. Please state your assumptions clearly.

Thanks in advance