Hi all, It's been a while since I've posted. I have a problem I'm stuck on. I'm trying to calculate how much time it will take for a tank to drain itself into another lower tank. I know the flow will be faster at start and slow down with the lowering liquid column. I'm trying to develop an equation for the height of liquid in terms of time so I can obtain the drainage time, the mean flow as well as the maximal flow in the pipe. Using the Bernoulli equation between point 2 (first tank's liquid level) and point 1 (just as the liquid exits the pipe - enters the second tank), I developed an equation relating the height of liquid in the first tank to the velocity in the pipe. My problem is both are functions of time. Here's my route so far : (direct : http://i1189.photobucket.com/albums/z434/Virgule99/Tankdrainage.jpg) So as you see, I have 2 terms that are functions of t : z(t) and V1(t). What now ?