exit velocity

1. n. [Drilling]
The speed the drilling fluid attains when accelerated through bit nozzles. The exit velocity is typically in the low-hundreds of feet per second. It has been reported that in certain shaly formations, an impingement velocity on the order of 250 feet per second is required to effectively remove newly created rock chips from the bottom of the hole. This impingement velocity is not, however, the same as the exit velocity, since the high-energy fluid jet loses velocity through viscous losses and conversions from kinetic energy to forms of potential energy occur once the fluid leaves the bit. For this reason, the well designer generally seeks to maximize the fluid velocity (or other measure of jet energy) to achieve maximum cleaning at the bottom of the hole.