I'm a student who is familiar with Calc 2 and is starting to learn MultiVariable calc.
I came across this article which seemed interesting for Fluid Flow:
http://www.ipt.ntnu.no/~kleppe/TPG4150/equations.pdf
There, this equation is shown.
All the variables are defined in the paper except for A and phi. Im assuming A is surface area, and I don't understand what Phi is.
$${uρA}_x − {uρA}_{x+ Δx} = \frac{∂}{∂t} {φAΔxρ}$$
What does phi represent here?