The behavior of the flows we are interested in is determined by the following set of three equations:
and u are the density and velocity at the location we
are interested in, and m is the known mass flux per unit area
at the cross section.
For example, for the problem of adding heat to a flow through a
constant area duct we discussed earlier, 
and u=u2
would be the density and velocity at the end of the duct. In that
example m would be the known mass flow 
entering the
duct. If we want, we can increase or decrease the value of m
by decreasing or increasing the exit area.
In the same example of heat addition to a constant area duct flow, I
would be the known value 
at the start of the
duct. In case of flow with friction, you also need to add the
frictional force (per unit cross-sectional area) to I. In either
case, I is again a known quantity.
We want to study the behavior of the solution 
for given
values of m,I,H. We will start simple in the next section by just
examining the effect of one variable, H, on one unknown, .After that we will do the same for three variables in three unknowns
in the final section.