Critical , Sub-critical and super critical flow

Rapidly Varied Flow

Critical , Sub-critical and super critical flow

The specific energy change with depth was plotted above for a constant discharge Q, it is also possible to plot a graph with the specific energy fixed and see how Q changes with depth. These two forms are plotted side by side below.

From these graphs we can identify several important features of rapidly varied flow.

For a fixed discharge:

1.   The specific energy is a minimum, E sc , at depth y c , This depth is known as critical depth .

2.      For all other values of E s there are two possible depths. These are called alternate depths. For

subcritical  flow y > yc

supercritical  flow y < yc

For a fixed Specific energy

1. The discharge is a maximum at critical depth, yc

2. For all other discharges there are two possible depths of flow for a particular Es

i.e. There is a sub-critical depth and a super-critical depth with the same Es

An equation for critical depth can be obtained by setting the differential of E  to zero: