As an object moves through a gas, the gas molecules are deflected
around the object. If the speed of the object is much less than the
speed of sound
of the gas, the density of the gas remains constant and the flow of
gas can be described by conserving momentum and energy.
speed of the object increases towards the speed of sound, we
on the gas. The density of the gas varies locally as the gas is
compressed by the object.
For compressible flows with little or small
flow turning, the flow process is reversible and the
The change in flow properties are then given by the
Isentropic means "constant entropy".
But when an object moves faster than the speed of sound,
and there is an abrupt decrease in the flow area,
the flow process is irreversible and the entropy increases.
Shock waves are generated. Shock waves
are very small regions in the gas where the
change by a large amount.
Across a shock wave, the static
increases almost instantaneously.
Because a shock wave does no work, and there is no heat addition, the
and the total temperature are constant.
But because the flow is non-isentropic, the
total pressure downstream of the shock is always less than the total pressure
upstream of the shock. There is a loss of total pressure associated with
a shock wave.
The ratio of the total pressure is shown on the slide.
Because total pressure changes across the shock, we can not use the usual (incompressible) form of
across the shock. The
and speed of the flow also decrease across a shock wave.
shock wave is perpendicular to the flow direction it is called a normal
shock. On this slide we have listed the equations which describe the change
in flow variables for flow across a normal shock.
The equations presented here were derived by considering the conservation of
for a compressible gas while ignoring viscous effects.
The equations have been further specialized for a one-dimensional flow
without heat addition.
The equations can be applied to the
two dimensional flow past a wedge for the following combination of
free stream Mach number M and wedge angle a :
The right hand side of all these equations depend only on the free stream
Mach number. So knowing the Mach number,
we can determine all the conditions associated with
the normal shock.
The equations describing normal shocks
were published in a NACA report
Here's a Java program based on the normal shock equations.
To change input values, click on the input box (black on white),
backspace over the input value, type in your new value. Then hit
the red COMPUTE button to
send your new value to the program.
You will see the output boxes (yellow on black)
change value. You can use either Imperial or Metric units and you can set default
conditions for the Earth or Mars.
Just click on the menu button and click on your selection.
For the given altitude input, the program computes the upstream
standard day conditions. The ratios are computed
specific heat ratio
inputs and the equations given in the figure. The
downstream conditions are computed from the upstream conditions and
If you are an experienced user of this simulator, you can use a
of the program which loads faster on your computer and does not include these instructions.
You can also download your own copy of the program to run off-line by clicking on this button:
There is more complete
program that is also available at this web site. The
program models the intersection and reflection of multiple shock waves.