As an object moves through the air, the air molecules near the
object are disturbed and move around the object. Aerodynamic
forces are generated between the gas and the object. The
magnitude of these forces depend on the shape of the object, the
speed of the object, the mass
of the air going by the object and on two other important properties
of the air; the **viscosity**, or stickiness, of the air and the
**compressibility**, or springiness, of the air. To properly model
these effects, aerodynamicists use **similarity parameters,**
which are ratios of these effects to other forces present in the
problem. If two experiments have the same values for the similarity
parameters, then the relative importance of the forces are being
correctly modeled. Representative values for the properties of air
are given on another
page, but the actual value of the parameter depends
on the state of the gas and on the altitude.

Aerodynamic forces depend in a complex way on the viscosity of the
air. As an object moves through the air, the air molecules stick to
the surface. This creates a layer of air near the surface (called a
boundary layer)
which, in effect, changes the shape of the
object. The flow turning reacts to the boundary layer just as it
would to the physical surface of the object. To make things more
confusing, the boundary layer may lift off or "separate" from the
body and create an effective shape much different from the physical
shape. And to make it even more confusing, the flow conditions in and
near the boundary layer are often **unsteady** (changing in time).
The boundary layer is very important in determining the drag
of an object. To determine and predict these conditions,
aerodynamicists rely on wind tunnel
testing and very sophisticated computer analysis.

The important similarity parameter for viscosity is the
**Reynolds number**. The Reynolds number expresses the ratio of
**inertial** (resistant to change or motion) forces to **viscous
**(heavy and gluey) forces and is given by the equation Re =
velocity x density x characteristic length/viscosity coefficient. If
the Reynolds number of the experiment and flight are close, then we
properly model the effects of the viscous forces relative to the
inertial forces. If they are very different, we do not correctly
model the physics of the real problem and will predict an incorrect
lift.

Aerodynamic forces also depend in a complex way on the compressibility of the air. As an object moves through the air, the air molecules move around the object. If the object passes at a low speed (typically less than 200 mph) the density of the fluid will remain constant. But for high speeds, some of the energy of the object goes into compressing the fluid and changing the density, which will alter the amount of resulting force on the object. This effect becomes more important as speed increases. Near and beyond the speed of sound (about 330 m/s or 700 mph), shock waves are produced that affect both the lift and drag of an object. Again, aerodynamicists rely on wind tunnel testing and sophisticated computer analysis to predict these conditions.

The important similarity parameter for compressibility is the Mach number, the ratio of the velocity to the speed of sound. So it is completely incorrect to measure a lift coefficient at some low speed (say 200 mph) and apply that lift coefficient at twice the speed of sound (approximately 1400 mph, Mach = 2.0). The compressibility of the air will alter the important physics between these two cases.

The effects of compressibility and viscosity on lift are contained in the lift coefficient and the effects on drag are contained in the drag coefficient. For propulsion systems, compressibility affects the amount of mass that can pass through an engine and the amount of thrust generated by a rocket or jet engine nozzle.

Go to...

- Beginner's Guide Home Page

*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *