This case, because of the nominal constantcy of drag coefficent
, is easier to compute - as long as the coefficient is known beforehand. On the other hand, to find
by some simple means (without wind tunnel tests or numerical solution of the whole flowfield) is practically out of question. For the simplest, two-dimensional case of the flat plate as shown in Fig.J-13,
 |
|
Fig.J-14 |
solution was sought by Kirchhoff in 1869, assuming stagnant fluid in the wake. His result,
= 0,88,
is much smaller than actual value
= 2,01. This suggests that fluid in the wake is not motionless and pressure there is lower. For plate of finite aspect ratio
the wake extent (and drag) is diminished by fluid inflow from sides. It is quite surprising how large this decrease in drag is.
With quite elongated plate of
= 10, the drag is just a little larger than for the square plate. In fact,
even with
= 50 the limiting value is still far away. This is a warning signal for anyone expecting "almost two-dimensional" results with finite spans in wind tunnels.
 |
 |
Fig.J-13 |
Also the results for axisymmetric bodies, as summarised in Fig.J-13, show clearly that even small changes in shape may have large effects upon the wake size and, as a consequence, upon the drag - which it is therefore possible to influence substantially by changing wall slope upstream from the separation point.
In some cases the reference area
is not clearly and unequivocally defined and it is a good idea to specify, instead of giving just
 |
 |
Fig.J-15 |
the product 
. Also in asuch cases as the aerodynamnics of car bodies
this product is a useful quantity because the car drag may be influenced both by decreasing
as well as decreasing the car frontal area. The product 
is there the best overall criterion of aerodynamic
quality.
An interesting aerodynamic behaviour is found in bodies with round blunt shape. They often exhibit a sudden substantial decrease of
at a certain critical Reynolds number.
Going to another page:
click
This is page Nr. J06 from textbook
Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author
to his address
tesar@fsid.cvut.cz
WWW server administrators: Jiri Kvarda, Zdenek Maruna
...... Contact: webmaster@vc.cvut.cz
Last change : 25.03.1997