14 Wind Tunnel Designs and Their Diverse Engineering Applications
Wexit
Length
Hexit
Hent
b
a/2/2 Flow direction
Went
Figure 6. Rectangular section diffuser.
Diffuser 3 guides the flow to the power plant which is strongly affected by flow separation. In
order to avoid it, the criterion imposing a maximum value of the semi-opening angle is
maintained here as well. The cross-sectional shape may change along this diffuser because it
must connect the exit of corner 2, whose shape usually resembles that of the test chamber, with
the entrance of the power plant, whose shape will be discussed later.
The same can be said about diffuser 4 because pressure oscillations travel upstream and
therefore may affect the power plant. Analogically to the previous case, it provides a connec‐
tion between the exit of the power plant section and the corner 3, which has a cross-section
shape resembling the one of the test chamber.
Diffuser 5 connects the corners 3 and 4. It is going to be very short, due to a low value of the
dynamic pressure, which will allow reducing the overall wind tunnel size. This will happen
mainly when the contraction ratio is high and the diffusion angle may be higher than 3,5°. It
can also be used to start the adaptation between the cross-section shapes of the tests section
and the power plant.
An accurate calculation of the pressure loss coefficient can be done with Idel´Cik´s (1969)
method. A simplified procedure, derived from the method mentioned above, is presented here
to facilitate a quick estimation of such coefficient.
Design Methodology for a Quick and Low-Cost Wind Tunnel 15
http://dx.doi.org/10.5772/54169
The pressure loss coefficient, with respect to the dynamic pressure in the narrow side of the
diffuser, is given by:
ζ = 4,0 · tan α / 2 · 4 tan α 2 · 1 - ( F0 2
F1 ) + ζf .
α being the average opening angle, F0 the area of the narrow section, F1 the area of the wide
section and where ζ f is defined as:
ζf =
0,02
8 · sin α / 2 1- ( )
F0 2
F1 .
3.5. Corners
Closed circuit wind tunnels require having four corners, which are responsible for more than
50% of the total pressure loss. The most critical contribution comes from the corner 1 because
it introduces about 34% of the total pressure loss. To reduce the pressure loss and to improve
the flow quality at the exit, corner vanes must be added. Figure 7 shows a typical wind tunnel
corner, including the geometrical parameters and the positioning of corner vanes.
The width and the height at the entrance, Went and Hent respectively, are given by the previous
diffuser dimensions. The height at the exit, Hexit, should be the same as at the entrance, but the
width at the exit, Wexit, can be increased, giving the corner an expansion ratio, Wexit/Went. This
parameter can have positive effects on the pressure loss coefficient of values up to approxi‐
mately 1,1. However, it must be designed considering specific geometrical considerations,
which will be discussed, in greater details in the general arrangement.
The corner radius is another design parameter and it is normally proportional to the width at
the corner entrance. The radius will be identical for the corner vanes. Although increasing the
corner radius reduces the pressure loss due to the pressure distribution on corner vanes, it
increases both the losses due to friction and the overall wind tunnel dimensions. According to
previous experience, it is recommended to use 0,25 Went as the value of the radius for corners
1 and 2, and 0,20 Went for the other two corners.
The corner vanes spacing is another important design parameter. When the number of vanes
increases, the loss due to pressure decreases, but the friction increases. Equal spacing is easier
to define and sufficient for all corners apart from corner 1. In this case, in order to minimise
pressure loss, the spacing should be gradually increased from the inner vanes to the outer ones.
The vanes can be defined as simple curved plates, but they can also be designed as cascade
airfoils, which would lead to further pressure loss reduction. In the case of low speed wind
tunnels the curved plates give reasonably good results. However, corner 1 may require to
further stabilise the flow and reduce the pressure loss. Flap extensions with a length equal to
the vane chord, as shown in Figure 7, is a strongly recommended solution to this problem.
Other parameters, such as the arc length of the vanes or their orientation, are beyond the scope
of this chapter. For more thorough approach the reader should refer to Idel´Cik (1969), Chapter
6. As mentioned above, the pressure loss reduction in the corners is very important. Therefore,
