| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
||
|---|---|---|
Test section or working section design is totally dependent on the requirements of each individual experiment. Many open-circuit blower tunnes have detachable working sections without exit diffusers, a very flexible arrangement. An exit diffuser must be fitted if the tunnel power factor is critical. The wide variety of blower tunnel working sections range from quasi-two-dimensional ones (for boundary layer studies or smoke tests) to ones with special features such as slotted walls and flexible roofs.
For tunnels which are used for testing models mounted in the working section, the working section size and shape are usually dictated by the need to minimize tunnel interference for a predetermined model size. The most popular cross-sectional shape for these working sections is a rectangle of about The flow out of a contraction often takes a distance equivalent to about 0.5 diameters before the non-uniformities are reduced to an acceptable level. Also, if a turbulence grid is installed, it may take up to10 – 15 mesh lengths before a homogeneous flow is obtained. These requirements often fix the minimum length of the working section. On the other hand, a very long working section (more than about 3 equivalent diameters) results in excessive boundary layer growth which could lead to separation in an exit diffuser. |
The inevitable boundary layer growth in the working section results in a static pressure drop in the axial direction. Any increase in cross-sectional area to compensate for this growth is usually accomplished by tapering the corner fillets (if installed) or by tapering the walls themselves, although the latter makes the mounting of the vertical traverse gears and balances more difficult. It is sometimes convenient to obstruct the outlet of the tunnel with a screen to produce an over-pressure in the working section, so that boundary layer control by suction, if necessary, is achieved by merely opening holes.
Most small wind tunnel working sections are made of plywood in timber frames. Drumming of panels leads not only to structural fatigue, but also to unsteadiness in the working section, and this must, therefore, be kept to a minimum, preferably by keeping the panel resonant frequencies above the rotor blade frequency at maximum speed. It is the deflection and vibrational properties and not the ultimate strength which decide the material size. Almost all working sections have removable side panels and it is suggested here that these be mounted on pinned hinges. This makes “single-handed” removal of these panels much easier, besides rendering them less susceptible to damage.
A blower tunnel working section without an exit diffuser gives almost as easy access to the model as does the open-jet arrangement sometimes used on return-circuit wind tunnels. |
|
2 to 1 ratio. If models like those of aircraft are to be tested, then it is usual for the large (span wise) dimension to be horizontal, for convenience in measuring forces.
