Unit I Layout of Open Circuit Wind Tunnels

General Features
All modern wind tunnels have four important components;

• the Effuser
• the Working or Test-section
• the Diffuser
• the Driving unit

The Effuser

This is a converging passage located upstream of the test-section. In this passage fluid gets accelerated from rest (or from very low speed) at the upstream end of it to the required conditions at the test-section.

In general, effuser contains honey-comb and wire gauze screens to reduce the turbulence and produce an uniform air stream at the exit. Effuser is usually referred to as contraction cone.

Test-section

Model to be tested is placed here in the air-stream, leaving the downstream end of the effuser, and the required measurements and observations are made. If the test-section is bounded by rigid walls, the tunnel is called a closed throat tunnel. If it is bounded by air at different velocity (usually at rest), the tunnel is called open jet tunnel. The test-section is also referred to as working-section.

Diffuser

The diffuser is used to re-convert the kinetic energy of the air-stream leaving the working-section into pressure energy, as efficiently as possible. Essentially it is a passage in which the flow decelerates.

Driving Unit

If there were no losses, steady flow through the test-section could continue for ever, once it is established, without the supply of energy from an external agency. But in practice, losses do occur, and kinetic energy

is being dissipated as heat in vorticity, eddying motion and turbulence. Moreover, as the expansion of the diffuser cannot continue to infinity, there is rejection of some amount of kinetic energy at the diffuser exit.

This energy is also converted to heat in mixing with the surrounding air.

To compensate for these losses, energy from an external agency becomes essential for wind tunnel operation. Since power must be supplied continuously to maintain the flow, the fourth essential component

namely, some form of driving unit is essential for wind tunnel operation. In low-speed tunnels this usually takes the form of a fan or propeller.

The overall length of the wind tunnel may be shortened, and the rejection of kinetic energy at the diffuser exit eliminated, by the construction of some form of return circuit. Even then the driving unit is necessary

to overcome the losses occurring due to vorticity, eddying motion and turbulence. The skin friction at the walls and other surfaces will be large since the velocity at all points in the circuit will be large (of the same

order as the test-section velocity). Also, a construction ahead of the test–section, is necessary if the turbulence at the test-section has to be low, and particularly if the velocity distribution has to be uniform. To achieve this, usually guide vanes are placed in the corners.