Wind Tunnels Basics

Module IV

Compressible Fluid Flow

VIII Semester

Wind Tunnels A wind tunnel is a laboratory facility for producing a jet of air or any other working substance that can be used for testing of aerodynamic models. Wind tunnels can be subsonic, transonic or supersonic, depending on the flow Mach number in the test section. Wind tunnels can also be classified as closed or open type wind tunnels. In open type wind tunnel the atmospheric air is taken in and exhausted to the atmosphere. A wind tunnel generally consists of a driving unit, a settling chamber, accelerating duct (nozzle), test section and a diffuser. The driving unit consists of a fan, blower or a compressor driven by an electric motor. The location of the driving unit depends on the type of the tunnel. The flow from the compressor/blower or a fan is settled in a large chamber called the settling chamber that is provided with wire gauzes and array of honeycombs to straighten the flow and remove irregularities in it. The settling chamber supplies the flow to the nozzle located down stream. This is designed to accelerate the flow from the nearly stagnation conditions of settling chamber to the required test section velocity. The nozzle discharges the jet into the test section. The model to be tested is fixed in the test section using suitable supports. A transparent window of strong glass is often provided on the sides of the test section. This facilitates in handling the models and the instruments and also permits the optical measurements in the flow over the model surfaces. The diffuser collects the flow from the test section and raises the pressure of the air for discharging it into the atmosphere or the return circuit.

Types of wind tunnels Blower tunnels The air from the blower or fan enters the settling chamber through a diffuser to reduce pressure loss. The contraction or the nozzle feeds the test section with an air jet at nearly ambient conditions. This is a special advantage of this tunnel since the models and the instruments can be easily manipulated during the operation of the tunnel.

Blower types are especially useful for conducting low speed (M<1) tests on cascades of turbine and compressor blades.

VENKITARAJ K P | Department of Mechanical Engineering, CE Adoor

1

Module IV

Compressible Fluid Flow

VIII Semester

Suction tunnels Suction type wind tunnels are open type tunnels and also used for subsonic testing of aerodynamic models. Air is sucked into the tunnel through a contraction (nozzle) by an exhaust fan. The air jet in the entire tunnel is slightly below atmospheric pressure. Therefore for a given velocity in the test section the power required to drive the fan is comparatively less.

Induction tunnels The flow in the test section is induced by introducing a high pressure air downstream of the test section through an annular nozzle. Such a tunnel is used when there is a small quantity of high pressure air is available.

The great advantage of an induction tunnel is that its operation is independent of the compressor characteristics. The compressors are used to fill the large reservoirs at the required pressure. The tunnel may be operated at any convenient time even when the power is not available to run the compressors. This type of tunnels can be designed for a wide range of Mach numbers both subsonic and super sonic (M<2.5) Supersonic wind tunnels In supersonic wind tunnels the Mach number in the test section is greater than unity. A supersonic wind tunnel can be of three types i. Continuous open circuit type ii. Blow down or intermittent type

VENKITARAJ K P | Department of Mechanical Engineering, CE Adoor

2

Module IV

Compressible Fluid Flow

VIII Semester

iii. Continuous closed circuit type. To obtain higher values of Mach number in the test section at lower gas velocities a suitable gas of higher molecular weight is used. In an ideal flow through the tunnel at the designed operating conditions, the flow is reversibly accelerated in the nozzle to supersonic velocity. But in the actual case, the flow through the wind tunnel encounters with shock waves. Open circuit supersonic tunnel (continuous flow type) Air is continuously supplied to the reservoir by a compressor. High pressure air is continuously expanded from the stagnation state ( po1, To1) to the test section pressure, p through the convergent-divergent nozzle.

The main disadvantage of an open type continuous supersonic wind tunnel is that a very considerable quantity of energy is thrown away into the atmosphere. This necessitates a higher pressure ratio and power for the compressor. In addition to this, the nuisance created by the high velocity jet exiting the tunnel is very high. Blow-down tunnels The operation of this type of tunnel is intermittent. This makes use of the high pressure air stored in large reservoirs. A quick opening valve between the reservoir and the nozzle is used to start the tunnel. The high pressure air is expanded to the atmosphere through the supersonic nozzle and the test section in an intermittent operation of a very short duration. Blow down concept can also be used by employing high vacuum in a reservoir downstream of the diffuser. The supply to the test section is from the infinite reservoir, thus the supply conditions are constant during the operation of the tunnel. Closed circuit supersonic tunnels ( Continuous type) A closed circuit continuous flow tunnel as shown in the figure below overcomes some of the limitations of an open type tunnel. The arrangement of the tunnel consists of a compressor, a convergent-divergent nozzle, test section, diffuser, cooler and a drier. VENKITARAJ K P | Department of Mechanical Engineering, CE Adoor

3

Module IV

Compressible Fluid Flow

VIII Semester

On account of the closed circuit, any gas besides air can be used in the tunnel. The diffuser recovers the kinetic energy of the jet leaving the test section. Thus the compressor receives air at a higher pressure, thereby reducing the compressor power required. The stagnation temperature of the air is continuously increasing due to the high rate of energy dissipation in the tunnels. So to keep the temperature constant a cooler is installed to cool the air before entering the compressor. The cooler can also be installed after the compressor, but the power requirement will be more as evident from the following figure.

Air from the reservoir is passed through the drier to avoid the moisture condensation in the tunnel. Since it is closed circuit type tunnel, all the air need not be passed through the drier. This helps in reducing the size of the drier used.

VENKITARAJ K P | Department of Mechanical Engineering, CE Adoor

4