Wind tunnel components
The important components of the wind tunnel are listed below;
Motor/Fan Driven unit : This is the air supply unit that drives the air flow in the wind tunnel. Typically, the fan is axial/centrifugal type and the axial fan is a better choice in the closed circuit tunnels since it produces a static pressure rise necessary to compensate for the total pressure loss in the rest of the circuit. The fans with higher ratio of tip speed to axial velocity generally produce the required pressure rise in a small blade area. The wind tunnels fitted with blower are generally driven by a centrifugal impeller of squirrel-cage type. While in operation, the fan draws air from the atmosphere through the honeycomb/screen section.
Fig. 8.1.2: Honeycomb structures for low speed wind tunnels.
Settling chamber and flow straightener : It mainly comprises of honeycomb and screens as combination. The main function is to reduce the turbulence and straighten the flow only in the axial direction. In principle, the air can enter to the tunnel from any directions. But, only the axial flow is desired in the test section. The main purpose of the screen is to reduce the turbulent intensity in the flow and not to allow any unwanted objects to enter the tunnels. The honeycomb can be made with cells or various shapes as shown in Fig. 8.1.2. These cells are aligned in the stream wise direction in the settling chamber thereby straightens the flow. The honeycomb has a longer length that reduces the transverse velocity components of the flow with minimal pressure drop in the stream wise direction. The minimum length required for this honeycomb is six to eight times the cell size. The number of screens required in the settling chamber depends on the flow quality requirement in the test section. Moreover, the power requirement is more when the number of screens is increased. The preferable length of the settling chamber is about 0.5 times the diameter of its inlet.
Contraction: The prime objective is to accelerate incoming flow from the settling chamber and supplies it to the test section at desired velocity. This section essentially reduces the cross-sectional velocity variation and maintains the flow uniformity. In general, small radius of curvature is used at the entry to this section and curvature of large radius is considered at the exit of the contraction section. However, the boundary-layer separation should be avoided at both the ends of this section. The contraction length is expected to be small so that large contraction area ratios are preferred.
Test Section: It is the basic element of wind tunnel on which all other designs are generally made. All the aerodynamic models are mounted in the test section when the tunnel is operated with desired flow velocity. Various shapes for the test section are considered for constructing the wind tunnel viz. hexagonal, octagonal, rectangle etc. The test section is generally designed on the basis of utility and aerodynamic considerations since cost of construction depends on the test section area. Length of the test section is mostly equal to major dimension of the cross-section of the same or twice of it. In addition, the test section should also be provided with facilities as per the testing requirement. The test section velocity is generally specified as percentage variation from the average of the cross-section. The ideal test section has steady uniform velocity at the inlet, no cross flow, less or no turbulence and less operating cost.
Diffuser: It is basically a duct with increase in area attached downstream of the test section. After the test section, it is desired that the air must pass smoothly out of the test section. So, this geometry is made to decrease the flow velocity and increase in pressure. In order to avoid flow reversal, the exit pressure should be higher than the atmospheric in case of open circuit wind tunnel. This is a very critical section in design since the incurred pressure rise reduces the power requirement for the wind tunnel which is proportional to the cube of velocity. Hence maximum pressure recovery to be achieved at least possible distance is the main objective of diffuser design. In general practice, the cone angle of the diffuse is 7º or less so as to avoid boundary layer separation.
Turning vanes: In a closed circuit wind tunnel, the air has to circulate in a controlled manner (Fig. 8.1.1-b). Typically, the corners of the wind tunnel are of two bends aligned 900 each other. These corners are provided with turning vanes for smooth passage of the flow. Chambered aerofoils of bent planes are generally accepted as the turning vanes. These vanes are purposely made as adjustable for smooth operation thereby avoiding under/over tuning.