**5. Diffuser design**

In general, modern wind tunnels are designed for very low turbulence levels require contrac‐ tion ratios of 12 to 16, in conjunction with up to six turbulence reduction screens. However, quite low turbulence levels may be obtained in wind tunnels with a contraction ratio of the order of 7:1, with four to six screens, and in conjunction with closely spaced vanes in the corner

The contraction ratio selected for the University of New South Wales tunnel produces

There is as yet, no established exact design method for octagonal section wind tunnel contrac‐ tions. Nevertheless, a design criterion common to all contraction is the avoidance of high wall curvature and large wall slope leading to possible adverse pressure gradients of strength

This problem is particularly critical at the contraction entrance [43 and 46] and modern wind tunnels no longer use very small radius of curvature at the inlet end as was favoured before 1940 [44, 64 and 65]. It has been shown theoretically [66] that in order to obtain a uniform velocity distribution at exit, the velocity increase along the contraction must be monotonic but this condition is incompatible with the need for a finite contraction length. Most methods of

**1.** Specification of an arbitrary contraction shape based on experience and/or the demands

**2.** A contraction shape given by the flow of a uniform stream about an arrangement of

**3.** Specification of velocity distribution along the contraction axis leading to a derived

**5.** Specification of the contraction boundary velocity distribution in the hodograph plane and transformation to the x, r plane so as to derive the contraction shape in axisymmetric

Details of these methods can be found in References 64 to 84. The method employed for the University of New South Wales tunnel was to sketch in the shape, keeping in mind the demands of the constructional material techniques selected and the requirements for satisfac‐ tory performance [44, 46, 48, 64, 66, 69 and 85]. The contraction length was first estimated from the fact that, for contraction ratios of the order of 6 to 10:1, the ratio [51], the length to major

The inlet and exit radii of curvatures are approximately 8 and 11 ft respectively for the University of New South Wales tunnel. The resultant contraction shape is very similar to that

or 0.022

upstream of the settling chamber, as for example, in the N.B.S. 4 ½ ft tunnel [45].

[B19] and of the mean RMS turbulence intensity by a factor of the order of [45] :

sufficient to cause flow separation in either the contraction or test section.

U'/UT= (2n/3+1/3n2)0.5 /n= 0.31

Taylor's alternative analysis suggests 0.4 to 0.8 [53]

36 Wind Tunnel Designs and Their Diverse Engineering Applications

design generally fall into of the five following categories:

of the constructional material

sources, sinks or vortex rings.

**4.** Conformal transformation techniques

or two-dimensional flow.

inlet dimension, lies within 0.8 to 1.2.

contraction shape

reduction in the percentage longitudinal velocity non-uniformities by a factor of 1/n2

As mentioned in section 1, space limitation prevented the fitting of a controlled rapid expan‐ sion and the achievement of the optimum contraction ratio of 12 to 16:1. When it is possible to fit such an arrangement, a variety of flow stabilization methods of varying suitability are available for wide angle diffusers [86-94].

Considerable data is also available for the conventional diffuser design [98-104 ]. Unfortu‐ nately, however, little of this information has direct application to the design of threedimensional octagonal section wind tunnel diffusers of any practical compact design must entail a certain amount of guess work or knowledge of previous experience in the selection of appropriate diffuser angles. For example, attempts to use the data of Ref D6 would indicate that for the large return diffuser of area ratio of 2.85:1, two-dimensional diffuser angles of up to 120 might be employed. However, experience with the square cross-section three-dimen‐ sional main return diffuser of the R.A.E. No. 2, 11 ½ ft x 8 ½ ft, wind tunnel1 indicated that equivalent cone angles of about 50 are satisfactory for this application. Shorter diffusers may employ somewhat larger angles and advantage has been taken of the fact in the design of the University of New South Wales tunnel where the equivalent cone angles used vary from 5.20 in the longest diffuser to a maximum of approximately 6 ¼0 in the shortest diffusers.

The first diffuser downstream of the test section is a particularly difficult design problem as the flow maldistribution caused by high lift and bluff models must be taken into account. Moreover, work by Willis [105] indicates that unsteady flow in the diffuser is responsible for a rise in a measured wall pressure spectra at low frequencies. The University of New South Wales tunnel has an essentially two-dimensional first diffuser with an included angle of 7 ¼ 0 and area ratio of 1.4:1 (equivalent cone angle of 3.40 ). Reference D6 indicates that a diffuser angle of up to 170 might be employed without separation for this diffuser.

Diffuser performance is also related to the inlet boundary layer thickness and free stream turbulence level [98, 99, 101-104). This makes the estimation of tunnel diffuser losses difficult. In the estimation shown in Table 1, the five diffusers contribute 37% of the tunnel loss, the first diffuser alone being about 14% of the tunnel loss. The design of the diffusion zone over the fan tail-fairing is a special problem and has been conveniently summarised by Russel and Wallis [106].

Diffuser numbers 4 and 5 of the University of New South Wales were built from ¾ inch thick exterior waterproof quality plywood with angle iron and 5 inch x 1 inch timber supporting frames. All sections are octagonal in shape as this permitted short length transitions to be made between the main components of the return circuit and circular fan ducting. The mitred sides of the octagons are constructed of 1/ inch ply mounted on 3 in x 2 in Oregon frames inside the main diffuser shell.

Diffuser No.1, the fan ducting and associated transitions are constructed from 16 gauge mild steel sheet which is reinforced with angle iron frames and rectangular bar steel frames and stringers.

Heavy Perspex windows and fluorescent lighting are fitted to enable easy visualisation of flow performance of the tunnel components. Each leg of the tunnel circuit between the turning vane cascades is provided with one or more quick opening doors for easy access. The doors are sealed with circular, foam rubber cord, formed into shape of an 'O' ring.
