The Aeronautical Journal: Volume 87 -

Longitudinal vortices in wind tunnel wall boundary layers
S. Mokhtari, P. Bradshaw
Published online by Cambridge University Press:

04 July 2016, pp. 233-236
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Several workers have found evidence for the presence of longitudinal vortices in wind tunnel wall boundary layers near the centre line of each wall, although the only actual publication appears to be the reproduction of Bearman's work in Ref. 3. The phenomenon usually observed is shown in Fig. 1: the boundary layer is considerably thickened near the centre line. (This symmetrical and localised perturbation should be distinguished from the effect of non-uniformities in the wind tunnel screens which apparently also leads to longitudinal vortices, randomly spaced across the width of the working section). The hypothetical vortex pattern is also shown in Fig. 1: it arises because lateral pressure gradients in the wind tunnel contraction naturally deflect the slow-moving fluid in the boundary layer towards the centre line more strongly than the main ‘inviscid’ part of the flow, implying the generation of longitudinal vorticity: this is secondary flow of Prandtl's first kind.

Heavy gas mixtures for wind tunnel use
S. K. J. Al-Ani, E. A. Johnson
Published online by Cambridge University Press:

04 July 2016, pp. 237-240
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The advantages of using heavy gases in wind tunnels—reduced power consumption and scale size — have been appreciated for some time. To be sure of correct scaling of the results from such a tunnel, the working fluid should have a specific-heat-ratio γ equal to that of air (γ = 1·4); such a fluid can be obtained as a mixture of a monatomic gas with a polyatomic one. Here, general principles for choosing such a mixture are discussed, and estimates are given for the savings in size and scale which may be expected (compared to a conventional tunnel), following earlier work of Chapman.

The Aeronautical Journal