THE task of building a wind tunnel for the requirements of Switzerland was somewhat complicated by the fact that the grant available for the purpose was strictly limited, a still greater difficulty being the need to be as economical as possible as regards space. It was, of course, desired to have a flow crosssection as large as possible, but it is inadvisable to compress to excess the other dimensions of a tunnel owing to the possible detrimental effect on the steadiness of the flow and the consequent errors in the measurements. We would certainly be faced with a difficulty if we were asked to arrange for a material decrease in the total length of the tunnel while maintaining the present jet unchanged, without reducing the speed or the steadiness. Comparison with the wind tunnel at Warsaw shows clearly that our design for Zurich has gone through a very thorough process of compression. Having regard to the noise factor, a fairly massive type of construction is necessary the precedent laid down by Prandtl was followed and concrete used. The question of cost interfered here and required the simplest possible inlays, which meant only simple curved surfaces. Excessively sharp angles being unfavourable, however, the solution was right angles with well bevelled corners. The quality of the air stream, as also the power requirements, were determined by careful measurements conducted in a model of the whole tunnel built accurately to a scale of 1 10. The test chamber is a rectangle, in section 3 by 21 metres, with bevelled corners. Tests can be conducted either with an open jet or with the chamber totally enclosed or half enclosed, the latter arrangement being a new departure. The jet is caught in a collector and deflected through a right angle downwards by baffles of strongly reinforced concrete. After a second deflection the stream is drawn through two fans set side by side. Each fan has a diameter of 25 metres with a maximum rate of revolution of 1,500 r.p.m., six blades elektron castings and a diffusor in front of the impeller, comprising a large number of welded blades which impart to the air a counter twist smoothing out all rotation of the stream so that after the fan parallel flow is ensured. The hub is well faired off. The blades are mounted on pivots, and their angle can be adjusted when the fan is at a standstill. The fans are driven by D.C. motors, with an output of 275 h.p.hr., mounted outside the tunnel. Their sense of rotation is the same, and they are synchronised by a continuous silk belt running over the couplings. This type of belt is the invention of the wellknown steam turbine expert, Dr. H. Zoelly. Their pliability ensures elimination of flapping even at very high peripheral velocities, and they are capable of transmitting very high powers.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Aug 1, 1935