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Aviation History
1931
1931 - 1323.PDF
FLIGHT, DECEMBER 18, 1931 which one has been familiar, in a general and perhaps somewhat vague sort of way, for many many years took on a new significance, and became illuminatingly " real." A few days ago Mr. Farren was good enough to send us a number of pencil sketches illustrating some of the particu- larly interesting and novel features brought out by the demonstrations, and these we have re-drawn to make them suitable for publication. These figures do not by a long way represent all the things Mr. Farren showed at the lecture, but perhaps they may serve to give some slight idea of the instructive advantages of the smoke apparatus. Referring to the sketches which Mr. Farren was kind enough to send us, Fig. 1 illustrates the demonstration of smooth flow past a cul-de-sac. By means of viscosity the main stream drives round an eddy in the " dead " region, and the smoke shows that there is a small amount of " mixing " of the two flows. In the next illustration, Fig. 2, is seen the flow in a rapidly-expanding pipe. It will be seen that there is a breakaway or " stalling," and it has been observed that this is generally unsymmetrical. Fig. 3 shows a " pipe " of constant cross-sectional area, but rapidly changing curvature of the boundaries. Such a pipe " flows full," suggesting that the breakaway shown in Fig. 2 is not due primarily to curvature. The type of flow seen in front of a flat plate is illus- trated in Fig. 4. This type of flow is closely similar to calculated " inviscid " flow, with a rapid rise of pressure along the stream near the centre of the upstream side. Fig. 5 contrasts strongly with Fig. 4 in the type of flow produced. In this case there is a smooth flat plate along the stream as well as the flat plate of Fig. 4 at right angles to the stream. The behaviour of the air stream has quite changed. There is a rise of pressure along the stream due to the presence of the flat plate at right angles to the stream, and the boundary layer is rolled up into two large eddies. In Fig. 6 is shown the breakaway of the flow past a circular cylinder. In the demonstration the formation of these eddies was clearly observable when the velocity of the airstream was changed suddenly. Fig. 7 shows the same cylinder, but with a guard shield- ing the lower half, and with means for rotating the cylin- der. We have here, of course, the Flettner rotor. Mr. Farren began with the cylinder stationary, when the streamlines had but a slight curvature, with an eddying region behind the cylinder, as in Fig. 6. He then started the cylinder rotating, and as the speed increased the stream- lines bent closer and closer to the upper half of the cylinder until, at full speed, they followed the contour without breaking away. The movement of the cylinder's upper surface gave the boundary layer the necessary energy to pre- fjf vent breakaway. The whole flow ' * past the upper half was closely r * similar to the calculated flow in the ideal or " inviscid " fluid of R the mathematician. Figs. 2 to 7, taken together, demonstrate that the origin of " stalling " or breakaway of the -y, air flow lies in the slowing up of y[) the boundary layer in the presence of a rise of pressure along the stream, and that it is not primarily due to the curva- ture of the surfaces. IQ The change of flow pattern with - Reynolds Number is illustrated in Figs. 8 to 11. In Fig. 8 is seen a large cylinder rotating at low speed, and the flow is of the " viscous " type. In Fig. 9 the same large cylinder is running at higher speed, and the wake is tif.xsli showing signs of instability. The "*~ rotational speed is still higher in Fig. 10, and the wake is broken up into a wave pattern. Fig. 11 shows a small cylinder running at high speed. The flow pattern is similar to that of Fig. 10, but the scale is reduced. This change in flow pattern occurs at Reynolds Number of about 100, i.e., a cylinder of | in diameter, with an air velocity of 1.5 ft. / sec. Initial stages of flow are shown in Figs. 12 and 13. For this demonstration Mr. Farren's throttle control of air velocity in the tunnel proved very useful, as he could accelerate and decelerate the rate of flow in almost an instant. Fig. 12 shows how eddies are formed at the edges of a flat plate whenever the flow is started or the speed changed suddenly. This should be compared with Fig. 4. In Fig. 13 is seen how eddies are formed where a pipe suddenly enlarges. The faint lines show the subsequent motion, which is ultimately a " jet " passing through a comparatively still " dead " region, but gradually mixing with it in the downstream direction. Flow past a Wing Tip.—Fig. 14 should help those who have not yet quite succeeded in obtaining a clear idea of the mechanism of induced drag. It will be observed that the streamlines do not flow across the wing in a direc- tion parallel to the chord, but form angles with the wing chord. Under the wing the streamlines are deflected slightly outwards, while above the wing they are deflected inwards. At the actual wing tip a vortice is formed. The actual demonstration was highly successful in show- ing these features of the flow. This type of flow, of course, applies to a wing of finite span only. The next figures, 15, 16 and 17, illustrate control of flow by removing the boundary layer by suction. Fig. 15 may be assumed to represent an obstruction to air flow. For instance, it could be a section through a hill. The well-known phenomenon of an up-current in front of the hill is evident, and the " stalled " part may be the lee of the hill. In the next illustration, Fig. 16, the boundary layer has been removed by suction, and now the air flow is seen to conform to the contour of the hill. The last figure, No. 17, shows how the air flow past a wing at large incidence is made conformable by removing the boundary layer by sucking air through the opening near the highest point on the wing. ® 1245
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