Coefficients for heat transfer from free rotating discs, at Reynolds numbers in excess of 4.8 x 105 have recently been obtained by Cobb and Saunders. These results are consistent with a limiting turbulent-flow heat transfer given by: —

Nu mean = 0.0172 Re0.80 . . (1)

The possibility of explosions in a duct or chambers, which may contain a mixture of an explosive vapour and air, is a hazard commonly occurring in industrial plant, often necessitating a very strong and costly form of construction which would otherwise not be required. Where the structure is not strong enough to withstand explosion pressures serious damage results, frequently with injury to personnel.

Published Works discuss the calculation of the natural frequencies of uniform beam systems, but not simple calculations on beams of arbitrary cross section. This note describes a simple method evolved for estimating the lowest natural frequency of an axial compressor blade, and compares its results with measurements and with other calculations. The logical extension of the method to calculating the frequency of any mode of any cantilever of arbitrary cross section is made in the Appendix.

Accurate representation of a model field in an electrolytic tank requires, among other things, that the electrical conductivity of the liquid should be everywhere constant. Einstein and Sander and Yates have discussed the importance of other possible inaccuracies, and have shown that by correct probe design, circuit design and choice of electrode materials, these may be made very small. This note describes very briefly some experiments, carried out in two shallow electrolytic tanks, which demonstrate that the principal error can be that introduced by variations of conductivity produced by non-uniformity of temperature distribution in the electrolyte. The point is of particular importance since the results also showed that the mechanical design of the tank can have a vital bearing on the achievement of uniform temperature.

A Phenomenon of interest in the study of high-speed aerodynamics concerns the flows generated around very slender bodies of revolution. It has been observed that these bodies, at moderate and large angles of attack, produce a kind of flow pattern that differs considerably from the ones assumed in perfect-fluid treatment according to the usual slender-body analysis.

Photographs reveal that the flow above the upper surface of the body contains two symmetrically disposed spiral vortex sheets which roll up and are carried down-stream. Viewed in respect to the stationary body, the shed vortices appear fixed. The vortices increase in strength as anybody cross plane moves rearward, being eventually discharged to form a Karman vortex street as viewed in the moving cross plane.

Appreciable forces can be obtained for steering a vehicle by the use of aerofoils whenever the relative air velocity is high and the air density is not too small. In practice this means velocities greater than 100 ft./sec. and altitudes below about 50,000 ft. Even when they are not in use for steering, aerofoils necessarily add to the drag of the vehicle.

In the July JOURNAL, page 528, Dr. H. Fessler describes a method of testing struts using links in tension to apply the load.

A much easier and cheaper method is to use sharpened rods as struts tested between hard flat plates. As the load increases the ends continually crush so that there is no restraint and the equivalent to a pin joint is obtained.

Eccentricities can easily be obtained by bending the rods and the amount can be checked by mounting between female centres and rotating. At least three specimens of each length should be tested. A strut curve consisting of failing stress plotted against length divided by radius of gyration can then easily be obtained for use with the material, without the use of theory.

Figure 1 is a set of S-N curves for DTD. 150, taken from Rotol Structures Department Report No. 337. It is difficult to estimate N accurately on the flat part of the curve, and estimates of fatigue life by different people can be very different. Fig. 1, although based on scanty test results, is typical of S-N curves.

The gustiness of the atmosphere is studied, among other reasons, because of its importance in producing loads on aeroplane structures much larger than the loads occurring in calm air.

Atmospheric turbulence is studied by observing the behaviour of aeroplanes passing through it, by observing the motion of balloons or other articles carried along by the wind itself, or by examining the fluctuations in velocity or direction occurring at a point fixed relative to the earth. The first of these methods is the most flexible and is the most convenient for aeronautical investigators.