Search

The combustion of spherical gas clouds may fall into two regimes, controlled either by buoyancy or by the intial velocity of the fuel gas. Dimensional analysis (confirmed by experiment) shows that the burning time is proportional to fuel mass to the power of one sixth for buoyancy control, or one third for velocity control, for fireballs large enough to be fully turbulent. The paper discusses the variation of diameter and rise height with time for buoyancy controlled fireballs. Experiments were performed using soap bubbles with up to 14g of fuel gas giving a quiescent release in wind-free conditions. Fuel masses covered a range of three orders of magnitude. Combustion was recorded on colour video, and analysed for burning time and for diameter and rise height versus time. Fireballs formed from over 0.5g of fuel resembled those observed in incidents with several tonnes of fuel. Initially, they formed a flattened sphere, with a turbulent vortex ring becoming visible in the later part of their life. The measurements of diameter and rise height versus time could be correlated in the way predicted from dimensional analysis. The rate of growth of fireball diameter was greatest just after ignition, when the rise velocity was smallest. This implies that the initial mixing of fuel with air occurred by gravitational instability rather than by shear-induced entrainment. Analysis of the motion of these fireballs showed that only a small proportion of the entrained air reacted with the fuel. Viscous effects could not be neglected for fireballs with less than about 0.5g of fuel. These differed in appearance from the larger fireballs, and did not obey the same correlations for burning time, rise height or diameter.

The merit of magnetic tape for storage of digital information and the benefits of abandoning linear in favour of two-state operation are briefly discussed. Limitations on the density of recorded information are reviewed to show the extreme precision called for in the manufacture of magnetic heads and tape guidance mechanisms. Design features made necessary by high speed and acceleration requirements are considered with reference to a recently developed high performance tape handler.

Negative-impedance convertors for lowpass active RC filters must respond down to zero frequency; any attendant drift is unwelcome. A relatively simple circuit is described wherein a balanced input stage feeds an unbalanced output stage, which, in turn, feeds back to both input terminals in the same phase.

A function generator for providing, in analogue, the mantissa of the logarithm of two numbers, given the logarithm of each, may be realised by the conductance of the series combination of a resistor and a semiconductor diode. Its application to a logarithmic digital/analogue convertor for serial inputs is described.

Special purpose digital apparatus is described for off-line correlation of time varying functions in analogue form. The necessary storage, repetition and relative scanning are afforded by an instrumentation magnetic tape recorder through the extension of its ‘closed loop’. A new value of lag coincides with the complete replacement of that length of tape contained within the ‘closed loop’, and a fresh value of correlation coefficient is printed out for each lag. The method takes up only two tracks, so hose remaining can be used in the normal acquisition role at any convenient speed, later transferring their signals at high speed to the analysis circuits.