It is both a privilege and a pleasure to be asked to contribute to an edition of the Aeronautical Journal to celebrate Professor Alec Young's 70th birthday. He has been my mentor for nearly a quarter of a century.

The subject of integral prediction methods for boundary layers is one with which Alec Young has long been associated. Starting with a method for laminar compressible flow he has guided the development of, amongst others, integral methods for compressible laminar flow with heat transfer, three-dimensional laminar flow, three-dimensional turbulent flow and flows with pressure gradients normal to the surface.

This paper attempts to describe the integral methods for compressible turbulent boundary layers which are most widely used in aerodynamic design and to discuss the application of inverse integral methods to three-dimensional separated flows.

As a member of the International group who set up the von Karman Institute for Fluid Dynamic (known at that time as the Training Centre for Experimental Aerodynamics — TCEA) Professor A. D. Young participated actively in the Institute's programme. After the death of Dr von Karman in 1963, he was nominated Chairman of the VKI Board of Directors, and still holds this position.

It is clear that the goals of the Institute, as envisaged by Dr von Karman, have been successfully met. After the first few challenging years under his leadership, it was essential to ensure that the scientific reputation of the Institute continued to be strengthened. The VKI is now recognised as a major establishment in its own right with a very high scientific reputation.

The objective of the present paper is to examine how the mathematical modelling of the overall aircraft system, in order to calculate its flight dynamics, has had to change to meet the requirements set by the type of problem posed. Some of these requirements reflect an increasing complexity in the aerodynamic design of the aeroplane. Others, a desire to improve the accuracy of the solutions to new and sometimes old problems.

In addition the basic difficulties raised by the fact that, strictly speaking, the aerodynamic forces and moments depend on the past as well as the present history of the motion.

A comparison is made of the aerodynamic formulations constructed from the character of the dynamics with those deduced, on the basis of a number of assumptions, from an analysis, which starts by considering the true nature of the aerodynamic forces and moments. In mathematical terms this is expressed by describing them as functionals of all the state variables.

This short note is intended as a measure of thanks to Alec Young for his encouragement and supervision of the author's PhD studies.

International air transport is a vital industry, integral to world trade, commerce and tourism. This industry has contributed substantially not only to worldwide economic growth, but also to political and sociological progress. However, in recent years the industry has fallen into chaos, to the point that many airlines find themselves either subject to unstable situations, or threatened by bankruptcies, or, in the case of state-owned airlines, in need of financial support from their governments. Competitive behaviour sometimes occurs on an under-the-table basis, interline co-operation has been deteriorating, and a clear international policy whose guidelines are accepted by everyone in the industry is absent. If the international airline industry is to continue to play a vital and dynamic role in contributing to the progress and prosperity of so many nations, then it is essential to adopt and implement policies aimed at eliminating the chaos in the industry, facilitating its healthy growth and stability, and developing international collaboration for the common good.

An analysis of results from reports on droplet evaporation indicated a marked difference in behaviour depending upon the suspension technique used. Few results were obtained with freely falling droplets, the condition which is of interest in aerial spraying operations. Test results obtained at Cranfield by dropping droplets down a climatic tower showed two unexpected results.

The diameter of water droplets reduced linearly with time until they reached a size of about 150 micrometers and then the rate of reduction in diameter increased by about 27%. It is shown that this change occurs when the fall Reynolds number of the droplet becomes less than about 4.

Above this Reynolds number the airflow is separated from the base of the droplet and the trapped air in the toroidal vortex becomes fully saturated and no evaporation occurs over the base region. Below a Reynolds number of 4 the flow is attached everywhere and evaporation occurs from the whole surface area.

When small amounts of molasses are added to a spray formulation the droplets evaporated initially in a way similar to water droplets. As a result of falling, a toroidal vortex motion is induced within the droplet. This causes the same small fraction of the volume of the droplet to cycle close to the surface.

Hence, this fraction of the volume looses all its water, becomes far more viscous as a result and the consequence is the toroidal motion almost stops. In effect the molasses forms a skin over the droplet surface inhibiting further evaporation of the remaining volume of water.

Transverse curvature effects in axi-symmetric flow on a circulai cylinder are discussed and it is deduced that for small δ/a, where δ is the boundary layer thickness and a is the cylinder radius, transverse curvature effects are small, for δ/a = 0(1) the effect is felt mainly in the outer region and for δ/a » 1, both the inner and the outer regions are affected. It is argued that the last case can only be achieved experimentally in the laboratory if the radius Reynolds number Ra= U1a/v is small and the streamwise Reynolds number Rx=U1xlv is large, implying x/a large. New experimental data are presented for this case. The data show that both the mean and turbulent flow field scale on outer variables throughout the layer and, for Ra≥455, exhibit Reynolds number independence. No logarithmic region is found in the mean profiles, a result which is echoed by the absence of a shoulder in the streamwise turbulence profiles which clearly evidence that the flow in the layer is fully turbulent. A flow model is suggested, in physical terms, to explain the very high turbulence levels found close to the cylinder at values of Ra which should, according to stability theory, be bordering on laminar flow.

Alec Young spent over 30 years in academic life. He has been active, not only in nurturing the teaching of and research in aeronautical engineering, first in the Aerodynamics Department at Cranfield College of Aeronautics, and then in the Department of Aeronautical Engineering at Queen Mary College, but also in participating in wider academic developments as Vice Principal of Queen Mary College and as Senator of the University of London. Hence, it is appropriate that in such a tribute, there should be an article on an academic topic. A review of undergraduate aeronautical engineering education, its past, present, and future, has been chosen. Such a review is timely as a contribution to the current debate on engineering education in general, initiated by the newly formed Engineering Council.

Experimental measurements of the separated flows around a range of tapered flat plates normal to a uniform stream are presented. Both the surface pressures and velocities in the wake are discussed. It turns out that even slight taper can modify the wake region of the flow significantly, and so produce a surprising degree of three-dimensionality into the overall flow structure. These experimental results point to the dangers in applying simple strip theories to highly separated flows.

Aerodynamic theory has evolved over the years to describe the flow fields around streamlined aircraft and their components, so as to provide, amongst other quantities, accurate estimates of the aerodynamic forces and moments.

It is a pleasure and indeed an honour to be invited to contribute this paper to the 70th Birthday Tribute to Professor A. D. Young, in recognition of his longstanding friendship and as a colleague of the author for many years.

It was Professor A. D. Young, at the College of Aeronautics in 1949, who gave the necessary guidance and encouragement to the author in starting research into aerodynamics noise, which was possibly one of the first attempts made in making a quantitative assessment of the noise from a turbulent air jet.

The paper discusses the early experimental work and its conclusions, and how it led to attempts to reduce jet noise through the use of vortex generators and the corrugated nozzle. The comparison between theory and experiment is commented on with reference to the early work on acoustic sources and the questions this comparison raised. A final section is included which attempts to close the gap in our understanding of jet noise generation.