The production of required velocity profiles in a duct or wind tunnel is a necessary part of much research aimed at understanding fluid behaviour. Perhaps the most obvious application is the simulation of wind gradients for the study of wind effects on structures, but equally important is the study of diffuser and duct behaviour when subjected to a variety of known and convenient velocity profiles. Furthermore, the effects of the variation in turbulence characteristics within a range of identical velocity profiles produced by different methods are not clearly understood.

In a recent Technical Note, Bradshaw offered additional thought on an earlier note by Markland and myself. In doing so he wrote of the efficiency of a diffuser which was confused with the pressure recovery coefficient, Cp. In view of the great significance when designing engine intakes, combustion chambers, nuclear reactor plant and even heating and ventilating systems of even minute improvements in duct efficiency, the distinction is further discussed.

In 1931 Peters showed that flow entry conditions had a marked effect on diffuser performance. Later, Winternitz and Ramsay indicated that the pressure recovery coefficient Cp of a conical diffuser might be regarded as a linear function of β(θ/d)i for values of β(θ/d)i up to 0·1. This note draws attention to the fact that diffusers with much larger values of this parameter are of interest and presents some experimental and theoretical results. Work is continuing along the lines indicated.

In the establishment of a fluid circuit's behaviour, it has been shown that the condition of the entry flow into each component of the circuit is of significance. This note indicatesthe problems encountered by a designer using diffusers at the present state of knowledge, and outlines a hopeful method of reducing diffuser pressure losses.

In order to determine the downstream consequences of the presence of small discrete surface discontinuities situated on otherwise smooth surfaces and subjected to six equilibrium two-dimensional adverse-pressure-gradient turbulent boundary-layer flows, these conditions were first established in a special-purpose wind tunnel. A surface discontinuity is small if it lies within the logarithmic region of the undisturbed boundary layer. Immediately downstream of such discontinuities flow separation ensues. After the subsequent reattachment, measurements were made of the downstream boundary-layer development. Even in strong adverse pressure gradients the local increments of momentum thickness caused by these roughness elements were well predicted by Gaudet & Johnson's zero-pressure-gradient correlation. With highly adverse pressure gradients it was found that these small surface discontinuities have little influence on the flow downstream. The essential outcome of this work is that the incremental drag of small roughness elements depends solely on local wall variables. Thus, when the pressure gradient is strongly adverse and the local skin friction is correspondingly small, the incremental drag of the roughness element becomes similarly small. After reattachment, it has an insignificant effect on the flow downstream of it.