This paper describes the results of an investigation of the flow over two cylinders in which the diameter of the upstream cylinder is half that of the downstream cylinder. Measurements were made of the surface pressure distribution around the downstream cylinder, and of the base pressures and Strouhal numbers of the two cylinders for separations between 1·32 D and 3·88 D, where D is the diameter of the downstream cylinder, and for angles of stagger between 0° and 45°.

With the cylinders in tandem two different flows were found, and it is suggested that there is a step change between the flows at a critical separation.

The behaviour of the measured quantities with a change of the angle of stagger is found, in general, to depend on whether the separation is less or greater than the critical separation. For each separation an angle of stagger is found at which the pressure distribution has a strong asymmetry over the rear of die cylinder. These pressure distributions are similar to each other, and they indicate the presence of an enhanced flow over the upper half of the cylinder. Also the lift force, for a given separation, then has its maximum value.

It is found that for separations greater than the critical separation the Strouhal numbers of the vortex shedding from the two cylinders are identical over a considerable range in the angle of stagger.

Currently there is renewed interest in the evaluation and reduction of steady wind tunnel wall interference, especially for large models.

Evaluation of previous predictions for perforated and slotted tunnels suggests that a hybrid slotted tunnel, ie a slotted tunnel with closed slats and perforated slots) should offer minimum corrections for upwash, flow curvature and solid blockage. This suggestion is confirmed by the present computer studies of a range of rectangular hybrid slotted tunnels.

The computer studies are for tunnel working section height to breadth ratios of 0·835 and 0·600 over the Mach number range from 0 to 0·85. Wings swept at 28° and 50°, with ratios of model span to tunnel breadth varying from 0 to 0·7, are considered. An idealised fuselage shape is used to predict solid and wake blockage corrections for the wall configurations selected on the basis of minimum upwash and curvature interference.

Propelled by the rapid advances in VLSI technology, supercomputers are now appearing with much larger real memory capacity than previously available. These new machines are expanding our ability to formulate very large-scale discrete models in order to discover non-linear dynamical flow behaviour. With a synergetic computational-analytical view point, we can begin to explore new and unexpected flow phenomena such as wave-like instabilities of vortices and even the onset of coherent large-scale turbulence brought about by vortex stretching.