The structure of a self-preserving turbulent plane jet exhausting into a slow-moving parallel airstream is studied. The investigation includes results of turbulence measurements and the structure is compared with that of a self-preserving plane wake. The results show that self-preservation is established at a distance of about thirty jet widths downstream of the jet nozzle and that, in the self-preserving region of the jet, the distributions of the turbulent intensities and shear stress across the jet are very similar to those found in the plane wake. The distribution of the intermittency factor, however, is found to be more like that found in an axi-symmetric jet than in a plane wake. The turbulent energy balance also shows important differences to that of the wake flow. The unsteady irrotational flow outside the turbulent shear layer is investigated and it is found that the experimental results agree with the predictions of the theories of Phillips (1955) and Stewart (1956). Some comments are also made on the eddy structure and the applicability of the simple theories of turbulence.

In an attempt to explain the discrepancies that have been observed in the spread of nominally axisymmetric jets, an experimental investigation has been carried out in which the effects of a number of factors which it was thought might be important to jet development have been studied. These factors included the nozzle boundary-layer thickness, turbulence level and convergence. However, over the limited range of the tests, it was found that none of these factors had a very strong influence on the jet development. By contrast, the insertion of small rectangular tabs into the jet flow on the nozzle perimeter was found to have a very profound effect on the jet development. In particular, it was found that just two tabs produced gross distortions in the jet development resulting in the jet almost splitting in two with high velocity regions on either side of the diameter joining the tabs. Some explanations for this effect based on further tests with wedges are put forward.In addition to the measurements of the mean flow field, a few spectrum and correlation measurements are reported for jets both from a clean nozzle and also from a nozzle with two tabs. In the former tests, evidence additional to the results of other experimenters was found for the existence of flow structures which have some coherence around the entire circumference of the jet. It has been suggested that these ‘vortex rings’ or ‘puffs’ may be of some importance in producing jet noise and it seems that the effect of inserting tabs is to prevent the occurrence of these structures.

The velocity measuring technique described in this paper consists of measuring the time of flight of a tracer of heated air from an electrically pulsed wire to one of two sensor wires which are operated as resistance thermometers. These sensor wires are at right angles to the pulsed wire and are placed one on either side of the pulsed wire. The instrument may be used in highly turbulent flows including regions in which flow reversals occur. The paper discusses the theoretical behaviour of the probe and the results of some calibration experiments.

The results of an experimental investigation into the development of a turbulent plane jet issuing into a parallel moving airstream are described. On the basis of a simple dimensional argument, it is shown that the results for the spread of jets with different ratios of jet nozzle to free-stream velocity can be collapsed into a single universal curve provided the effective origins of the various sets of data can be shifted. Evidence is found of a change in structure of the jet from a self-preserving plane jet flow near the origin of the flow towards a selfpreserving wake type of flow far downstream from the origin. This change of structure is compared with a prediction based on a simple application of Town-send's large-eddy hypothesis. It is shown that the hypothesis does not account for the way in which the jet structure changes and possible reasons for this are briefly discussed. Finally, some comments are made on the usefulness of the various theories of jet spreading.