On the spreading of a turbulent spot in the absence of a pressure gradient

Velocity measurements in the plane of symmetry of a turbulent spot are reported. The number of data points taken at various streamwise locations was adequate to map the ensemble-averaged flow field in a spot at a given instance. These results are compared with velocities taken in laboratory coordinates (i.e. at a given station with variable time), whereupon it is shown that the flow field in the spot depends either on the distance from its origin or on the time elapsed from its initiation. The two variables are related so that the flow may be transformed into either a time- or a space-independent problem. The dependence of the spot on the Reynolds number and on the surrounding laminar boundary layer isestablished. The effects of these parameters on the shape of the ensemble-averaged spot, its size, characteristic celerities, and relative rate of entrainment, are discussed.The present results indicate that a similarity approach based on ensemble-averaged data is severely limited. It might be used to predict the overall scales and flow field but much more sophisticated data-processing techniques are required to describe the structure of the spot.