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This investigation is based on a DNS of a steady self-preservingincompressible axisymmetric turbulent jet at a Reynolds number of 2400. The DNSdata enable accurate maps of the outer irrotational flow field, and also thevorticity field in the turbulent core of the jet. It is found necessary todefine two separate boundaries of the jet. The first is an inner boundary(turbulent/nonturbulent, T/NT), from where vorticity rises steeply towards tothe core. The second is an outer rotational/irrotational boundary, beyond whichthe flow may be considered irrotational. The velocity field beyond the outerboundary often has ordered, nearly irrotational circulatory motions. These canbe shown, in simpler cases, to be the velocity field induced by one or morevorticity elements in a coherent structure in the turbulent core. A detailedexamination of axial and diametral sections indicates that there are periodswhen there is a large inrush of ambient fluid into parts of the T/NT interface,which gets distorted into a gulf or well that can be both twisted and deep.Sections of these wells often appear as what may be called as lakes ofirrotational fluid in diametral sections of the jet flow. Part of the inrushingfluid crosses the T/NT interface within the well and is entrained into theturbulent core, by a process that can legitimately be called nibbling. Theduration of such an inrush process can be of the order up to 20 flow units andsuggests that entrainment can be an episodic process in which an inrush eventaccelerates ambient fluid even as it is pushed into a narrowing gulf, where itpenetrates the T/NT interface of the gulf by nibbling. In the turbulent roundjet, the entrainment burstiness is found to be of order 0.75, comparable to themomentum flux burstiness found in a turbulent boundary layer.

A DNS Study of entrainment in an axisymmetric turbulent jet as an episodic process