The term intermittency is applied to two distinct types of phenomena, externally and internally intermittent flows,  Chap. 7.2, Benzi and Biferale , Frisch , Chap. 8. Townsend  and Corrsin  observed in experiments on turbulent wakes and jets that turbulence away from the centre of the flow appeared as a random sequence of active bursts and quiescent intervals. Therefore, the flow field contains continuously changing subsets that are turbulent and subsets that are approximately non-turbulent (weakly turbulent), separated by an interface. This phenomenon is called external intermittency.