  In pipe, channel and boundary layer flows turbulence first occurs
intermittently in space and time: at moderate Reynolds numbers domains of
disordered turbulent motion are separated by quiescent laminar regions. Based
on direct numerical simulations of pipe flow we here argue that the spatial
intermittency has its origin in a nearest neighbor interaction between
turbulent regions. We further show that in this regime turbulent flows are
intrinsically intermittent with a well defined equilibrium turbulent fraction
but without ever assuming a steady pattern. This transition scenario is
analogous to that found in simple models such as coupled map lattices. The
scaling observed implies that laminar intermissions of the turbulent flow will
persist to arbitrarily large Reynolds numbers.
