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We have confirmed numerically that a subcritical laminar-turbulencetransition that belongs to directed percolation (DP) universality class occursin a purely two-dimensional (2D) simple Navier-Stokes (NS) flow without anywalls. The flow is called (extended) Kolmogorov flow which is governed by 2D NSequation in a doubly periodic box with a linear drag and a finite flow rate inthe direction in which Galilean invariance is broken. We examine the mechanismof DP class transition focusing on the role of the additional controlparameters: the drag coefficient and the flow rate. The drag kills coherentactive structures of the system size. The flow rate interferes the growth ofweak disturbances. These two effects control two essential and intrinsicelements of an absorbing state phase transition, i.e., the existence of anabsorbing state and the locality of active dynamical structures. We alsodiscuss what physically corresponds to the additional parameters in generalflow.

Subcritical laminar-turbulence transition with wide domains in simple two-dimensional Navier-Stokes flow without walls