  The proliferation of turbulence in subcritical wall-bounded shear flows
involves spatially localised coherent structures. Turbulent spots correspond to
finite-time nonlinear responses to pointwise disturbances and are regarded as
seeds of turbulence during transition. The rapid spatial decay of the turbulent
fluctuations away from a spot is accompanied by large-scale flows with a robust
structuration. The far field velocity field of these spots is investigated
numerically using spectral methods in large domains in four different flow
scenarios (plane Couette, plane Poiseuille, Couette-Poiseuille and a sinusoidal
shear flow). At odds with former expectations, the planar components of the
velocity field decay algebraically. These decay exponents depend only on the
symmetries of the system, which here depend on the presence of an applied
gradient, and not on the Reynolds number. This suggests an effective
two-dimensional multipolar expansion for the far field, dominated by a
quadrupolar flow component or, for asymmetric flow fields, by a dipolar flow
component.
