Oblique laminar-turbulent interfaces in plane shear flows
Résumé
Localized structures such as turbulent stripes and turbulent spots are typical features of transitional
wall-bounded flows in the subcritical regime. Based on an assumption for scale separation between large
and small scales, we show analytically that the corresponding laminar-turbulent interfaces are always
oblique with respect to the mean direction of the flow. In the case of plane Couette flow, the mismatch
between the streamwise flow rates near the boundaries of the turbulence patch generates a large-scale flow
with a nonzero spanwise component. Advection of the small-scale turbulent fluctuations (streaks) by the
corresponding large-scale flow distorts the shape of the turbulence patch and is responsible for its oblique
growth. This mechanism can be easily extended to other subcritical flows such as plane Poiseuille flow or
Taylor-Couette flow.