  Shear instabilities can be the source of significant amounts of turbulent
mixing in stellar radiative zones. Past attempts at modeling their effects
(either theoretically or using numerical simulations) have focused on idealized
geometries where the shear is either purely vertical or purely horizontal. In
stars, however, the shear can have arbitrary directions with respect to
gravity. In this work, we use direct numerical simulations to investigate the
nonlinear saturation of shear instabilities in a stably stratified fluid, where
the shear is sinusoidal in the horizontal direction, and either constant or
sinusoidal in the vertical direction. We find that, in the parameter regime
studied here (non-diffusive, fully turbulent flow), the mean vertical shear
does not play any role in controlling the dynamics of the resulting turbulence
unless its Richardson number is smaller than one (approximately). As most
stellar radiative regions have a Richardson number much greater than one, our
result implies that the vertical shear can essentially be ignored in the
computation of the vertical mixing coefficient associated with shear
instabilities for the purpose of stellar evolution calculations, even when it
is much larger than the horizontal shear (as in the solar tachocline, for
instance).
