  We numerically demonstrate that Lithium Niobate on Insulator (LNOI)
waveguides may support confined short-wavelength surface acoustic waves that
interact strongly with optical fields through backward stimulated Brillouin
scattering in both $Z$ and $X$-cut orientation. We conduct fully anisotropic
simulations that consider not only moving boundary and photoelastic forces, but
also roto-optic forces for the Brillouin interaction. Our results indicate that
photoelasticity dominates the Brillouin gain and can reach as high as
$G_{B}/Q_{m}$ = 0.43 W$^{-1}$m$^{-1}$ in standard ridge waveguides
