Linear bending wave propagation in laminar and turbulent discs

Abstract: Bending waves are perhaps the most fundamental and analytically tractable phenomena in warped disc dynamics. In this work we conduct 3D grid-based, numerical experiments of bending waves in laminar, viscous hydrodynamic and turbulent, weakly magnetised discs, capturing their behaviour in unprecedented detail. We clearly elucidate the theory from first principles, wherein the general Fourier-Hermite formalism can be simplified to a reduced framework which extends previous results towards locally isothermal discs. We obtain remarkable agreement with our laminar simulations wherein the tilt evolution is well described by the reduced theory, whilst higher order vertical modes should be retained for capturing the detailed disc twisting and internal velocity profiles. We then relax this laminar assumption and instead launch bending waves atop a magnetorotationally turbulent disc. Although the turbulence can be quantified with an effective $\alpha$ parameter, the bending waves behave distinctly from a classical viscous evolution and are readily disrupted when the turbulent velocity is comparable to the induced warping flows. This may have implications for the inclination damping rates induced by planet-disc interactions, the capture rate of black holes in AGN discs or the warped shapes assumed by discs in misaligned systems.