Effects of tilt on the orientation dynamics of the large-scale circulation in turbulent Rayleigh–Bénard convection

We experimentally test the effects of tilting a turbulent Rayleigh-B{e}nard convection cell on the dynamics of the large-scale circulation (LSC) orientation $\theta_0$. The probability distribution of $\theta_0$ is measured, and used to obtain a tilt-induced potential acting on $\theta_0$, which is used in a low-dimensional model of diffusion of $\theta_0$ in a potential. The form of the potential is sinusoidal in $\theta_0$, and linear in tilt angle for small tilt angles, which is explained by a simple geometric model of the vector direction of the mean buoyancy force acting on the LSC. However, the magnitude of the tilt-induced forcing is found to be two orders of magnitude larger than previously predicted. When this parameter is adjusted to match values obtained from the probability distribution of $\theta_0$, the diffusive model can quantitatively predict effects of tilt on $\theta_0$. In particular, tilt causes a change in potential barrier height between neighboring corners of a cubic cell, and changes in the barrier-crossing rate for $\theta_0$ to escape a corner are predicted with an accuracy of $\pm30\%$. As a cylindrical cell is tilted, the tilt-induced potential provides a restoring force which induces oscillations when it exceeds the strength of damping; this critical tilt angle is predicted within 20\%, and the prediction is consistent with measured oscillation frequencies. These observations show that a self-consistent low-dimensional model can be extended to include the dynamics of $\theta_0$ due to tilt. However, the underprediction of the effect of tilt on $\theta_0$ warrants revisiting the predicted magnitude.