Experiments for simultaneous horizontal $\left(h\right)$ and vertical $\left(v\right)$ vibration of granular materials show novel flow dynamics. We focus on moderate dimensionless accelerations $0<\mathrm{}{\Gamma }_{h,v}<1.6\mathrm{}$. Phenomena include the spontaneous formation of a static heap at ${\Gamma }_{\mathrm{hs}}\left({\Gamma }_v\right)$ when ${\Gamma }_v<1\mathrm{}$, convective flow for ${\Gamma }_{\mathrm{hc}}>{\Gamma }_{\mathrm{hs}}$, and a novel switching state for frequencies ${\omega }_v\ne {\omega }_h$. A simple friction model provides an approximate, but not exact, description of the steady states and the transition to convection.

• Received 5 February 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.794