  We experimentally investigate the response to perturbations of circular
symmetry for dense granular flow inside a three-dimensional right-conical
hopper. These experiments consist of particle tracking velocimetry for the flow
at the outer boundary of the hopper. We are able to test commonly used
constitutive relations and observe granular flow phenomena that we can model
numerically. Unperturbed conical hopper flow has been described as a radial
velocity field with no azimuthal component. Guided by numerical models based
upon continuum descriptions, we find experimental evidence for secondary,
azimuthal circulation in response to perturbation of the symmetry with respect
to gravity by tilting. For small perturbations we can discriminate between
constitutive relations, based upon the agreement between the numerical
predictions they produce and our experimental results. We find that the
secondary circulation can be suppressed as wall friction is varied, also in
agreement with numerical predictions. For large tilt angles we observe the
abrupt onset of circulation for parameters where circulation was previously
suppressed. Finally, we observe that for large tilt angles the fluctuations in
velocity grow, independent of the onset of circulation.
