  The results from a series of well characterised, unstable, miscible
displacement experiments in a Hele Shaw cell with a quarter five-spot
source-sink geometry are presented, with comparisons to detailed numerical
simulation. We perform repeated experiments at adverse viscosity ratios from 1
- 20 and Peclet numbers from 10$^4$ - 10$^6$ capturing the transition from 2D
to 3D radial fingering and experimental uncertainty. The open-access dataset
provides time-lapse images of the fingering patterns, transient effluent
profiles, and meta-information for use in model validation. We find the
complexity of the fingering pattern increases with viscosity ratio and Peclet
number, and the onset of fingering is delayed compared to linear displacements,
likely due to Taylor dispersion stabilisation. The transition from 2D to 3D
fingering occurs at a critical Peclet number that is consistent with recent
experiments in the literature. 2D numerical simulations with hydrodynamic
dispersion and different mesh orientations provide good predictions of
breakthrough times and sweep efficiency obtained at intermediate Peclet numbers
across the range of viscosity ratios tested, generally within the experimental
uncertainty. Specific finger wavelengths, tip shapes, and growth are hard to
replicate; model predictions using velocity dependent longitudinal dispersion
or simple molecular diffusion bound the fingering evolution seen in the
experiments, but neither fully capture both fine-scale and macroscopic
measures. In both cases simulations predict sharper fingers than the
experiment. A weaker dispersion stabilisation seems necessary to capture the
experimental fingering at high viscosity ratio, which may also require
anisotropic components. 3D models with varying dispersion formulations should
be explored in future developments to capture the full range of effects at high
viscosity ratio and Peclet number.

Fluid Dynamics