Irregular mixing due to a vortex pair interacting with a fixed vortex

•Irregular transport induced by the dipole–topography interaction is addressed.•Unbounded motion produces effective fluid particle mixing facilitating transport.•Bounded motion leads to a chaotically stirred vicinity of the topography.

The paper examines scalar advection caused by a point–vortex pair encountering a fixed point vortex in a uniform flow. The interaction produces two types of vortex motion. First is unbounded as the pair moves unrestrictedly after encountering the fixed vortex. The scalar exchanging between the pair's bubble and fixed vortex's neighbourhood is numerically estimated. Second is bounded as the pair's vortices periodically oscillate about the fixed vortex. The pair's periodic motion perturbs scalar motion causing a portion of scalar trajectories to manifest chaotic behaviour. We analyse scalar transport using Poincaré sections, which reveal regular and chaotic transport regions.