Results are presented from laboratory experiments on lateral intrusive flows driven by horizontal gradients of properties in double-diffusive systems. These are variations on the classical experiments in which salt stratified fluid was separated from a sugar stratified fluid by a removable barrier. With the removal of this barrier, lateral intrusions of sugar solution into the salt solution alternated in the vertical with salt intrusions into the sugar solution. In our present study, the fluid has continuous horizontal gradients of sugar, density-compensated by opposing horizontal gradients of salt, rather than the discontinuity of properties in the classical experiments.

Observations of these new experiments include: shadowgraphs, PTV (particle tracking velocimetry) showing the structure of the flow, and PIV (particle-image velocimetry) from which velocity vector fields on a vertical plane were obtained; from these, mean flows, Reynolds stresses, and associated momentum fluxes were computed.

These experiments have been conducted for three regimes of vertical stratification: (i) Salt-finger favourable (‘hot and salty’ above); (ii) Diffusive convection favourable (‘hot and salty’ below); (iii) Doubly stable (‘cold and salty’ below).

One of our main conclusions concerns the driving mechanism for the horizontal intrusive flow. Away from the front or ‘nose’ of the intrusions, the lateral flows are no longer tilted but are horizontal. Here, finger layers alternate in the vertical with convecting layers. In these convecting layers, vertical transport of horizontal momentum by the Reynolds stress plays a major role in maintaining the lateral motion against viscous dissipation.