Experiments and numerical investigations of wave propagation in thermal plumes

In laboratory experiments thermal plumes are created by injecting hot corn syrup into a column of cold syrup. The viscosity contrast is up to a factor of 1000. Solitary waves, that propagate upwards in the plume conduit, are generated by enhancing the injection rate for a few seconds. For the measurement of the thermal structure of the plume we have implemented a method based on the deflection of a laser beam passing through the plume. Continuous scanning provides a new radial temperature profile each second, which allows detailed studies of the thermal structure of solitary waves. A PIV - (particle image volecimetry) method provides the velocity structure of the thermal plume. Measurements were taken for plume heads, conduits and propagating waves. Comparison between experimental results and numerical 2-D axisymmetric simulations shows a good agreement of the temperature profiles and velocity fields in the plume conduit and waves. Because of thermal diffusion, the conduit widens with height, while its central temperature decreases. The solitary waves start with the same temperature as the unperturbed conduit, however, we find that the temperature in the waves decreases less rapide with rising height. This can be explained by the faster upward propagation and the trapping of fluid within the soliton. If solitary waves exists in mantle plumes, this would imply that they arrive at the bottom of the lithosphere with a larger excess temperature than what the plumes normally exhibits, which could explain strong variations of melt generation with time.