Stall and Collapse in Mantle Plumes: An experimental and numerical fluid dynamics perspective

Collapsing thermal plumes were investigated through experimental and numerical simulations. Collapsing plumes are an uncommon fluid dynamical phenomenon, usually observed when the heat source is removed. A series of fluid dynamical experiments were conducted on thermal plumes at a variety of \nohyphens{temperature} and viscosity contrasts, in a cubic plexiglas tank of inner side dimension 26.5cm and no-slip sides. The fluid was heated by a small 2cm diameter heater. Experimental fluids included Lyle’s Golden syrup and ADM’s Liquidose 436 syrup, which have strongly temperature-dependent viscosities and high Prandtl numbers (10$^{3}$-10$^{5}$ at \nohyphens{experimental} conditions). Visualisation techniques included white light shadowgraphs and Stereoscopic \nohyphens{Particle} Image Velocimetry (SPIV) of the tank's central plane. Temperature \nohyphens{contrasts} ranged from 3-60$^{\circ}$C, and two differing forms of collapse were identified. At very low temperature differences \enquote{stalled} collapse was observed, where the plumes stall in the lower third of the tank before collapsing. At temperature differences between 7-23$^{\circ}$C normal plume evolution occurred, until \enquote{lenticular} collapse developed between midway and two-thirds of the distance from the base of the tank. The \enquote{lens shape} originated in the top of the head and was present throughout collapse. At temperatures above $\Delta$T=23$^{\circ}$C, the plumes followed the expected growth and shape and the head flattened out at the top of the tank. Thermal collapse remains difficult to explain given experimental conditions (continuous \nohyphens{heating}). Instead, it is possible that small density differences arising from crystallisation at ambient \nohyphens{temperatures} changes plume buoyancy and therefore induces \enquote{lenticular} collapse. The \nohyphens{evolution} of the refractive index of the syrup through time to ascertain this possibility was measured. \nohyphens{Additionally}, SPIV revealed the presence of a large, downwelling, low velocity mass in the tank that inhibited the growth of low temperature difference \enquote{stalled} collapse plumes. In the mantle it is likely that the \enquote{stalled} collapse plumes would be unable to be detected by tomography because they would be unable to traverse far from the thermal boundary layer and would collapse back to the base. This would mean that they would have little impact on redistributing material in the mantle. The plumes in this \enquote{stalled} collapse regime had rise times comparable to diffusion times, which is an additional reason for the collapse. The \enquote{lenticular} collapse in the mantle could cause depletion of a deep-source and redistribute the material in the region where the plume began to collapse with some material flowing back to the base of the mantle. Numerical simulations using Fluidity\footnote{Fluidity, is an adaptive mesh finite element package} were undertaken to explore the parameter range where the two collapse phenomena were observed experimentally. These simulated plumes did not show signs of collapse in the purely thermal simulation but at temperature differences up to 14$^{\circ}$C the plumes stalled and were unable to ascend to the top of the tank. The aspect ratio of the tank was changed to explore the effect this had on plume stalling. At increased tank height the plume ascended further in the tank whilst the conduit radius remained constant. However, the very low temperature difference plumes remained unable to reach the upper surface of the tank. In contrast, when the tank width was increased the plumes ascended a little further in the tank but stalled at an earlier time and the plume conduit width generally increased. This implied that the tank width was inhibiting the growth of the plume marginally. Therefore, changing the aspect ratio of the tank does not inhibit the stalling of the simulated plumes and is unlikely to be influencing the experimental plumes growth, stalling and collapse

Mapping submarine glacial landforms using acoustic methods

The mapping of submarine glacial landforms is largely dependent on marine geophysical survey methods capable of imaging the seafloor and sub-bottom through the water column. Full global coverage of seafloor mapping, equivalent to that which exists for the Earth's land surface, has, to date, only been achieved by deriving bathymetry from radar altimeters on satellites such as GeoSat and ERS-1 (Smith & Sandwell 1997). The horizontal resolution is limited by the footprint of the satellite sensors and the need to average out local wave and wind effects, resulting in a cell size of about 15 km (Sandwell et al. 2001). A further problem in high latitudes is that the altimeter data are extensively contaminated by the presence of sea ice, which degrades the derived bathymetry (McAdoo & Laxon 1997). Consequently, the satellite altimeter method alone is not suitable for mapping submarine glacial landforms, given that their morphological characterization usually requires a much finer level of detail. Acoustic mapping methods based on marine echo-sounding principles are currently the most widely used techniques for mapping submarine glacial landforms because they are capable of mapping at a much higher resolution

Holographic interferometry of plasmas

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Research and Technology, 1989

Selected research and technology activities at Ames Research Center, including the Moffett Field site and the Dryden Flight Research Facility, are summarized. These accomplishments exemplify the Center's varied and highly productive research efforts for 1989

Applications of aerospace technology to petroleum exploration. Volume 2: Appendices

Participants in the investigation of problem areas in oil exploration are listed and the data acquisition methods used to determine categories to be studied are described. Specific aerospace techniques applicable to the tasks identified are explained and their costs evaluated