Laboratory flow tubes
A suite of laboratory tube systems for fluid-dynamic experiments, with pressure measurement from zero to one megasample per second, combined with imaging, displacement, force and acceleration sensing. Experiments can be carried out into pressures ranging from atmospheric down to 10 Pa. This allows experimental scaling of the decompression expansion of volcanic bubbles as they ascend the conduit. The model 'conduit' system can also incorporate geometry and inclination, factors that must be present volcanically but are rarely modelled either experimentally or numerically. Expansion and geometry are key factors in the behaviour of fluid dynamic seismic sources.
Images of gas slugs approaching the liquid surface in laboratory experiments. In (a), a short slug (c. 14mg of gas) is expanding slowly in an experiment carried out at atmospheric pressure (c. 0.1 MPa). The image is backlit through the flow tube and the slug outline is predominantly shown by refracted light from the gas-liquid interface. The sketch to the right indicates the position of the liquid surface, slug outline and exterior of the tube (dashed lines). Although the shape of the slug is horizontally distorted by refraction through the circular tube, the standard morphology under mixed viscous and inertial control (a domed nose and base) can be easily observed. In (b), a similar image shows the nose of a much longer slug (c. 1 mg of gas) expanding to a surface pressure of 319 Pa (the apparent decrease in image resolution is caused by the wider angle camera lens used in order to capture more of the long slug body). At this point in time, the slug nose was moving upward at c. 2.4 m/s. Note the longer curved length of the nose and the upward doming of the liquid surface, testifying to the dynamic nature of the slug expansion.
Still images taken from digital video (a) without and (b) with the bubble nose ascending in a jet-like manner immediately after slug disruption in the flare. To the right of the images, a sketch of the slug outline in is given. Note the unstable and dynamically controlled shape of the bubble nose as well as its irregular base, indicative of turbulence. The lower section of the original slug, still in the smaller tube, also has an irregular but flat (rather thandomed) upper surface but retains its steady state base. Pressure transducer positions are labelled
Rapid decompression of complex fluids such as a mixture of gum rosin and an organic solvent like acetone, or preferably diethyl ether, allow analogue study of degassing magmas. The key scaling factor here is the dependence of the liquid viscosity on the volatile content. Explosive fragmentation can be studied over short timescales, and conduit flow beneath dome-building eruptions on longer timescales.
