Mapping Densities in Analogue Laboratory Turbulent Plumes Using Dye Concentration

Abstract:

Changing tephra concentration in volcanic eruption columns is difficult to measure in the field due

to fluid opacity. The bulk fluid erupted may be higher density than the surrounding atmosphere at

the vent and then transition to positive buoyancy through the ingestion and heating of ambient air;

thus, the concentration of the plume fluid as it rises is critical to determining whether the material

rises in a sustained plume or collapses into a pyroclastic density current. We evaluate the changing

concentration of an analogue plume via tracer dye intensity and relate it to plume radius expansion and

vent distance. To calibrate our concentration metric, we calculated the density and dye concentration

of pre-determined tracer-water mixtures. The density of the solution was directly measured using a

micropipette and high precision balance. The calculated density falls within the standard error of the

measured density for each step. Five photographs were taken of each concentration using a mounted

Ex-FH100 digital camera with identical lighting. Using a MATLAB script, the RGB (Red-Green-Blue) color

value was extracted from five pixels located at the same coordinates in each image, confirming that

there was no inherent error caused by the camera and that the RGB value was the same across an

entire image. We created a color map to convert from the RGB color value of a pixel in an image to

its corresponding concentration. This method algorithm can then be applied to an analogue volcanic

tank model, using the color variations in the plume eddies to determine the tracer concentration, and

thereby density distribution, in the plume.