Experimental Study of Near-Field Entrainment of Moderately Overpressured Jets

Abstract

Particle image velocimetry (PIV) experiments have been conducted to study the velocity flow fields in the developing flow region of high-speed jets. These velocity distributions were examined to determine the entrained mass flow over a range of geometric and flow conditions, including overpressured cases up to an overpressure ratio of 2.83. In the region near the jet exit, all measured flows exhibited the same entrainment up until the location of the first shock when overpressured. Beyond this location, the entrainment was reduced with increasing overpressure ratio, falling to approximately 60% of the magnitudes seen when subsonic. Since entrainment ratios based on lower speed, subsonic results are typically used in one-dimensional volcanological models of plume development, the current analytical methods will underestimate the likelihood of column collapse. In addition, the concept of the entrainment ratio normalization is examined in detail, as several key assumptions in this methodology do not apply when overpressured.

Coupled fluid and solid evolution in analogue volcanic vents

“…At steady state, K = 2.6, M = 1, and Re = 467,000. Steady state data is from Solovitz et al [], acquired at K = 2.55 ± 0.1.…”

“…At the laboratory conditions, the speed of sound c = 343 m/s. In addition, Figure displays the steady state profile for a jet at similar conditions, as acquired by Solovitz et al []. Note that no data are shown closer than two diameters from the pipe due to illumination issues near the base plate wall.…”

“…Shortly afterward, the centerline velocities showed oscillations, which indicated the presence of compressions and expansions in shock cells. This was not representative of steady state, necessarily, even though centerline speeds were on the same order as those seen in earlier experiments at this condition [ Solovitz et al , ]. At t* = 1720, the axial speeds matched the steady state data to within the uncertainty up to almost four diameters downstream of the exit.…”

“…Supersonic speeds were generated as the initially choked, sonic flow expanded and accelerated through the flared, eroded sample. Correspondingly, the centerline speed at this instant was higher than the steady state speeds for the rigid pipe [ Solovitz et al , ], as observed in Figure . At the applied pressure, isentropic flow analysis [ Saad , ] determines the measured area increase of 22.5% would not be sufficient to decrease the pressure to atmospheric.…”

The Condition of Eruption Column Collapse: 2. Three‐Dimensional Numerical Simulations of Eruption Column Dynamics

“…However, the flow of the eruption column near the vent may deviate substantially from a self‐similar jet/plume (e.g., Carazzo et al, ; Suzuki & Koyaguchi, ). Recent experimental studies have indicated that the efficiency of entrainment varies with height even for idealized jets/plumes such as single‐phase gas jets (e.g., Solovitz et al, ). Therefore, the limitations of the entrainment hypothesis need to be assessed.…”