Near-exit flow physics of a moderately overpressured jet

Abstract

The developing region of high-speed jets is studied using particle image velocimetry methods. Ensemble-averaged and fluctuating velocity profiles were measured at a range of exit pressures, from a subsonic pressure-balanced case to an overpressured condition 3.2 times atmospheric. When pressure-balanced, the mean flow structure showed gradual development to a bell-shaped profile at approximately 8 diameters downstream, but the turbulent Reynolds stresses were far below self-similar levels. When overpressured, the mean structure displayed a series of compressions and expansions, including a normal shock, and the flow was far from Gaussian after 8 downstream diameters. The turbulent stresses were more suppressed than in the pressure-balanced jet, with little change exhibited in the jet core except very near the normal shock. Instantaneous vorticity contours also showed that the shear layer was divided into two bands at overpressure. This suggests that the turbulent eddies driving entrainment in the near-exit region were substantially weaker than in the self-similar region, which would result in lower mass flow from the ambient.

Flow Development and Entrainment in Turbulent Particle‐Laden Jets

“…Upon closer examination, there were slight differences, particularly when considering the higher‐inertia nickel particles. Both air and hollow glass‐laden jets reached a fully developed, Gaussian profile by x/D = 5, which agreed with previous measurements for single‐phase jets (Mi et al., 2007; Saffaraval & Solovitz, 2012). At this modest Stokes number, the particle inertia does not significantly modify the mean velocity development.…”

“…The nondimensional length of this final pipe was sufficient for fully developed turbulent conditions at the exit (Kays & Crawford, 1993) for typical test speeds (∼10 to 50 m/s). Previous measurements of the exit velocity profile in air matched the expected logarithmic dependence seen when fully developed (Saffaraval & Solovitz, 2012). The pipe exit was placed at the center of a rigid, horizontal plate with length and width of 61 cm each.…”

“…The number of image pairs was selected to permit reasonable accuracy for both mean and turbulent velocity statistics while maintaining a sufficient particle flow throughout. Previous experiments (Saffaraval & Solovitz, 2012) demonstrate good convergence when using more than 500 image pairs, and we were beyond this total.…”

Coupled fluid and solid evolution in analogue volcanic vents

“…Experimental study of volcanic vent formation has been much more limited, especially for high-speed flows. Laboratory studies have considered rigid structures, such as supersonic jet nozzles [Seiner, 1984;Alkislar et al, 2003], overpressured nozzles [Yüceil et al, 2000], or overpressured pipe flow [Solovitz et al, 2011;Saffaraval and Solovitz, 2012]. Rock erosion and failure have been examined experimentally for other physical systems, such as crater development [Mastin and Pollard, 1988] and avalanches [Iverson et al, 2004].…”

Effects of Nozzle Pressure Ratio and Nozzle-to-Plate Distance to Flowfield Characteristics of an Under-Expanded Jet Impinging on a Flat Surface

“…The mean axial velocity outside the jet core reduced gradually and became flatter in the region further downstream, indicating the entrainment mechanisms outside the jet. For the low-speed cases of under-expanded free jet, the maximum jet velocity is along the centerline, the flow entrainment was driven by the centerline jet velocity, which transferred the flow energy to the turbulent eddies in the jet shear layers (Saffaraval and Solovitz [40]). For the high-speed cases, the jet velocity remained supersonic in the core region while on either side, the surrounding velocity is subsonic.…”