Dynamics of Sand–Water Coaxial Jets in Stagnant Water

Abstract

A series of laboratory experiments was conducted to study the dynamics of sand–water coaxial jets in stagnant water. The effects of the velocity ratio of the annular to the core jets on the sand concentration, velocity, mixing properties, and energy distribution of sand–water coaxial jets were studied. In comparison to sand jets, the existence of carrier fluid in sand–water coaxial jets decreased the concentration fluctuations of sand particles. Proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) techniques were utilized to study the low-rank dynamic behavior on flow entrainment and particle oscillations. It was indicated that the first two POD modes have the most coherent and energetic quantitative flow features during the entrainment process. The energy contribution of the first five POD modes was reduced by 50% in sand–water coaxial jets with relatively low velocity ratios of Ru<0.35. The SPOD analysis revealed that particle oscillations can be characterized by the Kelvin–Helmholtz type wave instabilities, and sand–water coaxial jets with small velocity ratios (i.e., Ru<1.24) had weaker vortex shedding and particle velocity fluctuations. The SPOD analysis indicated that most of the turbulence kinetic energy was concentrated in the low-frequency modes and coaxial jets with high-velocity ratios. The power spectral density (PSD) of concentration signals demonstrated that the dominant frequency of vortex shedding increased by increasing the radial distance from the jet axis. The PSD analysis of sand concentration for Ru=0.62 and 0.74 was consistent with the Kolmogorov scaling law at dominant frequencies ranging between 30 and 40 Hz.