Homogeneous and isotropic turbulence modulation by small heavy ($St\sim 50$) particles

Abstract

The interaction of a dilute dispersion of small heavy particles with homogeneous and isotropic air turbulence has been investigated. Stationary turbulence (at Taylor micro-scale Reynolds number of 230) with small mean flow was created in a nearly spherical sealed chamber by means of eight synthetic jet actuators. Two-dimensional particle image velocimetry was used to measure global turbulence statistics in the presence of spherical glass particles that had a diameter of 165 $\umu$m, which was similar to the Kolmogorov length scale of the flow. Experiments were conducted at two different turbulence levels and particle mass loadings up to 0.3. The particles attenuated the fluid turbulence kinetic energy and viscous dissipation rate with increasing mass loadings. Attenuation levels reached 35–40% for the kinetic energy (which was significantly greater than previous numerical studies) and 40–50% for the dissipation rate at the highest mass loadings. The main source of fluid turbulence kinetic energy production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra energy. The sink of fluid energy in the chamber was due to the ordinary viscous dissipation and extra dissipation caused by particles. The extra dissipation was greatly underestimated by conventional models.

Particle-resolved direct numerical simulation of homogeneous isotropic turbulence modified by small fixed spheres

“…The very large dissipation rate near particles supports the view expressed by Hwang & Eaton (2006) and Tanaka & Eaton (2010) for heavy particles, namely that such particles, like grids, act as localized dampers of the turbulence motion. To quantify how much of the total amount of dissipation in the flow occurs near the particles, we define V r as the set of all points in the flow domain within distance r of a particle centre, and we define The function c v is the fraction between the volume of V r and the total flow volume, while c is the fraction between the dissipation that occurs in V r and the total dissipation.…”

“…Particle force and particle-induced dissipation in simulations L128, L64 and the point-particle simulations discussed in § 4.5. (Hwang & Eaton 2006;Tanaka & Eaton 2010) and the present particle-resolved DNS L128.…”

“…The phenomenon has been observed experimentally in pipe flows (Tsuji et al 1984), channel flows (Kulick, Fessler & Eaton 1994;Kussin & Sommerfeld 2002) and stationary homogeneous turbulence (Hwang & Eaton 2006;Tanaka & Eaton 2010). These works clearly show that the turbulence kinetic energy of the carrier phase of a turbulent flow laden with small particles is lower than in the corresponding unladen flow.…”

“…Stochastic forcing The forcing in the experiments by Hwang & Eaton (2006) and Tanaka & Eaton (2010) was caused by jet actuators driven by sine waves at random frequencies. In view of these experiments, it is a logical step to simulate (particle-laden) homogeneous isotropic turbulence forced by a number of Fourier modes at random frequencies.…”

“…Compared to the amount of experimental data on macroscopic turbulence modification, there are only a few experimental results on the modification of the small-scale turbulence. High-resolution experiments of turbulent air flow laden with small solid particles were performed by Tanaka & Eaton (2010), who refined the experiments performed by Hwang & Eaton (2006), and observed that the turbulence dissipation rate was augmented in a roughly spherical region around the particles. The experiments reported in these two papers were performed for stationary homogeneous isotropic turbulence, forced by synthetic jet actuators (woofers) driven with sine waves with random frequencies.…”