Two-way interaction between solid particles and homogeneous air turbulence: particle settling rate and turbulence modification measurements

Abstract

This paper describes experiments on small solid particle settling behaviour in stationary homogeneous isotropic air turbulence. We present here a new methodology using a recently developed cruciform apparatus: a large horizontal cylindrical vessel equipped with a pair of counter-rotating fans and perforated plates at each end is used to generate stationary near-isotropic turbulence in the core region between the two perforated plates and a long vertical vessel is used to supply heavy descending particles from its top. This novel experimental design, without the unwanted influences from the injection of particles, the mean flow, and the decay of turbulence, allows direct imaging and velocity measurements of the two-way interaction between heavy particles and homogeneous isotropic turbulence. Consequently, the spatiotemporal responses of both fluid turbulence and particle settling can be determined by high-speed digital particle image velocimetry and accelerometry, together with the wavelet transform analysis for the first time. Hence, experimental information on and thereby understanding of the particle settling rate, preferential accumulation, and turbulence modification due to the presence of the particles is obtained.We found that the particle settling velocity (${V}_{s})$ is much greater than the terminal velocity (${V}_{t})$ in still fluid for which the value of (${V}_{s}\,{-}\, {V}_{t})$ reaches a maximum of 0.13$u^\prime $ when the Stokes number $\hbox{\it St}\,{ =}\,\tau_{p}/\tau_{k}\,{\approx}\,$1 and ${V}_{t}/u^\prime \,{\approx}\,$0.5 at $\hbox{\it Re}_{\lambda }\,{=}\,$120 and $\hbox{\it Re}_{p} \,{<}\,$1, in good agreement with previous numerical results, where $\tau _{p}$ is the particle's relaxation time, $\tau _{k}$ is the Kolmogorov time scale, $u^\prime$ is the energy-weighted r.m.s. turbulent intensity, and $\hbox{\it Re}_{\lambda}$ and $\hbox{\it Re}_{p}$ are the Reynolds numbers based on the Taylor microscale ($\lambda$) and the mean diameter of particles, respectively. Non-uniform particle concentration fields are observed and most significant when $\hbox{\it St}\,{\approx}\,$1.0, at which the particle clusters accumulate preferentially around the outer perimeter of small intense banana-shaped vortical structures. These clusters can turn and stretch banana-shaped vortical structures toward the gravitational direction and thus significantly increase the mean settling rate especially when $\hbox{\it St}\,{ =}\,1$. From spatiotemporal analysis of the flatness factor, it is found that the characteristic length and time scales of these preferential particle clusters are related to the spacing between the adjacent intense vorticity structures of the order $\lambda$ and the time passage of these clustering structures of the order $\tau _{k}$, respectively. By comparing the average frequency spectra between laden (heavy particle) and unladen (neutral particle) turbulent flows over the measurement field at a fixed $\hbox{\it Re}_{\lambda }\,{=}\,$120, turbulence augmentation is found for most frequenci...

Droplet growth in warm turbulent clouds

“…Aliseda et al (2002) and Yang and Shy (2005) show enhanced particle-settling velocities which are consistent with the numerical work of Wang and Maxey (1993). Gerashchenko et al (2011) show the strong effects of gravity on the selective transport of particle sizes by changing the orientation of the apparatus in the gravitational field.…”

“…The numerical values of the 2-D RDFs were not in quantitative agreement with earlier DNS but this was attributed to the reduction in dimension of the measurement (Holtzer and Collins, 2002). Yang and Shy (2005) also show that, for 0.3 ≤ St ≤ 1.9, clustering is greatest for St ∼ 1. Salazar et al (2008) andde Jong et al (2010) study (experimentally) the full 3-D RDF for 0.21 ≤ St ≤ 0.6 and the former also provide excellent confirmation of their clustering measurements at the small scales using DNS.…”

Turbulent collision of inertial particles: Point-particle based, hybrid simulations and beyond

“…The increased settling occurs selectively for particles with a certain range of terminal velocity and inertial response time (Wang and Maxey, 1993;Davila and Hunt, 2001). The results of Wang and Maxey (1993) have been validated and extended in several experimental studies (Aliseda et al, 2002;Yang and Shy, 2005;Kawanisi and Shiozaki, 2008). Fig.…”

Fluid–particle correlated motion and turbulent energy transfer in a two-dimensional particle-laden shear flow

“…However, this is the region where particle clustering may be expected and one reason for the observed negligible turbulence modification could again be due to balancing out the opposing effects of terms 1a and 2a of eq.(3). Yang & Shy (2005) observed turbulence production at the Kolmogorov scales for all flow conditions together with particle clustering. Although they did not quantify preferential concentration, their study shows experimentally that term 2a may have the ability to model turbulence production.…”

“…More recently, Yang & Shy (2005) investigated counter-rotating fan generated turbulence in an air flow with settling particles and observed turbulence augmentation, although the particle sizes were equal or smaller by approximately a factor of two compared to the Kolmogorov scale. Additionally, they found an increased settling rate due to particle-turbulence interaction, when compared to the still air terminal velocity of particles.…”