Experimental investigation of the effects of particle near-wall motions on turbulence statistics in particle-laden flows

Abstract

Experiments on particle-free and particle-laden flows with the same incoming velocity were conducted in a horizontal wind tunnel. Three cases of particle-laden flows with different degrees of particle near-wall motions and similar particle volume fractions, including top-release particles, local-laying sand beds and global-laying sand beds, were designed to investigate the effects of collision bounces and impact splashing on turbulence statistics. The top-released particles accelerate the fluid during gravitational settling, but weaken the intensity and reduce the probability of ‘ejection’ and ‘sweep’ events. This leads to a weakened Reynolds stress and a decreased scale of the outer spectral peak at the centre of the logarithmic region, indicating a concentration of energy at small scales. In contrast, the collision bounce and impact splashing slow the fluid but promote the ‘ejection and sweep’ cycle with larger intensity, and thus enhance the Reynolds stress. Meanwhile, the bouncing and splashing generate ascending particles that transport the kinetic energy upwards, resulting in more energetic very-large-scale motions further from the wall. This study reveals the importance of particle motions to turbulence, and contributes to a further insight into the interactions between particles and turbulence in two-phase flows with erodible surfaces.

The modulation of coherent structures by the near-wall motions of particles

“…The experiments were carried out in the multifunctional environmental wind tunnel laboratory at Lanzhou University, and the experimental set-up was described in detail by Liu, Feng & Zheng (2022). Therefore, only a brief description is provided in this section.…”

“…In the two-phase flows, size-selected sand particles from the Tengger Desert, with a density ρ p = 2600 kg m −3 and diameter d p = 203 ± 95 μm (mean ± standard deviation) are used in this study. The particle diameter ranges from 30 to 480 μm and the particle size distribution can be found in Liu et al (2022). Here, we calculate the particle parameters using the average size of the particles (Zheng et al 2021;Zhu et al 2021;Liu et al 2022).…”

“…The particle diameter ranges from 30 to 480 μm and the particle size distribution can be found in Liu et al (2022). Here, we calculate the particle parameters using the average size of the particles (Zheng et al 2021;Zhu et al 2021;Liu et al 2022). The bulk volume fractions of particles in case 2-case 4 that are obtained by the ratio of the particle volume to the total field of view are 1.1 × 10 −4 , 1.4 × 10 −4 and 1.5 × 10 −4 , respectively.…”

“…These errors are consistent with the particle-laden turbulent boundary layer (TBL) result in Zhu et al (2019) (1 %) and channel flow in Tay et al (2015) (3 %), suggesting the reliability of the present experimental data. For the particle phase, a high-accuracy hybrid PTV-PIV algorithm was employed to calculate the position and velocity of the particles (detailed in Liu et al 2022). The uncertainties of particle mean velocities in all particle-laden flows are less than 1 % when the image numbers reach the order of 10 5 .…”

Direct numerical simulation of particle-laden flow in an open channel at

“…To the best of the authors’ knowledge, this is the highest-Reynolds-number particle-laden channel flow case simulated with DNS, presumably due to the rapidly increasing computational cost with higher Reynolds numbers. Liu, Feng & Zheng (2022) experimentally investigated the effects of collision bounces and impact splashing on turbulence statistics in an open-loop wind tunnel at with multiple modes of particle introduction (both released from above as well as eroded from below). They found that top-released particles attenuate the Reynolds stress and weaken the VLSMs, whereas bouncing and splashing generate ascending particles that transport the kinetic energy upwards, resulting in more energetic VLSMs.…”