Attenuation of Wall-Bounded Turbulence by Heavy Finite-Size Particles

Centrala begrepp

The presence of shedding vortices around particles that cannot follow the ambient fluid motions leads to additional energy dissipation, which reduces the turbulent energy production from the mean flow, resulting in the attenuation of wall-bounded turbulence.

Sammanfattning

The authors conducted direct numerical simulations (DNS) of turbulent channel flow laden with finite-size solid particles to investigate the attenuation mechanism of wall-bounded turbulence due to heavy small particles.
Key highlights:

When particles cannot follow the swirling motions of wall-attached vortices, vortex rings are shed around the particles.
These shedding vortices lead to additional energy dissipation, reducing the turbulent energy production from the mean flow.
The degree of turbulence attenuation is more significant when the Stokes number of particles is larger or particle size is smaller.
The authors propose a method to quantitatively predict the degree of turbulence attenuation without using DNS data by estimating the additional energy dissipation rate in terms of particle properties.

Statistik

The mean streamwise velocity and its gradient are not significantly modulated by the addition of particles.
The Reynolds stress is attenuated more significantly as the Stokes number of particles gets larger.

Citat

"When particles cannot follow the ambient fluid; namely, when the particle relaxation time is longer than the swirling time of the wall-attached vortices at the particles' existing height, vortex rings are shed from the particles."
"The presence of such shedding vortices results in a significant turbulence attenuation. This is because they produce the additional energy dissipation, which bypasses the energy production from the mean flow to turbulent vortices."

Viktiga insikter från

Attenuation mechanism of wall-bounded turbulence by heavy finite-size particles

by Yutaro Motoo... på arxiv.org 10-03-2024

https://arxiv.org/pdf/2410.01192.pdf

Djupare frågor

How would the turbulence attenuation mechanism change if the particles were not spherical but had a different shape?

The turbulence attenuation mechanism would likely be influenced significantly by the shape of the particles. Non-spherical particles, such as ellipsoids or irregularly shaped objects, would alter the flow dynamics around them due to their different drag characteristics and wake formations. For instance, elongated particles may experience different forces in the flow direction compared to spherical particles, potentially leading to enhanced alignment with the flow and reduced shedding of vortices. This could result in a lower additional energy dissipation rate, as the particles might be able to follow the swirling motions of the wall-attached vortices more effectively. Conversely, particles with complex geometries could create more intricate wake patterns, potentially increasing the turbulence attenuation by generating additional shedding vortices. The overall effect on turbulence modulation would depend on the balance between these competing influences, necessitating a reevaluation of the Stokes number and its implications for energy dissipation in wall-bounded turbulence.

What would be the effect of particle-particle interactions on the turbulence attenuation in a more concentrated regime?

In a more concentrated regime, particle-particle interactions would play a crucial role in the turbulence attenuation mechanism. As the volume fraction of particles increases, the likelihood of collisions and interactions among particles rises, which can lead to the formation of clusters or aggregates. These interactions can enhance the energy dissipation due to increased drag forces and the creation of additional wake structures. The collective behavior of particles may also lead to a more pronounced alteration of the flow field, potentially enhancing the turbulence attenuation effect. However, if the particle concentration becomes too high, it could lead to a reduction in the effective volume available for fluid flow, potentially increasing the mean flow velocity and altering the turbulence characteristics. This complex interplay between particle interactions and turbulence dynamics would require careful consideration of the particle concentration, size, and shape to fully understand its impact on turbulence modulation.

How could the insights from this study on wall-bounded turbulence be applied to understand turbulence modulation in other flow configurations, such as homogeneous isotropic turbulence or turbulent boundary layers?

The insights gained from this study on wall-bounded turbulence can be extrapolated to other flow configurations, such as homogeneous isotropic turbulence and turbulent boundary layers, by focusing on the fundamental principles of turbulence modulation through particle interactions. In homogeneous isotropic turbulence, the mechanisms of energy dissipation and turbulence attenuation due to particle presence would still apply, albeit with different flow characteristics. The uniformity of the turbulence may lead to a more consistent interaction between particles and the turbulent eddies, potentially simplifying the analysis of energy dissipation rates.
In turbulent boundary layers, the effects of wall proximity and shear flow would still be relevant. The findings regarding the Stokes number and its influence on the ability of particles to follow the flow dynamics can inform predictions about how particles of varying sizes and densities will behave in boundary layer flows. Additionally, the concept of shedding vortices and their contribution to energy dissipation can be applied to understand how particles influence the structure of boundary layer turbulence, potentially leading to enhanced or reduced drag in engineering applications.
Overall, the study's emphasis on the relationship between particle properties, energy dissipation, and turbulence attenuation provides a valuable framework for analyzing turbulence modulation across various flow configurations, facilitating a deeper understanding of the complex interactions between particles and turbulent flows.