The decay of isotropic turbulence carrying non-spherical finite-size particles

2019 ◽  
Vol 875 ◽  
pp. 520-542 ◽  
Author(s):  
Lennart Schneiders ◽  
Konstantin Fröhlich ◽  
Matthias Meinke ◽  
Wolfgang Schröder
Keyword(s):  
Energy Balance ◽  
Kinetic Energy ◽  
Energy Budget ◽  
Particle Motion ◽  
Particle Shape ◽  
Dissipation Rate ◽  
Isotropic Turbulence ◽  
Finite Size ◽  
Spherical Particles ◽  
The Impact

Direct particle–fluid simulations of heavy spheres and ellipsoids interacting with decaying isotropic turbulence are conducted. This is the rigorous extension of the spherical particle analysis in Schneiders et al. (J. Fluid Mech., vol. 819, 2017, pp. 188–227) to $O(10^{4})$ non-spherical particles. To the best of the authors’ knowledge, this represents the first particle-resolved study on turbulence modulation by non-spherical particles of near-Kolmogorov-scale size. The modulation of the turbulent flow is precisely captured by explicitly resolving the stresses acting on the fluid–particle interfaces. The decay rates of the fluid and particle kinetic energy are found to increase with the particle aspect ratio. This is due to the particle-induced dissipation rate and the direct transfer of kinetic energy, both of which can be substantially larger than for spherical particles depending on the particle orientation. The extra dissipation rate resulting from the translational and rotational particle motion is quantified to detail the impact of the particles on the fluid kinetic energy budget and the influence of the particle shape. It is demonstrated that the previously derived analytical model for the particle-induced dissipation rate of smaller particles is valid for the present cases albeit these involve significant finite-size effects. This generic expression allows us to assess the impact of individual inertial particles on the local energy balance independent of the particle shape and to quantify the share of the rotational particle motion in the kinetic energy budget. To enable the examination of this mechanistic model in particle-resolved simulations, a method is proposed to reconstruct the so-called undisturbed fluid velocity and fluid rotation rate close to a particle. The accuracy and robustness of the scheme are corroborated via a parameter study. The subsequent discussion emphasizes the necessity to account for the orientation-dependent drag and torque in Lagrangian point-particle models, including corrections for finite particle Reynolds numbers, to reproduce the local and global energy balance of the multiphase system.

scholarly journals Virtual motion of real particles

2010 ◽  
Vol 650 ◽  
pp. 1-4 ◽  
Author(s):  
G. TRYGGVASON
Keyword(s):  
Kinetic Energy ◽  
Numerical Simulations ◽  
Turbulent Kinetic Energy ◽  
Multiphase Flows ◽  
Isotropic Turbulence ◽  
Finite Size ◽  
Direct Numerical Simulations ◽  
Spherical Particles ◽  
Length Scale ◽  
Turbulent Eddies

Direct numerical simulations are rapidly becoming one of the most important techniques to examine the dynamics of multiphase flows. Lucci, Ferrante & Elghobashi (J. Fluid Mech., 2010, this issue, vol. 650, pp. 5–55) address several fundamental issues for spherical particles in isotropic turbulence. They show the importance of including the finite size of the particles and discuss how particles of a size comparable to the largest length scale at which viscosity substantially affects the turbulent eddies (i.e. the Taylor microscale) always increase the dissipation of turbulent kinetic energy.

scholarly journals The impact of particle shape on friction angle and resulting critical shear stress: an example from a coarse-grained, steep, megatidal beach

2013 ◽  
Vol 1 (1) ◽  
pp. 1187-1208 ◽  
Author(s):  
N. Stark ◽  
A. E. Hay ◽  
R. Cheel ◽  
C. B. Lake
Keyword(s):  
Shear Stress ◽  
Particle Motion ◽  
Particle Shape ◽  
Critical Shear Stress ◽  
Friction Angle ◽  
Sediment Dynamics ◽  
Tidal Range ◽  
Strong Increase ◽  
Elliptic Plate ◽  
The Impact

Abstract. The impact of particle shape on the friction angle, and the resulting critical shear stress on sediment dynamics, is still poorly understood. In areas characterized by sediments of specific shape, particularly non-rounded particles, this can lead to large departures from the expected sediment dynamics. The steep slope (1:10) of the mixed sand-gravel beach at Advocate Harbour was found stable in large-scale morphology over decades, despite a high tidal range of ten meters or more, and strong shorebreak action during storms. The Advocate sand (d < 2 mm) was found to have an elliptic, plate-like shape. Exceptionally high friction angles of the material were determined using direct shear, ranging from φ &amp;approx; 41–46°, while the round to angular gravel was characterized by φ = 33°. The addition of 25% of the elliptic sand to the gravel led to an immediate increase of the friction angle to φ = 38°. Furthermore, re-organization of the particles occurred during shearing, being characterized by a short phase of settling and compaction, followed by a pronounced strong dilatory behavior and an accompanying strong increase of shear stress. Long-term shearing (24 h) using a ring shear apparatus led to destruction of the particles without re-compaction. Finally, submerged particle mobilization was simulated using a tilted tray in a tank. Despite a smooth tray surface, particle motion was not initiated until reaching tray tilt angles of 31° and more, being 7° steeper than the latest gravel motion initiation. In conclusion, geotechnical laboratory experiments quantified the important impact of the elliptic, plate-like shape of Advocate Beach sand on the friction angles of both pure sand and sand-gravel mixtures. The resulting effect on initiation of particle motion was confirmed in tilting tray experiments. This makes it a vivid example of how particle shape can contribute to the stabilization of the beachface.

Modulation of isotropic turbulence by particles of Taylor length-scale size

2010 ◽  
Vol 650 ◽  
pp. 5-55 ◽  
Author(s):  
FRANCESCO LUCCI ◽  
ANTONINO FERRANTE ◽  
SAID ELGHOBASHI
Keyword(s):  
Isotropic Turbulence ◽  
Volume Fraction ◽  
Density Ratio ◽  
Particle Diameter ◽  
Finite Size ◽  
Rate Of Change ◽  
Spherical Particles ◽  
Length Scale ◽  
Frequency Spectra ◽  
Scale Size

This study investigates the two-way coupling effects of finite-size solid spherical particles on decaying isotropic turbulence using direct numerical simulation with an immersed boundary method. We fully resolve all the relevant scales of turbulence around freely moving particles of the Taylor length-scale size, 1.2≤d/λ≤2.6. The particle diameter and Stokes number in terms of Kolmogorov length- and time scales are 16≤d/η≤35 and 38≤τp/τk≤178, respectively, at the time the particles are released in the flow. The particles mass fraction range is 0.026≤φm≤1.0, corresponding to a volume fraction of 0.01≤φv≤0.1 and density ratio of 2.56≤ρp/ρf≤10. The maximum number of dispersed particles is 6400 for φv=0.1. The typical particle Reynolds number is of O(10). The effects of the particles on the temporal development of turbulence kinetic energy E(t), its dissipation rate (t), its two-way coupling rate of change Ψp(t) and frequency spectra E(ω) are discussed.In contrast to particles with d < η, the effect of the particles in this study, with d > η, is that E(t) is always smaller than that of the single-phase flow. In addition, Ψp(t) is always positive for particles with d > η, whereas it can be positive or negative for particles with d < η.

Kinetic Simulations for Analysing the Wall Collision Process of Non-Spherical Particles

2002 ◽  
Author(s):  
M. Sommerfeld
Keyword(s):  
Particle Shape ◽  
Fluid Dynamic ◽  
Impact Angle ◽  
Distribution Functions ◽  
Spherical Particles ◽  
Collision Process ◽  
Wall Collision ◽  
Particle Velocities ◽  
The Impact ◽  
Particle Shapes

In wall-bounded gas-solid flows the wall collision process plays an important role and may be strongly affected by wall roughness and particle shape. The modelling of the particle-wall collision mostly relies on the assumption of spherical particles. To extend such models appropriately for non-spherical particles, two-dimensional kinetic simulations were performed for different particle shapes. This implies, that the particle translational and angular motion is calculated by considering the particle shape, however neglecting fluid dynamic effects. The change of the particle velocities during the impact and rebound process was calculated by solving the impulse equations together with Coulombs law of friction. The simulations were performed for a given initial particle velocity by varying impact angle and initial angular velocity. The results for 2000 particle wall collisions allowed us to derive the distribution functions of the impact parameters required to describe the wall collision process for non-spherical particles correctly. Moreover, other wall collision properties, such as rebound angle and velocity ratios could be determined. Finally also a comparison with measurements was possible.

scholarly journals The impact of particle shape on the angle of internal friction and the implications for sediment dynamics at a steep, mixed sand–gravel beach

2014 ◽  
Vol 2 (2) ◽  
pp. 469-480 ◽  
Author(s):  
N. Stark ◽  
A. E. Hay ◽  
R. Cheel ◽  
C. B. Lake
Keyword(s):  
Internal Friction ◽  
Particle Motion ◽  
Particle Shape ◽  
Sediment Dynamics ◽  
Tidal Range ◽  
Strong Increase ◽  
Angle Of Internal Friction ◽  
Elliptic Plate ◽  
The Impact ◽  
Gravel Beach

Abstract. The impact of particle shape on the angle of internal friction, and the resulting impact on beach sediment dynamics, is still poorly understood. In areas characterized by sediments of specific shape, particularly non-rounded particles, this can lead to large departures from the expected sediment dynamics. The steep slope (1 : 10) of the mixed sand–gravel beach at Advocate Harbour is stable in large-scale morphology over decades, despite a high tidal range of 10 m or more, and intense shore-break action during storms. The Advocate sand (d < 2 mm) was found to have an elliptic, plate-like shape (Corey Shape Index, CSI ≈ 0.2–0.6). High angles of internal friction of this material were determined using direct shear, ranging from φ ≈ 41 to 49°, while the round to angular gravel was characterized as φ = 33°. The addition of 25% of the elliptic plate-like sand-sized material to the gravel led to an immediate increase in friction angle to φ = 38°. Furthermore, re-organization of the particles occurred during shearing, characterized by a short phase of settling and compaction, followed by a pronounced strong dilatory behavior and an accompanying strong increase of resistance to shear and, thus, shear stress. Long-term shearing (24 h) using a ring shear apparatus led to destruction of the particles without re-compaction. Finally, submerged particle mobilization was simulated using a tilted tray submerged in a water-filled tank. Despite a smooth tray surface, particle motion was not initiated until reaching tray tilt angles of 31° and more, being &amp;geq;7° steeper than for motion initiation of the gravel mixtures. In conclusion, geotechnical laboratory experiments quantified the important impact of the elliptic, plate-like shape of Advocate Beach sand on the angles of internal friction of both pure sand and sand–gravel mixtures. The resulting effect on initiation of particle motion was confirmed in tilting tray experiments. This makes it a vivid example of how particle shape can contribute to the stabilization of the beach face.

scholarly journals Turbulent kinetic energy balance in the atmospheric surface layer over a tropical region

MAUSAM ◽  
2021 ◽  
Vol 48 (1) ◽  
pp. 15-22
Author(s):  
B. PADMANABHAMURTY ◽  
PIALI CHAKRABORTY
Keyword(s):  
Surface Layer ◽  
Energy Balance ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Energy Budget ◽  
Atmospheric Surface Layer ◽  
Tropical Region ◽  
Turbulent Kinetic Energy Budget ◽  
Similarity Relations ◽  
Kinetic Energy Budget

    ABSTRACT. The various tenns of the turbulent kinetic energy budget in the surface layer over Jodhpur, India have been worked out and compared with established similarity relations. The turbulent production and dissipation tend to balance under moderately unstable conditions for most of the runs considered for investigation.    

scholarly journals Influence of the particle shape on the impact force of lahar on an obstacle

2021 ◽  
Vol 249 ◽  
pp. 03010
Author(s):  
Rime Chehade ◽  
Bastien Chevalier ◽  
Fabian Dedecker ◽  
Pierre Breul
Keyword(s):  
Particle Shape ◽  
Urban Areas ◽  
Large Scale ◽  
Distinct Element ◽  
Fluid Velocity ◽  
Solid Particles ◽  
Impact Pressure ◽  
Spherical Particles ◽  
Coupling Scheme ◽  
The Impact

Lahars represent natural phenomena that can generate severe damage in densely populated urban areas. The evaluation of pressures generated by these mass flows on constructions (buildings, infrastructure…) is crucial for civil protection and assessment of physical vulnerability. The existing tools to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the estimation of mechanical efforts. A discrete numerical model is developed for evaluating debris flow (DF) impact pressures at the local scale of one structure. The large-sized solid particles are modelled explicitly using Distinct Element Method (DEM) and the fine-grained solid particles are integrated in a fluid phase which generates two effects on the movement of particles, i.e. buoyancy and drag. Fluid velocity field and the fluid free surface are obtained from Computational Fluid Dynamics (CFD) code then imported in the DEM simulation in a one way coupling scheme. In this paper, the influence of particle shape on the impact forces generated on the obstacle is investigated: spherical particles and polygonal rigid blocks (r-blocks) are considered. The shape of the particle influences the contact surface and therefore the impact pressure. With an angular shape and several facets like r-blocks, the impact pressure on an obstacle is more important for a flow with the same characteristics.

Direct particle–fluid simulation of Kolmogorov-length-scale size particles in decaying isotropic turbulence

2017 ◽  
Vol 819 ◽  
pp. 188-227 ◽  
Author(s):  
Lennart Schneiders ◽  
Matthias Meinke ◽  
Wolfgang Schröder
Keyword(s):  
Kinetic Energy ◽  
Strain Rate ◽  
Viscous Dissipation ◽  
Dissipation Rate ◽  
Isotropic Turbulence ◽  
Bulk Flow ◽  
Small Scale ◽  
Length Scale ◽  
Kolmogorov Length Scale ◽  
Scale Size

The modulation of decaying isotropic turbulence by 45 000 spherical particles of Kolmogorov-length-scale size is studied using direct particle–fluid simulations, i.e. the flow field over each particle is fully resolved by direct numerical simulations of the conservation equations. A Cartesian cut-cell method is used by which the exchange of momentum and energy at the fluid–particle interfaces is strictly conserved. It is shown that the particles absorb energy from the large scales of the carrier flow while the small-scale turbulent motion is determined by the inertial particle dynamics. Whereas the viscous dissipation rate of the bulk flow is attenuated, the particles locally increase the level of dissipation due to the intense strain rate generated near the particle surfaces due to the crossing-trajectory effect. Analogously, the rotational motion of the particles decouples from the local fluid vorticity and strain-rate field at increasing particle inertia. The high level of dissipation is partially compensated by the transfer of momentum to the fluid via forces acting at the particle surfaces. The spectral analysis of the kinetic energy budget is supported by the average flow pattern about the particles showing a nearly universal strain-rate distribution. An analytical expression for the instantaneous rate of viscous dissipation induced by each particle is derived and subsequently verified numerically. Using this equation, the local balance of fluid kinetic energy around a particle of arbitrary shape can be precisely determined. It follows that two-way coupled point-particle models implicitly account for the particle-induced dissipation rate via the momentum-coupling terms; however, they disregard the actual length scales of the interaction. Finally, an analysis of the small-scale flow topology shows that the strength of vortex stretching in the bulk flow is mitigated due to the presence of the particles. This effect is associated with the energy conversion at small wavenumbers and the reduced level of dissipation at intermediate wavenumbers. Consequently, it damps the spectral flux of energy to the small scales.

Study of Local Turbulence Profiles Relative to the Particle Surface in Particle-Laden Turbulent Flows

2015 ◽  
Vol 138 (4) ◽  
Author(s):  
Lian-Ping Wang ◽  
Oscar G. C. Ardila ◽  
Orlando Ayala ◽  
Hui Gao ◽  
Cheng Peng
Keyword(s):  
Kinetic Energy ◽  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Model Development ◽  
Particle Surface ◽  
Finite Size ◽  
Solid Particles ◽  
Energy Profiles ◽  
Turbulence Transport ◽  
Boltzmann Method

As particle-resolved simulations (PRSs) of turbulent flows laden with finite-size solid particles become feasible, methods are needed to analyze the simulated flows in order to convert the simulation data to a form useful for model development. In this paper, the focus is on turbulence statistics at the moving fluid–solid interfaces. An averaged governing equation is developed to quantify the radial transport of turbulent kinetic energy when viewed in a frame moving with a solid particle. Using an interface-resolved flow field solved by the lattice Boltzmann method (LBM), we computed each term in the transport equation for a forced, particle-laden, homogeneous isotropic turbulence. The results illustrate the distributions and relative importance of volumetric source and sink terms, as well as pressure work, viscous stress work, and turbulence transport. In a decaying particle-laden flow, the dissipation rate and kinetic energy profiles are found to be self-similar.

scholarly journals Inertia–Gravity Waves Breaking in the Middle Atmosphere at High Latitudes: Energy Transfer and Dissipation Tensor Anisotropy

2020 ◽  
Vol 77 (9) ◽  
pp. 3193-3210
Author(s):  
Tiago Pestana ◽  
Matthias Thalhammer ◽  
Stefan Hickel
Keyword(s):  
Reynolds Number ◽  
Energy Transfer ◽  
Energy Dissipation ◽  
Kinetic Energy ◽  
Gravity Waves ◽  
Energy Budget ◽  
Dissipation Rate ◽  
Scaling Law ◽  
Energy Dissipation Rate ◽  
Inertia Gravity Waves

Abstract We present direct numerical simulations of inertia–gravity waves breaking in the middle–upper mesosphere. We consider two different altitudes, which correspond to the Reynolds number of 28 647 and 114 591 based on wavelength and buoyancy period. While the former was studied by Remmler et al., it is here repeated at a higher resolution and serves as a baseline for comparison with the high-Reynolds-number case. The simulations are designed based on the study of Fruman et al., and are initialized by superimposing primary and secondary perturbations to the convectively unstable base wave. Transient growth leads to an almost instantaneous wave breaking and secondary bursts of turbulence. We show that this process is characterized by the formation of fine flow structures that are predominantly located in the vicinity of the wave’s least stable point. During the wave breakdown, the energy dissipation rate tends to be an isotropic tensor, whereas it is strongly anisotropic in between the breaking events. We find that the vertical kinetic energy spectra exhibit a clear 5/3 scaling law at instants of intense energy dissipation rate and a cubic power law at calmer periods. The term-by-term energy budget reveals that the pressure term is the most important contributor to the global energy budget, as it couples the vertical and the horizontal kinetic energy. During the breaking events, the local energy transfer is predominantly from the mean to the fluctuating field and the kinetic energy production is in balance with the pseudo kinetic energy dissipation rate.

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