Internal Stresses and Breakup of Porous Aggregates in Homogeneous Isotropic Turbulence

2014 ◽  
Author(s):  
Marco Vanni
Keyword(s):  
Numerical Simulation ◽  
Isotropic Turbulence ◽  
Internal Stresses ◽  
Length Scale ◽  
Homogeneous Isotropic Turbulence ◽  
Stokesian Dynamics ◽  
Disordered Structure ◽  
Kolmogorov Length Scale ◽  
Primary Particles ◽  
Internal Forces

The stresses acting on aggregates smaller than the Kolmogorov length scale in homogeneous isotropic turbulence were estimated by a two-scale numerical simulation. The fluid dynamics at the scales larger than the Kolmogorov length scale was calculated by a Direct Numerical Simulation of the turbulent flow, in which the aggregates were modeled as point particles. Then, we adopted Stokesian Dynamics to evaluate the phenomena governed by the smooth velocity field of the smallest scales. At this level the disordered structure of the aggregates was modeled in detail, in order to take into account the role that the primary particles have in generating and transferring the internal stress. From this result, it was possible to evaluate the internal forces acting at intermonomer contacts and determine the occurrence of breakup as a consequence of the failure of intermonomer bonds. The method was applied to disordered aggregates with isostatic and highly hyperstatic structures, respectively.

A general self-preservation analysis for decaying homogeneous isotropic turbulence

2015 ◽  
Vol 773 ◽  
pp. 345-365 ◽  
Author(s):  
L. Djenidi ◽  
R. A. Antonia
Keyword(s):  
Velocity Structure ◽  
Isotropic Turbulence ◽  
Initial Conditions ◽  
Longitudinal Velocity ◽  
Length Scale ◽  
Grid Turbulence ◽  
Homogeneous Isotropic Turbulence ◽  
Kolmogorov Length Scale ◽  
Velocity Structure Function ◽  
Turbulence Data

A general framework of self-preservation (SP) is established, based on the transport equation of the second-order longitudinal velocity structure function in decaying homogeneous isotropic turbulence (HIT). The analysis introduces the skewness of the longitudinal velocity increment, $S(r,t)$ ($r$ and $t$ are space increment and time), as an SP controlling parameter. The present SP framework allows a critical appraisal of the specific assumptions that have been made in previous SP analyses. It is shown that SP is achieved when $S(r,t)$ varies in a self-similar manner, i.e. $S=c(t){\it\phi}(r/l)$ where $l$ is a scaling length, and $c(t)$ and ${\it\phi}(r/l)$ are dimensionless functions of time and $(r/l)$, respectively. When $c(t)$ is constant, $l$ can be identified with the Kolmogorov length scale ${\it\eta}$, even when the Reynolds number is relatively small. On the other hand, the Taylor microscale ${\it\lambda}$ is a relevant SP length scale only when certain conditions are met. The decay law for the turbulent kinetic energy ($k$) ensuing from the present SP is a generalization of the existing laws and can be expressed as $k\sim (t-t_{0})^{n}+B$, where $B$ is a constant representing the energy of the motions whose scales are excluded from the SP range of scales. When $B=0$, SP is achieved at all scales of motion and ${\it\lambda}$ becomes a relevant scaling length together with ${\it\eta}$. The analysis underlines the relation between the initial conditions and the power-law exponent $n$ and also provides a link between them. In particular, an expression relating $n$ to the initial values of the scaling length and velocity is developed. Finally, the present SP analysis is consistent with both experimental grid turbulence data and the eddy-damped quasi-normal Markovian numerical simulation of decaying HIT by Meldi & Sagaut (J. Turbul., vol. 14, 2013, pp. 24–53).

scholarly journals Direct Numerical Simulation in Two Dimensional Homogeneous Isotropic Turbulence Using Spectral Method

2013 ◽  
Vol 5 (3) ◽  
pp. 435-445
Author(s):  
M. S. I. Mallik ◽  
M. A. Uddin ◽  
M. A. Rahman
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Flow Field ◽  
Direct Numerical Simulation ◽  
Spectral Method ◽  
Isotropic Turbulence ◽  
Qualitative Behavior ◽  
Two Dimensional ◽  
Homogeneous Isotropic Turbulence ◽  
Decaying Turbulence

Direct numerical simulation (DNS) in two-dimensional homogeneous isotropic turbulence is performed by using the Spectral method at a Reynolds number Re = 1000 on a uniformly distributed grid points. The Reynolds number is low enough that the computational grid is capable of resolving all the possible turbulent scales. The statistical properties in the computed flow field show a good agreement with the qualitative behavior of decaying turbulence. The behavior of the flow structures in the computed flow field also follow the classical idea of the fluid flow in turbulence. Keywords: Direct numerical simulation, Isotropic turbulence, Spectral method. © 2013 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. doi:http://dx.doi.org/10.3329/jsr.v5i3.12665 J. Sci. Res. 5 (3), 435-445 (2013)  

Micro-structure and Lagrangian statistics of the scalar field with a mean gradient in isotropic turbulence

2003 ◽  
Vol 474 ◽  
pp. 193-225 ◽  
Author(s):  
G. BRETHOUWER ◽  
J. C. R. HUNT ◽  
F. T. M. NIEUWSTADT
Keyword(s):  
Scalar Field ◽  
Isotropic Turbulence ◽  
Numerical Study ◽  
Compressive Strain ◽  
Scalar Fields ◽  
Scalar Dissipation ◽  
Small Scale ◽  
Length Scale ◽  
Random Forcing ◽  
Kolmogorov Length Scale

This paper presents an analysis and numerical study of the relations between the small-scale velocity and scalar fields in fully developed isotropic turbulence with random forcing of the large scales and with an imposed constant mean scalar gradient. Simulations have been performed for a range of Reynolds numbers from Reλ = 22 to 130 and Schmidt numbers from Sc = 1/25 to 144.The simulations show that for all values of Sc [ges ] 0.1 steep scalar gradients are concentrated in intermittently distributed sheet-like structures with a thickness approximately equal to the Batchelor length scale η/Sc½ with η the Kolmogorov length scale. We observe that these sheets or cliffs are preferentially aligned perpendicular to the direction of the mean scalar gradient. Due to this preferential orientation of the cliffs the small-scale scalar field is anisotropic and this is an example of direct coupling between the large- and small-scale fluctuations in a turbulent field. The numerical simulations also show that the steep cliffs are formed by straining motions that compress the scalar field along the imposed mean scalar gradient in a very short time period, proportional to the Kolmogorov time scale. This is valid for the whole range of Sc. The generation of these concentration gradients is amplified by rotation of the scalar gradient in the direction of compressive strain. The combination of high strain rate and the alignment results in a large increase of the scalar gradient and therefore in a large scalar dissipation rate.These results of our numerical study are discussed in the context of experimental results (Warhaft 2000) and kinematic simulations (Holzer & Siggia 1994). The theoretical arguments developed here follow from earlier work of Batchelor & Townsend (1956), Betchov (1956) and Dresselhaus & Tabor (1991).

A physical mechanism of the energy cascade in homogeneous isotropic turbulence

2008 ◽  
Vol 605 ◽  
pp. 355-366 ◽  
Author(s):  
SUSUMU GOTO
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Direct Numerical Simulation ◽  
Transfer Rate ◽  
Physical Mechanism ◽  
Isotropic Turbulence ◽  
Scale Space ◽  
Energy Cascade ◽  
Homogeneous Isotropic Turbulence ◽  
Moderate Reynolds Number

In order to investigate the physical mechanism of the energy cascade in homogeneous isotropic turbulence, the internal energy and its transfer rate are defined as a function of scale, space and time. Direct numerical simulation of turbulence at a moderate Reynolds number verifies that the energy cascade can be caused by the successive creation of smaller-scale tubular vortices in the larger-scale straining regions existing between pairs of larger-scale tubular vortices. Movies are available with the online version of the paper.

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