Effect of Selective Modification of Turbulence on Two-Equation Models for Particle-Laden Turbulent Flows

1994 ◽  
Vol 116 (4) ◽  
pp. 778-784 ◽  
Author(s):  
K. D. Squires ◽  
J. K. Eaton
Keyword(s):  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Large Mass ◽  
Momentum Exchange ◽  
Two Phase ◽  
Heavy Particles ◽  
Turbulent Vortex ◽  
Simulation Results ◽  
Grid Points ◽  
Using Data

The effect of selective modification of turbulence by particles on K–ε models for dilute two-phase flows has been evaluated using data from direct numerical simulation of particle-laden isotropic turbulence. Simulation results were obtained using as many as 643 grid points and up to 106 particles. The ratio of the particle time constant to large-eddy turbulence time scale varied from 0.14 to 1.50 and particle mass loadings of 0.0, 0.1, 0.5, and 1.0 were used in the simulations. Simulation results demonstrate that the balance between enstrophy production by turbulent vortex stretching and viscous destruction is disrupted by momentum exchange with the particle cloud. Selective modification of the turbulence by lighter particles results in a significant attenuation of enstrophy production by turbulent vortex stretching. The decrease in enstrophy production causes the model constant Cε2 to increase for large mass loading. Heavy particles are found to act as a sink of enstrophy for all mass loadings used in the simulations. Preferential concentration of lighter particles by turbulence, however, can generate vorticity fluctuations, especially at higher mass loadings. For these cases conventional modeling of the destruction of dissipation by particles term in the ε equation requires that Cε3 be negative.

Effects of roughness on particle dynamics in turbulent channel flows: a DNS analysis

2014 ◽  
Vol 739 ◽  
pp. 465-478 ◽  
Author(s):  
Barbara Milici ◽  
Mauro De Marchis ◽  
Gaetano Sardina ◽  
Enrico Napoli
Keyword(s):  
Turbulent Flow ◽  
Turbulent Flows ◽  
Three Dimensional ◽  
Wall Region ◽  
Wall Roughness ◽  
Turbulent Structures ◽  
Two Phase ◽  
Heavy Particles ◽  
Turbulent Channel Flows ◽  
Lagrangian Tracking

AbstractDeposition and resuspension mechanisms in particle-laden turbulent flows are dominated by the coherent structures arising in the wall region. These turbulent structures, which control the turbulent regeneration cycles, are affected by the roughness of the wall. The particle-laden turbulent flow in a channel bounded by irregular two-dimensional rough surfaces is analysed. The behaviour of dilute dispersions of heavy particles is analysed using direct numerical simulations (DNS) to calculate the three-dimensional turbulent flow and Lagrangian tracking to describe the turbophoretic effect associated with two-phase turbulent flows in a complex wall-bounded domain. Turbophoresis is investigated in a quantitative way as a function of the particle inertia. The analysis of the particle statistics, in term of mean particle concentration and probability density function (p.d.f.) of wall-normal particle velocity, shows that the wall roughness produces a completely different scenario compared to the classical smooth wall. The effect of the wall roughness on the particle mass flux is shown for six particle populations having different inertia.

Tensor geometry in the turbulent cascade

2017 ◽  
Vol 835 ◽  
pp. 1048-1064 ◽  
Author(s):  
Joseph G. Ballouz ◽  
Nicholas T. Ouellette
Keyword(s):  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Energy Cascade ◽  
Three Dimensions ◽  
Inner Product ◽  
Transfer Energy ◽  
Average Value ◽  
Energy Cascades ◽  
Using Data ◽  
Highly Turbulent Flows

The defining characteristic of highly turbulent flows is the net directed transport of energy from the injection scales to the dissipation scales. This cascade is typically described in Fourier space, obscuring its connection to the mechanics of the flow. Here, we recast the energy cascade in mechanical terms, noting that for some scales to transfer energy to others, they must do mechanical work on them. This work can be expressed as the inner product of a turbulent stress and a rate of strain. But, as with all inner products, the relative alignment of these two tensors matters, and determines how strong the energy transfer will be. We show that this tensor alignment behaves very differently in two and three dimensions; in particular, the tensor eigenvalues affect the inner product in very different ways. By comparing the observed energy flux to the maximum possible if the tensors were in perfect alignment, we define an efficiency for the energy cascade. Using data from a direct numerical simulation of isotropic turbulence, we show that this efficiency is perhaps surprisingly low, with an average value of approximately 25 % in the inertial range, although it is spatially heterogeneous. Our results have implications for how the stress and strain-rate magnitudes influence the flux of energy between scales, and may help to explain why the energy cascades in two and three dimensions are different.

scholarly journals Intermittency in the relative separations of tracers and of heavy particles in turbulent flows

2014 ◽  
Vol 757 ◽  
pp. 550-572 ◽  
Author(s):  
L. Biferale ◽  
A. S. Lanotte ◽  
R. Scatamacchia ◽  
F. Toschi
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Exit Time ◽  
Three Dimensional ◽  
Stokes Number ◽  
Inertial Range ◽  
Relative Dispersion ◽  
Heavy Particles ◽  
Self Similar

AbstractResults from direct numerical simulations (DNS) of particle relative dispersion in three-dimensional homogeneous and isotropic turbulence at Reynolds number $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{Re}}_{\lambda } \sim 300$ are presented. We study point-like passive tracers and heavy particles, at Stokes number $\mathit{St}=0.6,1$ and 5. Particles are emitted from localised sources, in bunches of thousands, periodically in time, allowing an unprecedented statistical accuracy to be reached, with a total number of events for two-point observables of the order of ${10^{11}}$. The right tail of the probability density function (PDF) for tracers develops a clear deviation from Richardson’s self-similar prediction, pointing to the intermittent nature of the dispersion process. In our numerical experiment, such deviations are manifest once the probability to measure an event becomes of the order of – or rarer than – one part over one million, hence the crucial importance of a large dataset. The role of finite-Reynolds-number effects and the related fluctuations when pair separations cross the boundary between viscous and inertial range scales are discussed. An asymptotic prediction based on the multifractal theory for inertial range intermittency and valid for large Reynolds numbers is found to agree with the data better than the Richardson theory. The agreement is improved when considering heavy particles, whose inertia filters out viscous scale fluctuations. By using the exit-time statistics we also show that events associated with pairs experiencing unusually slow inertial range separations have a non-self-similar PDF.

Effects of Particles Collision on Separating Gas–Particle Two-Phase Turbulent Flows

2013 ◽  
Vol 39 (3) ◽  
pp. 2353-2361
Author(s):  
Sihao Lv ◽  
Weihua Yang ◽  
Xiangli Li ◽  
Guohui Li
Keyword(s):  
Turbulent Flows ◽  
Two Phase

scholarly journals Serum C-reactive protein metabolite (CRPM) is associated with incidence of contralateral knee osteoarthritis

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Anne-Christine Bay-Jensen ◽  
Asger Bihlet ◽  
Inger Byrjalsen ◽  
Jeppe Ragnar Andersen ◽  
Bente Juhl Riis ◽  
...  
Keyword(s):  
Phase Iii ◽  
Response To Treatment ◽  
C Reactive Protein ◽  
Two Phase ◽  
Knee Oa ◽  
Reactive Protein ◽  
Inflammatory Phenotype ◽  
The Mean ◽  
Using Data

AbstractThe heterogeneous nature of osteoarthritis (OA) and the need to subtype patients is widely accepted in the field. The biomarker CRPM, a metabolite of C-reactive protein (CRP), is released to the circulation during inflammation. Blood CRPM levels have shown to be associated with disease activity and response to treatment in rheumatoid arthritis (RA). We investigated the level of blood CRPM in OA compared to RA using data from two phase III knee OA and two RA studies (N = 1591). Moreover, the association between CRPM levels and radiographic progression was investigated. The mean CRPM levels were significantly lower in OA (8.5 [95% CI 8.3–8.8] ng/mL, n = 781) compared to the RA patients (12.8 [9.5–16.0] ng/mL, n = 60); however, a significant subset of OA patients (31%) had CRPM levels (≥ 9 ng/mL) comparable to RA. Furthermore, OA patients (n = 152) with CRPM levels ≥ 9 ng/mL were more likely to develop contra-lateral knee OA assessed by X-ray over a two-year follow-up period with an odds ratio of 2.2 [1.0–4.7]. These data suggest that CRPM is a blood-based biochemical marker for early identification OA patients with an inflammatory phenotype.

scholarly journals On the different contributions of coherent structures to the spectra of a turbulent round jet and a turbulent boundary layer

2001 ◽  
Vol 448 ◽  
pp. 367-385 ◽  
Author(s):  
T. B. NICKELS ◽  
IVAN MARUSIC
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Flows ◽  
Coherent Structures ◽  
Isotropic Turbulence ◽  
Structural Models ◽  
Spectral Measurements ◽  
Round Jet ◽  
Good Account ◽  
Single Size

This paper examines and compares spectral measurements from a turbulent round jet and a turbulent boundary layer. The conjecture that is examined is that both flows consist of coherent structures immersed in a background of isotropic turbulence. In the case of the jet, a single size of coherent structure is considered, whereas in the boundary layer there are a range of sizes of geometrically similar structures. The conjecture is examined by comparing experimental measurements of spectra for the two flows with the spectra calculated using models based on simple vortex structures. The universality of the small scales is considered by comparing high-wavenumber experimental spectra. It is shown that these simple structural models give a good account of the turbulent flows.

The Effects of Turbulence Length Scale on the Performance of Piezoelectric Harvesters

2015 ◽  
Author(s):  
Yiannis Andreopoulos ◽  
Amir H. Danesh-Yazdi ◽  
Oleg Goushcha ◽  
Niell Elvin
Keyword(s):  
Turbulent Flows ◽  
Power Output ◽  
Isotropic Turbulence ◽  
Spatial Scales ◽  
Mechanical Energy ◽  
Relative Size ◽  
Analytical Description ◽  
Length Scale ◽  
Linear Effect ◽  
Turbulence Length

Turbulent flows carry mechanical energy distributed over a range of temporal and spatial scales and their interaction with a thin immersed piezoelectric beam results in a strain field which generates electrical charge. This energy harvesting method can be used for developing self-powered electronic devices such as flow sensors. In the present experimental work, various energy harvesters were placed in a turbulent boundary layer or inside a decaying flow field of homogeneous and isotropic turbulence. The role of large instantaneous turbulent structures in this rather complex fluid-structure interaction is discussed in interpreting the electrical output results. The forces acting on the vibrating beams have been measured dynamically and a theory has been developed which incorporates the effects of mean local velocity, turbulence intensity, the relative size of the beam’s length to the integral length scale of turbulence, the structural properties of the beam and the electrical properties of the active piezoelectric layer to provide reasonable estimates of the mean electrical power output. Experiments have been carried out in which these fluidic harvesters are immersed first in inhomogeneous turbulence like that encountered in boundary layers developing over solid walls and homogeneous and isotopic turbulence for which a simplified analytical description exists. It was found that there is a non-linear effect of turbulence length scales on the power output of the fluidic harvesters.

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