scholarly journals Energy cascades in active-grid-generated turbulent flows

2019 ◽  
Vol 4 (10) ◽  
Author(s):  
D. O. Mora ◽  
E. Muñiz Pladellorens ◽  
P. Riera Turró ◽  
M. Lagauzere ◽  
M. Obligado
Keyword(s):  
Turbulent Flows ◽  
Active Grid ◽  
Energy Cascades

scholarly journals High-beta turbulence in two-dimensional magnetohydrodynamics

1976 ◽  
Vol 16 (2) ◽  
pp. 181-191 ◽  
Author(s):  
David Fyfe ◽  
David Montgomery
Keyword(s):  
Magnetic Fields ◽  
Turbulent Flows ◽  
Fluid Dynamic ◽  
Fluid Velocity ◽  
Navier Stokes ◽  
Magnetic Islands ◽  
Dissipation Energy ◽  
Energy Cascades ◽  
Wide Range ◽  
Current Filaments

Incompressible turbulent flows are investigated in the framework of ideal magnetohydrodynamics. All the field quantities vary with only two spatial dimensions. Equilibrium canonical distributions are determined in a phase space whose co-ordinates are the real and imaginary parts of the Fourier coefficients for the field variables. In the geometry considered, the magnetic field and fluid velocity have variable x and y components, and all field quantities are independent of z. Three constants of the motion are found (one of them new) which survive the truncation in Fourier space and permit the construction of canonical distributions with three independent temperatures. Spectral densities are calculated. One of the more novel physical effects is the appearance of macroscopic structures involving long wavelength, self-generated, magnetic fields (‘magnetic islands’) for a wide range of initial parameters. Current filaments show a tendency toward consolidation in much the same way that vorticity filaments do in the guiding-centre plasma case. In the presence of finite dissipation, energy cascades to higher wavenumbers can be accompanied by vector potential cascades to lower wavenumbers, in much the same way as, in the fluid dynamic (Navier-Stokes) case, energy cascades to lower wavenumbers accompany enstrophy cascades to higher wavenumbers. It is suggested that the techniques may be relevant to theories of the magnetic dynamo problem and to the generation of megagauss magnetic fields when pellets are irradiated by lasers.

scholarly journals Beyond Kolmogorov cascades

2019 ◽  
Vol 867 ◽  
Author(s):  
Bérengère Dubrulle
Keyword(s):  
Turbulent Flows ◽  
Large Scale ◽  
Scale Space ◽  
Energy Cascade ◽  
Cloud Formation ◽  
Small Scale ◽  
Weak Formulation ◽  
Anisotropic Flow ◽  
Energy Cascades ◽  
Homogeneity Hypothesis

The large-scale structure of many turbulent flows encountered in practical situations such as aeronautics, industry, meteorology is nowadays successfully computed using the Kolmogorov–Kármán–Howarth energy cascade picture. This theory appears increasingly inaccurate when going down the energy cascade that terminates through intermittent spots of energy dissipation, at variance with the assumed homogeneity. This is problematic for the modelling of all processes that depend on small scales of turbulence, such as combustion instabilities or droplet atomization in industrial burners or cloud formation. This paper explores a paradigm shift where the homogeneity hypothesis is replaced by the assumption that turbulence contains singularities, as suggested by Onsager. This paradigm leads to a weak formulation of the Kolmogorov–Kármán–Howarth–Monin equation (WKHE) that allows taking into account explicitly the presence of singularities and their impact on the energy transfer and dissipation. It provides a local in scale, space and time description of energy transfers and dissipation, valid for any inhomogeneous, anisotropic flow, under any type of boundary conditions. The goal of this article is to discuss WKHE as a tool to get a new description of energy cascades and dissipation that goes beyond Kolmogorov and allows the description of small-scale intermittency. It puts the problem of intermittency and dissipation in turbulence into a modern framework, compatible with recent mathematical advances on the proof of Onsager’s conjecture.

scholarly journals Exploring the capabilities of active grids

2021 ◽  
Vol 62 (6) ◽  
Author(s):  
Lars Neuhaus ◽  
Frederik Berger ◽  
Joachim Peinke ◽  
Michael Hölling
Keyword(s):  
Turbulent Flows ◽  
Wind Tunnels ◽  
Flow Structures ◽  
Generation Process ◽  
Active Grid ◽  
Reduced Frequency ◽  
Dynamic Damping ◽  
Induced Flow ◽  
Characteristic Features ◽  
High Reynolds

Abstract Active grids are commonly used in wind tunnels to generate turbulence with different characteristic features. In contrast to the common objective to generate turbulence with a very high Reynolds number, this work focuses on a method of blockage induced flow design for the generation of special flow structures. Particularly, we aim to investigate the underlying constraints of this excitation method. For this purpose, the scale dependency of the excitation is studied by clearly defined structures such as periodic sinusoidal velocity variations, velocity steps, and single gusts. It is shown that the generation process is limited by the reduced frequency of the active grid motion. For low values of reduced frequencies the imprinted flow structures remain undamped, whereas for higher reduced frequencies they are damped. This insight leads to the constraint that the active grid motion needs to be modified to compensate for the underlying dynamic damping effects. Thus, the inserted energy has to be increased for the corresponding reduced frequencies. This finding can be transferred to the generation of turbulent flows, for which an exemplary adaption is shown . Graphic abstract

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 Span effect on the turbulence nature of flow past a circular cylinder

2019 ◽  
Vol 878 ◽  
pp. 306-323 ◽  
Author(s):  
Bernat Font Garcia ◽  
Gabriel D. Weymouth ◽  
Vinh-Tan Nguyen ◽  
Owen R. Tutty
Keyword(s):  
Circular Cylinder ◽  
Turbulent Flows ◽  
Large Scale ◽  
Three Dimensional ◽  
Small Scale ◽  
Flow Past ◽  
Energy Cascades ◽  
Flow Evolution ◽  
Scale Structures ◽  
Small Scale Structures

Turbulent flow evolution and energy cascades are significantly different in two-dimensional (2-D) and three-dimensional (3-D) flows. Studies have investigated these differences in obstacle-free turbulent flows, but solid boundaries have an important impact on the cross-over from 3-D to 2-D turbulence dynamics. In this work, we investigate the span effect on the turbulence nature of flow past a circular cylinder at $Re=10\,000$. It is found that even for highly anisotropic geometries, 3-D small-scale structures detach from the walls. Additionally, the natural large-scale rotation of the Kármán vortices rapidly two-dimensionalise those structures if the span is 50 % of the diameter or less. We show this is linked to the span being shorter than the Mode B instability wavelength. The conflicting 3-D small-scale structures and 2-D Kármán vortices result in 2-D and 3-D turbulence dynamics which can coexist at certain locations of the wake depending on the domain geometric anisotropy.

Film Condensation and Aerosol Deposition in Turbulent Flows with Noncondensable Gases

1997 ◽  
Vol 28 (4-6) ◽  
pp. 277-288
Author(s):  
Leonid I. Zaichik ◽  
Bulat I. Nigmatulin ◽  
Vladimir M. Alipchenkov ◽  
V. A. Belov
Keyword(s):  
Turbulent Flows ◽  
Aerosol Deposition ◽  
Film Condensation ◽  
Noncondensable Gases
Sign in / Sign up

Export Citation Format

Share Document