63 research outputs found
Localization of unresolved regions in the selective large-eddy simulation of hypersonic jets
A method for the localization of the regions where the turbulent fluctuations are unresolved is applied to the selective large-eddy simulation (LES) of a compressible turbulent jet of Mach number equal to 5. This method is based on the introduction of a scalar probe function f which represents the magnitude of the twisting-stretching term normalized with the enstrophy [1]. The statistical analysis shows that, for a fully developed turbulent field of fluctuations, the probability that f is larger than 2 is zero, while, for an unresolved field, is finite. By computing f in each instantaneous realization of the simulation it is possible to locate the regions where the magnitude of the normalized stretching-twisting is anomalously high. This allows the identification of the regions where the subgrid model should be introduced into the governing equations (selective filtering). The results of the selective LES are compared with those of a standard LES, where the subgrid terms are used in the whole domain. The comparison is carried out by assuming as high order reference field a higher resolution Euler simulation of the compressible jet. It is shown that the selective LES modifies the dynamic properties of the flow to a lesser extent with respect to the classical LE
Philofluid turbulent flow database
A set of velocity and passive scalar fields and their statistics coming from direct numerical simulations and large-eddy simulations. The database includes: shearless mixings in two a three dimensions, turbulent channel flow, cavity flow. Username and password to access the netdisks is provided upon request
Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations
Plasma velocity and magnetic field measurements from the Voyager 2 mission
are used to study solar wind turbulence in the slow solar wind at two different
heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to
provide information on the radial evolution of this turbulence. The magnetic
helicity and the cross-helicity, which express the correlation between the
plasma velocity and the magnetic field, are used to characterize the
turbulence. Wave number spectra are computed by means of the Taylor hypothesis
applied to time resolved single point Voyager 2 measurements. The overall
picture we get is complex and difficult to interpret. A substantial decrease of
the cross-helicity at smaller scales (over 1-3 hours of observation) with
increasing heliocentric distance is observed. At 5 AU the only peak in the
probability density of the normalized residual energy is negative, near -0.5.
At 29 AU the probability density becomes doubly peaked, with a negative peak at
-0.5 and a smaller peak at a positive values of about 0.7. A decrease of the
cross-helicity for increasing heliocentric distance is observed, together with
a reduction of the unbalance toward the magnetic energy of the energy of the
fluctuations. For the smaller scales, we found that at 29 AU the normalized
polarization is small and positive on average (about 0.1), it is instead zero
at 5 AU. For the larger scales, the polarization is low and positive at 5 AU
(average around 0.1) while it is negative (around - 0.15) at 29 AU.Comment: 14 pages 5 figures. Accepted for publication on European Journal of
Mechanics B/Fluids (5/8/2015
Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU
Fluctuations in the flow velocity and magnetic fields are ubiquitous in the
Solar System. These fluctuations are turbulent, in the sense that they are
disordered and span a broad range of scales in both space and time. The study
of solar wind turbulence is motivated by a number of factors all keys to the
understanding of the Solar Wind origin and thermodynamics. The solar wind
spectral properties are far from uniformity and evolve with the increasing
distance from the sun. Most of the available spectra of solar wind turbulence
were computed at 1 astronomical unit, while accurate spectra on wide frequency
ranges at larger distances are still few. In this paper we consider solar wind
spectra derived from the data recorded by the Voyager 2 mission during 1979 at
about 5 AU from the sun. Voyager 2 data are an incomplete time series with a
voids/signal ratio that typically increases as the spacecraft moves away from
the sun (45% missing data in 1979), making the analysis challenging. In order
to estimate the uncertainty of the spectral slopes, different methods are
tested on synthetic turbulence signals with the same gap distribution as V2
data. Spectra of all variables show a power law scaling with exponents between
-2.1 and -1.1, depending on frequency subranges. Probability density functions
(PDFs) and correlations indicate that the flow has a significant intermittency.Comment: 14 pages, 7 figures. Discussion improved since the previous versio
Large fluctuations of the nonlinearities in isotropic turbulence. Anisotropic filtering analysis
Using a NavierâStokes isotropic turbulent field numerically simulated in a box with a discretization of
10243 (Biferale et al., 2005), we show that the probability of having a stretchingâtilting larger than a
few times the local enstrophy is low. By using an anisotropic kind of filter in the Fourier space, where
wavenumbers that have at least one component below a threshold or inside a range are removed, we
analyze these survival statistics when the large, the small inertial or the small inertial and dissipation
scales are filtered out. By considering a flow obtained by randomizing the phases of the Fourier modes,
and applying our filtering techniques, we identified clearly the properties attributable to turbulence.
It can be observed that, in the unfiltered isotropic NavierâStokes field, the probability of the ratio
(|Ï·âU|/|Ï|2) being higher than a given threshold is higher than in the fields where the large scales were
filtered out. At the same time, it is lower than in the fields where the small inertial and dissipation range
of scales is filtered out. This is basically due to the suppression of compact structures in the ranges that
have been filtered in different ways. The partial removal of the background of filaments and sheets does
not have a first order effect on these statistics. These results are discussed in the light of a hypothesized
relation between vortical filaments, sheets and blobs in physical space and in Fourier space. The study
in fact can be viewed as a kind of test for this idea and tries to highlight its limits. We conclude that a
qualitative relation in physical space and in Fourier space can be supposed to exist for blobs only. That is
for the near isotropic structures which are sufficiently described by a single spatial scale and do not suffer
from the disambiguation problem as filaments and sheets do.
Information is also given on the filtering effect on statistics concerning the inclination of the strain rate
tensor eigenvectors with respect to vorticity. In all filtered ranges, eigenvector 2 reduces its alignment,
while eigenvector 3 reduces its misalignment. All filters increase the gap between the most extensional
eigenvalue âšÎ»1â© and the intermediate one âšÎ»2â© and the gap between this last âšÎ»2â© and the contractile
eigenvalue âšÎ»3â©. When the large scales are missing, the modulus of the eigenvalue 1 becomes nearly
equal to that of the eigenvalue 3, similarly to the modulus of the associated components of the enstrophy
production
Dimensionality influence on passive scalar transport
We numerically investigate the advection of a passive scalar through an interface placed inside a decaying shearless turbulent mixing layer. We consider the system in both two and three dimensions. The dimensionality produces a different time scaling of the diffusion, which is faster in the two-dimensional case. Two intermittent fronts are generated at the margins of the mixing layer. During the decay these fronts present a sort of propagation in both the direction of the scalar flow and the opposite direction. In two dimensions, the propagation of the fronts exhibits a significant asymmetry with respect to the initial position of the interface and is deeper for the front merged in the high energy side of the mixing. In three dimensions, the two fronts remain nearly symmetrically placed. Results concerning the scalar spectra exponents are also presented
"Philofluid" turbulent flow database
A set of velocity and passive scalar fields and their statistics coming from direct numerical simulations and large-eddy simulations. The database includes: shearless mixings in two a three dimensions, turbulent channel flow, cavity flow. Username and password to access the netdisks is provided upon request
IMPACT OF TURBULENCE MODELING ON FLUID/SOLID HEAT TRANSFER INSIDE INDUSTRIAL AUTOCLAVES
This work is centred on the analysis of the impact of different turbulence
modeling approaches on the fluid/solid heat exchange inside a commercial size autoclave.
This project proposes itself to be a first step towards the optimization of the turbulent
flow inside this kind of machinery to improve the curing treatment of Carbon-Fiber Reinforced
Plastics (CFRP). The setup of the CFD simulations includes the presence of
a metallic sample object inside the autoclave, where air will be recirculated with velocity,
pressure and temperature typically adopted for this type of treatments. The analysis
takes advantage of parallel CFD simulations, conducted by using the open-source software
openFOAM v2106. Two turbulence models have been adopted: one is the well-known
Reynolds-Average Navier-Stokes approach (RANS), which is currently used to model the
turbulence inside this type of machinery. The second one is the Delayed Detached Eddy
Simulations (DDES), which allows the full resolution of the majority of turbulent scales
around the sample object. First, we propose the difference between the local heat flux distribution
at the air/solid interface computed by using RANS and DDES, next we analyse
the overall heat flux entering the sample object: the resolution of the turbulent scales does
not influence the local heat flux only, but also the overall heat flux entering the object; an
average increase of 35% is reported when the velocity fluctuations are neglected. Future
steps of the research foresee the analysis of the heat flux and temperature distributions on
the surface of realistic shapes and common-use CFRP. Afterwards, the autoclave design
will be optimized by adding multiple inlets and aerodynamic devices to guarantee a more
homogeneous heat flux distribution on the surface of realistic shapes of actual CFRP
Energy and water vapor transport in a turbulent stratified environment
We present direct numerical simulations about the transport of kinetic energy and unsaturated water vapor across a thin layer which separates two decaying turbulent flows with different energy. This interface lies in a shearless stratified environment modeled by means of Boussinesqâs approximation. Water vapor is treated as a passive scalar (Kumar et al. 2014). Initial conditions have Fr2 between 0.64 and 64 (stable case) and between -3.2 and -19 (unstable case) and Re_lambda = 250. Dry air is in the lower half of the domain and has a higher turbulent energy, seven times higher than the energy of moist air in the upper half. In the early stage of evolution, as long as Fr^2 > 1, stratification plays a minor role and the flows follows closely neutral stratification mixing. As the buoyancy terms grows, Fr2 ~ O(1), the mixing process deeply changes. A stable stratification generates a separation layer which blocks the entrainment of dry air into the moist one, characterized by a relative increment of the turbulent dissipation rate compared to the local turbulent energy. On the contrary, an unstable stratification sligthy enhances the entrainment. Growth-decay of energy and mixing layer thichness are discussed and compared with laboratory and numerical experiments
Energy and water vapor transport across a simplified cloud-clear energy air interface
We consider a simplified physics of the could interface where condensation, evaporation and radiation are neglected and momentum, thermal energy and water vapor transport is represented in terms of the Boussinesq model coupled to a passive scalar transport equation for the vapor. The interface is modeled as a layer separating two isotropic turbulent regions with different kinetic energy and vapor concentration. In particular, we focus on the small scale part of the inertial range of the atmospheric boundary layer as well as on the dissipative range of scales which are important to the micro-physics of warm clouds. We have numerically investigated stably stratified interfaces by locally perturbing at an initial instant the standard temperature lapse rate at the cloud interface and then observing the temporal evolution of the system. When the buoyancy term becomes of the same order of the inertial one, we observe a spatial redistribution of the kinetic energy which produce a concomitant pit of kinetic energy within the mixing layer. In this situation, the mixing layer contains two interfacial regions with opposite kinetic energy gradient, which in turn produces two intermittent sublayers in the velocity fluctuations field. This changes the structure of the field with respect to the corresponding non-stratified shearless mixing: the communication between the two turbulent region is weak, and the growth of the mixing layer stops. These results are discussed with respect to Large Eddy Simulations data for the Planetary Boundary Layers
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