71 research outputs found
Development and implementation of turbulence models in the combustion code Ares
This report describes the R&D activities carried out under the ENEA-MURST programme , objective 4. Three more turbulence models (the RNG K – ε, the one-equation Spalart & Allmaras and the Wilcox K – ω) have been implemented into the Ares combustion code. They represent state-of-the-art models that have demonstrated over the past decade their superior accuracy , robustness as well as ease of implementation with respect to the class of K – ε models. The first chapter describes the models formulations. In chapter 2 the three models have been validated against three well known test cases. Particular attention has been dedicated to coupling the one-equation turbulence model by Spalart & Allmaras to the TFC premixed combustion model, for two computed turbulence scales are needed to evaluate the turbulent flame velocity and one-equation models provide one turbulent scale only. For validating the correct models implementations, two simple cold test cases have been chosen, namely the turbulent boundary layer over a flat plate, and a well documented turbulent flow over a backward facing step. Finally the Moreau combustor test case have been used for the validation of the models for premixed combustion flow. The state-of-the-art turbulence models implemented should allow the combustion code Ares to increase its ability to correctly compute complex turbulent premixed reactive flows in real combustors, which is the objective of the next project tasks
On wind-wave interaction phenomena at low Reynolds numbers
After decades of research efforts, wind-wave interaction mechanisms have been recognized as extremely elusive. The reason is the complex nature of the problem, which combines complex coupling mechanisms between turbulent wind and water waves with the presence of multiple governing parameters, such as the friction Reynolds number of the wind, the water depth and the wind fetch. As shown unequivocally here, the use of suitable flow settings allows us to reduce the complex problem of wind-wave interaction to its essential features, mainly as a function of the sole friction Reynolds number of the wind. The resulting numerical solution allows us to study the interactions between water and air layers with their own fluid properties, and to unveil very interesting features, such as an oblique wave pattern travelling upstream and a wave-induced Stokes sublayer. The latter is responsible for a drag reduction mechanism in the turbulent wind. Despite the simulated flow conditions being far from the intense events occurring at the ocean-atmosphere interface, the basic flow phenomena unveiled here may explain some experimental evidence in wind-wave problems. Among other things, the wave-induced Stokes sublayer may shed light on the large scatter of the drag coefficient data in field measurements where swell waves of arbitrary directions are often present. Hence the present results and the developed approach pave the way for the understanding and modelling of the surface fluxes at the ocean-atmosphere interface, which are of overwhelming importance for climate science
An integrated approach for the analysis and modeling of road tunnel ventilation. Part II: Numerical model and its calibration
The present work represents the second and final part of a twofold study aiming at the definition and validation of an integrated methodology for the analysis and modeling of road tunnel ventilation systems. A numerical approach is presented, based on the Finite Volume integration of the 1D mechanical and thermal energy conservation equations on a network of ducts, representing the ventilation system of the 11.6 km long Mont Blanc Tunnel. The set of distributed and concentrated loss coefficients, representing dissipation of mechanical energy by friction in each part of the ventilation system, is calibrated against a rich experimental dataset, collected throughout a dedicated set of in situ tests and presented in the first part of the work. The calibration of the model is carried out by means of genetic optimization algorithms. Predictions of the flow field using the calibrated parameters are in remarkable agreement with the experimental data, with an overall RMS error of \ub1 0.27 m/s, i.e. of the same order of the accuracy of the measurement probes. Further validation against a selection of field data recorded by the tunnel monitoring and control system is brought forward, highlighting the robustness and potential general applicability of the proposed approach
Variability of the transitional T Tauri star T Chamaeleontis
We characterize the physical properties of T Chamaeleontis, a transitional T
Tauri star showing UX Ori-type variability, and of its associated disc, and
probe possible effects of disc clearing processes. Different spectral
diagnostics are examined, based on a rich collection of optical high- and
low-resolution spectra. We determine radial and projected rotational
velocities, and measure equivalent widths of the Li I (6708 A) line and of the
most prominent emission lines (e.g. Ha, Hb and [OI] 6300A); we analyse shape
changes of photospheric lines via bisector-method, while variability in Ha and
Hb is inspected through line-profile correlation matrices. The strength of the
Ha and Hb emission is highly variable and well correlated with that of the [OI]
lines, as well as with Av variations of over three magnitudes. Variations up to
nearly 10 km/s in the radial velocity of the star are measured on analogous
time-scale, but with no apparent periodicity. SED modelling confirms the
existence of a gap in the disc. Variable circumstellar extinction is pointed
out as responsible for the conspicuous variations observed in the stellar
continuum flux and for concomitant changes in the emission features by contrast
effect. Clumpy structures, incorporating large dust grains and orbiting the
star within a few tenths of AU, obscure episodically the star and, eventually,
part of the inner circumstellar zone, while the bulk of the hydrogen lines
emitting zone and outer low-density wind region traced by the [OI] remain
unaffected. Coherently with this scenario, the detected radial velocity changes
are also explainable in terms of clumpy materials transiting and partially
obscuring the star.Comment: 38 pages, 32 figures. To appear in the Astronomy and Astrophysics
main journa
Extreme Energy Cosmic Rays (EECR) Observation Capabilities of an "Airwatch from Space'' Mission
The longitudinal development and other characteristics of the EECR induced
atmospheric showers can be studied from space by detecting the fluorescence
light induced in the atmospheric nitrogen. According to the Airwatch concept a
single fast detector can be used for measuring both intensity and time
development of the streak of fluorescence light produced by the atmospheric
shower induced by an EECR. In the present communication the detection
capabilities for the EECR observation from space are discussed.Comment: 3 pages (LaTeX). To appear in the Proceedings of TAUP'9
UV and EUV Instruments
We describe telescopes and instruments that were developed and used for
astronomical research in the ultraviolet (UV) and extreme ultraviolet (EUV)
regions of the electromagnetic spectrum. The wavelength ranges covered by these
bands are not uniquely defined. We use the following convention here: The EUV
and UV span the regions ~100-912 and 912-3000 Angstroem respectively. The
limitation between both ranges is a natural choice, because the hydrogen Lyman
absorption edge is located at 912 Angstroem. At smaller wavelengths,
astronomical sources are strongly absorbed by the interstellar medium. It also
marks a technical limit, because telescopes and instruments are of different
design. In the EUV range, the technology is strongly related to that utilized
in X-ray astronomy, while in the UV range the instruments in many cases have
their roots in optical astronomy. We will, therefore, describe the UV and EUV
instruments in appropriate conciseness and refer to the respective chapters of
this volume for more technical details.Comment: To appear in: Landolt-Boernstein, New Series VI/4A, Astronomy,
Astrophysics, and Cosmology; Instruments and Methods, ed. J.E. Truemper,
Springer-Verlag, Berlin, 201
New MACRO results on atmospheric neutrino oscillations
The final results of the MACRO experiment on atmospheric neutrino
oscillations are presented and discussed. The data concern different event
topologies with average neutrino energies of ~3 and ~50 GeV. Multiple Coulomb
Scattering of the high energy muons in absorbers was used to estimate the
neutrino energy of each event. The angular distributions, the L/E_nu
distribution, the particle ratios and the absolute fluxes all favour nu_mu -->
nu_tau oscillations with maximal mixing and Delta m^2 =0.0023 eV^2. A
discussion is made on the Monte Carlos used for the atmospheric neutrino flux.
Some results on neutrino astrophysics are also briefly discussed.Comment: Invited Paper at the NANP03 Int. Conf., Dubna, 200
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