884 research outputs found
Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics
We present results of numerical simulation of inviscid thick accretion disks
and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH)
technique for this purpose. Formation of thick disks are found to be preceded
by shock waves travelling away from the centrifugal barrier. For a large range
of the parameter space, the travelling shock settles at a distance close to the
location obtained by a one-and-a-half dimensional model of inviscid accretion
disks. Occasionally, it is observed that accretion processes are aided by the
formation of oblique shock waves, particularly in the initial transient phase.
The post-shock region (where infall velocity suddenly becomes very small)
resembles that of the usual model of thick accretion disk discussed in the
literature, though they have considerable turbulence. The flow subsequently
becomes supersonic before falling into the black hole. In a large number of
cases which we simulate, we find the formation of strong winds which are hot
and subsonic when originated from the disk surface very close to the black hole
but become supersonic within a few tens of the Schwarzschild radius of the
blackhole. In the case of accretion of high angular momentum flow, very little
amount of matter is accreted directly onto the black hole. Most of the matter
is, however, first squeezed to a small volume close to the black hole, and
subsequently expands and is expelled as a strong wind. It is quite possible
that this expulsion of matter and the formation of cosmic radio jets is aided
by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press
Smoothed Particle Hydrodynamic Simulations of Viscous Accretion Discs Around Black Holes
Viscous Keplerian discs become sub-Keplerian close to a black hole since they
pass through sonic points before entering into it. We study the time evolution
of polytropic viscous accretion discs (both in one and two dimensional flows)
using Smoothed Particle Hydrodynamics. We discover that for a large region of
the parameter space, when the flow viscosity parameter is less than a critical
value, standing shock waves are formed. If the viscosity is very high then the
shock disappears. In the intermediate viscosity the disc oscillates very
significantly in viscous time-scale. Our simulations indicate that these
centrifugally supported high density region close to a black hole plays an
active role in the flow dynamics, and consequently, the radiation dynamics.Comment: MNRAS style 6 pages of output, macros included. MNRAS (submitted
A site selection model to identify optimal locations for microalgae biofuel production facilities in sicily (Italy)
The lack of sustainability and negative environmental impacts of using fossil fuel resources for energy production and their consequent increase in prices during last decades have led to an increasing interest in the development of renewable biofuels. Among possible biomass fuel sources, microalgae represent one of the most promising solutions. The present work is based on the implementation of a model that facilitates identification of optimal geographic locations for large-scale open ponds for microalgae cultivation for biofuels production. The combination of a biomass production model with specific site location parameters such as irradiance, geographical constraints, land use, topography, temperatures and CO2 for biofuels plants were identified in Sicily (Italy). A simulation of CO2 saved by using the theoretical biofuel produced in place of traditional fuel was implemented. Results indicate that the territory of Sicily offers a good prospective for these technologies and the results identify ideal locations for locating biomass fuel production facilities. Moreover, the research provides a robust method that can be tailored to the specific requirements and data availability of other territories. © Research India Publications
Activity cycles in members of young loose stellar associations
Magnetic cycles have been detected in tens of solar-like stars. The
relationship between the cycle properties and global stellar parameters is not
fully understood yet.
We searched for activity cycles in 90 solar-like stars with ages between 4
and 95 Myr aiming to investigate the properties of activity cycles in this age
range.
We measured the length of a given cycle by analyzing the long-term
time-series of three activity indexes. For each star, we computed also the
global magnetic activity index that is proportional to the amplitude of
the rotational modulation and is a proxy of the mean level of the surface
magnetic activity. We detected activity cycles in 67 stars. Secondary cycles
were also detected in 32 stars. The lack of correlation between and
suggest that these stars belong to the Transitional Branch and that
the dynamo acting in these stars is different from the solar one. This
statement is also supported by the analysis of the butterfly diagrams.
We computed the Spearman correlation coefficient between ,
and different stellar parameters. We found that is
uncorrelated with all the investigated parameters. The index is
positively correlated with the convective turn-over time-scale, the magnetic
diffusivity time-scale , and the dynamo number , whereas
it is anti-correlated with the effective temperature , the
photometric shear and the radius at which
the convective zone is located.
We found that is about constant and that decreases with the
stellare age in the range 4-95 Myr. We investigated the magnetic activity of AB
Dor A by merging ASAS time-series with previous long-term photometric data. We
estimated the length of the AB Dor A primary cycle as .Comment: 19 pages , 15 figures, accepte
Lower limit for differential rotation in members of young loose stellar associations
Surface differential rotation (SDR) plays a key role in dynamo models. SDR
estimates are therefore essential for constraining theoretical models. We
measure a lower limit to SDR in a sample of solar-like stars belonging to young
associations with the aim of investigating how SDR depends on global stellar
parameters in the age range (4-95 Myr). The rotation period of a solar-like
star can be recovered by analyzing the flux modulation caused by dark spots and
stellar rotation. The SDR and the latitude migration of dark-spots induce a
modulation of the detected rotation period. We employ long-term photometry to
measure the amplitude of such a modulation and to compute the quantity
DeltaOmega_phot =2p/P_min -2pi/P_max that is a lower limit to SDR. We find that
DeltaOmega_phot increases with the stellar effective temperature and with the
global convective turn-over time-scale tau_c. We find that DeltaOmega_phot is
proportional to Teff^2.18pm 0.65 in stars recently settled on the ZAMS. This
power law is less steep than those found by previous authors, but closest to
recent theoretical models. We find that DeltaOmega_phot steeply increases
between 4 and 30 Myr and that itis almost constant between 30 and 95 Myr in a 1
M_sun star. We find also that the relative shear increases with the Rossby
number Ro. Although our results are qualitatively in agreement with
hydrodynamical mean-field models, our measurements are systematically higher
than the values predicted by these models. The discrepancy between
DeltaOmega_phot measurements and theoretical models is particularly large in
stars with periods between 0.7 and 2 d. Such a discrepancy, together with the
anomalous SDR measured by other authors for HD 171488 (rotating in 1.31 d),
suggests that the rotation period could influence SDR more than predicted by
the models.Comment: 23 pages, 15 figures, 5 tables,accepted by Astronomy and Astrophysic
Numerical and experimental analysis of micro HAWTs designed for wind tunnel applications
In this paper the authors describe a design and optimization process of micro HAWTs using a numerical and experimental approach. An in-house 1D BEM model was used to obtain a first geometrical draft. It allowed to quickly optimize blade geometry to maximize energy production as well. As these models are quite sensitive to airfoil coefficients, above all at low Reynolds numbers, an accurate 3D CFD model was developed to support and validate the 1D BEM design, analyzing and fixing the discrepancies between model output. The 3D CFD model was developed and optimized using ANSYS Fluent solver and a RANS transition turbulence model. This allowed to correctly reproduce the transition and stall phenomena that characterize the aerodynamic behavior of micro wind turbines, solving the issues related to low Reynolds flows. The procedure was completed, thus building two micro HAWTs with different scales, testing them in the subsonic wind tunnel of the University of Catania. Wind tunnel features, experimental set-up and testing procedures are presented in the paper. Through the comparison of numerical CFD and experimental test results, a good compatibility was found. This allowed the authors to analyze and compare numerical calculation results and verify blockage effects on the prototypes as well
Flow similitude laws applied to wind turbines through blade element momentum theory numerical codes
This paper deals with the analysis of the per- formance of different wind turbines using the Similitude Theory. Wind turbine performance was determined as a function of geometrical similarity coefficient, which is related to all parameters of the Similitude Theory. There- fore, a mathematical model simplification is possible in the 'in similitude' wind turbines comparison. The mathemati- cal model for wind turbine performance is based on BEM Theory, and its efficacy was verified several times by comparing different wind turbine experimental data. The original mathematical model was modified to take into account Similitude Theory parameters. The model is able to determine which wind turbine is most suited to particular design specification. This work presents power and torque curves, power and torque coefficients as functions of rotational speed and wind velocity. All the results are function of the geometrical similarity coefficient. With this methodology it is possible to maximize the power coeffi- cients of a wind turbine, and it is possible to identify a family of wind turbines, geometrically different, but with the same high performances
HAWT Design and Performance Evaluation: Improving the BEM Theory Mathematical Modelsâ
Abstract This paper presents an improved numerical code based on BEM theory, implemented to evaluate the performance of a HAWT (Horizontal Axis Wind Turbines). This numerical code is a 1-D code characterized by fast processing times and reliable results. The critical aspects of the codes based on the BEM theory are widely known in scientific literature. In this paper, the authors explain how to resolve these aspects. One of these is represented by the radial flow along the blades. Radial flow is a 3-D flow, but can be dealt with inside a 1-D code only using a mathematical expedient. This expedient was tested and validated for the Riso test turbine LM 8.2 (with the NACA 63 x -2xx airfoil series along the blades). Radial flow along the blades is taken into account, thus increasing the experimental C L distribution in the stalled aerodynamic region, based on CFD 3D results. The mathematical equation adopted to describe the C L distribution of the NACA 63 x -2xx airfoil is a fifth order logarithmic polynomial. With this numerical code, the mechanical power curve of the Riso test turbine has been calculated, and then compared with the experimental curve found in scientific literature
Numerical and experimental analysis of micro HAWTs designed for wind tunnel applications
In this paper the authors describe a design and optimization process of micro HAWTs using a numerical and experimental approach. An in-house 1D BEM model was used to obtain a first geometrical draft. It allowed to quickly optimize blade geometry to maximize energy production as well. As these models are quite sensitive to airfoil coefficients, above all at low Reynolds numbers, an accurate 3D CFD model was developed to support and validate the 1D BEM design, analyzing and fixing the discrepancies between model output. The 3D CFD model was developed and optimized using ANSYS Fluent solver and a RANS transition turbulence model. This allowed to correctly reproduce the transition and stall phenomena that characterize the aerodynamic behavior of micro wind turbines, solving the issues related to low Reynolds flows. The procedure was completed, thus building two micro HAWTs with different scales, testing them in the subsonic wind tunnel of the University of Catania. Wind tunnel features, experimental set-up and testing procedures are presented in the paper. Through the comparison of numerical CFD and experimental test results, a good compatibility was found. This allowed the authors to analyze and compare numerical calculation results and verify blockage effects on the prototypes as well
Design of a vertical-axis wind turbine: how the aspect ratio affects the turbine's performance
This work analyses the link between the aspect ratio of a vertical-axis straight-bladed (H-Rotor) wind turbine and its performance (power coefficient). The aspect ratio of this particular wind turbine is defined as the ratio between blade length and rotor radius. Since the aspect ratio variations of a vertical-axis wind turbine cause Reynolds number variations, any changes in the power coefficient can also be studied to derive how aspect ratio variations affect turbine performance. Using a calculation code based on the Multiple Stream Tube Model, symmetrical straight-bladed wind turbine performance was evaluated as aspect ratio varied. This numerical analysis highlighted how turbine performance is strongly influenced by the Reynolds number of the rotor blade. From a geometrical point of view, as aspect ratio falls, the Reynolds number rises which improves wind turbine performance
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