7,143 research outputs found
Tiltrotor CFD part I: validation
This paper presents performance analyses of the model-scale ERICA and TILTAERO tiltrotors and of the full-scale XV-15 rotor with high-fidelity computational fluids dynamics. For the ERICA tiltrotor, the overall effect of the blades on the fuselage was well captured, as demonstrated by analysing surface pressure measurements. However, there was no available experimental data for the blade aerodynamic loads. A comparison of computed rotor loads with experiments was instead possible for the XV-15 rotor, where CFD results predicted the FoM within 1.05%. The method was also able to capture the differences in performance between hover and propeller modes. Good agreement was also found for the TILTAERO loads. The overall agreement with the experimental data and theory for the considered cases demonstrates the capability of the present CFD method to accurately predict tiltrotor flows. In a second part of this work, the validated method is used for blade shape optimisation
Variations of the solar granulation motions with height using the GOLF/SoHO experiment
Below 1 mHz, the power spectrum of helioseismic velocity measurements is
dominated by the spectrum of convective motions (granulation and
supergranulation) making it difficult to detect the low-order acoustic modes
and the gravity modes. We want to better understand the behavior of solar
granulation as a function of the observing height in the solar atmosphere and
with magnetic activity during solar cycle 23. We analyze the Power Spectral
Density (PSD) of eleven years of GOLF/SOHO velocity-time series using a
Harvey-type model to characterize the properties of the convective motions in
the solar oscillation power spectrum. We study then the evolution of the
granulation with the altitude in the solar atmosphere and with the solar
activity. First, we show that the traditional use of a lorentzian profile to
fit the envelope of the p modes is not well suitable for GOLF data. Indeed, to
properly model the solar spectrum, we need a second lorentzian profile. Second,
we show that the granulation clearly evolves with the height in the photosphere
but does not present any significant variation with the activity cycle.Comment: Paper accepted in A&A. 7 pages, 4 figures, 2 table
Analysis of the acoustic cut-off frequency and HIPs in six Kepler stars with stochastically excited pulsations
Gravito-acoustic modes in the Sun and other stars propagate in resonant
cavities with a frequency below a given limit known as the cut-off frequency.
At higher frequencies, waves are no longer trapped in the stellar interior and
become traveller waves. In this article we study six pulsating solar-like stars
at different evolutionary stages observed by the NASA Kepler mission. These
high signal-to-noise targets show a peak structure that extends at very high
frequencies and are good candidates for studying the transition region between
the modes and the interference peaks or pseudo-modes. Following the same
methodology successfully applied on Sun-as-a-star measurements, we uncover the
existence of pseudo-modes in these stars with one or two dominant interference
patterns depending on the evolutionary stage of the star. We also infer their
cut-off frequency as the midpoint between the last eigenmode and the first peak
of the interference patterns. By using ray theory we show that, while the
period of one of the interference pattern is very close to half the large
separation the other, one depends on the time phase of mixed waves, thus
carrying additional information on the stellar structure and evolution.Comment: Accepted for publication in A&A. 14 pages, 28 figure
Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations
Accurate determination of the rotation rate in the radiative zone of the sun
from helioseismic observations requires rotational frequency splittings of
exceptional quality as well as reliable inversion techniques. We present here
inferences based on mode parameters calculated from 2088-days long MDI, GONG
and GOLF time series that were fitted to estimate very low frequency rotational
splittings (nu < 1.7 mHz). These low frequency modes provide data of
exceptional quality, since the width of the mode peaks is much smaller than the
rotational splitting and hence it is much easier to separate the rotational
splittings from the effects caused by the finite lifetime and the stochastic
excitation of the modes. We also have implemented a new inversion methodology
that allows us to infer the rotation rate of the radiative interior from mode
sets that span l=1 to 25. Our results are compatible with the sun rotating like
a rigid solid in most of the radiative zone and slowing down in the core (R_sun
< 0.2). A resolution analysis of the inversion was carried out for the solar
rotation inverse problem. This analysis effectively establishes a direct
relationship between the mode set included in the inversion and the sensitivity
and information content of the resulting inferences. We show that such an
approach allows us to determine the effect of adding low frequency and low
degree p-modes, high frequency and low degree p-modes, as well as some g-modes
on the derived rotation rate in the solar radiative zone, and in particular the
solar core. We conclude that the level of uncertainties that is needed to infer
the dynamical conditions in the core when only p-modes are included is unlikely
to be reached in the near future, and hence sustained efforts are needed
towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19
figure
Tiltrotor CFD part II: aerodynamic optimisation of tiltrotor blades
This paper presents aerodynamic optimisation of tiltrotor blades with high-fidelity computational
fluid dynamics. The employed optimisation framework is based on a quasi-Newton
method, and the required high-fidelity flow gradients were computed using a discrete adjoint
solver. Single-point optimisations were first performed, to highlight the contrasting requirements
of the helicopter and aeroplane flight regimes. It is then shown how a trade-off blade
design can be obtained using a multi-point optimisation strategy. The parametrisation of the
blade shape allowed to modify the twist and chord distributions, and to introduce a swept tip.
The work shows how these main blade shape parameters influence the optimal performance
of the tiltrotor in helicopter and aeroplane modes, and how a compromise blade shape can
increase the overall tiltrotor performance. Moreover, in all the presented cases, the accuracy
of the adjoint gradients resulted in a small number of flow evaluations for finding the optimal
solution, thus indicating gradient-based optimisation as a viable tool for modern tiltrotor
design
Investigating magnetic activity of F stars with the it Kepler mission
The dynamo process is believed to drive the magnetic activity of stars like
the Sun that have an outer convection zone. Large spectroscopic surveys showed
that there is a relation between the rotation periods and the cycle periods:
the longer the rotation period is, the longer the magnetic activity cycle
period will be. We present the analysis of F stars observed by Kepler for which
individual p modes have been measure and with surface rotation periods shorter
than 12 days. We defined magnetic indicators and proxies based on photometric
observations to help characterise the activity levels of the stars. With the
Kepler data, we investigate the existence of stars with cycles (regular or
not), stars with a modulation that could be related to magnetic activity, and
stars that seem to show a flat behaviour.Comment: 2 pages, 1 figure, proceedings of IAU Symposium 302 'Magnetic fields
through stellar evolution', 25-30 August 2013, Biarritz, Franc
Detection of periodic signatures in the solar power spectrum. On the track of l=1 gravity modes
In the present work we show robust indications of the existence of g modes in
the Sun using 10 years of GOLF data. The present analysis is based on the
exploitation of the collective properties of the predicted low-frequency (25 to
140 microHz) g modes: their asymptotic nature, which implies a quasi
equidistant separation of their periods for a given angular degree (l). The
Power Spectrum (PS) of the Power Spectrum Density (PSD), reveals a significant
structure indicating the presence of features (peaks) in the PSD with near
equidistant periods corresponding to l=1 modes in the range n=-4 to n=-26. The
study of its statistical significance of this feature was fully undertaken and
complemented with Monte Carlo simulations. This structure has a confidence
level better than 99.86% not to be due to pure noise. Furthermore, a detailed
study of this structure suggests that the gravity modes have a much more
complex structure than the one initially expected (line-widths, magnetic
splittings...). Compared to the latest solar models, the obtained results tend
to favor a solar core rotating significantly faster than the rest of the
radiative zone. In the framework of the Phoebus group, we have also applied the
same methodology to other helioseismology instruments on board SoHO and ground
based networks.Comment: Proceedings of the SOHO-18/GONG2006/HELAS I: Beyond the spherical Su
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