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SPICA—A Large Cryogenic Infrared Space Telescope: Unveiling the Obscured Universe
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- P. R. Roelfsema, H. Shibai, L. Armus, D. Arrazola, M. Audard, M. D. Audley, C.M. Bradford, I. Charles, P. Dieleman, Y. Doi, L. Duband, M. Eggens, J. Evers, I. Funaki, J. R. Gao, M. Giard, A. di Giorgio, L. M. González Fernández, M. Griffin, F. P. Helmich, R. Hijmering, R. Huisman, D. Ishihara, N. Isobe, B. Jackson, H. Jacobs, W. Jellema, I. Kamp, H. Kaneda, M. Kawada, F. Kemper, F. Kerschbaum, P. Khosropanah, K. Kohno, P. P. Kooijman, O. Krause, J. van der Kuur, J. Kwon, W. M. Laauwen, G. de Lange, B. Larsson, D. van Loon, S. C. Madden, H. Matsuhara, F. Najarro, T. Nakagawa, D. Naylor, H. Ogawa, T. Onaka, S. Oyabu, A. Poglitsch, V. Reveret, L. Rodriguez, L. Spinoglio, I. Sakon, Y. Sato, K. Shinozaki, R. Shipman, H. Sugita, T. Suzuki, F. F. S. van der Tak, J. Torres Redondo, T. Wada, S. Y. Wang, C. K. Wafelbakker, H. van Weers, S. Withington, B. Vandenbussche, T. Yamada, I. Yamamura
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- Journal:
- Publications of the Astronomical Society of Australia / Volume 35 / 2018
- Published online by Cambridge University Press:
- 28 August 2018, e030
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Measurements in the infrared wavelength domain allow direct assessment of the physical state and energy balance of cool matter in space, enabling the detailed study of the processes that govern the formation and evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions revealed a great deal about the obscured Universe, but were hampered by limited sensitivity.
SPICA takes the next step in infrared observational capability by combining a large 2.5-meter diameter telescope, cooled to below 8 K, with instruments employing ultra-sensitive detectors. A combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With mechanical coolers the mission lifetime is not limited by the supply of cryogen. With the combination of low telescope background and instruments with state-of-the-art detectors SPICA provides a huge advance on the capabilities of previous missions.
SPICA instruments offer spectral resolving power ranging from R ~50 through 11 000 in the 17–230 μm domain and R ~28.000 spectroscopy between 12 and 18 μm. SPICA will provide efficient 30–37 μm broad band mapping, and small field spectroscopic and polarimetric imaging at 100, 200 and 350 μm. SPICA will provide infrared spectroscopy with an unprecedented sensitivity of ~5 × 10−20 W m−2 (5σ/1 h)—over two orders of magnitude improvement over what earlier missions. This exceptional performance leap, will open entirely new domains in infrared astronomy; galaxy evolution and metal production over cosmic time, dust formation and evolution from very early epochs onwards, the formation history of planetary systems.
On sound generation by the interaction between turbulence and a cascade of airfoils with non-uniform mean flow
- I. EVERS, N. PEAKE
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- Journal:
- Journal of Fluid Mechanics / Volume 463 / 25 July 2002
- Published online by Cambridge University Press:
- 31 July 2002, pp. 25-52
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The sound generated by the interaction between a turbulent rotor wake and a stator is modelled by considering the gust response of a cascade of blades in non-uniform, subsonic mean flow. Previous work by Hanson & Horan (1998) that considers a cascade of flat plates at zero incidence is extended to take into account blade geometry and angle of attack. Our approach is based on the work of Peake & Kerschen (1997), who calculate the forward radiation due to the interaction between a single vortical gust and a cascade of flat plates at non-zero angle of attack. The extensions completed in this present paper are two-fold: first we include the effects of small but non-zero camber and thickness; and second we produce uniformly valid approximations which predict the upstream radiation near modal cut-off. The thin-airfoil singularity in the steady flow at each leading edge is crucial in our model of the sound generation. A new analytical expression for the coefficient of this singularity is derived via a sequence of conformal mappings, and it turns out that in our asymptotic limit this is the only quantity which needs to be calculated from the steady flow in order to predict time-averaged noise levels. Once the response to a single gust has been completed, we use Hanson & Horan (1998)'s approach to determine the response to an incident turbulent spectrum, and find that as well as the noise corresponding to the auto-correlation of the gust velocity component normal to the blade, there is also a contribution from the cross-correlation of the normal and tangential velocities. Predictions are made of the effects of blade geometry on the upstream acoustic power level. The blade geometry can have a very significant effect on the noise generated by interaction with a single gust, with changes of up to 10 dB from the flat-plate noise levels. However, once these gust results have been integrated over a full incident turbulence spectrum the effects of the geometry are rather smaller, although still potentially significant, leading to changes of up to about 2 dB from the flat-plate results. The implication of all this is that the blade geometry can have a significant effect on the tonal noise components generated by rotor–stator interaction (i.e. by single harmonic gusts), but that the broadband part of the noise spectrum is relatively unaffected.
Noise generation by high-frequency gusts interacting with an airfoil in transonic flow
- I. EVERS, N. PEAKE
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- Journal:
- Journal of Fluid Mechanics / Volume 411 / 25 May 2000
- Published online by Cambridge University Press:
- 25 May 2000, pp. 91-130
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The method of matched asymptotic expansions is used to describe the sound generated by the interaction between a short-wavelength gust (reduced frequency k, with k [Gt ] 1) and an airfoil with small but non-zero thickness, camber and angle of attack (which are all assumed to be of typical size O(δ), with δ [Lt ] 1) in transonic flow. The mean-flow Mach number is taken to differ from unity by O(δ2/3), so that the steady flow past the airfoil is determined using the transonic small-disturbance equation. The unsteady analysis is based on a linearization of the Euler equations about the mean flow. High-frequency incident vortical and entropic disturbances are considered, and analogous to the subsonic counterpart of this problem, asymptotic regions around the airfoil highlight the mechanisms that produce sound. Notably, the inner region round the leading edge is of size O(k−1), and describes the interaction between the mean-flow gradients and the incident gust and the resulting acoustic waves. We consider the preferred limit in which kδ2/3 = O(1), and calculate the first two terms in the phase of the far-field radiation, while for the directivity we determine the first term (δ = 0), together with all higher-order terms which are at most O(δ2/3) smaller – in fact, this involves no fewer than ten terms, due to the slowly-decaying form of the power series expansion of the steady flow about the leading edge. Particular to transonic flow is the locally subsonic or supersonic region that accounts for the transition between the acoustic field downstream of a source and the possible acoustic field upstream of the source. In the outer region the sound propagation has a geometric-acoustics form and the primary influence of the mean-flow distortion appears in our preferred limit as an O(1) phase term, which is particularly significant in view of the complicated interference between leading- and trailing-edge fields. It is argued that weak mean- flow shocks have an influence on the sound generation that is small relative to the effects of the leading-edge singularity.