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The absolute temperature of the cosmic microwave background (CMB) has been measured at a frequency of 1280 MHz. The observation was made with a modified version of the L-band receiver used in the Giant Metre wavelength Radio Telescope (GMRT): the feed horn was replaced by a corrugated plate and the receiver was placed on the ground, directed at zenith, and shielded from ground radiation by an aluminium screen with corrugated edges. Novel techniques have been adopted for ?reducing and cancelling unwanted contributions to the system temperature of the receiver and ?calibrating the contributions from the feed assembly and receiver. The thermodynamic temperature of the CMB is estimated to be 3.45 ± 0.78 K. 相似文献
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Predicting the velocity within the ship’s propeller jet is the initial step to investigate the scouring made by the propeller jet. Albertson et al. (1950) suggested the investigation of a submerged jet can be undertaken through observation of the plain water jet from an orifice. The plain water jet investigation of Albertson et al. (1950) was based on the axial momentum theory. This has been the basis of all subsequent work with propeller jets. In reality, the velocity characteristic of a ship’s propeller jet is more complicated than a plain water jet. Fuehrer and Römisch (1977), Blaauw and van de Kaa (1978), Berger et al. (1981), Verhey (1983) and Hamill (1987) have carried out investigations using physical model. Current paper reviews the state-of-art of the equations used to predict the time-averaged axial, tangential and radial components of velocity within the zone of flow establishment and the zone of established flow of a ship’s propeller jet. 相似文献
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Saumyadip Samui Raghunathan Srianand Kandaswamy Subramanian 《Monthly notices of the Royal Astronomical Society》2009,398(4):2061-2068
We present a semi-analytical model of star formation which explains simultaneously the observed ultraviolet (UV) luminosity function (LF) of high-redshift Lyman break galaxies (LBGs) and LFs of Lyman α emitters. We consider both models that use the Press–Schechter (PS) and Sheth–Tormen (ST) halo mass functions to calculate the abundances of dark matter haloes. The Lyman α LFs at z ≲ 4 are well reproduced with only ≲10 per cent of the LBGs emitting Lyman α lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed LF at z > 5 can be reproduced only when we assume that nearly all LBGs are Lyman α emitters. Thus, it appears that 4 < z < 5 marks the epoch when a clear change occurs in the physical properties of the high-redshift galaxies. As Lyman α escape depends on dust and gas kinematics of the interstellar medium (ISM), this could mean that on an average the ISM at z > 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman α photons. At z > 5 , the observed Lyman α LF are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high-redshift galaxies. In particular, up to z = 6.5 , we do not see the effect of evolving intergalactic medium opacity on the Lyman α escape from these galaxies. 相似文献
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A. Aghaee P. Petitjean R. Srianand C. S. Stalin R. Guimarães 《Journal of Astrophysics and Astronomy》2010,31(2):59-79
In recent years, the Lyman-α forest in quasar spectra has been used, together with N-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters
to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the
latter, one has first to fit the quasar continuum. We have observed 20 high redshift and highly luminous QSOs (m
V ≤ 17.5 and 2.40 ≤ z
em ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC
at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample.
We have measured the amount of absorption, known as the flux decrement, DA, in the Lyman-α forest for these different methods and compared the results. In addition, we have compared DA values measured along the same lines of sight observed at high and intermediate spectral resolutions. We discuss the systematics
resulting from the use of automatic continuum fitting methods. 相似文献
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Wei-Haur Lam Gerard Hamill Desmond Robinson Srinivasan Raghunathan 《Ocean Engineering》2010,37(14-15):1380-1388
The present paper was aimed at presenting the time-averaged velocity and turbulence intensity at the initial plane from a ship's propeller. The flow characteristics of a ship's propeller jet are of particular interest for the researchers investigating the jet induced seabed damage as documented in the previous studies. Laser Doppler Anemometry (LDA) measurements show that the axial component of velocity is the main contributor to the velocity magnitude at the initial plane of a ship's propeller jet. The tangential component contributes to the rotation while the radial component which contributes to the diffusion, are the second and third largest contributors to the velocity magnitude. The maximum tangential and radial velocity components at the initial plane are approximately 82% and 14% of the maximum axial velocity component, respectively. The axial velocity distribution at the initial plane shows two peaked ridges with a low velocity core at the rotation axis. The turbulence intensity distribution shows a three-peaked profile at the initial plane. 相似文献
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S. Narayanan N. Raghunathan U. B. Nayak A. K. Lahiri 《International Journal of Mineral Processing》1983,10(4):309-317
The effect of surface-active agents in improving the grinding efficiency for haematite, expressed as a measure of reduction ratio, has been studied. Drying of the feed before grinding has been found to improve the grinding efficiency. The effect of surfactant additions on hardness, microhardness, crushing strength and wet grinding of haematite has shown that dispersants are better than flocculants, and each dispersant has a critical concentration at which the grinding efficiency is maximum. 相似文献
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