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We discuss the feasibility of the detection of the 21 cm forest in the diffuse intergalactic medium (IGM) with the radio telescope LOFAR. The optical depth to the 21 cm line has been derived using simulations of reionization which include detailed radiative transfer of ionizing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV versus X-ray heating) we do not expect to distinguish them by means of observations of the 21 cm forest. Because the presence of a strong X-ray background would make the detection of the 21 cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the X-ray background and the thermal history of the Universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z ≳ 12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high-density pockets of gas could also be detected at z ∼ 7 along preferential lines of sight. [ABSTRACT FROM PUBLISHER]

ABSTRACT We investigate the thermal history of the intergalactic medium (IGM) in the redshift interval z = 1.7-3.2 by studying the small-scale fluctuations in the Lyman α forest transmitted flux. We apply a wavelet filtering technique to 18 high-resolution quasar spectra obtained with the Ultraviolet and Visual Echelle Spectrograph, and compare these data to synthetic spectra drawn from a suite of hydrodynamical simulations in which the IGM thermal state and cosmological parameters are varied. From the wavelet analysis we obtain estimates of the IGM thermal state that are in good agreement with other recent, independent wavelet-based measurements. We also perform a reanalysis of the same data set using the Lyman α forest flux probability distribution function (PDF), which has previously been used to measure the IGM temperature-density relation. This provides an important consistency test for measurements of the IGM thermal state, as it enables a direct comparison of the constraints obtained using these two different methodologies. We find the constraints obtained from wavelets and the flux PDF are formally consistent with each other, although in agreement with previous studies, the flux PDF constraints favour an isothermal or inverted IGM temperature-density relation. We also perform a joint analysis by combining our wavelet and flux PDF measurements, constraining the IGM thermal state at z = 2.1 to have a temperature at mean density of T0/[103 K] = 17.3 ± 1.9 and a power-law temperature-density relation exponent γ = 1.1 ± 0.1 (1σ). Our results are consistent with previous observations that indicate there may be additional sources of heating in the IGM at z < 4. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a measurement of the Lyman α flux probability distribution function (PDF) obtained from a set of eight high-resolution quasar spectra with emission redshifts in the range 3.3 ≤ z≤ 3.8. We carefully study the effect of metal absorption lines on the shape of the PDF. Metals have a larger impact on the PDF measurements at lower redshift, where there are relatively fewer Lyman α absorption lines. This may be explained by an increase in the number of metal lines that are blended with Lyman α absorption lines towards higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium (IGM) at earlier times. We also provide a new measurement of the redshift evolution of the effective optical depth, τeff, at 2.8 ≤ z≤ 3.6, and find no evidence for a deviation from a power-law evolution in the log (τeff)-log (1 + z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the IGM at z≃ 3. By comparing the PDF to state-of-the-art cosmological hydrodynamical simulations, we place constraints on the temperature of the IGM and compare our results with previous measurements of the PDF at lower redshift. At redshift z= 3, our new PDF measurements are consistent with an isothermal temperature-density relation, T= T0Δγ− 1, with a temperature at the mean density of T0= 19 250 ± 4800 K and a slope γ= 0.90 ± 0.21 (1σ uncertainties). In comparison, joint constraints with existing lower redshift PDF measurements at z < 3 favour an inverted temperature-density relation with T0= 17 900 ± 3500 K and γ= 0.70 ± 0.12, in broad agreement with previous analyses. [ABSTRACT FROM AUTHOR]

In this paper, the effects of fuel injection parameters on combustion-induced noise in a small single-cylinder direct-injection diesel engine used in a generator set are described. A range of fuel injection parameters, including injection timing and injection quantity, was explored to understand their influence on combustion-induced noise by studying their effect on in-cylinder pressure development. Fourier analysis was performed to understand the effects of changing combustion characteristics on in-cylinder pressure levels in different frequency ranges. In addition, time–frequency analysis was used to understand the changes in the combustion chamber resonance effects under different injection settings. Finally, overall engine noise levels were measured to confirm the changes as predicted by the in-cylinder pressure analysis. Results suggest that retarding main injection has the largest impact on reducing noise. When using split injection, earlier preinjection and larger preinjection quantities tend to result in lower noise. [ABSTRACT FROM PUBLISHER]

Acoustical holography procedures make high-resolution visualization possible via estimation of the sound intensity on surfaces closer to the sources than the near-field measurement surface. Another source localization technique, beamforming, has been used to estimate the direction of arrival of sound from sources that typically lie in the far-field. However, little work has been done using beamforming as a visualization technique based on near-field measurements. As a result, the performance of beamforming and acoustical holography in terms of source resolution capabilities has not been directly compared when using near-field measurements. In this work, point source beamforming was used to visualize sources based on near-field measurements. Acoustic intensity estimated from beamformed pressure measurements was compared with the absolute intensity estimated using acoustical holography techniques. In addition to noise-free, anechoic simulations, cases of measurement pressure with random noise-were generated and used to compare source resolution accuracy of acoustical holography and beamforming techniques in the presence of measurement noise. It was found that intensity estimated using acoustical holography provided the clearest image of sources when the measurement surface was conformal with the source geometry. However, sources can be resolved more accurately using near-field beamforming than acoustical holography at high frequencies when the sources are not located perfectly on a surface conformal with the measurement geometry. [ABSTRACT FROM AUTHOR]