This work demonstrates that a hybrid approach for linear thermoacoustic stability analysis that combines the Linearized Navier–Stokes Equations (LNSE) with a global Flame Transfer Function (FTF), ...generates spurious entropy waves when used to model acoustically forced premixed flames. The inability of the global FTF to account for the effects of flame movement is identified as the root cause of this unphysical behavior. Utilization of a local FTF, which resolves unsteady heat release on scales comparable to the reaction zone of the flame, suppresses the spurious entropy perturbations. This affirms that fine-grained resolution of the spatio-temporal distribution of heat release rate fluctuations in the combustion zone is required to model the movement of the flame front, even for acoustically and convectively compact flames. As an alternative to hybrid models, a Linearized Reactive Flow (LRF) approach is employed, which extends the LNSE by the linearized species transport equations as well as the reaction mechanism. Such a monolithic approach inherently accounts for the locally resolved flame dynamics, including the movement of the flame front, and does not require an external model for the flame-flow interaction. Thus the LRF eliminates the need for the cumbersome identification of a local FTF. Two configurations of lean premixed methane-air flames, i.e. a freely propagating 1D flame and a 2D flame anchored in a duct, are considered for validation. All results obtained with linearized modeling approaches and conclusions deduced thereof are validated against high resolution CFD results with excellent quantitative accuracy.
ABSTRACT We report on radio and X-ray observations of the only known repeating Fast Radio Burst (FRB) source, FRB 121102. We have detected six additional radio bursts from this source: five with the ...Green Bank Telescope at 2 GHz, and one at 1.4 GHz with the Arecibo Observatory, for a total of 17 bursts from this source. All have dispersion measures consistent with a single value (∼559 pc cm−3) that is three times the predicted maximum Galactic contribution. The 2 GHz bursts have highly variable spectra like those at 1.4 GHz, indicating that the frequency structure seen across the individual 1.4 and 2 GHz bandpasses is part of a wideband process. X-ray observations of the FRB 121102 field with the Swift and Chandra observatories show at least one possible counterpart; however, the probability of chance superposition is high. A radio imaging observation of the field with the Jansky Very Large Array at 1.6 GHz yields a 5 upper limit of 0.3 mJy on any point-source continuum emission. This upper limit, combined with archival Wide-field Infrared Survey Explorer 22 m and IPHAS H surveys, rules out the presence of an intervening Galactic H ii region. We update our estimate of the FRB detection rate in the PALFA survey to be FRBs sky−1 day−1 (95% confidence) for peak flux density at 1.4 GHz above 300 mJy. We find that the intrinsic widths of the 12 FRB 121102 bursts from Arecibo are, on average, significantly longer than the intrinsic widths of the 13 single-component FRBs detected with the Parkes telescope.
The multielectron reduction of small molecules (e.g., CO2) is a key aspect of fuel synthesis from renewable electricity. Transition metals have been researched extensively in this role due to their ...intrinsic redox properties and reactivity, but more recently, strategies that forego transition metal ions for p-block elements have emerged. In this vein, we report an analogue of boranthrene (9,10-diboraanthracene) stabilized by N-heterocyclic carbenes and its one- and two-electron oxidized congeners. This platform exhibits reversible, two-electron redox chemistry at mild potentials and reacts with O2, CO2, and ethylene via formal 4+2 cycloaddition to the central diborabutadiene core. In an area traditionally dominated by transition metals, these results outline an approach for the redox activation of small molecules at mild potentials based on conjugated, light element scaffolds.
Phasor diagrams are utilized to plot and analyze thermoacoustic modes in a duct with ideal closed/open acoustic boundary conditions that contains a velocity-sensitive flame. Inspection of phasor ...diagrams that represent fluctuations of velocity, pressure, heat release rate and characteristic wave amplitudes at the flame elicits characteristic features of marginally stable, intrinsic thermoacoustic (ITA) modes: the sign of velocity fluctuations and the sign of the gradient of pressure fluctuations change across the flame. These sign changes result from a reversal of direction of the velocity phasor across the flame, affected by unsteady heat release exactly out-of-phase with respect to upstream velocity fluctuations and of sufficient strength. Unlike alternative methods proposed for the identification of ITA modes, the proposed categorization does not involve a parameter sweep, but relies on an analogy with the structure of ITA modes in an anechoic environment. The phasor diagram also elucidates that continuous transitions from acoustic to ITA modes and vice versa can be achieved by an increase or decrease of the gain of the flame transfer function. The representation of acoustic wave propagation in terms of phasors facilitates the formulation of a compact dispersion relation of the thermoacoustic configuration under consideration. The value of the transitional gain of the flame transfer function, where acoustic/ITA transitions occur, may be deduced easily from the dispersion relation. A plot of solution branches of acoustic and ITA modes illustrates how the respective mode transitions, and how they can be related to the well-known quarter wave modes of a closed/open resonator. It is observed that under variation of transfer function gain and phase, marginally stable eigenmodes may occur at almost any frequency, not only at frequencies that correspond to the quarter-wave modes.
The dynamic magnetosphere of Swift J1818.0–1607 Lower, M E; Johnston, S; Shannon, R M ...
Monthly notices of the Royal Astronomical Society,
03/2021, Letnik:
502, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Radio-loud magnetars display a wide variety of radio-pulse phenomenology seldom seen among the population of rotation-powered pulsars. Spectropolarimetry of the radio pulses from these ...objects has the potential to place constraints on their magnetic topology and unveil clues about the magnetar radio emission mechanism. Here, we report on eight observations of the magnetar Swift J1818.0–1607 taken with the Parkes Ultra-Wideband Low receiver covering a wide frequency range from 0.7 to 4 GHz over a period of 5 months. The magnetar exhibits significant temporal profile evolution over this period, including the emergence of a new profile component with an inverted spectrum, two distinct types of radio emission mode switching, detected during two separate observations, and the appearance and disappearance of multiple polarization modes. These various phenomena are likely a result of ongoing reconfiguration of the plasma content and electric currents within the magnetosphere. Geometric fits to the linearly polarized position angle indicate we are viewing the magnetar at an angle of ∼99○ from the spin axis, and its magnetic and rotation axes are misaligned by ∼112○. While conducting these fits, we found the position angle swing had reversed direction on MJD 59062 compared to observations taken 15 d earlier and 12 d later. We speculate this phenomena may be evidence the radio emission from this magnetar originates from magnetic field lines associated with two co-located magnetic poles that are connected by a coronal loop.
Microalgae have the potential to become microbial cell factories for lipid production. Their ability to convert sunlight and CO2 into valuable lipid compounds has attracted interest from cosmetic, ...biofuel, food and feed industries.
In order to make microalgae-derived products cost-effective and commercially competitive, enhanced growth rates and lipid productivities are needed, which require optimization of cultivation systems and strain improvement. Advances in genetic tool development and omics technologies have increased our understanding of lipid metabolism, which has opened up possibilities for targeted metabolic engineering. In this review we provide a comprehensive overview on the developments made to genetically engineer microalgal strains over the last 30 years. We focus on the strategies that lead to an increased lipid content and altered fatty acid profile. These include the genetic engineering of the fatty acid synthesis pathway, Kennedy pathway, polyunsaturated fatty acid and triacylglycerol metabolisms and fatty acid catabolism. Moreover, genetic engineering of specific transcription factors, NADPH generation and central carbon metabolism, which lead to increase of lipid accumulation are also reviewed.
•30 years of genetic engineering for increasing microalgal lipid content is reviewed.•Strategies used to enhance PUFAs and TAGs production are provided.•Engineering of NADPH and central metabolism, TFs and FA catabolism are also revised.
The influence of Intrinsic Thermoacoustic (ITA) feedback on the combustion noise spectrum produced by a confined, turbulent, premixed, swirl flame is investigated. The analysis is based on the ...understanding that sound is generated by unsteady heat release resulting from turbulent fluctuations on the one hand, and from the response of the flame to incoming acoustic perturbations on the other. The former effect is described by a source term for combustion noise, i.e. a spectral distribution of unsteady heat release rate, the latter by the flame transfer function. Both quantities are identified from time series data for fluctuating velocity and heat release rate, generated with large eddy simulation of premix swirl burner. The combustion noise source term and the flame transfer function are then introduced in an acoustic network model of the test rig in order to compute the spectral distribution of the sound pressure level at a certain location in the combustion chamber. Results for the noise spectrum are in good agreement with experiment, showing a broadband component and well-defined peaks. The frequencies of the peaks correspond to either acoustic cavity or ITA resonances. The acoustic network model is used for parametric studies, where the acoustic reflection coefficient at the combustor exit is varied. Remarkably, it is found that the magnitude of the ITA peak increases with decreasing values of the acoustic reflection coefficient, and vice versa. Furthermore, the influence of combustion chamber length on resonance frequencies is explored. It is observed that the frequency of the ITA resonance is insensitive to combustor length. This behaviour is observed qualitatively also in experiments.
We present new 0.6-10 GHz observations of the binary neutron star merger GW170817 covering the period up to 300 days post-merger, taken with the upgraded Karl G. Jansky Very Large Array, the ...Australia Telescope Compact Array, the Giant Metrewave Radio Telescope and the MeerKAT telescope. We use these data to precisely characterize the decay phase of the late-time radio light curve. We find that the temporal decay is consistent with a power-law slope of t−2.2, and that the transition between the power-law rise and decay is relatively sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated) outflow, but is instead the classic signature of a relativistic jet. This provides strong observational evidence that GW170817 produced a successful jet, and directly demonstrates the link between binary neutron star mergers and short-hard gamma-ray bursts. Using simple analytical arguments, we derive constraints on the geometry and the jet opening angle of GW170817. These results are consistent with those from our companion very long baseline interferometry paper, reporting superluminal motion in GW170817.
•A multi-fidelity approach is proposed to identify flame frequency response.•This approach delivers accurate, efficient, and robust identification results.•The approach is further validated using ...dataset of an actual test rig.
Display omitted
Accurate, robust, and efficient identification of flame frequency response (FFR) plays a crucial role in thermoacoustic instability prediction, analysis and control. In order to extract the FFR from high-fidelity numerical simulation time series data, two methods are currently used in the community, which are based on harmonic excitation or broadband excitation, respectively. The former can produce quite accurate FFR estimates even in the presence of significant noise, but only at discrete frequencies; the latter method, which combines broadband forcing and system identification techniques, provides the complete FFR over the frequency range of interest, but may introduce increased levels of uncertainties in the identified results. The present study aims to fully exploit the respective strengths, while avoiding the weaknesses of the two aforementioned methods by proposing a multi-fidelity approach that merges FFR identification results from a short time broadband excitation (low-fidelity) and harmonic excitations at a few select frequencies (high-fidelity). The proposed approach is realized via a machine-learning technique called “Multi-fidelity Gaussian Process.” Our case study demonstrates that the proposed multi-fidelity approach can effectively assimilate the global trend provided by the low-fidelity results and local estimates provided by the high-fidelity results, thus leading to a globally accurate and robust FFR identification even in the presence of strong noise. In addition, we investigate the impact of the number and locations of harmonic forcing frequencies on the performance of the proposed approach. Finally, we employ the proposed multi-fidelity framework to identify the FFR of a turbulent premixed swirl burner test rig based on previously published data, which further highlights the capability and flexibility of the proposed approach in real applications.
Abstract
The 44.7 ms X-ray pulsar in the supernova remnant G12.82−0.02/HESS J1813−178 has the second highest spin-down luminosity of known pulsars in the Galaxy, with
E
̇
=
5.6
×
10
37
erg s
−1
. ...Using the Green Bank Telescope, we have detected radio pulsations from PSR J1813−1749 at 4.4–10.2 GHz. The pulse is highly scattered, with an exponential decay timescale
τ
longer than that of any other pulsar at these frequencies. A point source detected at this position by Dzib et al. in several observations with the Jansky Very Large Array can be attributed to the pulsed emission. The steep dependence of
τ
on observing frequency explains why all previous pulsation searches at lower frequencies failed (
τ
≈ 0.25 s at 2 GHz). The large dispersion measure, DM = 1087 pc cm
−3
, indicates a distance of either 6.2 or 12 kpc according to two widely used models of the electron density distribution in the Galaxy. These disfavor a previously suggested association with a young stellar cluster at the closer distance of 4.8 kpc. The high X-ray measured column density of ≈10
23
cm
−2
also supports a large distance. If
d
≈ 12 kpc, HESS J1813−178 would be one of the most luminous TeV sources in the Galaxy.
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