Simultaneous dual-frequency radio observations of S5 0716+714: A search for intraday variability with the Korean VLBI Network

This study aims to search for the existence of intraday variability (IDV) of BL Lac object S5 0716+714 at high radio frequencies for which the interstellar scintillation effect is not significant. Using the 21-meter radio telescope of the Korean VLBI Network (KVN), we present results of multi-epoch simultaneous dual-frequency radio observations. Single-dish observations of S5 0716+714 were simultaneously conducted at 21.7 GHz (K-band) and 42.4 GHz (Q-band), with a high cadence of 30-60 minute intervals.We observed four epochs between December 2009 and June 2010. Over the whole set of observation epochs, S5 0716+714 showed significant inter-month variations in flux density at both the K- and Q-bands, with modulation indices of approximately 19% for the K-band and approximately 36% for the Q-band. In all epochs, no clear intraday variability was detected at either frequency. The source shows monotonic flux density increase in epochs 1 and 3 and monotonic flux density decrease in epochs 2 and 4. In the flux density increasing phases, the flux densities at the Q-band increase more rapidly. In the decreasing phase, no significant flux density difference is seen at the two frequencies. The situation could be different close to flux density peaks that we did not witness in our observations. We find an inverted spectrum with mean spectral indices of -0.57+-0.13 in epoch 1 and -0.15+-0.11 in epoch 3. On the other hand, we find relatively steep indices of +0.24+-0.14 and +0.17+-0.18 in epochs 2 and 4, respectively. We conclude that the frequency dependence of the variability and the change of the spectral index are caused by source-intrinsic effects rather than by any extrinsic scintillation effect.Comment: 6 pages and 4 figures and 4 table

Characteristics of Azimuthal Thermoacoustic Instabilities in Multi-Nozzle Can Combustors

To satisfy stringent NOx emission restrictions, modern gas turbines have developed toward lean, premixed combustion systems, which introduce a new challenge called 'thermoacoustic instability'. This instability limits the turbine's operating conditions, reduces hardware lifetimes, and eventually destroys the combustor hardware. Therefore, the importance of understanding thermoacoustic phenomena in gas turbine combustors has increased sharply. This study focuses on azimuthal instability in a multi-nozzle can combustor. This azimuthal mode can be decomposed into two counter-rotating waves where they can either compete and potentially suppress one of them (spinning wave) or coexist (standing wave), depending on the operating conditions. To identify its modal nature (standing vs spinning), multiple pressure sensors must be installed around the circumference. This study first addresses the question of how to optimally locate the sensors for identifying the acoustic mode. With this sensor configuration, the study demonstrates the experimental results showing how the instability amplitude and modal nature vary with operating conditions, such as thermal power and azimuthal non-uniformities. As a final step, this study develops a low order modeling that captures important experimental observations. The contributions from each works are essential to monitor the azimuthal thermoacoustic instability, provide mitigation strategy, and develop a model for stability margin. We believe that the works presented here will be greatly beneficial to combustion systems experiencing thermoacoustic issues.Ph.D

Bis[(2,2-dimethyl­propano­yloxy)meth­yl] {[2-(6-amino-9H-purin-9-yl)eth­oxy]meth­yl}phospho­nate–succinic acid (2/1)

The title compound, C20H32N5O8P·0.5C4H6O4, is composed of two 9-{2-[bis­(pivaloyloxymeth­oxy)phosphinylmeth­oxy]eth­yl}adenine, commonly known as adefovir dipivoxil (AD), mol­ecules linked to the carb­oxy­lic acid groups of succinic acid (SA). The asymmetric unit contains one mol­ecule of AD and half a mol­ecule of SA, which sits on an inversion center. Both adenine units in the two AD mol­ecules make AD–SA N—H⋯O and SA–AD O—H⋯N hydrogen bonds to SA. In addition, the inter­molecular AD–AD N—H⋯O—P hydrogen bond serves to stabilize the cocrystal. There is also a π–π stacking inter­action [inter­planar spacing 3.34 (19) Å] between adjacent inversion-related adenine groups