Helicity hardens the gas

A screw generally works better than a nail, or a complicated rope knot better than a simple one, in fastening solid matter, but a gas is more tameless. However, a flow itself has a physical quantity, helicity, measuring the screwing strength of the velocity field and the degree of the knottedness of the vorticity ropes. It is shown that helicity favors the partition of energy to the vortical modes, compared to others such as the dilatation and pressure modes of turbulence; that is, helicity stiffens the flow, with nontrivial implications for aerodynamics, such as aeroacoustics, and conducting fluids, among others

N′-[6-(3,5-Dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetra­zin-3-yl]butano­hydrazide

In the title compound, C11H16N8O, the tetra­zine and pyrazole rings form a dihedral angle of 48.75 (2)°. In the crystal, N—H⋯O and N—H⋯N hydrogen bonds link the mol­ecules into layers parallel to (101)

Color Behavior Of BL Lacertae Object OJ 287 During Optical Outburst

This paper aims to study the color behavior of the BL Lac object OJ 287 during optical outburst. According to the revisit of the data from the OJ-94 monitoring project and the analysis the data obtained with the 60/90 cm Schmidt Telescope of NAOC, we found a bluer-when-brighter chromatism in this object. The amplitude of variation tends to decrease with the decrease of frequency. These results are consistent with the shock-in-jet model. We made some simulations and confirmed that both amplitude difference and time delay between variations at different wavelengths can result in the phenomenon of bluer-when-brighter. Our observations confirmed that OJ 287 underwent a double-peaked outburst after about 12 years from 1996, which provides further evidence for the binary black hole model in this object.Comment: 25 pages, 13 figure