1,688 research outputs found
Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows
The cooling flow problem is one of the central problems in galaxy clusters,
and active galactic nucleus (AGN) feedback is considered to play a key role in
offsetting cooling. However, how AGN jets heat and suppress cooling flows
remains highly debated. Using an idealized simulation of a cool-core cluster,
we study the development of central cooling catastrophe and how a subsequent
powerful AGN jet event averts cooling flows, with a focus on complex
gasdynamical processes involved. We find that the jet drives a bow shock, which
reverses cooling inflows and overheats inner cool core regions. The shocked gas
moves outward in a rarefaction wave, which rarefies the dense core and
adiabatically transports a significant fraction of heated energy to outer
regions. As the rarefaction wave propagates away, inflows resume in the cluster
core, but a trailing outflow is uplifted by the AGN bubble, preventing gas
accumulation and catastrophic cooling in central regions. Inflows and trailing
outflows constitute meridional circulations in the cluster core. At later
times, trailing outflows fall back to the cluster centre, triggering central
cooling catastrophe and potentially a new generation of AGN feedback. We thus
envisage a picture of cool cluster cores going through cycles of
cooling-induced contraction and AGN-induced expansion. This picture naturally
predicts an anti-correlation between the gas fraction (or X-ray luminosity) of
cool cores and the central gas entropy, which may be tested by X-ray
observations.Comment: Slightly revised version, accepted for publication in MNRAS. 14
pages, 10 figure
Adhesion at Diamond /Metal Interfaces: A Density Functional Theory Study
To understand the basic material properties required in selecting a metallic interlayer for enhanced adhesion of diamondcoatings on the substrates, the interfaces between diamond and metals with different carbide formation enthalpies (Cu, Ti, and Al) are studied using density functional theory. It is found that the work of separation decreases, while the interface energy increases, with the carbide formation enthalpy ΔHf (Tiys (Ti\u3eCu\u3eAl), is needed to achieve a higher overall interface strength. In addition, when the surface energy is larger than the interface energy, a wetted diamond/metal interface is formed during diamondnucleation, providing the strongest adhesion compared to other growth modes. These results indicate that a strong carbide-forming ability and a large surface energy of the interlayer promote nucleation and enhance the adhesion and interface strength of the coating/substrate system
Predicting the Hydrogen Pressure to Achieve Ultralow Friction and Diamondlike Carbon Surfaces from First Principles
Hydrogen atmosphere can significantly change the tribological behavior at diamond and diamondlike carbon (DLC) surfaces and the friction-reducing effect depends on the partial pressure of hydrogen. We combined density functional theory modeling and thermodynamic quantities to predict the equilibrium partial pressures of hydrogen at temperature T, PH2 (T), for a fully atomic hydrogen passivated diamondsurface. Above the equilibrium PH2 (T), ultralow friction can be achieved at diamond and DLC surfaces. The calculation agrees well with friction tests at various testing conditions. We also show that PH2 (T) increases with temperature; therefore, the temperature effect observed in friction tests should first be treated as an equilibrium factor rather than a kinetic factor
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