Maps Interchanging f-Structures and their Harmonicity

We study some remarkable classes of metric f-structures on differentiable manifolds (namely, almost Hermitian, almost contact, almost S-structures and K-structures). We state and prove the necessary condition(s) for the existence of maps commuting such structures. The paper contains several new results, of geometric significance, on CR-integrable manifolds and the harmonicity of such maps.     

Space-times carrying a quasirecurrent pairing of vector fields

A quasirecurrent pairing of vector fields {X₁, X₂} (defined in [1]) is investigated on a space-time in two cases: (1)X ₁ is spacelike andX ₂ is timelike; (2) X₁ is null andX ₂ space-like. The physical interpretation of these vector fields is given.     

Effects of fuel properties on self-ignition and flame-holding performances in SCRAM jet combustor for n-alkane fuels

Combustion characteristics of liquid hydrocarbon fuels are studied in a model combustor of SCRAM jet engines. The Mach number and total pressure of main flow in the combustor are 2.0 and 0.38 MPa, respectively, and the total temperature is varied from 1800 to 2400 K. Five kinds of n-alkane fuels such as n-heptane, n-octane, n-decane, n-tridecane and n-hexadecane are employed in experiments. Fuels are injected with a carrier nitrogen gas perpendicular to the mail flow in the combustor and the self-ignition behavior is investigated. The results show that the liquid fuels with lower carbon number have better self-ignition performance. This suggests that physical properties of liquid fuels such as volatility have a dominant effect on the self-ignition. The flame-holding behavior is investigated with the addition of pilot hydrogen to carrier nitrogen gas. The critical equivalence ratio at which the stable combustion keeps after cut-off of the pilot hydrogen is obtained. The relationship between the critical equivalence ratio and carbon number of fuel shows that fuels with the carbon numbers from 8 to 10 have the best flame-holding performance among the tested fuels. These experimental results can be expressed qualitatively by the simplified analysis with the concept of physical and chemical induction times.     

Some submanifolds of a parakähler manifold

SiaM una sottovarietà totalmente ombelicale di una varietà parakähleriana $\tilde M$ . SeM è anche debolmente antiolomorfa, le curvature bisezionali, ordinarie e normale, diM sono legate da una elegante relazione, da cui discendono interessanti conseguenze. L'ultimo risultato del lavoro si riferisce alle sottovarietà parakäleriane delle varietà a curvatura sezionale costante.