A class of almost contact metric manifolds and double-twisted products

We determine the Chinea-Gonzales class of almost contact metric manifolds locally realized as double-twisted product manifolds of an open interval and an almost Hermitian manifold, by means of two smooth positive functions. We also give an explicit expression for the cosymplectic defect of any manifold in the considered class and derive several consequences in dimensions 2n+1>3. Explicit formulas for two algebraic curvature tensor fields are obtained. In particular cases, this allows to state interesting curvature relations.