This review embodies the theoretical investigation of three types of problems viz., a general model for the Technique of Reconstitution, Penetrative convection in fluid and porous layers under different constraints like, rotation, salinity gradient, nonlinear temperature profile etc., and thermosolutal instability in homogenous and heterogeneous layers in the presence of coupled molecular diffusion.The aim of the present investigation is to provide the qualitative as well as the quantitative features of the phenomena e.g. about the form as the flow pattern, the size of the convective cell, the temperature, salinity and velocity distributions, the formation of horizontally long convection cells, occurrence of subcritical motions, the construction of the evolution equation, the amplitude etc., In Type I, in order to elucidate the properties of reconstituted equations by applying the technique to a highly complicated system of nonlinear equations , a general model consisting of coupled three nonlinear differential equations are considered. In Type II, five models are discussed where the boundaries have fixed-heat and salt flux conditions. In Type III, the boundaries are of free-slip type.
