1. A method executed in a computer system with at least one physical computing device for producing a model of a combined physical system having physical quantities by representing physical quantities of said combined physical system in terms of a combined set of partial differential equations, said method comprising: representing at least one of a plurality of physical systems as two or more selected application modes modeling physical quantities of said at least one of said plurality of physical systems, wherein said application modes are configured to model the physical quantities for at least one of structural mechanics properties, fluid dynamic properties, electromagnetic properties, chemical reaction properties, acoustic properties, and heat transfer properties of said physical system; using a first physical computing device, determining a set of partial differential equations for each of said two or more selected application modes, parameters of said partial differential equations being physical quantities of corresponding ones of said plurality of physical systems; using said first physical computing device or a second physical computing device, forming said combined set of partial differential equations using said determined sets of partial differential equations associated with said one of said plurality of physical systems; and outputting to a display device or a communication device said model of said combined physical system based on said combined set of partial differential equations for said two or more selected application modes for said one of said plurality of physical systems, whereby said model represents a mathematical expression of said physical quantities of said combined physical system.