Physical Properties in Fluid Dynamics

Physical properties of a fluid can be described within the context of transport analogies for all the transport processes. Numerical solutions to fluid dynamics problems require that the viscosity and the density p are known. Under isothermal conditions, if the fluid is Newtonian and incompressible, both of these physical properties are constants that depend only on the fluid, not the flow conditions. The viscosity p, is the molecnlar transport property that appears in the 

Numerical solutions to simple thermal energy transport problems in the absence of radiative mechanisms require that the viscosity fi, density p, specific heat Cp, and thermal conductivity k are known. Fourier s law of heat conduction states that the thermal conductivity is constant and independent of position for simple isotropic fluids. Hence, thermal conductivity is the molecular transport property that appears in the linear law that expresses molecular transport of thermal energy in terms of temperature gradients. The thermal diffusivity a is constructed from the ratio of k and pCp. Hence, a = kjpCp characterizes diffusion of thermal energy and has units of length /time. 

The binary molecular diffusion coefficient, ab has units of length /time and characterizes the microscopic motion of species A in solvent B, for example. Hab is also the molecular transport property that appears in the linear law that relates diffusional fluxes and concentration gradients. In this respect, the same quantity, Bab. represents a molecular transport property for mass transfer and a diffusion coefficient. This is not the case for the other two transport processes. 

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