Mean Age

For open systems, users can automatically calculate the transient mean age distribution using the Calculate Mean Age command. After selecting this option from the Menu, users will be prompted to select an age source from among the inlets, a calculation start time, and an age diffusion coefficient. The age diffusion coefficient is related to the molecular self-diffusion coefficient. The spatial variation in the mean age is printed to volume and slice output files. The mean age at each outlet is printed to output.

The transient mean age represents two physical values. First, it represents the average age of molecules at a given point inside the tank, relative to the time these molecules entered the system via the selected inlet. Second, the represents the mean residence time of molecules at a given point inside the tank. For systems with a single outlet, the mean age at the outlet is equal to the volume averaged mean-age. The volume averaged mean age is also equal to the mean residence time of the system.

Start Time

s | Time at which to begin the mean age calculation.

Diffusion Coefficient Type

There are two options for defining the species diffusion coefficient: Constant and UDF.

Constant

The diffusion coefficient is a constant value that is uniform through the simulation domain.

Diffusion Coefficient

m ² /s | Diffusion coefficient of the scalar through the base fluid. Typical liquid-liquid diffusion coefficients range from \(10^{-9}\) to \(10^{-8}\) \(m^2/s\), depending on temperature and composition. Typical gas-gas diffusion coefficient ranges from \(10^{-6}\) to \(10^{-5}\), again depending on temperature and composition.

UDF

The diffusion coefficient is a user-defined function that can be time-varying and non-uniform.

Diffusion Coefficient UDF

m ² /s | This UDF defines the local species diffusion coefficient. One output must be defined within the UDF: a floating-point variable named D. This output variable defines the local species diffusion coefficient within the fluid. This is a voxel-based local UDF, calculated on a voxel-by-voxel basis using the local fluid properties.

Download Sample File: Diffusion Coefficient

Note

Scalar fields can be susceptible to numerical diffusion. Numerical diffusion occurs when the simulated fluid presents a higher diffusivity than the physical fluid. The effects of numerical diffusion can be minimized by reducing the simulation resolution, applying a flux limiter, or increasing the scalar field update interval. When running scalar fields in a simulation, resolution tests should be performed to ensure that the effects of numerical diffusion are negligibly small. This point is discussed in Application of flux limiters to passive scalar advection for the lattice Boltzmann method

Selected Inlet

Selected inlet for mean age calculation.