Time scales

Time scales#

It is important to understand the time scales involved in the process of simulating a case and post processing it.

Firstly, the raw results are generated from a CFD simulation using a solver. Each solver may have a different time and spatial scales. The common time and spatial scales can be listed as:

  • LBM scale

  • Prototype scale (Fluent and OpenFOAM)

  • Full scale (Real scale)

  • Normalized scale (Independent)

Output from solvers use LBM or prototype scale, and must be transformed to a normalized scale when generating the coefficients.

The normalization (\(t^*\)) uses the Convective Scale Time (\(CST\)):

\[t^* = \frac{t_{solver}}{CST_{solver}}\]

And to convert normalized time scale to full scale:

\[t_{FS} = t^*CST_{FS}\]

To calculate the CST for time scale transformations, the following expression can be used:

\[CST = \frac{L_{carac}}{U_{H}}\]

Where \(L_{carac}\) is the characteristic length scale, and \(U_H\) is the velocity at the interest height in the current scaling.