The `yPlus`

value is an important index to consider when choosing the grid size for the near-wall region. How about the outer region?

# The Rule-of-Thumb Estimation

There is a rule-of-thumb estimation for outer region flows using the ratio of **integral length scale** to **grid length scale**,

$$R_l = \frac{l_0}{\Delta} , $$

where

$$l_0 = \frac{k^{1.5}}{\varepsilon}, \quad \mathrm{or} \quad l_0 = \frac{k^{0.5}}{C_{\mu}\omega} \quad \mathrm{where} \quad{}C_{\mu} = 0.09, \\\Delta \approx{}\sqrt[3]{\mathrm{Cell \, \,Volume}}\quad \mathrm{for \,\,modest\,\,aspect\,\,ratios.}$$

It is recommended that $R_l > 5-10$, where **5** is the lower limit for RANS/URANS/VLES simulations and **10** for LES/DES.

# How to Check the Resolution of a Simulation in ParaView

Use two filters, `Cell Volume`

and `Calculator`

.

- First, use the
`Cell Volume`

filter to get the`Volume`

scalar field. - Then use the
`Calculator`

filter to calculate the $R_l$ field, e. g.`k^0.5 / 0.09 / omega / (Volume)^0.3333`

- Plot the contour of the $R_l$ field, and use the clapping tool, the
`Threshold`

filter, to show only the region where $0 < R_l < 5$, which is under-resolved.