# Hyperbolic tangent function to create stretched grid

Hyperbolic tangent function $${\rm tanh}$$ is often used to generate the stretched structured grid.

In this blog post, I will introduce some examples I have found in the references.

 Example #1 [1]

y_j = \frac{1}{\alpha}{\rm tanh} \left[\xi_j {\rm tanh}^{-1}\left(\alpha\right)\right] + 1\;\;\;\left( j = 0, …, N_2 \right), \tag{1}

with

\xi_j = -1 + 2\frac{j}{N_2}, \tag{2}

where $$\alpha$$ is an adjustable parameter of the transformation $$(0<\alpha<1)$$ and $$N_2$$ is the grid number of the direction. As shown in the following figure, the grids are more clustered towards the both ends as the parameter $$\alpha$$ approaches 1.

 Example #2 [2]

y_j = 1 -\frac{{\rm tanh}\left[ \gamma \left( 1 – \frac{2j}{N_2} \right) \right]}{{\rm tanh} \left( \gamma \right)}\;\;\;\left( j = 0, …, N_2 \right), \tag{3}

where $$\gamma$$ is the stretching parameter and $$N_2$$ is the number of grid points of the direction.

 Grid Images

Coming soon.

 References

[1] H. Abe, H. Kawamura and Y. Matsuo, Direct Numerical Simulation of a Fully Developed Turbulent Channel Flow With Respect to the Reynolds Number Dependence. J. Fluids Eng 123(2), 382-393, 2001.
[2] J. Gullbrand, Grid-independent large-eddy simulation in turbulent channel flow using three-dimensional explicit filtering. Center for Turbulence Research Annual Research Briefs, 2003.

## Author: fumiya

CFD engineer in Japan

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