文件列表:

box2d/
box3d/
boxTurb2D/
integralLength.py

# Using built-in background turbulence generator ## Physics The OpenFOAM (seems to) uses a Passot-Pouquet spectrum. $E(k) = E_a (\frac{k}{k0})^4 e^{-2(\frac{k}{k0})^2}$ Two parameters need to be set, in constant/boxTurbDict, which are $E_a$ and $k_0$ - $E_a$ will influence the scale of velocity, but not the distribution. - $k_0$ is the characteristic wave number. Adjust $k_0$ will change the scale of the turbulence. - The Reynolds number $Re = \frac{ul}{\nu}$. Adjust $E_a$ will allow you to adjust the velocity. Adjusting $k_0$ will affect the integral length scale. The dynamic viscosity is a property of the fluid itself. ## Integral length calculation The integral length $l$ is calculated by $\int_{0}^{+\infty}k^{-1}E(k)dk /\int_{0}^{+\infty}E(k)dk$, suggested by https://en.wikipedia.org/wiki/Integral_length_scale. It is evident that $E_a$ will not influence the $l$. To make the calculation of $l$ easier, a Python script is offered here (An installation of Python 3 is required). Run ```bash python integralLength.py ``` Please note that an integral to positive infinity might not be numerically stable, to specify a upper limit, try ```bash python integralLength.py ``` ## Generation Strategy Generate a initial turbulent velocity field with in a domain larger than your target domain. And map the velocity back to your target domain! ## 2D generator OpenFOAM does not provide a 2D initial turbulence generator. But we can easily implement one. I offered one, modified from "boxTurb". The velocity in the $z$ direction is set to 0. To compile it, go to the boxTurb2D folder and run ```bash wmake ``` Run ```bash boxTurb2D ``` to generate the 2D background turbulence. A 2D geometry is required. Configure $E_a$ and $k_0$ in constant/boxTurbDict See the box2d case for more information. ## 3D generator OpenFOAM provides a built-in 3D initial turbulence generator. See the test case. Run ```bash boxTurb ``` to generate the 3D background turbulence. Configure $E_a$ and $k_0$ in constant/boxTurbDict See the box3d case for more infomation. ## Something to note - The domain to generate the turbulence should be a box. - The number of cells in each direction should be $2^n$, for $n \in \mathbb{N}$. ## Relevant functions in OpenFOAM The spectrum is implemented in https://cpp.openfoam.org/v10/Ek_8H_source.html. The generator is implemented in https://cpp.openfoam.org/v10/turbGen_8C_source.html. Please note that K is a mesh with wave number information.

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