Hi,
I am using transient simulation in fluent. In the inlet boundary condition, I have to put the value of turbulent length scale. How can I decide it for wind turbine blade model?
Thank You.
Hello,
It is common to set the turbulence length scale to a certain percentage of a typical dimension of the problem. For example, at the inlet to a turbine stage a typical turbulence length scale could be say 5% of the channel height.
When the inlet flow is bounded by walls with turbulent boundary layers, the turbulence length scale can be estimated (approximately) from the inlet boundary layer thickness. For codes using a turbulence length-scale based on the mixing-length like Fluent does, you can use 0.4 of the inlet boundary layer thickness for your turbulence length scale.
I have seen a few formulas to calculate the turbulence length scale otherwise, with one being l = (C_mu)^3/4* ((k^3/2)/ε), where C_mu is a model constant, for example for k-epsilon it is 0.09, k is the turbulence kinetic energy and epsilon is the rate of dissipation of turbulent kinetic energy.
Note that the Fluent manual provides the following guideline for estimating the turbulence length scale: L = 0.4 delta_0.99, where delta_0.99 is a representative boundary layer thickness. Depending on how sophisticated an answer you need, you can estimate the boundary layer thickness via correlations for a flat plate (i.e., by neglecting any effects of the pressure gradient), e.g., delta/x = 0.38/Rex^0.2, or by using a public domain airfoil code such as XFOIL. If your airfoil surface is significantly rough, then you will need to apply a correction to the estimated thickness for a smooth surface. In practice, it would be best for you to assess the sensitivity of your metric of interest (e.g., airfoil drag or lift) by varying the length scale.
As a preferred option, note that most prominent turbulence models have a typical range of values for turbulence field quantities at the inflow and you should use those values -- see, for instance, Eqs. H-175 to H-179 from https://cfl3d.larc.nasa.gov/Cfl3dv6/V5Manual/TurbEq.pdf (please be mindful of the non-dimensionalizations used by each code). The caveat on evaluating the sensitivity of your answers to the assumed values applies in this case as well.
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