Deciding mesh size and time step in Actuator Line Model simulations

Hello everyone,
According to my (quite short) experience with ALM simulations of wind turbines, determination of the cell size in rotor plane and time step size plays an important role in the convergence and agreement with experimental measurements. There are many turbines in various sizes and operating at various speeds but I have hypothesized the following as key parameters:

  • In a time step; linear distance traveled by the blade tip or angular travel of the rotor should not exceed a certain value,
  • Cell size in the rotor region should be decided on a “per rotor radius” basis (should this also depend on the number of airfoil sections defined?)
  • Question: Should blade tip travel more than one cell in a time step?
  • Question: Should the cell size in rotor plane depend on the sectional chord length for an accurate relative velocity / AoA calculation?

Of course, this depends on whether LES or RANS is employed (in which I am more interested in RANS solutions).

Would you agree with these or add some other “rules of thumb”, or have some such values you consider in your simulations according to your experience so far? It would be the most valuable.

Kind regards,
Hüseyin Can Önel

Hello Hüseyin Can Önel,

I have a question on the account of this item:

Particularly interested in the implication. What is meant by a certain value? Is it a constant value which is calculated experimentally or is it a variable which can be calculated due to specific conditions (rotor dimensions, turbine power, its kind, etc.). Is there a formula for this value?

Thank you,
Renee Degutis

Hi Huseyin.Can.Onel :
I recently implemented the actuator line method into a lab FDM-code and may be quite appropriate to answer your question. :mrgreen:

Mesh size should fulfill that there are approximately 30-60 grid points per blade(rotor radius) and time step is derived by restricting the blade tip do not travel over one mesh size length.
Actuator line method usually coupled with LES in wind farm simulations, so the Co<1 criterion must be satisfied. Wind farm simulations should run a sufficient long physical time( hundreds of seconds ) for the wake turbulence to be fully developed, so 30 grids per blade can be a reasonable one.

Dr.JHA systematically investigated the various parameters in actuator line method and their effects one accuracy in paper(see attachment file):
Guidelines for Volume Force Distributions Within Actuator Line Modeling of Wind Turbines on Large-Eddy Simulation-Type Grids ,2014,Journal of Solar Energy Engineering-transactions of The Asme. volume 136,issue 3.

Hope it helps you. 
By Lin.Yang

sol_136_03_031003.pdf (2.01 MB)