Learning Forum

[ahmad.hijazi05]

Greetings,
I am new to this forum.
So I am working on a 2D transient simulation of a vertical axis wind turbine. I tried first to run the simulation at tip speed ratio (TSR 2.58). Things were fine, but when I moved to TSR 1, I am getting a very low results in comparison with experimental results in terms of comparing the power coefficient Cp. I am using the coupled pressure-velocity solver (it is pressure based type) so I noticed that in the solution control panel, there is a flow courant number that is 200 by default. I changed it to 1, and run the simulation. I got close results to experimental.
I then tried to change this Flow courant number to 0.5, I got different results than the 1 flow courant number case.
My question is what is the recommended flow courant number that can be used, and is using a 1 courant number wrong ?
thank you in advance.

[Amine Ben Hadj Ali]

That courant number has nothing to do with the time step size or the temporal evolution of your run. It is a numerical key to add more stability /diagonal dominance into the stiffness matrix. We generally to try to keep that number as high as possible if the run is stable enough.

 

It seems for me that your results are lacking deep convergence.

[ahmad.hijazi05]

I dont know why at low tip speed ratio (TSR=1), I am getting very low results in comparison with experimental results. It could be the time step size ( I am using a time step equivalent to 0.25 degrees), I will try to decrease it and see what I will get while keeping flow courant number 200 in stead of setting it to 1. 

[ahmad.hijazi05]

I lowered the time step size (previous one was 0.25 degrees, new one 0.05 degrees) but I am still getting very low result in comparison with the experimental data (turbine solidity=0.265). Once I set flow courant number to 1, It is giving me better results and also residuals are converging to 10^-3. So can I work with flow courant number of 1 or I am building my work on wrong setup ?