Learning Forum

[mohalinasre2001]

Hello Everyone,

I am a master's degree student working on my thesis. I am running a 3D transient simulation with coupled solver and IDDES turbulence model. The problem is that I am struggling in choosing the approriate CFL number found in Solution Controls. It's 200 by default, but it gives wrong power coefficient. I tried numbers like 1, 0.7, 0.5 and I realised the following: the more I decrease the CFL number, the larger the power coefficient gets. So I am confused on what to choose as my CFL, because for each Tip Speed Ratio it's different. i tried tunning it for each TSR, but I feel I am doing it wrong because I am trying to validate the setup vs. experimental data and I should not change settings for each TSR.

I would appreciate if someone can help me get around this because I am running out of time. I am willing to include the name of whoever helps me finds a solution in my thesis Acknowledgements section.

Thank you in advace!

 

[mohalinasre2001]

Side Notes:

• y+ values for the blades are below 1. 
• Mesh quality is decent

[Essence]

Hello,

You need not add my name in your thesis. It's totally fine.

What is the equation of power coefficients you are using? Are you sure that CFL number is the only one which is impacting the results? There can be other factors which may be affecting the results.

[mohalinasre2001]

Hello!

I am using the moment coefficient defined in report definitions. I am averaging it over a complete turbine rotation, and then multiplying it by TSR to get power coefficient (Cp=Cm* TSR). So basically CFL in solution controls is affecting the moment coefficient, which in turn affects the power coefficient.

And yes after trying several simulations while changing CFL number only, the moment coefficient was changing such that: increasing CFL makes Cm-avg smaller, while decreasing it makes it larger!

Thank you for your help!

[Essence]

CFL/Courant number in Solution controls is used to define the stability of the residuals. How do the residuals look? Are they converging for each time step?

[mohalinasre2001]

Yes they are converging well. I set maximum iterations per timestep to 60. The continuity residual falls below a limit of 10^-3 while the rest fall below 10^-5 without a problem. 

[mohalinasre2001]

This is why I am confused! But I feel like 60 iterations is a high number for residuals to converge.. It depends on the TSR, for hight TSRs (1, 1.25), it takes around 40 iterations, while at low TSRs (0.25, 0.5) it takes about 60+ iterations to converge.

 

[mohalinasre2001]

I made this graph to show you how increasing CFL slightly moves the moment coefficient curve downwards, which in result leads to decrease the average moment coefficient. Each curve is taken from a seperate identical simulation with different CFL as shown. They are all taken at the same time (5th revolution of the VAWT).

[Essence]

Thanks for your response.

Could you please use lower time steps (use equations to determine the time step)? You can also reduce the number of iterations to 20 - 25 maybe.

[mohalinasre2001]

I did a timestep sensitivity study, I tried azimuthal increments of 0.6 deg and 1.2 deg for turbine revolutions, and they gave the same moment coefficient curve. I even tried lower timesteps (0.1 and 0.25 deg) but I got a totally wrong solution. So I don't think lower timesteps will help. 
Based on the paper I am referencing in my thesis (Experimental and numerical investigation of a three-dimensional
 vertical-axis wind turbine with variable-pitch), they used 1.2 deg increments, and it worked fine with LES turbulence model.

[Essence]

Thanks for trying out the suggestions. Could you share the screenshot of the viscous models panel? Please be careful about the reference values too. Since they are used to determine the moment coefficients. And moreover, please check if the moment report definitions and that from the "Forces" in Results section match or not.

[mohalinasre2001]

Concerning the reference values, they are all correct (area = height x diameter of turbine) (length = radius of turbine) 

And for the turbulence model, please find attached a screenshot showing the panel:

But I didn't quite understand this "please check if the moment report definitions and that from the "Forces" in Results section match or not." 

Thank you so much for your help!