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Import Polar and Effect on LLFVW Results
Hi,
I am doing the aerodynamic analysis of a 5 helical blade VAWT with the LLFVW model. At the beginning I was using the polars given by XFoil and the power curves, despite largely overpredicting power output when compared to high fidelity cfd solver, had a “normal shape”, almost like a bell curve. However, when I tried to use the aerodynamic data given by ANSYS, the power curves droped but are all mostly flat across different RPM with a sudden spike before dropping. I checked the Reynolds Number encountered by the blades ( 5E4 and 1E6) which is within the range I have simulated with ANSYS (1E4 – 5E6).
I am not sure what the problem is:
Does it have anything to do with the vortex modeling parameters? (currently 0.05% for both core radius)
Should I have more angles of attack to smooth out the polar curves? (For lower Reynolds, I currently have from 0º to 30º with 5º increments, and after that with 30º jumps for the full rotation. For Reynolds between 1E5 and 5E6 the angular jumps after stall are 10º and not 30º)
Thank you,
Pedro
Hello Pedro,
some brief comments from my side:
When comparing the CFD data with the XFoil generated results, make sure that the wake is long enough so that the Cp converges to the final value. Did you perform a sensitivity study on wake length?
Regarding the polar data from ANSYS: Could you post a plot of the data here? Its tough to comment on it, solely based on your description. Also, did you assign the correct Reynolds number to the polars when you imported them?
BR,
David
Thank you for the response.
Regarding the convergence, I assumed the mean value after 10 full rotations once the wake reaches the end of the domain (checked by the plateauing of number of vortex filaments). I did not perform a sensitivity study for the wake length. The total length has been 9.5 revolutions (0.5/2/3/4), which I thought was far enough to not interfere with the turbine.
Yes, the Reynolds numbers are correct based on the .plr file and I checked again when assigning the multipolar.
Please find attached a sketch of both Cp curves and one of the polars from CFD for the lower Reynolds where the jumps in angle of attack are higher (Do you think I should increase the number of data points?)
Once again, thank you for your reply,
Pedro Gonçalves
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Hi Pedro,
depending on the operating point the required wake length for a fully converged Cp value can actually be larger than 9.5 revolutions. I would test the simulation with up to 18 revolutions of developed wake.
Looking at the ANSYS generated polar that you have attached I would assume that this is causing the “low performance” CP values.
The data appears to be very coarse and doesnt show the typical “drag bucket” associated with airfoil polars. As a result the glide ratio under which the airfoil is operating will be very low and hence the overall turbine performance. Even though I understand that generating such highly resolved polar data by means of CFD simulation can be quite time consuming you would need to resolve the polar much higher (especially in the area around 0° AoA) to be suitable for simulations with QBlade, or any other code relying on the blade element or lifting line theory.
BR,
David
