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Reference Frame for Tangential and Normal Forces in LLFVW
Dear David,
I’m new to using QBlade and I have a few questions! I’m currently running simulations of a 3-bladed H-Darrieus turbine using LLFVW, and I want to compare the results with those from a Vortex Particle Method (VPM) code.
I’ve exported all the results from QBlade, but I have a major doubt regarding the reference frames used for the tangential (Tangential_Force_Blade_1_PAN_) and normal (Normal_Force_Blade_1_PAN_) force components for each blade element.
- Are these forces reported in the local reference frame of the blade, or in the global/turbine-fixed frame?
This is important because, in the VPM code, I calculate CT and CP based on the Fx, Fy, and Fz forces in the blade reference frame, and then I transform them into the global frame.
Unfortunately, while I get a very good match in CT between the two methods, the CP values differ significantly. I’m using the same airfoil polars in both tools to ensure consistent input conditions, but I’m still unsure how QBlade actually computes CP and CT, especially in LLFVW.
To better understand the discrepancy, I’d like to verify whether the forces acting on the blades are truly comparable between the two simulations.
Thanks in advance for any clarification you can provide!
Thanks in advance for any clarification you can provide!
Hi,
the tangential and normal forces are defined relative to the direction of rotation: the tangential force acts along the tangent to the circle of rotation, while the normal force acts perpendicular to it. These directions are determined locally for each blade, as they vary with the blade’s position along its rotational path.
The coefficients Cp (power coefficient) and Ct (thrust coefficient) are calculated by summing the tangential and normal forces, respectively, over all blades and then normalizing them appropriately.
Best regards,
David
Thank you so much for the reply!
I’m so sorry but I have another question, this time related to the vortex modelling parameter: what’s the difference between the fixed bound core radius and the initial wake core radius? I tried different values of them and I noticed that changing the first one the CP value that I get from my VAWT simulation is so affected but I cannot say why.
Thanks in advance for your time!
