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Variable correspondance
Good afternoon,
I am simulating a VAWT and controlling the blade pitch using the SIL python interface. I am interested in extracting the following variables:
- Lift Coefficient
- Drag Coefficient
- Moment Coefficient
- Radial Coefficient
- Tangential Coefficient
I define the radial and tangential forces as shown in the attached image.
The associated dimensional forces would also be nice (but I only seem to be able to get the tangential one as shown below).
When extracting data from the simulation I am using the following function calls:
tangential_force = qblade_sim.getCustomData_at_num(f”Total Tangential Load Blade 1 [N]”.encode(), 0, 0)lift_force_coefficient = qblade_sim.getCustomData_at_num(f”Lift Coefficient (at section) Blade 1 [-]”.encode(), 0, 0)drag_force_coefficient= qblade_sim.getCustomData_at_num(f”Drag Coefficient (at section) Blade 1 [-]”.encode(), 0, 0)radial_force_coefficient = qblade_sim.getCustomData_at_num(f”Normal Force Coefficient (at section) Blade 1 [-]”.encode(), 0, 0)tangential_force_coefficient = qblade_sim.getCustomData_at_num(f”Tangential Force Coefficient (at section) Blade 1 [-]”.encode(), 0, 0)moment_coefficient = qblade_sim.getCustomData_at_num(f”Moment Coefficient (at section) Blade 1 [-]”.encode(), 0, 0)
Are these the right variables (in particular for Cr and Ctheta)?
Can I get the associated dimensional forces for all of these quantities?
I am also having a hard time identifying in which coordinate system each of these quantities is defined. Would it be possible to clarify if the drawing in the attached image is clear?
Thank you very much for your time,
Best regards,
Serge
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Hi Serge,
Your drawing looks mostly correct. The only point that seems problematic is the definition of the lift and drag directions. At each airfoil section, these directions are defined with respect to the local inflow velocity: lift acts perpendicular to the local inflow, and drag acts parallel to it. They are not defined relative to the global inflow direction, as indicated in your figure.
Your choice of variables seems mostly consistent. For the moment coefficient, however, I would recommend using the reprojected:
“Moment Coefficient (PAxis) (at section) BLD_1 [-]”.
This quantity represents the moment coefficient reprojected onto the blade pitch axis. The standard airfoil moment coefficient Cm, is defined about the quarter-chord location, 0.25c.
To obtain dimensional quantities, you can also extract the total velocity at the station and dimensionalize the sectional loads as follows:
dLift = Cl * 0.5 * V^2 * rho * chord
dDrag = Cd * 0.5 * V^2 * rho * chord
dMoment = Cm_pitch * 0.5 * V^2 * rho * chord * chord
Here, V is the local total velocity at the station, rho is the fluid density, and c is the local chord.
These quantities are sectional values, that is, loads per unit blade span (or per unit blade length).
Best regards,
David
