Parasitic torque struts
This document describes the analytical test used to validate the computation of torque loss due to drag on the struts for a vertical axis wind turbine (VAWT) in Ashes.
1 Test Description
The test evaluates the parasitic torque generated by aerodynamic drag forces acting on the struts of a vertical axis wind turbine. The struts connect the rotor blades to the central shaft and experience drag forces as they rotate through the air.
The simulations are run with loads only analysis, meaning the RPM is kept constant throughout the simulation.
The test consists of 7 load cases that systematically vary one parameter at a time to validate the implementation across different operating conditions. The baseline configuration uses:
- Rotor radius: 3 m
- Strut radius: 0.075 m
- Rotor speed: 72 rpm
- Wind speed: 10 m/s
- Aerodynamic drag coefficient: 1.0
- Struts scheme: Top-bottom
2 Load Cases
The following table summarizes the 7 load cases used in this validation test. Each load case varies a single parameter from the baseline configuration to test the sensitivity and accuracy of the parasitic torque calculation:
| Load Case | Duration [s] | Drag Coefficient [-] | Analysis Type | RPM [rpm] | Struts Scheme | Rotor Radius [m] | Strut Radius [m] | Wind Speed [m/s] |
|---|---|---|---|---|---|---|---|---|
| 1 | 10 | 1.0 | Loads only | 72 | Top-bottom | 3 | 0.075 | 10 |
| 2 | 10 | 1.0 | Loads only | 100 | Top-bottom | 3 | 0.075 | 10 |
| 3 | 10 | 1.0 | Loads only | 72 | Top-bottom | 3 | 0.075 | 12 |
| 4 | 10 | 0.5 | Loads only | 72 | Top-bottom | 3 | 0.075 | 10 |
| 5 | 10 | 1.0 | Loads only | 72 | Top-bottom | 2 | 0.075 | 10 |
| 6 | 10 | 1.0 | Loads only | 72 | Top-bottom | 3 | 0.1 | 10 |
| 7 | 10 | 1.0 | Loads only | 72 | Middle | 3 | 0.075 | 10 |
The parameters varied across the load cases are:
- Load Case 2: Increased rotor speed (100 rpm vs. 72 rpm)
- Load Case 3: Increased wind speed (12 m/s vs. 10 m/s)
- Load Case 4: Reduced drag coefficient (0.5 vs. 1.0)
- Load Case 5: Reduced rotor radius (2 m vs. 3 m)
- Load Case 6: Increased strut radius (0.1 m vs. 0.075 m)
- Load Case 7: Different struts scheme (Middle vs. Top-bottom)
3 Analytical Solution
The parasitic torque due to drag on the struts can be calculated analytically based on the drag force acting on cylindrical struts rotating through the air. The derivation of the equation to compute the parasitic torque is give in the
Parasitic torque on struts
document.
The analytical torque is computed for each load case and serves as the reference solution for validation.
4 Results
A 10 s simulation is carries out. The test is considered passed if the results from Ashes are within 0.1% of the analytical solution for all load cases.
The report with the results can be found here: