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Table of Contents

Momentum Model

This section explains the various inputs related to the calculation of steady induction and wake rotation using actuator disc theory. Various correction models are then presented for computation of the steady flow induction at the blade based on the Blade Element Momentum Theory. The induction calculation is only relevant for turbine in operational state and is switched off based on a tip speed ratio threshold specified by the user.

Actuator Disc Momentum Model

The following options are given:

  • Drag induction: When enabled, the drag coefficient will be included in the induction calculations. This mainly influences the BEM solution near the root of the blade, where the drag force is significant.

  • Structural velocity induction: When enabled, the axial structural velocity will be included in the induction calculations. This option is enabled by default.

  • Momentum theory: This allows selection of the way that mass flow is calculated. In axial flow conditions there are no differences between the two methods.

    • Glauert: Considered more accurate for yawed flow and is the recommended option.

    • Axial: Legacy pre-4.8 sub-model. Not recommended and should only be used for testing purposes.

Engineering Corrections

A traditional BEM code assumes that the turbine operates in a steady and uniform flow field with no yaw misalignment. In reality, wind turbines do operate in yaw, hence a Skew wake correction model is included to better predict the aerodynamic loads when operating at high yaw misalignments. Furthermore, it is assumed that the induction in each annulus is invariant with azimuth angle. This assumption does not hold near the tip and root of the blade for a rotor with a finite number of blades, therefore the Tip/Hub loss models are used.

  • Skew wake correction model:

    • Glauert: Recommended skew wake correction model for all conditions. It is not recommended to disable the Glauert model in yaw misalignments in excess of 10 degrees. For idling or parked rotors this model has no influence as the induction calculation is disabled.

    • Disabled: A pre-4.8 legacy option. This is not recommended.

  • Tip speed ratio thresholds for inflow: Specify tip speed ratio thresholds beyond which the induction calculations both steady and dynamic will be switched off.

  • Tip/Hub loss: Enable or disable corrections for tip and hub loss.

  • Simplified aerofoil orientation: This is a pre-4.8 compatibility and it is not recommended to use this option. When not selected the blade aerofoil orientation is defined by the blade element method. The aerofoil orientation will therefore account for the curvature of the blade including geometric prebend and also blade deflection. When selected, it is assumed that the aerofoil remains perpendicular to the blade root Z-axis such that it ignores blade prebend and the influence of blade deflection.

Last updated 25-09-2024