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AeroDyn Overview

Introduction

This section presents a very brief synopsis of AeroDyn, using information sourced directly from the program documentation, reproduced with the permission of NREL. Refer to the NREL AeroDyn documentation for further information.

AeroDyn performs aerodynamic calculations for simulations of horizontal axis wind turbines. It calculates the aerodynamic lift, drag, and pitching moment of aerofoil sections along the wind turbine blades, by dividing each blade into a number of segments along the span. Equipped with information about the turbine geometry, operating condition, blade-element velocity and location, and wind inflow from InflowWind, it calculates the various forces for each segment, which may then be used to compute distributed forces on the turbine blades.

Aerodynamic calculations within AeroDyn are based on the principles of actuator lines, where the three-dimensional (3D) flow around a body is approximated by local two-dimensional (2D) flow at cross sections, and the distributed pressure and shear stresses are approximated by lift forces, drag forces, and pitching moments lumped at a node in a 2D cross section. Analysis nodes are distributed along the length of each blade and tower, the 2D forces and moment at each node are computed as distributed loads per unit length, and the total 3D aerodynamic loads are found by integrating the 2D distributed loads along the length. The actuator line approximations restrict the validity of the model to slender structures and 3D behaviour is either neglected, captured through corrections inherent in the model (e.g., tip-loss, hub-loss, or skewed-wake corrections), or captured in the input data (e.g., rotational augmentation corrections applied to aerofoil data).

AeroDyn consists of four submodels: (1) rotor wake/induction, (2) blade aerofoil aerodynamics, (3) tower influence on the wind local to the blade nodes, and (4) tower drag. Nacelle, hub, and tail-vane wind influence and loading, aeroacoustics, and wake and array effects between multiple turbines in a wind plant, are not yet available (AeroDyn v15).

Rotor Wake/Induction

For operating wind turbine rotors, AeroDyn calculates the influence of the wake via induction factors based on the quasi-steady Blade-Element/Momentum (BEM) theory, which requires an iterative nonlinear solver (implemented via Brent’s method). By quasi-steady, it is meant that the induction reacts instantaneously to loading changes. The induction calculation, and resulting inflow velocities and angles, are based on flow local to each analysis node of each blade, based on the relative velocity between the wind and structure (including the effects of local inflow skew, shear, turbulence, tower wind disturbances, and structural motion, depending on features enabled). The Glauert’s empirical correction (with Buhl’s modification) replaces the linear momentum balance at high axial induction factors. In the BEM solution, Prandtl tip-loss, Prandtl hub-loss, and Pitt and Peters skewed-wake are all 3D corrections that can optionally be applied. When the skewed-wake correction is enabled, it is applied after the BEM iteration. Additionally, the calculation of tangential induction (from the angular momentum balance), the use of drag in the axial-induction calculation, and the use of drag in the tangential-induction calculation are all terms that can optionally be included in the BEM iteration (even when drag is not used in the BEM iteration, drag is still used to calculate the nodal loads once the induction has been found). The wake/induction calculation can be bypassed altogether for the purposes of modeling rotors that are parked or idling, in which case the inflow velocity and angle are determined purely geometrically. Dynamic wake that accounts for induction dynamics as a result of transient conditions are not yet available (AeroDyn v15).

Blade AErOfoil Aerodynamics

The blade aerofoil aerodynamics can be steady or unsteady. In the steady model, the supplied static aerofoil data—including the lift-force, drag-force, and optional pitching-moment coefficients versus angle of attack (AoA)—are used directly to calculate nodal loads. The unsteady aerofoil aerodynamic (UA) models account for flow hysteresis, including unsteady attached flow, trailing-edge flow separation, dynamic stall, and flow reattachment. The UA models can be considered as 2D dynamic corrections to the static aerofoil response as a result of time-varying inflow velocities and angles. Three semi-empirical UA models are available: the original theoretical developments of Beddoes-Leishman (B-L), extensions to the B-L developed by González, and extensions to the B-L model developed by Minnema/Pierce. While all of the UA models are documented in the AeroDyn manual, the original B-L model is not yet functional (AeroDyn v15). The aerofoil-, Reynold’s-, and Mach-dependent parameters of the UA models may be derived from the static aerofoil data. These UA models are valid for small to moderate AoA under normal rotor operation; the steady model is more appropriate under parked or idling conditions. The static aerofoil data is always used in the BEM iteration; when UA is enabled, it is applied after the BEM iteration and after the skewed-wake correction. The interpolation of aerofoil data based on Reynolds number or aerodynamic-control setting (e.g., flaps) is not yet available (AeroDyn v15).

Tower Influence

The influence of the tower on the wind local to the blade is based on a potential-flow and/or a tower shadow model. The potential-flow model uses the analytical potential-flow solution for flow around a cylinder to model the tower dam effect on upwind rotors. In this model, the freestream (undisturbed) wind at each blade node is disturbed based on the location of the blade node relative to the tower and the tower diameter, including lower velocities upstream and downstream of the tower, higher velocities to the left and right of the tower, and cross-stream flow. The Bak correction can optionally be included in the potential-flow model, which augments the tower upstream disturbance and improves the tower wake for downwind rotors based on the tower drag coefficient. The tower shadow model can also be enabled to account for the tower wake deficit on downwind rotors. This model includes an axial wind deficit on the freestream wind at each blade node dependent on the location of the blade node relative to the tower and the tower diameter and drag coefficient, based on the work of Powles. Both tower-influence models are quasi-steady models, in that the disturbance is applied directly to the freestream wind at the blade nodes without dynamics, and are applied within the BEM iteration.

Tower Drag

The wind load on the tower is based directly on the tower diameter and drag coefficient and the local relative wind velocity between the freestream (undisturbed) wind and structure at each tower analysis node (including the effects of local shear, turbulence, and structural motion, depending on features enabled). The tower drag load calculation is quasi-steady and independent from the tower influence on wind models.

AERODYN OUTPUT Parameters

A list of all possible output parameters for the AeroDyn module is presented below. The names are grouped by meaning, but can be requested in any order you see fit. BαNβ, refers to output node β of blade α, where α is a number in the range [1,3] and β is a number in the range [1,9], corresponding to entry β in the List of Blade Output Node Numbers. TwNβ refers to output node β of the tower and is in the range [1,9], corresponding to entry β in the List of Tower Output Node Numbers.

Refer to the schematics for a graphical illustration of the Tower Geometry, Blade Geometry - Side View, Blade Geometry - Front View and the Blade Local Coordinate System.

List of all possible output parameters from AeroDyn