# Aerodynamics

## General

### BEM module

Choose which BEM module you want to use.

**Options:**

** Hansen** (default):

**AeroDyn**:

### Hub loss correction

Reduces the aerodynamic loads due to the presence of the hub. The effect of this correction increases close to the root of the blade.

The hub loss is calculated according to Prandtl's correction factor (see Steady BEM)

**Default value:**True**Unit:**—

### Tip loss correction

Reduces the aerodynamic loads following Prandtl's correction factor. The effect of this correction increases close to the tip of the blade.

The tip loss is calculated according to Prandtl's correction factor (see Steady BEM)

**Default value:**True**Unit:**—

### Excluded stations at tip

The number of stations excluded at the tip. Numerical problems related to tip elements can occur especially if they are close to the tip. This parameter will exclude the specified number of stations at the tip - and the corresponding sections. If the number is equal to or greater than the total number of stations then all stations are excluded. Default: 0**Default value:**0**Unit:**—**Range:**0 — 1000

### Glauert correction method

For high (axial) induction, the theoretical BEM is no longer useful. Thus, a Glauert correction method can (and should) be applied. There are two different algorithms (methods) available.

The corrections are described in detail in the Glauert correction document.

**Options:**

** Empirical** (default):

**No correction**:

No correction is applied. This will give wrong results for high indiction and should only be used for comparison.

**Wilson and Walker**:

Glauert correction published by Wilson and Walker(1984). Also, referenced in Spera(2009) and Spera(1994).

### Critical Glauert's value

Glauert's correction is applied if the axial induction factor is above this value - the critical axial induction ac. The correction factor is then calculated according to Spera(2009).

Recommended value: 0.33 ≤ ac ≤ 0.4. Only used (and visible) for the Wilson and Walker option for Glauert correction method.

More information about the Wilson and Walker algorithm is given in the Glauert correction document.

**Default value:**0.33**Unit:**—**Range:**0.01 — 1

### Tower shadow

Enables/disables the inclusion of a correction of the windspeed due to the presence of the tower. For upwind wind turbines, a simple potential flow model is used. For downwind wind turbines, the Powles model is used.

The theory for the potential flow model is given here. The theory for the Powles model is given here.

The tower shadow modeling is only valid for vertical towers.

**Default value:**True**Unit:**—

### Shadow radius

Not relevant for circular towers. The equivalent radius used for reducing the wind speed due to the presence of the tower.**Default value:**2.5**Unit:**$\text{m}$**Range:**0 — 1000

### Movement of structure

When calculating aerodynamic drag on the support structure, the loads can be implemented assuming a fixed, partly moving, or a (fully) moving structure. Default: Fixed. This setting does not affect the loads on the blades.

**Options:**

** Fixed** (default):

Drag loading is calculated only based on air particle (wind) velocity at the initial (start) position of the structure. This is more numerically stable and is usually OK when the movement is relatively small.

**Partly**:

Drag loading is calculated based on air particle (wind) velocity at the first iteration of a time step. However, the load is not updated during nonlinear iterations. The main reason to use this instead of 'Moving', is that it is faster.

**Moving**:

Drag loading is calculated based on both air particle velocity at the current position and time and the structural position and velocity at the current time. This causes numerical problems more often, but typically gives more accurate results if successful (every real structure moves, even if it is so little that it is not observed). Moving should be used for floating wind turbines as relatively large movements are expected.

### Reduce load due to partly submergence

Reduce loading for an element that is partly below the ground or sea surface. If not checked, then the loading on a surface element is 100% or 0%.

**Default value:**1**Unit:**—

### Aero drag coef. of circular cs.

The drag coefficient for the support structure that is exposed to wind. The drag coefficient is (for now) treated as a constant. Should be/will be dependent on Reynolds number at a later stage.

**Default value:**1**Unit:**—**Range:**0 — 10

### Aero drag on the Nacelle/hub?

A drag load can be added to the nacelle. For the purpose of calculating the magnitude of the drag load the Nacelle is given a pseudo radius of a pseudo sphere. Thus, the direction of the drag load is the same as the direction of the undisturbed relative wind at the center of mass of the nacelle.

More information about the theory can be found in the theory manual, in the Aerodynamic Loads on Nacelle/hub section

**Default value:**False**Unit:**—

### Drag coefficient for nacelle

Drag coefficient for the nacelle.

**Default value:**1**Unit:**—**Range:**0 — 10

### Pseudo radius for drag (of nacelle)

The drag on the nacelle is calculated as if it were a sphere with this (pseudo) radius. It's called pseudo because it is not a physical sphere.

Getting a good/acceptable estimate for the drag and drag coefficient of the nacelle (including the hub) is very complicated and is thus rarely available. So,instead using the drag on a virtual sphere is considered a engineering rule-of-thumb approach to take the drag into account. Using our engineering judgement, we suggest using a virtual (pseudo) sphere of similar size as the nacelle.

**Default value:**1**Unit:**$\text{m}$**Range:**0 — 100

## Blade modeling

### Force center

Specifies where the areodynamic force (lift + drag) is applied at the blade station. The aerodynamic loads will also be calculated at the same position.

**Options:**

** Aerodynamical reference point** (default):

The force is applied at the aerodynamical reference point, which is specified in the blade's shape file. This is the recommended option, as it gives the most correct loading.

**Elastic center**:

The force is applied at the elastic center, i.e at the structural element's node.

### Apply moments

Enables aerodynamic moments at each blade station, computed from the Cm coefficient.

**Default value:**True**Unit:**—

### Shear center offsets applied

If enabled, the offset/eccentricity from the neutral axis/elastic center of the cross-sectional shear center is taken into account. A non-zero shear offset will produce a torque/pitching moment on the blade structure if the aerodynamic forces are not applied in the shear center. The eccentricities are specified in the blade structural data. If this option is disabled, the shear eccentricities are modeled as being zero.

**Default value:**True**Unit:**—

### Mass center offsets applied

If enabled, the offset/eccentricity from the neutral axis/elastic center of the cross section's mass center is taken into account. The eccentricities are specified in the blade structural data. If this option is disabled, the mass eccentricities are modeled as being zero--i.e the mass center is located at the neutral axis/elastic center. Gravity loads will be applied in the mass center.

**Default value:**True**Unit:**—

### Include rotational mass/inertia

If enabled, the structural rotational mass (also called inertia) is included at each blade station. If disabled, only the translational masses are included.

**Default value:**False**Unit:**—

## Advanced

### BEM mode

Enables either the steady or unsteady BEM algorithm. The recommended algorithm is the unsteady, which applies the generalized dynamic wake method.

**Options:**

**Steady**:

Steady conditions

** Unsteady** (default):

Unsteady conditions

### Incoming wind for induction

Decides which wind vector is used when the axial induction factor is calculated. Default: Whole

**Options:**

** Whole** (default):

The whole incoming wind vector is used (only modified due to tower shadow). This is recommended.

**Parallel**:

The component of the incoming wind that is parallel with the rotor plane normal is used.

### Polar interpolation scheme

Sets the scheme for interpolation of airfoil polars.

**Options:**

**Linear interpolation**:

Polars are interpolated linearly to generate polars for other Reynolds numbers.

** Logarithmic interpolation** (default):

Polars are interpolated logarithmically to generate polars for other Reynolds numbers.