# II - Position of the velocity sensor

Collection of accurate wind speed data is one of the most problematic elements in conducting wind turbine power performance tests. It allows us to send a signal to the control systems in order to stop the turbine when the wind velocity is too hight, in order to prevent material damage.

The owners of wind farms and turbine manufacturers have shown interest in the use of anemometers placed on the platform for the wind speed data collection, rather than using a meteorological tower upstream of the turbine because of the low cost of this solution.

However, the most significant problem with this practice is that the wind flow is disturbed by the rotor and the nacelle and the wind speed measurements gathered by an anemometer located at the rear of the nacelle does not exactly represent the speed of undisturbed wind upstream of the rotor. This problem can be avoided if the measures can be adjusted.

This second part of the project concerns the modeling of flow at the surface of a wind turbine nacelle in order to find the optimal position for an anemometer.

An optimal position means a spot where the flow is the most uniform, and the less turbulent possible. That would make a correlation between the inlet velocity $V_{\infty}$ and the velocity measured by the anemometer $V_{nacelle}$ with the less residual possible.

Key Parameters:

First we have to sort out the parameters influencing the measurement, which are:

• the nacelle geometry,
• the boundary layer around the nacelle,
• the near wake generated by the rotor and the nacelle,
• the atmospheric boundary layer,
• the pitch angle and the position of the anemometer on the nacelle.

The effect of the nacelle geometry will be studied as a first step (a 2D simulation without the nacelle), then a 3D simulation with the presence of the rotor will be carried out:

# a) Simulation of the nacelle (2D)

In this part, we will simulate the wake of the nacelle

We already know that the sensor will be placed on the upper part of the nacelle, so we will focus our study on this upper domaine.

# Sketch of the Domain - Region - Physics Continuum

Sketch

First we create a sketch of the fluid domain.

Right click 3D-CAD Models $\rightarrow$ New

Then we creat a new sketch on the XY plane,

Then we use the entities shown above to creat our sketch.

Then we create a domain with the following dimensions :

$L_H \simeq$ 8 x hight of the nacelle

$L_{upstream} \simeq$  lenght of nacelle

$L_{downstream} \simeq$ 2 x lenght of the nacelle

Then we make an extrude of this sketch, in order to be able to make the simulation :

The domain looks like this:

Then we rename each boundary by right clicking on the surface $\rightarrow$ rename.

The region

Then we create the region:

In Parts node ,right click on Domain $\rightarrow$ Assign part to regions

Physics continuum

It is now important to define each kind of Boundary Condition:

Then we have to create a physics continuum by right clicking on Continua $\rightarrow$ New $\rightarrow$ Physics Continuum and select the physic models required by the simulation, here we shose:

# Mesh

Mesh

Now we have to create the mesh,

right click continua $\rightarrow$ New $\rightarrow$ Mesh Continuum, to choose models right click on Models $\rightarrow$ Select Models

The mesh consists of predominantly non-uniform rectangular cells with fine resolution near the nacellein order to capture the boundary around the nacelle. This will allow us to optimize the computing time.

a zoom on the nacelle region shows the refined mesh in this part:

We can also make the mesh finer on a selected block, as the following figure shows:

Finally the mesh is generated by successively clicking on the the following buttons

# Simulations - Results

Results

we follows similar steps to the first part in order to get the results,

We can create a line probe to get the velocity values on this line,

Derived $\rightarrow$ Parts $\rightarrow$ New Part $\rightarrow$ Probe $\rightarrow$ Line...

We plot the streamlines afterwards,

Derived $\rightarrow$ Parts $\rightarrow$ New Part $\rightarrow$ Streamline...

Velocity field is represented in the following figure for an upstream velocity of $10m/s$

A zoom on the nacelle:

We plot here the velocity measured by the sensor $s_1$ as a function of the velocity upstream the rotor:

• Position of sensor $s_1$: $0,5\ m$ above the pod and $0,5\ m$ from the end,
• calibration equation: $y=-0,176+1,044 x$
• the residual norme for this method: $1,6\ .10^{-4}$.

One can notice that the residual is very tiny, because we didn't simulate the rotor which is the most important source of turbulence.
Once we simulate the rotor, one can plot the calibration equation for different points above the nacelle, and find the point with the less residual norme.

# b) Simulation of the rotor (3D)

In this part, we will simulate the wake of the rotor.

# Domain - Region

Domain

First we create the nacelle 3D and a box containing this nacelle, then we substract the nacelle from the box:

Then we define the Bondary Conditions as done in the previous part,

# Mesh - Results

Mesh

We follow exactly the same procedure as in the previous part in order to creat the mesh, and here we create the virtual disk which represent the rotor,

it is explained in the first part how to create a virtual disk,

Results of this 3D simulation couldn't be exploited because of lack of time.

# Conclusion

Conclusion

From this study, we propose the ideal position to place our anemometer:

• in the vertical symmetry plane of the nacelle (by means of geometric symmetry),
• vertically, far from the nacelle's boundary layer,
• not too far from the nacelle (because the wake created by the profiled part of the blades is more turbulent than the wake created by the cylindrical part of it),
• horizontally, far from the blades that create an important wake.